HomeMy Public PortalAboutSan Francisco Area Geologic MapsTi QLE 1. GENERALIZED D ESCRIPTION OF ENGINEERING PR0PERTL' J s OF ?AP UN ITS
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N AME AND
MAP SY MBOL
LITHOLOGY
WEATHERING
SOIL DEVELOPMENT
ALTERATION
PERMEABILITY
Artificial
fill
(Oaf)
Mostly du ne sand, but in cludes silt, clay, rock wa ste,
man made debris, an d o rganic waste.
Non e.
High. except where
clayey materials pre -
dominate.
WORKA BILITY
SLOPE STABILITY
EARTHQUA KE STABILITY
Gen erally easy to remo ve except
locally where tan gles of ship
timbers and other manmade de-
bris are enco untere d.
Generally ]ow becau se most fills are un ce-
men ted and lie n ear or below the water
table.
Poor to fa ir. Most movemen t where
thick, po orly co mpac ted, and over-
lying soft bay mud an d clay. Least
shaking where thin, well compatgd,
and overlyin g firm materia ls.
Lan dslide
deposits Va riable. Ro ck pieces of all sizes in mud an d clay
(QI, Qio, Qty) matrix .
Allu viu m
(Qat)
No ne or very little.
Variable, but gen erally
high.
Silty clayey mediu m -grained san d; fine- to medium- No well -developed so ils; top 1 to
grained san d; clayey silt; some pebbles. Grades to 2 feet of older alluvium has
slope debris and ravin e fill. Older alluvium is rich in abu ndant plant fragments an d
comminu ted pla nt fragments. Bedding obscure. higher silt conten t than lo wer
portions.
Beach
deposits
(Qrb, Qob)
Well -sorted, mediu m- to co arse -grained sand. A few
srnallegra vel beaches.
Non e.
Moderate; high in clean
sa nd.
High.
Slope debris
a nd
ra vine fill
(Qsr)
U nsorted rock fragmen ts, gravel, sand, silt, and clay
in various propo rtions.
Bay mu d an d
cla y
(Qm)
Dun e san d
(Qd)
Silty, ca rbonaceous clay with v ery mino r amounts of
sand; loca lly, len ses of sand, peat, or shell fragments.
Soft near top, moderately stiff at depth. Plastic and
swellin g whe n wetted; shrinks a nd becomes hard
when dried.
U niform fine- to mediu m -gra ined san d.
G enera lly none, but a slight
weatherin g of top 2 to 4 feet
seen in older material on Marin
Pen insula.
N on e.
Slight to non e. Most grain s
coa ted with iron oxide. Minor
amounts of carbona ceo us plan t
ma tter dissemin ated locally in
top 2 to 3 feet.
Va riable, but generally
moderate to low.
Imp
erviou s, e is r,1 t o
P ex ce
pt
t
san d lenses. Gen er-
ally belo w water
table.
High. Abov e wa ter
table in most places.
Colma
formatio n
(Qc)
Fin e- to mediu m -grain ed sa nd with minor amou nts of
clay. Eve nly spaced horizon tal or nearly horizon tal
bedding or continuo us inclined beddin g. R arely
massiv e.
Moderate to slight soil dev elop-
ment. ; in pla ces soil iden tifie d
on ly by presence of organic
matter an d increase in silt and
cla y con tent.
Mo derate to high.
Found abov e and be -
low wa ter table.
Serpen tine
(sp, spm)
Mo stly sheared, s!ickensided rock con ta ining hard
sphero idal kn obs of urrsher,red serpen tine. Rarely
massiv e and tou gh, spm.
Soil generally absen t or less than
a foo t thick. Locally, weather-
ing and hydrothermal altera-
tion produ ced qua ntities of
high -swelling montmorillo nite
clay.
Low to moderate.
Coarse- to fine-grained, equigranular to diab asic igne-
Gabbro-diabase ous ro ck that occurs as segregations in serpen tine.
(gd) Weathered rock is speckled brown and ora nge,
ranges from crumbly to mode rately hard.
Sandstone
(KJ ss)
To ugh nonporous fin e- to co arse -grain ed, thick -bedded
graywacke. Altered rock is brown or oran ge and de-
creases in soundn ess with in creasing altera tion . In-
cludes thin sla ty shale an d siltston e len ses an d joint
fillings, and on A ngel Islan d a few conglo merate
beds, 10 feet thick. Semischistose on An gel Island.
Observ ed weathering depth is less
than 3 feet, D euteric or hydro -
thermal alteration is gen eral,
even in tou gh ro ck.
Low, except w here
fractured.
:w0 {MAI . - __ l -_ Pte- -• -. ,--,. .- .'rr-.yr.
Maximum depths of wea thering
observed, 60 feet; average 30
feet. Soils well developed lo -
cally, with thick B horizon of
sandy clay. Maximum soil
thickn ess observed, 13 feet.
Genera lly low; moder-
ate to high in so me
a ltered rock an d in
fractu red rock.
Shale
and
thin -bedded
sa ndstone
(KJsh)
Radiola rian
c hert
a nd sha le
(KJc)
ti it-IIM..
Greenstone
(KJg)
Sha le in terbedded with fine-grained sandston e. Beds
gen erally 2 to 5 inc hes thick; paper -thin laminations
loca lly common. Sheared rock is slaty or redu ced to
soft, ma shed materia l contain ing hard no du les.
Altered, she ared shale plastic when wet.
Maximu m depth of weathering
o bse rved, 30 feet. Well -devel-
ope d soils with thick clayey B
ho rizon.
Lo w.
Meta mo rphic
socks
(Kim)
Shea red
rocks
1 Hard, fine- to coarse -grained sc hist, gneiss, an d
granu lite.
1 Wide .ones of in ten sely shea red rocks of the } ancleca n Moderate to well -develo ped so il.
grou p; gen erally in cludes serpen tine. Predom in antly Hydro therma l altera tio n
Alternate beds of hard chert, 1 w 5 in ches thick, and
soft shale as mu ch as one-half in ch thick. Includes
massive chert with brecciated structue e. Hydro -
thermally altered chert is elcsely fractured and
• splintery.
Apha nitic to mediu m -grained v olcan ic rocks. Pre-
domin antly ba salt flows, a gglomerates, and tuffs.
Pillow la vas, loca lly interbedded with radiola ria n
chest, are commo n. Most expo sures are soft,
crumbly, a ltered rock; hard, tou gh u naltered rock
limited to excavation s an d deep cuts. Some ro ck
hydrothermally altered to roft cla y gha f swells
when wet.
Slight. Shown in some pla ces by
raveling of chert along joints. Lo w, ex cept where
H ydrothermal alteration pro - fra ctured.
Trou nced in some places.
Maximum depth of weathering
o bserved. 40 feet. Soil well de-
veloped and reddish bro wn, or
grayish o ran ge in color. H ydro -
thermal altera tio n common.
Lo w, except where
fractu red.
Modera te to slight.
(K.+u) hard rock frugrnentsin so ft, waxy, and crumbly matrix. common .
Fat nds of dry ma terial per cubic too t of o rigin al ma te ria l
2elwif:: ation used by U.3 Bure.:u or Ree kr. ;atio ' and Carpe of Engineers,
U :i. Army. So': 11 ir.ci D npt, of tb 1 ^.c. rio r f}u reau of R er .Saret;on, 1953,
Ur. irlct dui, .gas:i!ic nri!.r. , r•:r,, 2C p,; 2) Co rp.; of X'agi::�'zr• J. S. A rmy.
l`"°• Cne - `: ':1 r'K • .ti.�n - ;n , VistP1-- ,- aya R
Low, except where
fractured.
Va ria ble, but generally ea sy to
e xcav ate a nd compact.
Ea sily removed with han d or power
equipmen t. Gen erally easy to
compact, except where organic
con tent is high.
Co mpacts rapidly; co mpa ction den-
sity increased by water floodin g.
Easily exca vated. Owing to high
wa ter table, deep ex cav ations
require pu mping.
Cut slopes in landslide deposits generally
un stable. Althou gh some u ndistu rbed
natu ral slopes of landslide depo sits are
stable fo r man y yea rs, sliding ma y be
reactu ated up on slight cha nges in stress
or strength con ditio ns.
Cla yey ma teria l stan ds in steep or vertical
cuts for sev era l mon ths when dry. Su bject
to se ve re gullying. Sandy alluv iu m un -
stable in steep cu ts.
Generally unstable and free ru nnin g, espe-
cially on slopes grea ter than abou t 30° .
Susceptible to wind and ra in erosion. Ex-
cava tion walls mo re than 2 or 3 feet in
height requ ire suppo rt.
Low.
Modera te.
Probably mo dera te.
SH EARING STREN GTH
FOUN DATION CONDITION S
POSSIBLE
OR
REPORTED USE
UNIT WEIGHT!
(pounds per
cu bic foot)
UNIFIED S0IL2
CLASSIFICATIO N
GROUP SYMBOL
3enerally moderate shearing strength, but exceedingly
variable dependin g on method of pla ceme nt, age,
thickness, un derlying material, and history following
placement, such as gro un d -water condition s, lo adin g,
etc. In 1906 earthqu ake the greatest da ma ge to stru c-
tures was inflicted in areas of a rtificia l fill overlying
bay mud a nd clay along east shore of city.
Variable within wide
limits.
Va ri able withi n wide
limits.
Gen erally unsu ita ble fer foun dation s.
moderate to high in sandy alluviu m; low where deposits
are predominan tly clay and silt or high in plan t frag-
ments. To sa feguard foun dation s, clay -filled surfa ce
and subsurface cha nnels should be adequately drained.
M oderate to high shea ring strength where co nfin ed. Sus-
ceptible to wave erosion on beach.
Rockfalls possible local so urce of pervi-
ous fill, riprap, etc.
Su rface material possible sou rce of top-
so il for lawns and ga rdens.
Blending sand for concrete aggreg ate;
fill.
Variable .
105 to 110 .
100 to 140 .
Variable. GC to CH .
SM, SC, SW, OL .
SP .
Variable, but gen erally easily com-
pacted an d ex cavated with power
equipmen t.
Stan ds in steep to vertical cu ts for sev era l
months when dry. Generally un stable and
prone to slidin g when wet. Gullying severe.
H ighly compressible, difficult to
co mpact. Excav ated with power
equ ipment, such as clamshell
bucket. Sheet
piling requ ired fo r
excavatio
ns in so ft fluid mud.
Modern me thods of placing arti-
ficial fill push aside su rface lay-
ers so as to a vo id trapping un-
stable flu id mu d belo w the fill.
Compacts rapidly; compa ction
den sity increased by water flood-
ing. Easily exca va ted.
Gen erally u nstable; where abo ve water
table, has moderate stability at 1:1 cu t
slo pe for several mon ths during dry season.
Genera lly un stable and free runnin g, espe-
cially on slopes grea ter than abou t 30°.
Su sceptible to win d an d ra in erosion. Lag-
ging required to support excava tion walls
more tha n 2 or 3 feet high.
Moderate.
Variable. Deposits with relatively high clay con tent are
soft and plastic when wet. Sa ndy and gra velly deposits
underlying unsta ble clays give adequate suppo rt for
fou ndation piles.
Low. Structu res erected on a rtificia l fill
overlyin g bay mud and clay seve rely
dama ged in earthqua kes of 1866,
1868, and 1906.
Probably moderate.
.. ,ow shearin g strength. Structu res on mu d successfully
supported by piles or caissons. An older bay cla y, lying
below Co lma fo rmation , is firm and preconso lidated in
most places a nd has moderate shea rin g strength.
Moderate to high shearing strength when c onfined.
Easily compacted. Easily exca -
vated by hand or power e quip-
ment.
Fair to good, except for silt- and cla y -free
layers, which are unstable in cut slopes
greater than 30° . E . cav ated v ertical fa ce s
stand for several wee ks to severa l mo nths
when dry. Slopes of 30° to 35° common ly
sta biliz ed by protective cover of su ccu -
len t plants.
Probably modera te to high.
FilL
104 to 124.
CL, SM, SC, r arely GC
and GM.
Although very p oor quality, b ay mud
and clay have been used for fill be-
hind p art of Embarcadero sea wall.
Recent shell deposits and underlyi ng
clay and m ud dredged from bay f or
m anufacture of cement clinker .
Blended with better quality clay to
make structural clay prod ucts. May
be s uit able for m aking f oundry sa nd .
Go od q uality fill. Admixed with clay to
m ake foundry sa nd . Sm all ton nages
used as blending sa nd in co ncret e
aggregate.
43 to 98 . Older bay cl ay
underlying Colma
formatio n: great er
than 100 .
65 to 102 at surface;
110 compacted.
Voderate to high shearing strength, appro ximately 3 to 6
times grea ter than firm bay clay. Used for pile and
caisson support.
Shea red serpentine removed readily
with power equ ipment. Massive
serpen tin e ma y requ ire blasting.
G ood qu ality fill.
105 to 130.
CL, CH.
Mostl y SP; some is SC,
SM .
Massiv e serpen tine stable in steep or verti-
cal cu ts. Cu t slo pes in sheared serpentine
sho uld n ot exceed 1:1. Nodules of hard
serpen tine tend to fall o ut of sheared
matrix.
High.
hea ring strength high in massive rock but decre ases
with increasing proportion of sh eari ng and alteration.
Thoroughly she ared and altered serpenti ne has low
shearing strength . Veins of soft, altered m aterial in
hard serpentine may present special problems.
Ge nerally requires blastin g except
in shallow weathered zon e,
A lte red rock easily han dled by
po wer equipmen t; fresh to mod-
erately fresh rock remov ed with
difficulty a nd u su ally requires
blasting.
High.
Fresh or moderately fresh rock stable in ver-
tica l cu ts. B locks ma y fall from vertical
fa ces cut in jo inted sandston e. Moderately
sheared an d fra:_dured sandstone stable in
cu t slopes of 55°. Ba dly altered, sheared,
fractu red san dston e tends to slump an d
slide, especially when wet; cut slopes
shou ld be low angled an d well drained.
H igh.
