U.S. Department of the Interior
U.S. Geological Survey

Geology Of Palo Alto 30 X 60 Minute Quadrangle, California:  A Digital Database

By E.E. Brabb, R.W. Graymer, and D.L. Jones

Pamphlet Derived From Digital Open-File Report 98-348


Geologic Explanation

Introduction

This map database represents the integration of previously 
published and unpublished maps by several workers (see 
Sources of Data index map on Sheet 2 and the 
corresponding table below) and new geologic mapping and 
field checking by the authors with the previously 
published geologic map of San Mateo County (Brabb and 
Pampeyan, 1983) and Santa Cruz County (Brabb, 1989, 
Brabb and others, 1997), and various sources in a small 
part of Santa Clara County.  These new data are released 
in digital form to provide an opportunity for regional 
planners, local, state, and federal agencies, teachers, 
consultants, and others interested in geologic data to have 
the new data long before a traditional paper map is 
published.  The new data include a new depiction of 
Quaternary units in the San Francisco Bay plain 
emphasizing depositional environment, important new 
observations between the San Andreas and Pilarcitos 
faults, and a new interpretation of structural and 
stratigraphic relationships of rock packages 
(Assemblages).

Scope and Purpose of this report

The purpose of this new map is to compile the best 
available data on the identity and distribution of bedrock 
units, surficial deposits, and geologic structures at a 
regional scale, and to integrate those data into a digital 
spatial database.  The digital nature of the product is 
important because it allows the geologic data to be easily 
combined with other data to produce derivative products 
(some of which are discussed below), allows new 
information to be readily integrated into the map database, 
makes transfer of data to users easy via the Internet, and 
facilitates production of a high-quality paper maps.  Some 
of the benefits that can be derived from these geologic data 
are discussed below.

1.  Materials properties and geologic maps.  The 
character of the materials beneath the ground surface affect 
engineering use of the land.  For example, expansivity of 
the materials may affect the stability of foundations, 
permeability determines the success of water wells and 
suitability of the ground for septic systems, and ease of 
excavation is related to hardness of the material and 
fracture spacing.  Such material properties can be 
delineated in a regional way based on lithologic units 
shown on geologic maps.  The characterization of several 
materials properties of geologic map units has been done 
in the San Francisco Bay region by Ellen and Wentworth 
(1995) for hillside materials, and Helley and others (1979) 
for flatland (Quaternary) materials.  Our geologic map can 
be used in combination with the reports mentioned above 
to give the most up-to-date depiction of the distribution of 
materials properties that affect land use engineering.
2.  Earthquake shaking and geologic maps.  The 
intensity of earthquake shaking has been shown to depend 
strongly on the distribution of geologic units, the 
intensity increasing with decreasing firmness of ground 
materials.  In the San Francisco Bay region, the 
relationship between intensity and geologic units was first 
calculated by Borcherdt and others (1975).  More recently, 
earthquake shaking maps have been prepared for the region 
for a number of hypothetical earthquakes (Perkins and 
Boatwright, 1995), that integrate the effect of the geologic 
units on the calculated intensity.  These intensity maps 
are based on the most detailed geologic maps that were 
available during their study, but could be improved by 
integration of the most up-to-date geologic mapping 
available, such as this report.
3.  Liquefaction potential and geologic maps.  
Liquefaction during earthquakes happens in areas underlain 
by loose sand and silt that is saturated with water.  The 
distribution of these areas can be delineated by geologic 
and hydrologic mapping.  This mapping was first done for 
the San Francisco Bay region by Youd and others (1973), 
and later by Dupre and Tinsley (1980), and Tinsley and 
Holtzer (1990).  Most recently, Sowers and others (1992) 
have developed a technique that combines geologic 
mapping of surficial deposits with other mapped factors to 
produce a map of liquefaction potential.  The method of 
that report is entirely compatible with the new mapping 
of surficial units in this report, and therefore this report 
could be used in combination with other factors to produce 
a liquefaction susceptibility map for the map area.
4.  Ground water pollution and geologic maps.  In 
urban areas, such as the San Francisco Bay region, the 
contamination of ground water by introduction of 
pollution from the surface is a major problem (Howard, 
1997).  Because the flow of groundwater pollutants is 
governed by the porosity and permeability of materials 
shown on geologic maps, areas that are vulnerable to rapid 
contamination of groundwater can be identified.  In the 
southern San Francisco Bay region, for example, Helley 
(1986) showed that pollutants tended to follow high-
porosity Holocene stream channel and levee deposits, like 
those shown on our map.
5.  Relating landslides to geologic maps. Although 
the distribution of mapped landslides is a good first order 
indicator of future landslide activity in the San Francisco 
Bay region (Nilsen and Turner, 1975), a better analysis of 
the landslide hazard is provided by a statistical approach 
that involves more than a single factor, and yields a 
quantitative prediction of landslide potential.  Work by 
Brabb and others (1972) in San Mateo County provided 
the first such study using two factors, lithologic 
distribution, taken from a geologic map, and slope, in 
addition to landslide distribution.  GIS capabilities now 
allow a much more sophisticated analysis, using many 
factors and a variety of statistical methods (see Soeters and 
van Westin, 1996, for a discussion of statistical methods).  
At least one of the factors required for any such analysis, 
lithologic distribution, is provided solely by a geologic 
map.  In addition, some, but not all, of the additional 
factors, such as distance from an active fault or orientation 
of strata, may be derived from a geologic map.  Clearly, 
accurate geologic maps are a prerequisite to sophisticated 
studies of landslides.
6.  Location of mineral hazards and geologic maps.  
Certain naturally occurring minerals can pose a hazard to 
human health or to the environment when disturbed by 
human development.  An example is the naturally 
occurring mercury deposits in the San Francisco Bay 
region (J. Rytuba, USGS, written comm., 1997).  
Because hazardous minerals are in many places associated 
with a specific geologic unit, such as mercury with silica-
carbonate rock in serpentinite (Crittenden, 1951), geologic 
maps that show the distribution of those units can be used 
to regionally delineate the potential hazard. 
7.  Seismic velocity.  Accurate seismic velocity models 
are required to precisely locate earthquake epicenters, 
which can be used to map fault location, geometry, and 
activity.  Seismic velocity is proportional to the density 
of the rock in the crust, which in turn is dependent on the 
rock type.  However, most seismic velocity models are 
constructed by calculation based on travel times of seismic 
waves from various sources.  Construction of a superior 
model may be possible by combining the regional 
distribution of geologic materials at the surface provided 
by a geologic map with data from well logs, and 
aeromagnetic, gravity, and seismic data..
8.  Earthquake faults and geologic maps.  Although 
regional geologic maps cannot take the place of large-scale 
fault-rupture hazard maps (Hart, 1988), they do show 
faults in their geologic context.  Recent work by the 
authors along the Hayward fault in the east San Francisco 
Bay area has shown that studying active faults in their 
geologic context reveals a much more complex history of 
faulting and a wider zone of Holocene active faults than 
previously thought (Graymer, Jones, and Brabb, 1995).  
In addition, other recent work based on the geologic 
context of faults in the San Francisco Bay region (Jones 
and others, 1995, Jayko and Lewis, 1996) has shown that 
a component of stress in the area is perpendicular to 
known active faults, suggesting the possibility of active 
thrust and blind-thrust faults.  The 1989 Loma Prieta 
earthquake may have occurred on such a fault.  Blind-
thrusts, in areas without intensive seismic 
reflection/refraction studies, such as most urban areas, 
must be mapped by studying the regional structure, as 
there is no surface rupture.
9.  Education and scientific inquiry.  In addition to 
applications related to engineering and geologic hazards, 
regional geology can be a subject of intellectual curiosity 
and academic inquiry.  This map represents the state of 
knowledge of the geology of the area at the time of 
publication, and can be used by the academic community 
and by the public to understand the geology of the area at 
a regional scale.