High sheari ng strength.
eax u. 3'ide aY¢. �s... W - �- . •X v4+ 3'6S iL'- •"-'- .,
••�JiSL9¢iR
Nigh. Shearing stre ngth is high e xcept in badly sh attered and
altered rock.
Moderately she ared serpenti ne widely
used for fill; highly sheared a nd al-
tered rock is uns uitable. Although no
c ommercial -deposits kn ow n at present
in the quadrangle, serpenti ne is pos-
sible source of mag nesium, asbestos,
nickel, chromit e, m ercury, and jade .
Good qualityfillePossible limited sourc e
of co ncreteiggregate and l arge -size
riprap.
Fresh rock s uit able fcr good qu ality fill,
road metal, ripr ap, concrete aggre-
gate. M oderately altered rock may
be s uitable f or fill.
78 (sheared and alte red )
to 158 (massi ve) .
180 to 192.
r-'.CY horn-..- ...- ..
128 to 144.
Fresh rock moved with some diffi-
c ulty by power equ ipment; blast-
• ing required in some places.
Support required fo r excav atio n
walls and tun nels. Alte red rock
moved readily with power equ ip-
men t.
Steep cut slopes sta ble for long periods ex-
cept where extensively shea red, fractured,
and a ltered. Slidin g likely on bedding dip-
ping in same direction as cu t slo pe.
High in fresh rock. Probably mod erate Shearing strength high in fresh rock . Fo undations on
in thoroughly sheared a nd alt er ed b adly she ared. altered rock may req uire pile support.
rock .
Fresh rock a nd m uch moder ately al-
tered rock s uitable f or fill . Calci ned,
exp anded shale is so urce of light
weight aggregate. Shale used to make
commo n bricks . Possible raw m aterial
f or manufact uring cement clinker.
B edded chert • generally ca n be re-
mo ved by po wer equipment.
Massive chert may requ ire
bla sting.
Most a ltered greensto ne can be
ea sily ex cavated with power
equ ipmen t. Fresh green stone re-
quires bla stin g.
G enerally stable in steep cu ts, but sheared
an d hydrothermally altered zones may
slide. D ip slopes shou ld be cut at lower
a ngle tha n dip of beds.
Fresh an d mo derately fresh rock stable in
steep cuts, but lava pillows may fall out
cf weak matrix. Altered rock stable at 1:1
or gentler slopes.
High.
High .
R emov ed with difficu lty with po wer
equ ipment; ma y require boa ting.
Steep cut slopes are st able . Dip slopes should
be cut lower than a ngle of selristosity.
High .
Low.
Soft material easily ex cavated;
large, hard pieces may require
blasting.
Ge nerally low, especially whe n wet.
M oder ate.
High sheari ng stre ngth.
Shearing strength high in relatively fresh rock but de-
creases with incre asing alteratio n.
Chert, shale, and alter ed greenston e,
loc ally called "redrock", are used as
fill and ro ad metal. Suitability of chert
for c oncrete aggregate q uesti onable.
Moder ately altered gr eenstone with
associ ated chert and shale, kn own
locally as "redr ock", is used as fill and
road metal . Fills of badly alter ed
greenst on e are prone to slidi ng on
moderate or steep sl opes . Relati vely
fresh rock is p ossible source of c on-
crete aggreg ate and riprap.
High shearing stre ngth.
Good q uality fill, road metal, large -size
riprap, co ncrete aggregat e.
127 to 142.
113 (thoroughly al-
tered) to 185 (slightly
altered).
169 to 195 .
Matri x has low sheari ng strength . Large rock fragments
encount ered its exploratory bori ngs inay give false im-
Presion of sound fou ndatio n c onditions .
Used extensively for low q uality
78 to 110 (m atri x); 125
fill. to 170 (inclusio ns).
DIVISION Or MN.f?
OLAF P. JENKINS, CHIEF
STATE OF CALIFORNIA
DEPARTMENT OF NATURAL RESOURCES
Journal Vo1.51
No. 4 Plate 5
QUATERNARY
c
z
SYMBOL
Q01
01
Tp
Tm
Tv
=Tsl
gyrA
Tmz
qd
3 Jf
Qo l
Ku Ku
mz0
qd
4
EXPLANATION
SEDIMENTARY ROCKS
FORMATION AND ROCK TYPE
Alluvium and fill
Terrace deposits
Merced fm.
(marine sandstone, shale)
;Tsc ; J
Purisima tm.
(marine sandstone, shale, conglomerate)
Monterey fm.
(marine silicious and diatomaceous
shale, chert, sandstone)
Vaqueros fm.
(morme sandstone,
San Lorenzo fm.
(marine shale)
Butono sandstone
(marine)
Undifferentiated
(marine sondstone, shale)
Martinez fm.
(marine sandstone, shale)
Undifferentiated
(marine sandstone, shale, conglomerate)
Franciscan -Knoxville group
(dork marine sandstone, shale, chert;
minor basalt, diobase, and glaucophone
schist; Calera limestone member, Jca)
IGNEOUS ROCKS
Bosolt,andesite, diabose with limestone
inclusions
Serpentine
Franciscan -Knoxville group
(diobase, basalt)
Montara quartz diorite
(with inclusions of Paleozoic(?)
Gabilan limestone)
MINERAL PRODUCT OPERATIONS
Limestone (shells)
Magnesium salts
Mercury prospect
Solt
Quarries
Abandoned oil well
Abandoned dry hole
qd
)C36 01
Y'39
Santa Clara fm.
(non -marine gravel,
sand, clay)
47
64+t 63
t
LIST OF MINERAL DEPOSITS SHOWN ON MAP OF SAN MATEO COUNTY
Map
no.
3 3
¢ Cr
un c
Jf
\)Jd
TP
a`
•
e
0
Qa1
qd
Tv -
T.35.
ra PI
Oyster Pt
Jsp
Tv
Tp
41)-46
r}5.
4950 41,,-
54
fit 57'59
+ -
'55 - 56<'
Pur.s,mc reek
Oil a�'§o
7*66 69
T
P
'Qt
01
92
Q01
Ot
Tp
lo
Qt \'
Ku'
e
e
9C: 9,
Tp
57-5
Tp
Tm
Deposit
LIMESTONE (Shells)
1 Ideal Cement Co 11
MAGNESIUM SALTS
2 Marine Magnesia Division,
Merck & Co., Inc.
MERCURY
3 Emerald Lake
SALT
4 Leslie Terminal Co
STONE (Crushed Chert)
5 Casey
6 Middle Road
7 South Road 1
STONE (Crushed Conglom-
erate)
8 Archibald
STONE (Crushed Granite)
9 Canadas
10 El Granada
11 Miguel
Projected. ** Mt. Diablo Base and Meridian.
T. 3 S
T. 4 S.
San Mateo Pt
6
S 50 Ni c0
Tp
otf
a
63
Sec. T** R**
Map
no.
Deposit
STONE (Crushed Limestone)
-12(?) 4S 4W 12 California Aggregates
13 Hilltop 14 Marks Materials
23 3S 5W 15 Skyline Materials
STONE (Crushed Sandstone)
16 Brisbane
96 6S 4W* 17 Causeway
18 Dalys_
19 Edgewood Road
17 5S 3W 20 Golden West
21 Healy-Tibbitts
22 Holy Cross
17 45 4W* 23 San Mateo Development Co.
3 6S 4W* 24 San Mateo Development Co.
0(?) 6S 4W* 26 Southern Pacific
26 Vasquez
27 WPA
STONE (Crushed —Mint.)
9 BS 5W6 28 Canada Road (North)
29 Canada Road (South)
30 Kavanaugh
22 5S 6W 31 Macco
19 6S 6W* 32 North Road
18 5S 5W* 33 Sanitary Fill
34 South S.F. Land & Imp. Co.
36 Tyson
STONE —SANDS
(Asphalt Aggregate)
36 Duncan
37 Edgemar
38 Pacific Coast Aggregates, Inc.
39 Pilarcitos
40 Rockaway
41 State
(Foundry)
42 Brumley -Donaldson
43 Brumley -Donaldson
-94/c
OI
Tp
Tdb
Tp
Sec. T** R**
11
11
2
12
9
9
6
27
4
10
17
30
31
10
17
16
34
34
19
9
3
3
22
9
25
23
19
30
2
33
45
5S
4S
5S
3S
35
3S
55
3S
3S
3S
45
4S
9S
5S
4S
55
55
3S
45
5S
3S
3S
6S
5S
3S
3S
55
45
IS
6W*
5W*
6W*
5W*
5W*
5W*
5W*
4W*
5W*
5W*
5W*
4W*
4W*
5W*
5W*
5W*
4W*
4W*
6W*
5W*
4W*
5W
5W*
4W*
5W*
6W*
5W*
5W*
6W*
5W*
23 3S 5W*
9 4S 5W*
Tsc \,
\
Tsc
Tv
Tdb
EXPLORATORY WELLS DRILLED FOR OIL AND GAS
IN SAN MATEO COUNTY
Mo Total
MoD
T'6* R** Sec.
Name of company
and well
Date
started
Date
abon-
doned
depth
Geology
botton
44
5S
5W 33*
Sage, W. A., No. 1
1-22
pre -1925
2563
Miocene
46
6S
4W 17
Continental Oil Co., Bianchi 1
8-60
11-50
5850
Eocene
46
6S
6W 3*
Sage & Olsen
pre -1925
930
Miocene
47
6S
6W 8
(Shell Oil Co.) Cowell 1,
Thompson & McNichles __
2-27
1928
5905
Miocene
48
6S
5W 8
(Shell Oil Co.) Cowell 2,
Thompson & McNichles
4-28
1928
3689
Miocene
49
6S
5W 10*
McClintock, H. H., No. 1'
4-24
pre -1925
517
Montere
50
6S
5W 10*
Midstate Oil Co., No. 1
51
6S
5W 10
Poso-Moon Oil Co., No. 1
9-22
pre -1925
830
Miocene
52
6S
5W 11
Standard Oil Co., Std. -Rich.-
Cafferata-Pimental No. 1
8-52
10-52
5658
Eocene
53
6S
5W 15
Elk Hills Pool Oil Co., No. 1
3-23
1935
1114?
Miocene
64
6S
6W 15
Sage, Olsen & Blalock, No. 1
8-22
pre -1925
2100
Miocene
65
6S
5W 15
Skyline Oil & Refining Corp.,No. 1
4-28
56
6S
5W 15*
Willard, E. T., 2
9-46
1948
2422
Pliocene
57
65
5W 15*
Willard, E. T., 3
10-46
1948
2001
Purisim,
58
6S
5W 15
Willard, E. T., 1
10-46
1946
1896
Purisim,
59
6S
5W 15*
Willard, E. T., Sarah Wilson 4
9-48
1948
1610
60
6S
5W 16*
Berger & Caglieri, No. 1
11-21
pre -1925
1930
61
6S
5W 16
(Julian, C. C.) No. 4,
Thompson & McNichles
2-25
1925-26
1800
Miocene
62
6S
5W 16
(Stratton Pet. Corp.) No. 1,
Thompson & McNichles _ _
8-29
285
63
6S
5W 17
(A. & C. Oil Co.) No. 1,
Greater Santa Cruz Oil Co.
3-25
1927
2510
Miocene
64
6S
6W 17
Wilshire Oil Co., Cowell 3___
6-40
1940
1954
Pliocene
65
6S
5W 20*
Elk Hills Pool 011 Co., No. 1
pre -1926
66
6S
5W 21
Wilshire Oil Co., Inc., Cowell
1
8-37
1944
7982
Miocene
67
6S
5W 21*
(Shell Oil Co.) Butts 1,
Thompson & McNichles
1-28
1928
3369
68
6S
5W 21*
Thompson & McNickles,
Butts 2
69
6S
5W 22
Berger & Caglieri, No. 2
7-22
pre -1925
1765
70
6S
6W 22
Wilshire Oil Co., Inc., Cowell
2
9-38
1944
1604
Pliocene
71
7S
4W 7
Owsley, J. N., No. 1 -
pre -1925
625
Miocen,
72
7S
4W 7
Owsley, J. N., No. 2
pre -1925
275
Miocen,
73
7S
4W 7
Perkins well
1100
74
7S
4W 12
Richfield Oil Corp., Isenberg
1
9-51
10-51
4699
75
7S
4W 16
(La Honda Oilfields Assoc.)
No. 1 La Honda Oil & Gas Co.
4-21
pre -1934
3750?
Miocen,
at
y
?
6n
(i o0 kr
``rye
ti R° j,,, • I''
Rovenwoad Pt ��,po �,�
qo
0
eP
la II
'PALO ALTO
P60' \
Sand Pt
Zap
10.
K* # Sec
T R.
Name of company
and well
Date
started
ban Date_
done
cloned
Total
p de th
Geology ,
bottom
76
TS
4W 18
Elk Hills Pool Oil
Co., No. 1
6-25
pre -1925
77
7S
4W 18*
(La Honda Oilfields
Assoc.) No. 3 La
Honda Oil & Gas
Co. _
6-26
786
78
7S
4W 200
Elk Hills Pool Oil
Co., No. 2
pre -1925
436
Miocene
79
7S
4W 21
McKinney & Ellis
well
pre -1925
288
Miocene?
80
7S
4W 21
Northern Explor.
Co., No. 1
1-23
pre -1926
340
Miocene
81
7S
4W 21
Old Bell Well
1905?
1300
82
7S
4W 5*
E. B. Ralston Ranch,
Steele Broe. 1____
3-26
1926
980
83
7S
4W 26
Jergins Oil Co.,
YMCA 1 _
6-50
6-60
2510
84
7S
4W 26
Jergins Oil Co.,
YMCA 2
6-52
7-52
4735
Eocene
85
7S
5W 10
Texas Co., Ross 1
5-51
7-51
7111
86
75
5W 19
Bell and Burden,
Inc., Riskin 1_ _ _ _
7-51
9-51
4500
Gas and <
shows
87
7S
5W 13*
Sequoia Oil & Gas
3740'-395
Co., Souza 1
7-27
88
7S
5W 13*
Old O'Brien
89
7S
6W 13
Richfield Oil Corp.,
Lena Souza 1
7-44
1944
5211
Miocene?
90
7S
5W 15*
San Mateo Pet. Co._
_ _ _ _
pre -1925
480
Miocene
91
7S
5W 15*
Unity Oil Co., No. 1_
8-23
1925-26
600
Miocene?
92
7S
5W 21
Walker, Fred C. &
Albert Monyier,
Walker, Monyier,
Caughey 1
10-46
1947
3338
Pliocene
93
7S
5W 34
Texas Co., Mattei
Est. 1
6-51
8-51
5813
94
7S
5W 36
Texas Co., Steele 1_
9-61
1-52
8025
Miocene
95
8S
3W 10
Big Basin Paraffin
Oil Co.. No. 1-A _
pre -1925
360
Miocene
96
8S
3W 10
Royer, Frank W.,
No. 1
5-23
1935
1000
Miocene)
97
8S
4W 4
Texas Co., Blom-
quist 1
7-50
9-50
4957
Eocene
sand-
stone anc
98
9S
4W 19
Smuggler Divide
siltstone
Mining Co., No. 1
9-27
1930
900
Tv
Tdb
Tv
N
Plotted in center of section.
Tm
GEOLOGIC MAP
OF
*r Ml. Diablo Base and Meridian.
SAN MATEO COUNTY
CALIFORNIA
SHOWING LOCATIONS OF MINERAL
PRODUCT OPERATIONS
1955
2
5
it
Tm Tp
-
Tp
Scale in Miles
DIVISION OF MINES
IAN CAMPBELL, CHIEF
GORDON B. OAKESHOTT, DEPUTY CHIEF
STATE OF CALIFORNIA
DEPARTMENT OF NATURAL RESOURCES
GEOLOGIC MAP OF CALIFORNIA
SAN FRANCISCO SHEET
EXPLANATION
121 00
8 00
i
45
30'
7'00' —
124'00
45
30'
T
DEL1L BANK.
LF.
ri •� 9r
4-320 ft.)
rid
thnI!
seed.
mud'
shells
• - Mm
\ (-900 ft.)
i
(-2400
/
6
r
9000 .
TOPOGRAPHIC BASE MAP
Prepared by the Army Map Service (GUSX), Corps of Engineers, U. S. Army,
Washington, D. C. Compiled in 1956 from: United States Quadrangles,
1.24,000, U. 5. Geological Survey, 1947.50; California, 1:25,000, Army Map
Service, Sheets 1959 IV SE and 1959 IV SW, 1951; California, 1:50,000,
Army Map Service, 1946-51; USC&GS Charts 5402, 1941; 5502. 1947;
5533, 1953, 5532, 1947; 5520, 1935; 5531, 1950, and 5599, 1941,
Pla ni metric detail revised by photo -pia nimetnc methods. Horizontal and
vertical control by USGS and CE. Map field checked, 1956.
Land net prepared by U.S. Geological Survey
Submarine contours adapted from Shepard and Emery Special Paper No. 31,
Geol. Soc. America
m
:.500 --_
sand
15'
saud
--- 60n0 —
/Mm
• (-5700 ft.)
I
dUIDE SEAMOUNT
rand
Prohibited area
rind
POINT REYES 7 MI
123 OOYMu ar
Poem Reyez .'U 5 Coast Guard Bode
gel 9r Mu
mud
mud
sand
,11746.
71
qr
RH<<O/V a �/
'1'9T/ e North Farallon
ti11
IZTln.
'd
scud
shells
Muddle
Or Farallon
mr lv
3000,~
sand
Southeast Farallon
p
Ughl
O C
1
/ r
shells
i
sard
s1e16
43110
mud
.and
sand
amt
sand
0
sand
15'
123 00
Double Point
a
sand
Help
K
9olinas Poin
sand
45.
sand
1
R
lOokas
Kelp lrnux .Here H
B Y POIA
mud
sand
mua
sand
Lightship
,and
mud
N7cky Point - T '
• Gull Rock
•
Muir Beach
•
Tennessee Print
GUl F FT CRONINN
OF VIE Rofeo Laylxln
g rd l la d ,I 5lde: "a•
FARALL'QNES Po nr ao r cote Bud
Ligh 44)
• holden LP
shells gravel
Sane ''`.).'
Sane - MIL RE
•
FORT MILE/_
Point Lobos
-" Seal Rocks
• Golden Gat
Par
SAN FRANcISC
nu d
mid
sand
sand
rusel
FORT'
FUN$TON.
• TLS 1
•S iG
%Gm
Pu
Mussel Rock
>harpP
po
KJivN
Rockaway 4a
r
Shelter Crlwv .l
Paint San Pedro.
Ku"
De 9r:
Slide
Mantas Po M
Pml
H' It INn. I
Palm Pon
m
/ 9 4WPv
m( . q
Light Po,nd jn Pablo
Richmond San Rat... -
Quenbrs KJiv erry„
yll\C �!♦ `may `
a mua a S 0
r9>� f rnie Z 9ti y�' 1 °u `•
P 'nt and cnl 'rq'Pad,
tl 4s r> b 9 OS a ok
eh . a 7lA% 41aal C 4c * L�:K.Jt
° * cba of mud 0U O K, Getden.Gale
On nodo (TNW: Ge pb911whit 'n 1 Feld
Se s g o�•YA Ight of lf��, ace
Cfi� ; r a m mua 3`
AnH l �ultl 3 m 1pa , oc
I la d - w'
_
and � light ', N ° Emeryville NDe j
tens . itn NA LSTATIiG hot k ed1a,A%3f 6111 o 0•- „•y.
N
g crave N.Ight N NS O o r,g pd Yu %ml/
aCri etl MASO' 11
er B acolfrFPd .G
Vlol
U'Yei
•A
• ,d
ruipi
Redondo $aarh
Manhattan P AU
30'
Poi
0
0cl
MI
Om
a 3
ALAMEdA\5—'
NAVAL R L 1S -
u 1��
SC'tn ss Basin -}_
Misslml iin ,_ % '
liyk Polre.o Point �'
tor, ,'A7AMEDA
I• 'KJI ni 944 l'Seaplane ,gall lxa
?SF � Ianding
P 3, n' area
., AN FRANCISCO
15'
NA AL SHIPYARD
KJSAN MATED COUNTY
mud ,, mud
V,snafian Point
1r sbmle
Sierra Point
e Mgir Point mud SAN PRA
hells
lP6int San Bruno shell'
ouch San Francisco -
U S Coast Guard Au Slabon
Frankton P,.
Mr.
M11'n
Aro Nuevo Ininct
Ado Plops, Llond ) Ilaah''rn
lip, lu1,1
Rnp
SUBMARINE CONTOUR INTERVAL 300 FEET
Local depressions
Limit of danger, Reef
Racks Awash, Sunken
Foreshore flat
Intermittent or dry stream _
Marsh or swamp
1500
Mu"+1'T(�
Tank
mud Mulford GI
Mulford-�landm
Forbidden
'anchorageSCO
Forbldderr
anchorSAage i,Npbe
shell, Lapoing
l\ Y TORE
shells \
N. shells
shells
San Mateo
Bridge Boll)
Guano Island
Hayward Landm
y mud
Johnson Landing
mua
shells
Mr2 Mu\m re
c2)
CC mm�� O\utlM1ells--.
t 4 rib?
'eAII ',Itry
l�v /� `Red toil Po nf\
Ir+ nd R eels pPod\\
0,1
e,eyhuond Roc
,rind
El Jams Point
mud
\\ R i W Mm 9r IM
SANTA CRUZ 70 MI.
01
3 3
122'00'
8' 00'
T 2 N.
\Om
R. 2
TIN
TIS
Oal
0
T25
Pmlc
45'
-Mu
W
35
—KI
P
—Mu
Mm
K
T45
F
L
0
ANTA CRUZ 2 MI
30'
T55
t75
15' N
0
0
8
_0
1 95
U
0
Nf
0
z
W
0
MESOZOIC
U
0
w
0
5
z
Pmt' 0
° N
0t
o 1
T.10 5. d
LOCATION MAP FOR SAN FRANCISCO SHEET
.5515
• RENO
1 NEVADA
18'
Sr7FRANCISC
122° 2
SACRAMENTO LAKE•
CALIFORNIA
•
•SAN JOSE
SAN LUIS.
BAKERSFItIX
5
0
Scale 1:250,000
5 10
15
20 Statute Miles
5
5
10 15 20 25 30 Kilometers
CONTOUR INTERVAL 200 FEET
TRANSVERSE MERCATOR PROJECTION
GEOLOGIC MAP OF CALIFORNIA
OLAF P. JENKINS EDITION,
SAN FRANCISCO SHEET
COMPILATION BY CHARLES W. JENNINGS AND JOHN L. BURNETT 1961
Contact
(Dashed where approximately locatec!,
gradational or inferred)
Fault
(Dashed where approximately locatee;
dotted where concealed)
WILLIAMS HEINTZ MAP CORPORATION WASHINGTON 27, D. C.
INDEX TO GEOLOGI
(COMPLETE INDEX ON EXPLAN
1. Aarons, Bernard L., unpublished. 17.
3. Brabb, Sad E., unpubrished.
Dibblee, Thomas W., Jr., unpublished. 20.
5. Burchfield, B.C., unpublished. 21.
7, Clessen, James S., unpublished. 23.
8. CommingS, Jon C., unpublished. 20.
10. Oibblee, Thomas W., Jr., unpublished.
11. Coumani, G. I . , unpublished. 26.
Mack, John E., unpublished. 28.
13, Galloway, Alan J., unpublished.
14. Glenn, William. 1959. SO.
C MAPPING
ATORY DATA SHEET)
Hanna, G. Dallas, 1951.
Leo, G. W., unpublished.
Schlocker, J., Bonilla, M.G., and Radbruch,
Sheehan, J. R., unpublished.
Touring, R. M.. unpublished,
29. Union Oil Company, unpublished.
Untermenn, 8.R., unpublished.
QUATERNARY
1,
TERTIARY
0
CRETACEOUS
JURASSIC
2 rt
0
0
co
0
ce
cz
rc
r
SEDIMENTARY AND METASEDIMENTARY ROCKS IGNEOUS AND META -IGNEOUS ROCKS
Dune sand
Alluvium
Os
Qsc
Of
Ost
01
a9
Or
0m
QP
Stream channel
deposits
Fan deposits
Basin deposits
GREAT VALLE
Salt deposits
Quaternary lake deposits
Glacial deposits
Quaternary nonmarine
terrace deposits
Pleistocene marine and
marine terrace deposits
Pleistocene nonmarine
Plio-Pleistocene nonmarine
LP'H Undivided Pliocene nonmarine
Puc
Pu
Pmlc
Pod
Mc
�Muc
ICE
Wes
11111111
me
L
0
E
Epo
EP
Upper Pliocene nonmarine
Upper Pliocene marine
Middle and/or lower Pliocene
nonmarine
Tc
Middle and/or lower Pliocene marine
Undivided Miocene nonmarine
Upper Miocene nonmarine
Upper Miocene marine
Middle Miocene nonmarine
Middle Miocene marine
Lower Miocene marine
Oligocene nonmarine
Oligocene marine
Eocene nonmarine
Eocene marine
Paleocene nonmarine
Paleocene marine
Cenozoic nonmarine
Tertiary nonmarine
It __ Tertiary lake deposits
Ku
Ju
Tertiary marine
Undivided Cretaceous marine
Upper Cretaceous
marine
Lower Cretaceous
marine
Knoxville formation
Upper Jurassic
marine
Middle and/or Lower
Jurassic marine
Triassic marine
o
Lein
gr
bi
Pre -Cretaceous metamorphic m.
rocks (Is = limestone or dolomite) _
rocks
Paleozoic marine
(ls - limestone or dolomite
Permian marine
Undivided Carboniferous marine
Pennsylvanian marine
PM. Mississippian marine
Devonian marine
Silurian marine
Ordovician marine
Cambrian marine
Cambrian -Precambrian marine
Leg
roe
Undivided Precambrian
metamorphic rocks
Later Precambrian sedimentary
and metamorphic rocks
Earlier Precambrian metamorphic
rocks
Resent volcanic: (WV -rhyolite;
Pleistocene volcanic: opt -rhyolite:
Quaternary and/or Pliocene
cinder cones
Pliocene volcanic: Invr -rhyolite;
Pv0—andesite; P.4 -basalt;
Miocene volcanic: m.' -rhyolite;
m.5-pyroelastic rocks
$sP-pyroclastic rocks
Eocene volcanic: Es' -rhyolite;
E.° -pyroclastic rocks
Cenozoic volcanic: OTer-rhyolite;
Tertiary intrusive (hypabyssal)
- rib -basalt
Tertiary volcanic: Tvr -rhyolite;
Franciscan volcanic and
metavolcanie rocks
Mesozoic granitic rocks
Mesozoic baisie intrusive
rocks
Mesozoic ultrabasic
intrusive rocks
rocks
Pre -Cenozoic granitic and
metamorphic rocks
wvj Paleozoic metavolcanic rocks
Permian rnetavolcanic rocks
Low J Carboniferous metavolcanic rocks
Devonian metavoleanic rocks
Undivided pre -Devonian and
Devonian metavolcanic rocks
Undivided Precambrian
granitic rocks
HEAVY BORDER ON PDXES INDICATES UNITS THA T APPEAR ON THIS SHEET
STRATIGRAPHIC NOMENCLATURE—
SAN FRANCISCO SHEET
AGE
STATE
MAP
SYMBOL
STATE MAP UNIT
STRATIGRAPHIC UNITS AND CHARACTERISTIC LITHOLOGIES
(The formally named formations grouped within an individual State Map Unit,
are listed in stratigraphic sequence from youngest to oldest.)
CENOZOIC
TERTIARY QUATERNARY
r
Paleocene Eocene Oligocene Miocene Pliocene Pleistocene Recent
Qs
RECENT DUNE SAND
Dune sand, beach deposits.
Qal
RECENT ALLUVIUM
Alluvium; artificial fill; bay mud; salt marsh deposits; Temescal formation (includes Lawson's San Antonio formation) —alluvia!
fan deposits (largely Pleistocene).
Qt
QUATERNARY NONMARINE TERRACE DEPOSITS
Stream terrace deposits.
Qm
PLEISTOCENE MARINE DEPOSITS AND
MARINE TERRACE DEPOSITS
Colma formation —sand and clay (San Francisco Peninsula). Merritt sand —clayey, silty sand (East Bay area). Marine deposi-
tional and wave -cut terraces (non -marine deposits in part).
Qc
PLEISTOCENE NONMARINE SEDIMENTARY
DEPOSITS
Alameda formation —clay, sand and gravel (in part marine); Campus formation —clay, limestone, conglomerate, tuff, agglom-
erate, andesite, basalt flaws.
QP
PLIOCENE -PLEISTOCENE NONMARINE
SEDIMENTARY DEPOSITS
Santa Clara formation —gravel, sand, and clay.
Pu
UPPER PLIOCENE MARINE SEDIMENTARY
ROCKS
Merced formation1—sand, silt, and clay.
Pmlc
MIDDLE AND/OR LOWER PLIOCENE
NONMARINE SEDIMENTARY ROCKS
Mulholland formation —shale, siltstone, sandstone; Siesta formation —conglomerate, sandstone, mudstone, conglomerates of the
Moraga formation (bulk of formation is volcanic —see Pv); Orinda formation —conglomerate, sandstone, siltstone (in part
upper Miocene marine).
Pml
MIDDLE AND/OR LOWER PLIOCENE MARINE
SEDIMENTARY ROCKS
Purisima formation—congWneerate, sandstone and shale (in part upper Miocene).
P V
P vr
P V b
P V p
PLIOCENE VOLCANIC ROCKS:
UNDIFFERENTIATED
RHYOLITIC
BASALTIC
PYROCLASTIC
Moraga formation—andesite and basalt flows, rbyolite tug.
Leona rbyolite—pyritic rbyolite flows, domes, and dikes (may be lower or middle Pleistocene, Robinson, 1953). Northbrae
rbyolite—flows of rbyolite.
Bald Peak basalt —flows of basalt.
Pinole tuff —pumiceous stratified tuff '(some interbedded basalt in Las Trampas Ridge area).
Mu
UPPER MIOCENE MARINE SEDIMENTARY
ROCKS
San Pablo group (includes Neroly, Cierbo and Briones formations) —sandstone, shale, and conglomerate; Santa Margarita forma -
tion—white sandstone, shale (in part lower Pliocene).
Mm
MIDDLE MIOCENE MARINE SEDIMENTARY
ROCKS
Monterey group (includes Rodeo shale, Hambre sandstone, Tice shale, Oursan sandstone, Claremont shale, Sobrante sandstone),
Monterey formation undifferentiated in Santa Cruz Mountains.'
MI
LOWER MIOCENE MARINE SEDIMENTARY
ROCKS
Sandholdt formation—siltstone; Vaqueros formation —sandstone and siltstone.
M V b
MIOCENE VOLCANIC ROCKS:
BASALTIC
Basalt lows, flow breccias, pillow lavas; some tuffs.
OLIGOCENE MARINE SEDIMENTARY ROCKS
Kirker formation—tuffaceous sandstone; San Ramon formation—tnflaceous sandstone and shale; San Lorenzo formation —shale
and mudstone (Eocene in part).
E
EOCENE MARINE SEDIMENTARY ROCKS
Butane sandstone —sandstone and shale (Santa Cruz Mtns.). Markley formation —sandstone and shale (Diablo Range). "Tejon"
formation —sandstone, shale, conglomerate. Unnamed Eocene units (sandstone, shale, conglomerate).
Ep
PALEOCENE MARINE SEDIMENTARY ROCKS
Martinez formation of Lawson' —sandstone and shale (Eocene in part). Unnamed Paleocene unit (siltstone, sandstone, conglom-
erates) in Santa Cruz Mtns.
Tv
TERTIARY VOLCANIC ROCKS:
UNDIFFERENTIATED
Rhyolite, andesite, and basalt (offshore).
Ti b
TERTIARY INTRUSIVE (HYPABYSSAL)
ROCKS:
BASALTIC
Intrusive diabase and basalt sills (middle Miocene age). Includes some lava flows locally.
STRATIGRAPHIC NOMENCLATURE— Continued
AGE
STATE
MAP
SYMBOL
STATE MAP UNIT
STRATIGRAPHIC UNITS AND CHARACTERISTIC LITHOLOGIES
(The formally named formations grouped within an individual State Map Unit,
are listed in stratigraphic sequence from youngest to oldest.)
MESOZOIC
r
UNDIVIDED JURASSIC CRETACEOUS
K
UNDIVIDED CRETACEOUS MARINE
SEDIMENTARY ROCKS
Undifferentiated Cretaceous sedimentary rocks including some "Chico" sandstone, shale and conglomerate (San Leandro Hills).
Ku
UPPER CRETACEOUS MARINE SEDIMENTARY
ROCKS
"Chico" formation' —sandstone and shale. Pigeon Point formation —sandstone, siltstone, and conglomerate. Unnamed Upper
Cretaceous sandstone, shale, and conglomerate.
KI
LOWER CRETACEOUS MARINE SEDIMENTARY
ROCKS
Oakland conglomerate —coarse conglomerate and sandstone with some shale.
KJf
FRANCISCAN FORMATION
Franciscan formation —sandstone (graywacke), shale. chert, and conglomerate. Minor bodies of limestone, metamorphic rocks,
glaucophane schist, silica -carbonate rock, and greenstone.
KJfv
FRANCISCAN VOLCANIC AND METAVOLCANIC
ROCKS
Franciscan greenstone, basalt, diabase, and pyroclastic rocks.
gr
MESOZOIC GRANITIC ROCKS
Quartz diorite and granodiorite.
bi
MESOZOIC BASIC INTRUSIVE ROCKS
Gabbro (Santa Cruz Mountains).
ub
MESOZOIC ULTRABASIC INTRUSIVE ROCKS
Serpentine, peridotite, and pyroxenite (some gabbro, diabase and small areas of silica -carbonate rock included). Sheared Fran-
ciscan rocks including serpentine in San Francisco area.
Jk
KNOXVILLE FORMATION
Knoxville formation —shale, some sandstone, conglomerate.
m
Is
PRE -CRETACEOUS METAMORPHIC ROCKS,
UNDIFFERENTIATED, 1s = LIMESTONE AND/OR
DOLOMITE
Sur series —gneiss.
1.= Gabilan limestone and dolomite; marble and calcium silicate hornfels undifferentiated.
ms
PRE -CRETACEOUS METASEDIMENTARY ROCKS
Sur series —schist.
NOTES
1. Ranges from middle Pliocene through early Pleistocene (Glenn, 1959).
2. In the Santa Cruz basin west of Ben Lomond Mountain, the area shown as Mm is considered by some geologists to be lower Purisima (upper Miocene).
3. Martinez formation of Lawson is now known to range in age from Paleocene to middle Eocene. The Paleocene portion is called Vine Hill sandstone (Weaver, C. E., 1953, Eocene and Paleocene
deposits at Martinez, California: Univ. Wash. Pub. in Geol., vol. 7, pp 1-102).
4. The Chico mapped east of the San Andreas fault by Branner et al. (1909) is now considered to be Eocene, (Graham and Classen, 1955).
y rk .. . .'8.
4 rn •
pig3, N•
+YM f
44'�
!
+t t y •
5 1t
tt n g r s 3• I j 1 i 'dI &§ - R�
1 dik; i �Vy' sR �aF :'J s �r 4t } 'et3t,
}}�r
1,,�@jg dro ° ty(�,p • �• t..y,, jY, � T Ytra'.," a
l!'} V1. § Yy 5 L 9q !d A 1 � Md Y3" '•'
f 4s py t"1 .r l f=p5it, s
•
l r r r�r- �3-�• ,p sue,
ls \ v
Steeply dipping Cretaceous(?) strata at Point San Pedro, San Mateo County. Rocks consist of alternating black shale and buff
sandstone which display in detail many sedimentary structures formed by turbidity currents. Photo by C. W. Jennings and R. G.
Strand.
DIVISION OF MINES
Ian Campbell, Chief
STATE OF CALIFORNIA
Edmund G. Brown, Governor
DEPARTMENT OF NATURAL RESOURCES
DeWitt Nelson, Director
124°OO'
38°00'
37°00
124°00'
TOPOGRAPHIC QUADRANGLES
WITHIN THE SAN FRANCISCO SHEET
AVAILABLE FROM THE U.S. GEOLOGICAL SURVEY
1961
123°00'
123.00'
MT
TAMALPAIS
Q �P
0
%Cji
q-+
co
SAN`
FRANCISCO
lO +O
O Coco OP P5
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122°00'
.j�0 ca. 02 �J.F
14,
J �V
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v �q00
PO
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ST
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co
AT EC)
P
5 X000 P
toP � TP
HAYWARD
42- Q0�20�P
+
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0`
HALF MOON
BAY Osrco 0
PALO ALTO
co
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04
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—ANO NUEVOoBEN LOMOND—
is
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Aerial view of the Marin Peninsula northwestward toward the San Andreas fault and Point Reyes. The dissected hills in the
foreground, which culminate in Mt. Tamalpais in the right middleground, are composed of graywacke, shale, radiolarian chert,
greenstone, and serpentine of the Jurassic and Cretaceous Franciscan formation. The pronounced trench of the San Andreas fault
separates the Franciscan formation from Mesozoic granitic rocks and Tertiary sedimentary rocks on the Point Reyes Peninsula.
Photo by Aero Photographers, Sausalito.
38°00'
37°00'
122°00'
EXPLANATORY DATA
SAN FRANCISCO SHEET
GEOLOGIC MAP OF CALIFORNIA
OLAF P. JENKINS EDITION
Compiled by Charles W. Jennings, and John L. Burnett, 1961
INDEX TO GEOLOGIC MAPPING
USED IN THE COMPILATION OF THE
SAN FRANCISCO SHEET
123°00'
Point
Reyes
cs
13 31
North Farallon
Middle
Farallon a
18
a South
" Farallon
BOLINAS0
122°00'
38°00'
SAN RAFAEL
13 19 °J
MILL
VALLEY°
SAUSALITO 0
Point Bonita
RICHMOND
O
25
/--SA
TFRA°NCISCO
Shelter
Cove
14
Pillar Point
HALF MOON BAY°
\7
1—\\O
CONCORD
TIIWALNUT
�COREEK
OORINDA
ERKELEYI
2530AKLAND
0
OSAN BRUNO
0 SAN'MATEO
f28
Pigeon Point
Ano Nuevo Point
17
HAYWARD
°24
ORC
CITY
OPALO
L ALTO
I5
3
BIG
OBASIN
DAVENPORT
MOUNTAIN
VIEW
4
20 10
123°00'
1. Aarons, Bernard L., Geology of a portion of the Las
Trampas Ridge and Hayward quadrangles, California,
scale 1:24,000, University of California, Berkeley, un-
published M.S. thesis, 1958.
2. Bonilla, M. G., Geology of the San Francisco South quad-
rangle, California, scale 1:24,000, U. S. Geological Sur-
vey, report in preparation (1959).
3. Brabb, Earl E., Geology of the Big Basin area, Santa Cruz
Mountains, California, scale 1:24,000, Stanford Univer-
sity, unpublished PhD. thesis, 1960.
4. Branner, J. C., Newsom, J. F., and Arnold, R., 1909, De-
scription of the Santa Cruz quadrangle, California: U. 5.
Geological Survey Geol. Atlas, Santa Cruz Folio (no.
163), 11 pp., map scale 1:125,000. (Faults in southern
San Francisco Bay area from S. G. Taylor, Gravity
investigations of the southern San Francisco Bay area,
Stanford University, unpublished Ph.D. thesis, 1956).
Dibblee, Thomas W., Jr., Geologic map of a portion of
the Palo Alto quadrangle (northeast of San Andreas
fault), scale 1:62,500, unpublished (1959-60).
5. Burchfiel, B. C., Geology of the Two Bar Creek area,
Boulder Creek, California, scale 1:12,000, Stanford Uni-
versity, unpublished M.S. thesis, 1958.
6. Clark, C. W., 1917, The geology and ore deposits of the
Leona rhyolite: California Univ. Dept. Geol. Sci. Bull.,
vol. 10, pp. 361-382, pl. 27: Geologic map of Leona
rhyolite and adjacent formations, scale 1:62,500. (Modi-
fications by D. H. Radbruch, personal communication
1961, and J. E. Case, Geology of parts of the Berkeley
and San Leandro Hills, scale 1:24,000, University of
California, Berkeley, Ph.D. thesis in progress, 1961.)
7. Classen, James S., Geology of a portion of the Half Moon
Bay quadrangle, San Mateo County, scale 1 inch equals
about 1750 feet, Stanford University, unpublished M.S.
thesis, 1960.
8. Cummings, Jon C., Geology of the Langley Hill -Water-
man Gap area, Santa Cruz Mountains, California, scale
1:24,000, Stanford University, unpublished Ph.D. thesis,
1960.
9. Darrow, R. L., Geology of the northwest part of the
Montara Mountain' quadrangle, scale 1:24,000, Univer-
sity of California, Berkeley, unpublished M.A. thesis,
1951.
10. Dibblee, Thomas W., Jr., Geologic map of the Ben
Lomond quadrangle, California, scale 1:62,500, unpub-
lished (1947-49).
1 1. Doumani, G. I., 'Stratigraphy of the San Pablo group,
Contra Costa County, scale L24,000, University of Cali-
fornia, Berkeley, unpublished M.A. thesis, 1957.
12. Esser, R. W., The reconnaissance geology of a part of
the Woodside quadrangle, northeast of Skyline Boule-
vard, San Mateo County,. California, scale 1:24,000, Stan-
ford University, unpublished M.S. thesis, 1958.
Mack, John E., Reconnaissance geology of part of the
Woodside quadrangle, San Mateo County, scale 1:24,-
000, Stanford University, unpublished M.S. thesis, 1959.
13. Galloway, Alan J., Geologic map of part of the Drakes
Bay and Bolinas quadrangles, scale 1:24,000, unpublished
work in progress (1959-60), California Academy of
Sciences.
14. Glenn, William, 1959, Pliocene and lower Pleistocene of
the western part of the San Francisco Peninsula: Cali-
fornia University, Dept. Geol. Sci. Bull., vol. 36, no. 2,
pp. 147-198, fig. 3: Geologic map of the Pillar Point
Merced" formation area, scale 1 inch equals approxi-
mately '/ mile.
15. Graham, J. J., and Classen, W. J., 1955, A lower Eocene
foraminiferal faunule from the Woodside area, San
Mateo County, California: Contrib. from Cushman
Foundation for Foraminiferal Research, vol. VI, pt. 1,
38 pp., fig. 1: Sketch map of portion of Woodside area,
scale 1 inch equals about 1875 feet.
16. Hall, C. A., Brooks, S. A., and Jones, D. L., Geology of
the Pescadero-Ano Nuevo arca, western Santa Cruz
Mountains, San Mateo and Santa Cruz Counties, Cali-
fornia, scale 1:31,250, Stanford University, unpublished
report in Division of Mines file, 1953 (modified in part
by Union Oil Company maps, see no. 29).
Hall, C. A., Jones, D. L., and Brooks, S. A., 1959, Pigeon
Point formation of Late Cretaceous age, San Mateo
County, California: Am. Assoc. Petroleum Geologists
Bull., vol. 43, no. 12, pp. 2855-2865, fig. 2; Geologic
map of the Pigeon Point area, scale 1 inch equals about
2'44 miles.
17. Ham, C. K., 1952, Geology of Las Trampas Ridge, Berke-
ley Hills, California: California Div. Mines Special Rept.
22, 26 pp., pl. 1: scale 1:31,680.
18. Hanna, G. Dallas, 1951, Geology of the Farallon Islands:
California Div. Mines Bull. 154, pp. 301-309. (Identifica-
tion of offshore rocks by G. D. Hanna and C. W.
Chesterman, personal communication; Hanna, G. D.,
1952, Geology of the continental slope off central Cali-
fornia: Calif. Acad. Sci. Proc., Fourth Series, vol. 27,
no. 9, pp. 325-358; and Chesterman, C. W., 1952, De-
scriptive petrography of rocks dredged off the coast of
central California: Calif. Acad. Sci. Proc., Fourth Series,
vol. 27, no. 10, pp. 359-374).
19. Lawson, A. C., 1914, Description of the San Francisco
district; Tamalpais, San Francisco, Concord, San Mateo,
and Hayward quadrangles: U. S. Geological Survey
Geol. Atlas San Francisco Folio (no. 193), 24 pp., map
scale 1:62,500. (San Pablo fault from C. F. Tolman,
1931, Geology of upper S. F. Bay region with special
reference to a salt water barrier: California Div. Water
Resources Bull. 28, Appendix D, pp. 309-359, pl. D-9;
scale 1:80,000 . .Geology east of Decoro, on Hayward
15' quadrangle, mapped by C. J. Kundert, California
Division Mines, unpublished, 1955 . Faults in southern
San Francisco Bay area from S. G. Taylor, Gravity
investigations of the southern San Francisco Bay area,
Stanford University, unpublished Ph.D. thesis, 1956
Modifications on Tamalpais and San Mateo 15' quad-
rangle areas by J. Schlocker, written communication
1960. Northern extent of Hayward fault from J. E.
37°00'
122°00'
Case, Geology of parts of the Berkeley and San Leandro
Hills, scale 1:24,000, University of California, Berkeley,
Ph.D. thesis in progress, 1961).
20. Leo, G. W., Geology of the igneous and metamorphic
rocks of Ben Lomond Mountain, Santa Cruz County,
California, scale 1:24,000, Stanford University, Ph.D.
thesis in preparation (1959).
21. Lutz, G. C., 1951, The Sobrante sandstone: California
Univ. Dept. Geol. Sci. Bull., vol. 28, pp. 367-406, fig. 3,
scale 1:43,000.
22. Pease, M. H., Jr., Geology of the Sobrante anticline and
vicinity, Briones quadrangle, Contra Costa County, Cali-
fornia, scale 1:24,000, University of California, Berkeley,
unpublished M.A. thesis, 1953.
23. Radbruch, D. H., 1957, Areal and engineering geology of
the Oakland West quadrangle, California: U. S. Geo-
logical Survey Miscellaneous Geologic Investigations
Map 1-239, scale 1:24,000.
24. Robinson, G. D., 1956, Geology of the Hayward quad-
rangle, California: U. S. Geological Survey Geologic
Quadrangle Map G Q 88, scale 1:24,000.
25. Schlocker, J., Bonilla, M. G., and Radbruch, D. H., 1958,
Geology of the San Francisco North quadrangle, Cali-
fornia: U. S. Geological Survey Miscellaneous Geologic
Investigations Map 1-272, scale 1:24,000.
26. Sheehan, J. R., The structure and stratigraphy of north-
western Contra Costa County, California, scale 1:24,000:
University of California, Berkeley, unpublished M.S.
thesis, 1956.
27. Thomas, R. G., 1951, An example of re -intrusion of ser-
pentine: Am. Geophys. Union Trans., vol. 32, no. 3,
pp. 462-465, fig. 1, scale 1:36,000.
28. Touring, R. M., Structure and stratigraphy of the La
Honda and San Gregorio quadrangles, San Mateo
County, scale 1:24,000, Stanford University, unpublished
Ph.D. thesis, 1959.
29. Union Oil Company, Geologic maps of the Santa Cruz
basin, California, scale 1:24,000, unpublished (1952, 1954,
1957,1958).
30. Untermann, B. R., A study of the Wildcat fault in the
Berkeley Hills, scale 1 inch equals 500 feet. University
of California, Berkeley, unpublished M.A. thesis, 1934.
31. Weaver, C. E., 1949, Geology and mineral deposits of an
area north of San Francisco Bay, California: California
Division Mines Bulletin 149, 135 pp., pl. 9: Geologic
map of the Point Reyes quadrangle, scale 1:62,500.
For a complete list of published geologic maps of
this area see Division of Mines Special Report 52.
40454 4-61 6N SPO
MAP 1-272
DEPARTMENT OF THE INTERIOR
UNITED STATES GEOLOGICAL SURVEY
GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE, CALIFORNIA
By
J. Schlocker, M. G. Bonilla, and D. H. Radbruch
MISCELLANEOUS GEOLOGIC INVESTIGATIONS
MAP I-272
PUBLISHED BY THE U. S. GEOLOGICAL SURVEY
WASHINGTON. D.C.
1958
MAP 1-272
AP UNITS
SHEARING STRENGTH
FOUNDATION CONDITIONS
POSSIBLE
OR
REPORTED USE
UNIT WEIGHT'
(pounds per
cubic foot)
UNIFIED SOIL2
CLASSIFICATION
GROUP SYMBOL
Generally moderate shearing strength, but exceedingly
variable depending on method of placement, age,
thickness, underlying material, and history following
placement, such as ground -water conditions, loading,
etc. In 1906 earthquake the greatest damage to struc-
tures was inflicted in areas of artificial fill overlying
bay mud and clay along east shore of city.
Variable within wide
limits.
Variable within wide
limits.
Generally unsuitable for foundations.
Rockfalls possible local source of pervi-
ous fill, riprap, etc.
Variable.
Variable. GC to CH.
Moderate to high in sandy alluvium; low where deposits
are predominantly clay and silt or high in plant frag-
ments. To safeguard foundations, clay -filled surface
and subsurface channels should be adequately drained.
Surface material possible source of top
soil for lawns and gardens.
100 to 140.
SM, SC, SW, OL.
Moderate to high shearing strength where confined. Sus-
ceptible to wave erosion on beach.
Blending sand for concrete aggregate;
fill.
105 to 110.
SP.
Variable. Deposits with relatively high clay content are
soft and plastic when wet. Sandy and gravelly deposits
underlying unstable clays give adequate support for
foundation piles.
Fill.
104 to 124.
CL, SM, SC, rarely GC
and GM.
Low 43
shearing strength. Structures on mud successfully
supported by piles or caissons. An older bay clay, lying
below Colma formation, is firm and preconsolidated in
most places and has moderate shearing strength.
Although very poor quality, bay mud
and clay have been used for fill be-
hindLow
Recent part shellof depositsEmbarand sead wall.
deposits underlying
clay and mud dredged from bay for
manufacture of cement clinker.
Blended with better quality clay to
make structural clay products. May
be suitable for making foundry sand.
to 98. Older bay clay
underlying Colma
formation: greater
than 100.
CL, CH.
Moderate to high shearing strength when confined.
Good quality fill. Admixed with clay to
make foundry sand. Small tonnages
used as blending sand in concrete
aggregate.
65 to 102 at surface;
110 compacted.
SP
Moderate to high shearing strength, approximately 3 to 6
times greater than firm bay clay. Used for pile and
caisson support.
Good quality fill.
105 to 130.
ostl
MSM. tl
y SP; some is SC,
Shearing strength high in massive rock but decreases
with increasing proportion of shearing and alteration.
Thoroughly sheared and altered serpentine has low
shearing strength. Veins of soft, altered material in
hard serpentine may present special problems.
Moderately sheared serpentine widely
used for fill; highly sheared and al -
tered rock is unsuitable. Although no
commercial•deposits known at present
in the quadrangle, serpentine is pos-
sible source of magnesium, asbestos,
nickel, ehromite, mercury, and jade.
78(shearedandaltered)
to 158 (massive).
High shearing strength.
Good quality fill. Possible limited source
of concrete aggregate and large -size
riprap.
180 to 192.
Shearing strength is high except in badly shattered and
altered rock.
Fresh rock suitable for good quality fill,
road metal, riprap, concrete aggre-
gate. Moderately altered rock may
be suitable for fill.
128 to 144.
Shearing strength high in fresh rock. Foundations on
badly sheared, altered rock may require pile support.
Fresh rock and much moderately al-
tered rock suitable for fill. Calcined,
expanded shale is source of light
weight aggregate. Shale used to make
common bricks. Possible raw material
for manufacturing cement clinker.
127 to 142.
High shearing strength.
g
Chert, shale, and altered greenstone,
locally called "redrock", are used as
fill and road metal. Suitability of chert
for concrete aggregate questionable.
151 to 166.
Shearing strength high in relatively fresh rock but de-
creases with increasing alteration.
Moderately altered greenstone with
associated chert and shale, known
locally as "redrock", is used as fill and
road metal. Fills of badly altered
greenstone are prone to sliding on
moderate or steep slopes. Relatively
fresh rock is possible source of con-
crete aggregate and riprap.
113 (thoroughly al -
tered)to185(slightly
altered).
High shearing strength.
Good quality fill, road metal, large -size
riprap, concrete aggregate.
169 to 195.
Matrix has low shearing strength. Large rock fragments
encountered in exploratory borings may give false im-
pression of sound foundation conditions.
Used extensively for low quality fill.
78 to 110 (matrix); 125
to 170 (inclusions).
INTRODUCTION
San Francisco North is one of four quadrangles being
mapped by the U. S. Geological Survey as part of a
detailed engineering geologic study of the San Francisco
Bay area. This area was selected because its strategic
location, both as the focal point of development for a
large part of the west coast and as the gateway to trans-
pacific commerce, has made it one of the outstanding
centers of continuous expansion and construction in the
nation. This study in no way pretends to supplant de-
tailed site studies. Rather it tries to supply an accurate
background picture of the lithology and of the geologic
processes that change or modify the earth materials
involved.
As much of the quadrangle that does not lie beneath
the bay is hidden by streets, buildings, and other man-
made structures, geologic observations were confined
largely to undeveloped lots and to current excavations
for utility lines and building foundations and had to be
supplemented by data obtained from boreholes and
earlier foundation construction. Several thousand logs of
boreholes drilled by private firms as well as federal, state,
county, and municipal agencies provided data that were
invaluable in filling out the geologic story of the area.
The authors wish to acknowledge the contribution
made by their colleague Mrs. Helen D. Varnes in the
critical editing and revision of the manuscript.
LOCATION AND TOPOGRAPHY
The San Francisco North quadrangle includes part of
San Francisco Bay, the northern half of the city of
San Francisco, the southern tip of Marin and Tiburon
Peninsulas, Angel Island, Alcatraz Island, and part of
Treasure Island.
San Francisco Bay, which covers approximately half
of the quadrangle, is connected to the Pacific Ocean by
the Golden Gate channel, a deep narrow waterway be-
tween the San Francisco peninsula on the south and
Marin Peninsula on the north. The Golden Gate channel
and much of the bay represent drowned valleys of the
Pleistocene Sacramento River and its tributaries. Water
depths generally range from,100 to more than 400 feet
below mean sea level, although shallows of less than 60
feet are charted south of Rincon Point, north of Treasure
Island, southwest of Angel Island, in Richardson Bay,
and along a narrow shoal between Alcatraz Island and
Fort Point.
The physiographic history of the San Francisco penin-
sula is incompletely known. The present topography,
however, undoubtedly reflects both the intricate struc-
tural and lithologic patterns that underlie it and the
sustained, and often severe, tectonic activity which re-
peatedly interrupted and modified the geologic processes
shaping those patterns. In general, the city area is irregu-
larly hilly and rolling, rising on the south-central border
to a small group of rugged hills which reach heights
greater than 900 feet above mean sea level at several
points. Mount Sutro is 908 feet in altitude and South
Twin Peak is 922 feet in altitude. The strong north-
westerly trend of ridges and valleys, which is character-
istic of most of the Coast Ranges, is obscured in much of
this area although it is indicated by such minor features
as Russian and Telegraph Hills and the valley between
them, and Potrero Hill. The topography has an overall
aspect of early maturity combined with many youthful
features such as the sheer sea cliffs that border the Golden
Gate, and the bold slopes of Potrero Hill, Telegraph Hill,
Mount Sutro, and Twin Peaks. The rolling terrain of
Golden Gate Park and the moderate slopes in the central
part of the city represent in part a modification of the
original landforms by the deposit of tremendous quanti-
ties of dune sand.
The topography of Marin Peninsula, Belvedere Island,
Tiburon Peninsula, and Angel Island is generally more
youthful and rugged than that of San Francisco. Rounded
ridges and spurs and steep -sided V-shaped canyons are
characteristic of these areas, although a few of the larger
canyons widen near their mouths into flat -floored valleys.
Marin Peninsula has an average relief of 800 to 900 feet.
One ridge, half a mile west of Sausalito, is the highest
point in the quadrangle with an altitude slightly greater
than 1,125 feet.
The typical northwesterly structural and topographic
trend of northern California Coast Ranges is well shown
by Alcatraz Island, Richardson Bay, Belvedere Island,
and Corinthian Island. Although somewhat modified by
the erosion features of a large transverse stream, it can
also be found in the ridges of Marin Peninsula.
Angel Island is the upper portion of a drowned moun-
tain. Its rounded summit, 781 -foot Mount Ida, and the
flanking radial spurs have an overall aspect of topographic
maturity locally modified by steep youthful shorelines
and sharp ridge crests.
STRATIGRAPHY
The geologic formations of the San Francisco North
quadrangle fall into two distinct units that differ greatly
in age, lithology, and topographic expression. The older
unit forms the bedrock, which comprises the pre -Tertiary
sedimentary, igneous, and metamorphic rocks of the
Franciscan group, and the serpentine and gabbro-diabase
intrusives associated with them. The Franciscan group
includes massive and thick -bedded sandstones of the
graywacke type, shale, thin -bedded fine-grained sand-
stone, radiolarian chert, greenstones that are predomi-
nantly basaltic pillow lavas and pyroclastics, and a small
volume of metamorphic rocks. Assemblages of rock types,
very similar to the serpentine and Franciscan units, are
found in many parts of the world and appear to be typical
of the orogenically active portions of geosynclines.
Wherever the Franciscan group and its associated ser-
pentine and gabbro-diabase are exposed or lie near the
surface, they generally form prominent hills and ridges.
Complicated regional structure, great variation in
lithology over short distances, and the extensive blanket
of younger sediments make it difficult to determine the
actual thickness of the Franciscan group. On Marin
Peninsula, however, about 9,000 feet of the Franciscan
is exposed. Lack of fossil evidence likewise prevents ac-
curate age determination. A single fossil found in San
Francisco indicates that part of the group is Lower
Cretaceous. Evidence outside the quadrangle shows that
at least some of the Franciscan was deposited during
Jurassic time.
The second and younger unit is composed of largely
unconsolidated surficial deposits, of Pleistocene and
Recent age. These deposits are predominantly dune sand
and water -laid sand, mud, and clay, but include some
fairly extensive deposits of slopewash, artificial fill, allu-
vium, and landslide debris. Above sea level the surficial
deposits mantle and extensively modify the lower slopes
and fill the valleys between the bedrock hills; thickness
varies from a few feet to more than 100 feet. In the bay
itself, borings show that the pre -Tertiary bedrock is over-
lain by deposits of sand, clay, and mud ranging from 100
to 300 feet in thickness. In some of the channels cut in
the bay floor, bedrock (Louderback, 1961, p. 83) and
"rocky" bottom conditions (U. S. Coast and Geodetic
Survey Charts 5,532 and 5,533) have been reported, indi-
cating that the unconsolidated material is locally absent.
PRE -TERTIARY FORMATIONS
Franciscan group
Sandstone (KJss).-A thick -bedded graywacke sand-
stone of the Franciscan group is found in many parts of
the quadrangle but is best exposed on Marin Peninsula,
on Angel and Alcatraz Islands, and in the hills and cliffs
along the north border of San Francisco. The geologic
structure, though incompletely known, and the distribu-
tion of known exposures indicate that it probably under-
lies part of the bay and much of San Francisco that is
covered by dune sand. The thickest measured sections in
the quadrangle are 1,300 feet on Angel Island, 1,100 feet
on Marin Peninsula, and 600 feet on Telegraph Hill. In
general, the sandstone occurs in beds 10 or more feet
thick separated by thin lenses and beds of fissile hard
black shale. In some places, the massive sandstone layers
are separated by 2 to 4 feet of thinly bedded shale and
fine-grained sandstone. On Angel Island and Belvedere
Island, the graywacke sandstone includes several con-
glomerate beds, about 10 feet thick, composed of rounded
gravel- and cobble -size fragments.
The fresh sandstone is a hard, tough medium -gray
rock. When moderately weathered it becomes light brown
but is still fairly hard and tough. Highly weathered sand-
stone is pale orange, very friable, and crumbles readily
under finger pressure. The average depth of weathering
is less than 30 feet although in a few places the weathered
zone extends down 60 or 70 feet. The sandstone has been
badly fractured by randomly brierited joints that are 2
to 5 inches apart in most exposures, although locally the
joint spacing is a foot or greater. Most joint surfaces are
coated by clay minerals, iron oxide, or manganese oxide.
Calcite and quartz veins one -eighth inch to 3 inches thick
are also common.
Much of the sandstone on Angel Island and Belvedere
Island is sheared, generally parallel to the bedding plane,
and has a semischistose appearance with a crude cleav-
age. In other parts of the quadrangle, the sandstone com-
monly is sheared where it is interbedded with thick shales
and along faults and intrusive contacts. At many locali-
ties, this material consists of a soft, dark bluish -gray
matrix of thoroughly comminuted shale and sandstone
crossed by many shear surfaces and enclosing round, hard
masses of sandstone as much as 10 feet in diameter.
In composition and texture the sandstone can be classi-
fied as a graywacke. In most places it is a poorly sorted
mixture of angular and subangular grains ranging from
fine to very coarse sand in size. The commonest grain
materials are quartz, plagioclase feldspar, and fragments
of volcanic rock, chert, and shale. The matrix appears to
be mostly silt and very fine sand -size particles and is
commonly composed of chlorite, clay minerals, musco-
vite, biotite, carbonaceous materials, quartz, and feld-
spars. The carbonaceous matter is concentrated locally
into coal -like beds as much as an inch in thickness.
Thin sections of the semischistose sandstone on Angel
Island and Belvedere Island show that in many places a
large percentage of the material has been reconstituted
to form new minerals such as muscovite, chlorite, pum-
pellyite, zoisite, clinozoisite, glaucophane, tremolite-
actinolite, albite, and jadeite. The original minerals that
remain are deformed, shattered, or recrystallized.
It has been suggested by Taliaferro (1943, p. 190) that
materials for the sandstone and conglomerate were prob-
ably derived from a rugged mountainous land that lay
west of the present coastline and were deposited rapidly
by high -gradient streams in a marine basin. Fossils are
exceedingly rare. An ammonite found in the sandstone
cliffs of South Bay is Early Cretaceous in age. Cretaceous
fossils have been reported from Alcatraz Island (Schlocker,
Bonilla, and Imlay, 1954).
Shale and thin -bedded sandstone (KJsh).-The Francis-
can group on portions of Nob Hill, Telegraph Hill, and
Russian Hill comprises predominantly laminated silty
and clayey shales, interbedded with fine-grained sand-
stone. The sandstone beds are generally 2 to 5 inches
thick although laminations one -eighth of an inch and less
in thickness are common in the shales. Some sandstone
layers show graded bedding. These beds pass from a light-
colored fine-grained sandstone at their base to a darker
and even smaller grained material at the top. The section
of shale and thin -bedded sandstone on Russian Hill is
about 1,350 feet thick and appears to be the same as the
one exposed on the west side of Telegraph Hill. The Nob
Hill section is older than the preceding ones and is sepa-
rated from them by 700 feet of sandstone.
Unweathered shale and sandstone are hard but con-
siderably fractured. Weathered rock is moderately hard
to crumbly depending on the intensity of alteration. Fold-
ing and faulting have produced considerable shearing and,
in the vicinity of Taylor Street and Broadway, small-
scale isoclinal folding. Locally the shales have been
sheared parallel to the bedding. In a few exposures the
shale has been altered to slate with prominent slicken-
sided planes. Elsewhere -the shale is minutely crosscut by
randomly oriented slickensided surfaces so that the rock
crumbles at the touch.
Although the shale and thin -bedded sandstone unit of
the Franciscan is believed to be marine in origin, no fos-
sils have been found in it, and its age relative to the Lower
Cretaceous sandstone of the South Bay cliffs is not known.
Radiolarian chert and shale (KJ c).- The radiolarian
chert and shale of the Franciscan group consist of hard
chert layers 1 to 5 inches thick alternating with shale
partings as much as three -fourths of an inch in thickness.
This habit of bedding is remarkably constant even in
sections several hundred feet thick. Pinching and swelling
of individual beds is common and, in many exposures,
small-scale intricate contortions are typical. Locally the
chert is massive rather than thin bedded and occurs both
as isolated bodies and in association with the layered
chert and shale. Generally the chert is grayish red al-
though green and orange chert is found locally.
Throughout the quadrangle, the radiolarian chert and
shale form prominent hills and ridges. In San Francisco,
Sunset Heights, Corona Heights Playground, Mount
Olympus, Mount Sutro, and Twin Peaks are capped by
thick sections of chert. Isolated but prominent exposures
are also found in Golden Gate Park, at the Cross, on
Strawberry Hill, and in an abandoned quarry (Swan
Lake) southwest of the Conservatory. On Marin Penin-
sula, radiolarian chert and shale is exposed in several
thick sections of different ages over at least one-third of
the area and forms all the prominent ridges.
The bedded cherts consist of tiny interlocking grains of
quartz, fibrous chalcedony, and iron and aluminum
minerals in varying proportions. Radiolaria appear in most
of the red cherts as tiny, colorless, spherical or bell -shaped
masses. The chert beds were extensively fractured per-
pendicular to the bedding and most of the fractures later
filled with white quartz, which forms a slightly raised,
reticulated pattern on weathered surfaces. Joints and
other fractures are stained with manganese and iron
oxides. The massive chert is similar in composition al-
though much of it is a breccia of angular chert pieces in
a matrix of white and yellow quartz and chalcedony.
Locally, the massive chert is studded with concentric
bands of white quartz and red chert.
The bedded chert is hard but brittle and breaks readily
except along the quartz -filled fractures. The massive
chert is hard and tough and cannot be broken as readily
as the thin -bedded material.
Chert is resistant to weathering and mild hydrothermal
alteration. It becomes bleached through loss of iron and
manganese but retains its hardness. In areas of faulting,
where hydrothermal action was more intense, the altered
chert is generally very brittle and shatters easily.
On Angel Island and Belvedere Island some chert and
shale bodies near or within the greenstone and serpentine
are found in various stages of low-grade metamorphism
to glaucophane schist. Generally the altered chert has a
matrix of interlocking, sutured quartz crystals that en-
close rosettes and bundles of stilpnomelane and glauco-
phane crystals, and the associated shale partings are rep-
resented by subparallel bands of stilpnomelane and glau-
cophane. In a more advanced stage, garnet, in numerous
tiny colorless dodecahedral crystals, and sparse flakes of
muscovite are also present. Moreover, the quartz crystals
in the matrix are recrystallized and their interlocking
texture has been largely destroyed.
The radiolarian chert is a marine deposit probably
formed by the dehydration, compaction, and hardening
of colloidal silicic acid. Flocculation of the siliceous oozes
concentrated the admixed clay and other minerals that
later formed the thin shale beds.
Greenstone (KJg).-This rock name has long been used
for altered fine-grained igneous rocks that have an abun-
dance of greenish secondary minerals. Following the
practice of other geologists in this area, the term has
been broadened in this report to include all volcanic
flows, dikes, sills, plugs, agglomerate, and tuff associated
with the Franciscan group. Extensive deposits of green -
stone are found on Marin Peninsula, Belvedere Island,
Angel Island, and on Twin Peaks and the hills lying
between Twin Peaks and Valencia Street. In most ex-
posures it is a highly altered, soft, crumbly reddish -brown
rock. Moderately altered greenstone is closely fractured,
so that it breaks easily into pieces about a quarter of an
inch across. Relatively unaltered greenstone is generally
found only in a few deep cuts and excavations. The fresh
rock is hard, tough, and dark gray or greenish gray.
Except for a large sill on Angel Island, much of the
greenstone shows pillow structure and is in intimate asso-
ciation with the radiolarian chert. The greenstone evi-
dently represents submarine lava flows that pushed
through or covered the silicious oozes that later hardened
into chert. The greatest development of pillow -lava
greenstones is on Marin Peninsula where one section
approximately 1,600 feet thick extends from Horseshoe
Bay to Sausalito. Similar rock is also extensively exposed
on the south slopes of Twin Peaks below the radiolarian
chert. Most greenstone pillows are rough ellipsoids that
are rounded on top and flat or concave on the bottom,
conformable with the underlying pillow. The size varies
locally but most pillows average about 2% feet on the
longest diameter and 1 foot across the shortest. In a few
places the pillows are considerably smaller, ranging from
6 to 12 inches in diameter, and are spherical rather than
ellipsoidal. In some places the material between pillows
is limestone or hard, flinty chert, but generally it is a
soft sheared crumbly chloritic material.
Tuff and agglomerate are present sporadically and
commonly occur in layers as much as 10 feet thick inter-
bedded with the pillow lavas and, locally, with massive
greenstones. Some beds grade almost imperceptibly into
the graywacke sandstone. In general the tuff and agglom-
erate are more altered and softer than the pillow green -
stones with which they are associated.
Most greenstones appear to be basaltic in composition.
They range in grain size from aphanitic to a medium -
grained rock containing crystals 1 to 2 mm in size. Dia-
basic texture, which can still be recognized even in highly
altered rock, is common in the fine- and medium -grained
greenstone. In a few places, ophitic texture has been
observed. Generally the pillows are aphanitic, although
at some localities they are partly or wholly crystalline.
The aphanitic material is very dense and consists pre-
dominantly of microscopic spherulites of radiating crystal
fibers probably derived from glass. The composition of
the spherulites varies with the type of devitrification and
subsequent alteration, but most commonly includes albite-
oligoclase, chlorite, clinozoisite, leucoxene, ilmenite, and
magnetite.
The least altered specimens of crystalline pillow lava
are predominantly sodic labradorite and titanaugite. The
sill -like greenstone body on Angel Island and some
columnar -jointed and massive, structureless greenstones
on Marin Peninsula contain augite rather than titanau-
gite as the pyroxene. Even in the freshest greenstone on
Marin Peninsula and San Francisco peninsula, at least
10 percent of the pyroxenes are altered to nontronite and
chlorite although the plagioclase shows little change. The
reverse is true on Angel Island where the pyroxene is fresh
and the plagioclase is considerably altered. Highly weath-
ered greenstones are almost entirely converted to non-
tronite, or possibly chlorite or vermiculite. Some hydro -
thermally altered greenstones are converted to halloysite,
montmorillonite, and hydrous mica. Of the minor miner-
als in the fresh rock, magnetite and ilmenite are most
abundant. There are scattered crystals of skeletal pyrite
and olivine. The latter are nearly completely replaced by
nontronite, antigorite, and chlorite. Spherical amygdules
of nontronite and chlorite are common; prehnite amyg-
dules were found at one locality in San Francisco.
Fractures are coated with manganese and iron oxide or
nontronite or are filled with veins of calcite; calcite and
quartz; pumpellyite and quartz; chlorite, nontronite, and
apophyllite; or chlorite. On Angel Island and Belvedere
Island, glaucophane is found as veinlets in greenstone.
Locally the greenstone is found in several stages of low -
temperature metamorphism in which the plagioclase and
pyroxene are more or less replaced by lawsonite, chlorite,
and albite.
Metamorphic rocks (KJ m). - Several types of meta-
morphic rocks occur on Angel Island, Belvedere Island,
and Tiburon Peninsula. Except for one large body of
tremolite-actinolite schist on the southwest side of Angel
Island, exposures are small and widely scattered. The
only known occurrences in San Francisco are a few pods
of tremolite and talc enclosed in serpentine and some
large boulders of glaucophane schist in the water between
Point Lobos and Lands End.
Although volumetrically unimportant, the metamor-
phic rocks of the San Francisco North quadrangle are
notable for wide variations in mineralogy and f or the
fact that some glaucophane is present in nearly all ex-
posures except the tremolite-actinolite schist body. To
date, at least eight different mineral assemblages have
been recognized. The commoner minerals include tremo-
lite, actinolite, talc, quartz, glaucophane, muscovite, al -
bite, garnet, hornblende, stilpnomelane, chlorite, epidote,
and sphene.
Most rocks in this unit have developed a prominent
schistosity although some bodies have gneissic or granu-
lar texture. In some exposures metamorphism has pro-
gressed so far that nearly all the original structures,
textures, and minerals have been destroyed. On Angel
Island, Belvedere Island, and Tiburon Peninsula, how-
ever, partially metamorphosed chert, greenstone, sand-
stone, and shale beds clearly indicate that this unit was
derived from the Franciscan group. Most of the meta-
morphism apparently was accomplished by the introduc-
tion of magnesium, iron, aluminum, calcium, and other
elements in hydrothermal solutions although shearing
may have contributed locally. The tremolite-actinolite
schist on Angel Island is adjacent to a large serpentine
body and may have been metamorphosed by direct con-
tact as well as by hydrothermal action.
Serpentine (sp, spm), gabbro-diabase (gd), and sheared
rocks (KJ u). -Most of the serpentine in the quadrangle is
in a zone about 114 miles wide that extends from the
southeast corner of the quadrangle northwestward across
the city to Bakers Beach and Fort Point. The largest
exposures are on Potrero Hill. Other exposures are found
in Lincoln Park, at James D. Phelan Beach, below the
south tower of Golden Gate Bridge, and in shear zones
associated with the landslide west of Lands End. Uncor-
roborated reports indicate that 3,800 feet of the Twin
Peaks Tunnel was cut through serpentine. A tabular
westward -dipping serpentine body about 600 feet thick,
overlain by metamorphosed rocks of the Franciscan
group, crosses the southwest portion of Angel Island, and
there are small serpentine exposures along the southwest
shore of Belvedere Island and on Tiburon Peninsula. In
all places, the serpentine seems confined to shear and
fault zones. The serpentine shows considerable variation
in color although green or blue hues are commonest.
Most of the serpentine is badly sheared so that typical
exposures show spheroidal knobs of hard serpentine en-
cased in a polished, slickensided "rind" and lying in a
waxy and crumbly matrix of thoroughly sheared serpen-
tine. The proportion of large blocks to sheared matrix
varies widely. Most generally, the matrix constitutes 30
percent or more of the entire rock and, in some places,
bands of serpentine several hundred feet wide are so
mashed that there are few, if any, hard nodules more
than an inch in diameter. In a few exposures the serpen-
tine (spm), is largely massive with less than 10 percent
sheared matrix. The large blocks and the massive bodies
of serpentine are dense, hard, and tough. The sheared ser-
pentine is soft, friable, and more or less plastic when wet.
All the observed contacts between the serpentine
bodies and the Franciscan group are shear zones that
vary in width from place to place and generally involve
both formations. Some of these zones are so extensive
that they were mapped as a separate unit (sheared rocks,
KJ u).
The intense shearing of the serpentine has been at-
tributed to two causes. The serpentine, and, to some
extent, the peridotite from which it was derived were
somewhat fractured when they were squeezed into older
rocks in a relatively cold, semisolid state. Most of the
shearing, however, probably resulted from the repeated
crustal disturbances to which the San Francisco area has
been subjected, and which were localized to some extent
in inherently weak material such as the serpentine.
Serpentine consists predominantly of the serpentine
minerals, antigorite and chrysotile, and a dense amor-
phous material, serpophite, which is also a hydrated
magnesium silicate. Small chromite and magnetite grains,
locally with iron -bearing chlorite aureoles, are dissemi-
nated throughout the serpentine. Some unsheared serpen-
tine contains relict enstatite and olivine crystals. Tough
masses of green coarse -grained diopside (diallage) occur in
the serpentine exposures near the south base of Potrero
Hill.
Hydrothermal action was widespread along shear zones
within and bordering the serpentine. Locally, it deposited
swarms of magnesite veinlets. In many shear zones, the
serpentine is more or less replaced by: clay minerals;
quartz; chalcedony; opal; iron oxides; carbonates of mag-
nesium, calcium, or iron; talc; and several unidentified
minerals. Locally, hydrothermal action has entirely re-
duced the serpentine to a waxy, plastic material composed
almost entirely of trioctahedral montmorillonite.
In a few exposures on the San Francisco peninsula, the
serpentine contains small bodies of fine- to coarse -grained
gabbro and diabase (gd), which are composed largely of
saussuritized labradorite (An52), hornblende,and chlorite
derived from hornblende, and minor amounts of pyroxene,
apatite, sphene , and magnetite. On Angel Island, the
gabbro included in the serpentine is composed of andesine
(An,$), actinolite, pigeonite, and 3 to 5 percent apatite
and sphene.
SU R FICIAL DEPOSITS
Colma formation (Qc).-A sequence of unconsolidated
sand deposits, here named the Colma formation, have
been mapped together as a new formation„because they
are similar in appearance and physical properties and
they occupy approximately the same stratigraphic posi-
tion, although they may not all be contemporaneous. The
name is derived from the town of Colma, located 414 miles
south of the quadrangle near the center of the area in
which the formation is best developed (Bonilla, personal
communication). The Colma formation is described in
detail in a report on the geology of the San Francisco
South quadrangle now in preparation.
The Colma formation is found on Angel Island and at
several points on the San Francisco peninsula. In general,
it consists of moderately well sorted fine- to medium -
grained sand with minor amounts of clay, and is charac-
terized by well -developed, evenly spaced near -horizontal
bedding intercalated with layers of inclined "torrentially -
bedded" sand and locally with crumpled "slump" bedding.
In three exposures on the San Francisco peninsula, the
sands enclose clay beds 6 inches to 5 feet thick. A deeply
gullied badland topography commonly develops on the
Colma formation as it is soft and easily eroded.
The only deposits on Angel Island are along the south
and east shores between Quarry Point and Knox Point.
They are roughly horizontal beds 5 to 50 feet thick plas-
tered on the steep bedrock hills, from water level to
altitudes of about 200 feet. In the valley east of Knox
Point, they appear to be considerably thicker and to
extend below the present sea level.
Outcrops of the Colma formation in San Francisco are
generally small and widely scattered. Most of them are
obscured and at least partly destroyed by erosion and
construction activity. One of the best preserved outcrops
lies at the north end of Ocean Beach where the contact of
the Colma formation with overlying dune sand is well
exposed. The basal member overlies and grades into a
coarse rubbly material. Here the Colma formation is at
least 40 feet thick and appears to thicken and dip gently
southward under the dune sand of Golden Gate Park.
Prominent exposures are also found in the Presidio
Military Reservation and near the south border of the
quadrangle between Mount Sutro and Twin Peaks. The
latter appear to be the northernmost extension of a large
sand and silt deposit that covers more than 10 square
miles south of this quadrangle.
Records of boreholes and excavations indicate that
sand deposits, which may be the Colma formation, under-
lie bay mud and clay (Qm) along much of the bay shore
of San Francisco.
The sand grains are subangular to subround. About
75 percent have polished surfaces; the remainder are
pitted and frosted. Mineralogically, the sand consists
largely of approximately equal amounts of quartz and
feldspar with 5 to 10 percent chert and small varying
percentages of dark heavy minerals including magnetite,
ilmenite, sphene, amphiboles, epidote group minerals,
and zircon. In most exposures there are lensing streaks of
black heavy minerals and irregular undulating layers of
iron oxide cemented sand.
The combination of normal horizontal bedding, inclined
"torrential" bedding, and distorted slump bedding, and
the predominance of polished grain surfaces suggest that
these sands are predominantly water -laid. They are prob-
ably in part fresh -water alluvial and lake deposits, and in
part beach, dune, and estuarine deposits.
Present evidence indicates that the deposits assigned
to the Colma formation are Pleistocene in age. Radio-
carbon studies on a large juniper tree (probablyJuniperus
californica) found buried in sand deposits of the Colma
on Russian Hill indicate that these beds were laid down
at least 30,000 years ago. Similar deposits to the south
of the quadrangle overlie fossiliferous beds of proven late
Pliocene and early Pleistocene age. The sand deposits
underlying the bay mud and clay have been tentatively
correlated (Schlocker and Radbruch, in press; Radbruch,
in press) with thg Temescal formation and the Merritt
sand of Pleistocene age.
Dune sand (Qd).-Dune sand covers more than half of
the north part of the San Francisco peninsula. The only
other occurrence in the quadrangle is a small deposit on
the east shore of Angel Island. Prevailing westerly winds
swept the sand from Ocean Beach and Bakers Beach as
far eastward as Rincon Hill. Dune sand was deposited
more than 600 feet above sea level at Sunset Heights Park
and covers the 575 -foot summit of Buena Vista Park.
The western slopes of Mount Sutro, on the lee side of the
Sunset Heights Park ridge, are completely free of dune
sand. The extensive low area in the lee of the Mount
Sutro-Mount Olympus -Twin Peaks highland was also
protected from blowing sand. At the present time, wind
transportation of sand is confined by manmade barriers
to a narrow zone near Ocean Beach.
The dune sand varies in thickness from place to place.
Dunes 75 feet high were measured along Ortega Street
prior to development of that area in 1952, and the total
depth of sand was more than 100 feet. Dune sand
mantling the prominent hills and ridges is much thicker
on the east (lee) side than on the west and is thin or
absent on the crests.
The sand in general is fine to medium grained and is
well sorted, although locally it is coarse grained and only
moderately well sorted. Considerable local variation is
found in the shape and surface character of the grains.
Most sand grains are subround or subangular, and pol-
ished surfaces are commoner than dull pitted surfaces.
In all dune deposits, however, some well-rounded grains
and dull pitted surfaces are present and, in a few places,
are predominant. Quartz, feldspar, and chert in varying
proportions make up 90 to 95 percent of the dune sand,
except for a few exposures in which more than a third of
the grains are heavy minerals. The commonest heavy
minerals are hornblende, magnetite, ilmenite, epidote,
hypersthene, sphene, chromite, and clinozoisite; present
in sparse amounts are zircon, muscovite, glaucophane,
augite, and garnet. Magnetite is commonly concentrated
as thin black layers of lag sand in the troughs of ripples
near the base of the dunes.
In three widely separated localities an old soil or swamp
deposit of dark -gray organic clayey sand and plastic
sandy clay was found below the dune sand.
Bay mud and clay (Qm).-The bay mud and clay ap-
parently covers the entire bay bottom except in the
submarine tidal channels. The eastern margin of the city
of San Francisco is underlain by clay and mud deposits
that are more than 100 feet thick in places. Except for
temporary exposures in foundation excavations, the mud
and clay are almost everywhere concealed by water,
artificial fill, dune sand, or slope debris. Mud now being
deposited in the bay is very soft and has a high water
content. At depth it becomes a moderately stiff clay.
When loaded by artificial fill or heavy construction, the
surface mud and clay show appreciable compaction in
only a few years. Near Fifth and Folsom Streets, local
settlement of artificial fill over bay mud and clay has been
more than '7 feet in the last 60 years. Settling is often
uneven and causes serious cracking, tilting, and, infre-
quently, failure of entire structures built on the fill.
Heavy construction has also resulted in lateral movement
and upthrusting of the plastic mud and clay several
hundred feet away.
Partially consolidated bay mud and clay are generally
soft, massive, and olive gray to dark bluish gray. The
average composition is a silty clay consisting of 45 to
95 percent clay -size particles; the remainder is mostly
silt size with very minor amounts of fine sand. Samples
dried at room temperature and moderate humidity become
fairly tough and somewhat brittle but still contain
considerable water.
The high percentage of water and the plasticity of the
mud and clay are largely reflections of the mineralogy of
the mud and clay. The clay fraction of a sample obtained
from the foundation excavation for the Headworlcs
Building of the Southeast Sewage Treatment Plant, half
a mile south of the quadrangle, had the following compo-
sition: approximately one-third montmorillonite and
one-third micas, probably both hydrated and normal; the
remaining third is composed predominantly of a mixed -
layered montmorillonite-chlorite mineral and a kaolinitic
clay mineral of the halloysite-fireclay type, but also
includes minor amounts of quartz, feldspar, diatoms, and
sponge fragments. Both montmorillonite and the
montmorillonite-chlorite minerals are noted for their
capacity to adsorb water, expand, and become plastic.
The silt fraction had approximately the same mineralogy
but a larger proportion of nonclay minerals and contained
about 5 percent peaty and lignitic material.
The excavations at the Headworlcs Building and logs
of nearby boreholes indicate that the bay mud and clay
lie unconformably on the eroded surface of sand deposits
that may belong to the Colma formation.
Clays older than the bay mud and clay (Qm) also lie
below the sand deposits (see sec. C-0'). They are firmer
than the bay mud and clay and contain leas water. In
downtown San Francisco, a 22 -story building founded on
the older clay has settled about eight -tenths of a foot
in 20 years.
Slope debris and ravine fill (Qsr).-Many slopes are
mantled by unconsolidated debris derived from the
weathering of the bedrock. These deposits have been
mapped as separate units wherever they are known to be
more than 5 feet thick. Locally, this unit includes soils
developed on bedrock as well as minor amounts of
alluvial, eolian, and landslide materials. The slope debris
thickens progressively downhill to an observed maximum
of 18 feet; however, there are indications that it locally
attains thicknesses 2 or 3 times as great.
Most of the ravines are partly filled with material
derived from the adjoining slopes. These deposits range
from a few feet to more than 30 feet in thickness and are
locally interbedded with and grade into stream alluvium.
In many places throughout the quadrangle, road cuts in
gently rounded mature slopes have revealed deep, steep -
sided, youthful ravines that have been completely buried
in slope debris.
Slope debris and ravine fill are generally very poorly
sorted and consist of a mixture of angular rock fragments
in a matrix of sand, silt, and clay. Where the hills are
underlain by radiolarian chert or pillow greenstone, the
debris contains a relatively high percentage of coarse
gravel and cobble -size fragments. On many slopes in San
Francisco, where the bedrock is predominantly sandstone
and shale of the Franciscan group, most of the material is
in the coarse -silt to fine -sand size range with some clay
and coarse sand; there is little or no material coarser than
fine gravel. On Marin Peninsula, where slopes are gener-
ally steeper and transportation of surface materials more
rapid, the gravel fraction has a high percentage of chert
but also contains appreciable quantities of sandstone
and greenstone.
This unit probably includes deposits of different ages.
In many areas, the slope debris and ravine fill are
apparently the youngest materials. On some of the lower
slopes and along the wide, shelving shores, however, the
ravine fill is overlain by marine deposits and may be late
Pleistocene in age. Material identical with the modern
slope debris has also been observed below and interbedded
with the Colma formation.
Beach deposits (Qrb, Qob).-There are many beaches on
the west and north edge of San Francisco, on Angel
Island, and along the south edge of Marin Peninsula.
Most of them are small and are confined to coves and
inlets. The largest is Ocean Beach, which extends for
several miles south of Cliff House. The beach deposits
that at one time bordered nearly all the shore between
Fort Point and North Point are now largely buried under
artificial fill. Deposits on the smaller beaches probably do
not exceed 20 feet in thickness, but those along Ocean
Beach and Bakers Beach may be considerably thicker.
Most of the beach deposits are well -sorted medium- to
very coarse -grained sand. The sand grains are angular to
subangular with polished surfaces. They are predomi-
nantly quartz and feldspar with 10 to 15 percent of dark
minerals. The latter are mostly magnetite but include
ilmenite, hornblende, epidote, sphene, clinozoisite, biotite,
chromite, rutile, and some rock and shell fragments.
Gravel beaches are found in a few coves on both sides
of the Golden Gate and on Angel Island. These deposits
are predominantly coarse gravel and cobbles with some
pieces having a maximum diameter of 2 feet; sand and
fine gravel are present in minor amounts. The principal
materials are chert and greenstone; locally, there is an
appreciable percentage of concrete derived from excava-
tion and construction waste that had been dumped into
the sea.
In May 1952, an old beach sand (Qob) about 100 feet
above present mean sea level was uncovered in an exca-
vation on the Presidio Military Reservation east of
Bakers Beach. The sand is very fine to medium in grain
size but otherwise very similar to the modern beach
deposits. The bedding of the old deposit is very regular
and even and dips at a low angle towards Bakers Beach.
Alluvium (Qa O. -Several deposits of stream alluvium
were mapped on the west and southwest slopes of Twin
Peaks. They have a minimum thickness of 15 feet and
are interbedded with and grade laterally and vertically
into slope debris and ravine fill. All are clearly related to
modern or slightly older drainage systems. Similar allu-
vial deposits occur in many other parts of the quadrangle
but could not be mapped separately because they were
largely or entirely concealed by other surficial deposits
or by manmade structures and artificial fill.
Most of the alluvium is composed of medium -grained
sand mixed with silt and clay, although some silt -free
sand occurs locally. In general, the sand grains are sub -
angular to subround and consist mostly of quartz with
subordinate amounts of feldspar. Chert and greenstone
pebbles as much as an inch in diameter are abundant
in the alluvium near the heads of the valleys and are
sparsely scattered throughout the other parts of the
deposits.
Landslide deposits (Q I, Qlo, Qly). - The numerous
landslides in the quadrangle owe their existence to sev-
eral factors: an irregular hilly terrain that is underlain in
many areas by porous, unconsolidated surficial deposits
or by badly shattered bedrock; a relative abundance in
the above deposits of highly plastic and bentonitic clay;
occasional periods of prolonged rainfall; frequent earth-
quakes; and the continuous disturbance and alteration of
the original terrain by human agency.
Most of the landslide deposits are composed of a
heterogeneous, unstratified mixture of rock, sand, silt,
and clay in varying proportions. Some landslides occur
only in the unconsolidated surficial materials, but more
than half involve weathered or sheared bedrock as well.
Earthflow and debris flow are the commonest types of
movement. Many of the slides, however, are complex;
their upper portions are rockslides or debris slides and the
lower portions are earthfiows or debris flows. A few rock -
fall deposits are found below steep quarry faces.
Stabilized landslides or those in which only one period
of movement could be distinguished are shown as Q I.
Those in which an old slide has been partially reactivated
are mapped as Qlo for the stabilized parts and Qly for the
areas of more recent movement.
The largest landslides are in the sheared serpentine and
rocks of the Franciscan group around Lands End and in
the sheared serpentine south of the Golden Gate Bridge.
Wave action periodically reactivates some of these slides
by removing the supporting material at their bases.
Slides, many of them too small to be shown on the
geologic map, are numerous in the slope debris and ravine
fill (Qsr), especially along road cuts. The abundance of
ground water in the ravine fill is believed to be the pri-
mary cause for sliding in this material although the actual
movement may be triggered by some other factor. In the
densely built-up residential areas of the city, sliding of
weak materials may be produced by excessive watering
of lawns and gardens, by overloading, and by removal of
support from the base of the slope.
Artificial fill (Qaf).-Flat land has been at a premium
almost since the days when the Gold Rush first made
San Francisco a center for the growth and development
of that part of the west. The practice of creating artificial
land by dumping fill on the gently shelving tidal flats
along the eastern and northern margins of the peninsula
was begun before 1850 and is still continuing. At present,
more than 3 square miles of the most valuable land in
San Francisco originated in this way. The average thick-
ness of the fill north of China Basin is about 10 feet.
South of China Basin the fill locally reaches a maximum
thickness of about 60 feet.
The artificial fill shows extreme variations in composi-
tion. In the earliest attempts nearby dune sand and some-
times alluvium: were simply dumped on the soft we bay
mud, but it was soon found necessary to build a sea wall
to prevent the fill from being washed away. A wall now
extends more than 3 miles from Fort Mason to China
Basin and supports the Embarcadero. The first sections
were made by building a long ridge of rock and earth.
Subsequent sections were better designed and consist of
a concrete wall supported on piles set in a rock embank-
ment. For a long time, no restrictions were placed on the
type or quantity of fill. Spoil from excavations, debris
from quarries, dune sand, and mud dredged from the bay
were dumped indiscriminately (Am. Soc. of Civil Engi-
neers, 1932, p. 30). Some parts of the fill area were also
used for public dumps, so that fairly large deposits of
manmade debris were added to the mixture. After a
number of years, it was found that differential subsidence
caused many serious problems and that, in less than 60
years, parts of this fill had settled more than 7 feet.
Because of these troubles, construction of modern fills
should be preceded by careful study of the properties and
ground -water conditions of the fill foundation, and only
selected materials laid under controlled conditions should
be used.
The hilly portions of the quadrangle are dotted with
thousands of small fills made in the course of highway,
home, and other building development. Most are too
small to be mapped. Those shown are generally along the
downhill side of major roads or represent large deep
ravines that are used as dumps for rock and soil waste
from nearby excavations.
STRUCTURE
Although the structural history of the California Coast
Ranges is fairly well known, the record of the several
periods of orogeny that affected the area is very frag-
mentary in the San Francisco North quadrangle itself.
Part of this obscurity is due to absence of sediments
younger than the Franciscan group and older than the
surficial deposits, and part is due to the complexities of
the structures. The character of the Franciscan group
further contributes to the difficulties for it contains no
distinctive and persistent marker beds, fossils are almost
nonexistent, lateral variations in texture and lithology
are the rule, and usually no criteria can be found to dis-
tinguish tops and bottoms of beds. Moreover, a large part
of the bedrock is covered by water, surficial deposits, and
manmade structures.
The best evidence of repeated crustal movement is
found outside the quadrangle where the sedimentary
section is more complete, but it is shown in the quadrangle
by the high degree to which the Franciscan has been
faulted and sheared. Almost every exposure has one or
more sets of slickensided slip planes and gouge zones.
Most of these fractures are of very limited extent and
show little or no displacement. The faults shown on the
map are believed to have some measure of persistence,
and probably many more, of equal or greater importance,
are omitted.
The marked contrast in the lithology and local struc-
tures on Marin Peninsula and San Francisco peninsula
suggest that a major fault exists in the Golden Gate
channel but no supporting evidence was found with the
possible exception of the westward -trending shear zone
at Point Diablo. Likewise, stretches of more or less straight
coast lines bordered by precipitous cliffs suggest fault
control origin for part of the bay shoreline, but again
positive field evidence is lacking.
On Marin Peninsula, the immense section of southwest-
ward -dipping sediments and greenstone apparently form
the limb of a large anticline whose axis lies in Richardson
Bay (see sec. B-13'). The principal structure on Angel
Island is a broad syncline whose axis plunges north-
westward to Hospital Cove.
In San Fran cisco, bedding attitudes and the disposition
of an apparently persistent unit of shale and thin -bedded
sandstone of the Franciscan group suggest a syncline in
which the axis plunges northwestward along Columbus
Avenue between Nob Hill and Telegraph Hill. Lawson
(1914, p. 15; 1915, p. 115), on the contrary, believed that
this same structure is the southeastern extension of the
Richardson Bay anticline. Northeasterly dips on Alcatraz
Island and southwesterly dips on Telegraph Hill indicate
opposing flanks of a large anticline. Meager structural
data indicate that there are one or more large folds west
of the Columbus Avenue syncline. The thick greenstone
and chert section on Twin Peaks appears to be part of a
northward -plunging anticline. The structural and strati -
graphic relations of the bedrock in the Twin Peaks -Mount
Sutro hills with bedrock in the area between Rincon
Hill -Telegraph Hill and Point Lobos are not known at
present.
No definite conclusions have been reached on the
structural relations of the serpentine bodies in the Fort
Point and Potrero Hill areas. Lawson (1914, p. 6; 1915,
p. 48) believed that they were parallel sill -like bodies.
Recent evidence indicates that they occupy a major
shear zone.
SEISMICITY
The San Francisco North quadrangle is part of a highly
active seismic belt. Earth tremors are frequent though
seldom severe, and all structures should be designed to
withstand them.
On a map showing epicenters of earthquakes that
occurred during the periods 1930 to 1941 and 1947 to
1948 (Byerly, 1951, p. 159), none appear in the quadrangle
but there are several in nearby areas, especially along the
well-known San Andreas fault zone. The San Andreas
fault crosses the Pacific Ocean shore several miles south
of the San Francisco North quadrangle and is believed to
lie below the ocean floor 3 or 4 miles west of Ocean Beach.
The 1906 earthquake vibrations, which caused enormous
destruction of life and property, resulted from a sudden
displacement along this zone.
Despite recurring seismid activity in recent years, no
field evidence of recent fault movement was found in the
quadrangle. Earthquakes severe enough to cause serious
damage to buildings in San Francisco have occurred only
3 times since 18.50: in 1865, 1868, and 1906. Each time the
greatest damage was in the artificial land built on old
tidal marshes, although these areas are farther from the
San Andreas fault than the less damaged portions of the
city. The least damage was found in hilly areas where
buildings were founded on or near bedrock. These facts
make it apparent that all construction must take into
consideration the reaction of the various earth materials
to seismic action. A rough appraisal of current ideas on
the earthquake stability of the geologic units in this
quadrangle is included in table 1.
SELECTED BIBLIOGRAPHY
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North Berkeley Hills, Calif.: Am. Jour. Sci., v. 252,
no. 10, p. 614-626.
Byerly, Perry, 1961, History of earthquakes in the San
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Palache, Charles, 1894, The lherzolite-serpentine and
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Calif.: U. S. Geol. Survey Misc. Geol. Inv. Map 1-264.
Ransome, F. L., 1894, The geology of Angel Island: Univ.
Calif. Pub., Bull. Dept. Geol., v. 1, no. 7, p. 193-240.
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1954, Preliminary bedrock -surface map of the San
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INTERIOR -GEOLOGICAL SURVEY. WASHINGTON 0 C -1966 666491
Far sale by U. S. Geological Survey, price $100