NOTE

Uses of this map database are limited by compilation scale 
and content.  The map is not intended for site specific 
studies of any sort  The fault maps of the California 
Division of Mines and Geology (see Hart, 1988 for an 
index) should be used for site specific studies of fault 
activity, whereas the work of Wentworth and others, 
1997, provide a regional depiction of landslides and Pike, 
1997, provides an index of more detailed work.

Stratigraphy

Lithologic associations in the Palo Alto 30 X 60 minute 
quadrangle are divided into ten assemblages (see Index Map 
on Sheet 2).  As defined in Graymer, Jones, and Brabb 
(1994), assemblages are large, fault - bounded blocks that 
contain a unique stratigraphic sequence.  The stratigraphic 
sequence differs from that of neighboring assemblages by 
containing different rock units, or by different stratigraphic 
relationship among similar rock units.  These 
stratigraphic differences represent changes in depositional 
conditions in one or more large depositional basins.  The 
current adjacent location of the different assemblages 
reflects the juxtaposition of different basins or parts of 
basins by large offsets along the faults that bound the 
assemblages.
	In general, the Tertiary strata in the map area rest 
with angular unconformity on three complexly deformed 
Mesozoic rock complexes.  One of these Mesozoic 
complexes is made up of  the Coast Range ophiolite, 
which includes serpentinite, gabbro, diabase, and basalt; 
keratophyre which is closely associated with the ophiolite; 
and overlying Great Valley sequence of Jurassic and 
Cretaceous age.  This complex represents the accreted and 
deformed remnants of Jurassic oceanic crust, overlying arc 
volcanic rocks, and a thick sequence of turbidites.  In the 
map area rocks of this complex are only definitely present 
in small, fault-bounded slivers at the base of the Portola 
Valley Assemblage, although the large unit of diabase and 
gabbro (db) along the San Andreas fault in the Woodside 
Assemblage may also be part of the complex.
	The second Mesozoic complex is the Franciscan 
complex, which is composed of weakly to strongly 
metamorphosed graywacke, argillite, limestone, basalt, 
serpentinite, chert, and other rocks.  The rocks of the 
Franciscan complex in the area were probably Jurassic 
oceanic crust and pelagic deposits overlain by Upper 
Jurassic to Upper Cretaceous turbidites.  Although 
Franciscan rocks are dominantly little metamorphosed, 
high-pressure, low-temperature metamorphic minerals are 
common within the complex (Bailey, Irwin, and Jones, 
1964).  High-grade metamorphic blocks in sheared but 
relatively unmetamorphosed argillite matrix (Blake and 
Jones, 1974) reflects the complicated history of the 
Franciscan.  The complex was subducted beneath the 
Coast Range ophiolite, at least in part, during Late 
Cretaceous or early Tertiary time, after the deposition of 
the Franciscan sandstone containing  Campanian (Late 
Cretaceous) fossils that crops out in Marin County.  
Because the Franciscan was subducted under the complex 
containing the Coast Range Ophiolite, the contact 
between the two Mesozoic complexes is everywhere 
faulted (Bailey, Irwin, and Jones, 1964), and the 
Franciscan complex presumably underlies the entire area 
east of the Pilarcitos fault.
	The third Mesozoic complex is the Salinian 
complex, which is composed of granitic plutonic rocks, 
and inferred gabbroic plutonic rocks at depth, overlain in 
places by Cretaceous strata.  It is separated from the 
combined Franciscan, Coast Range ophiolite, Great 
Valley sequence Mesozoic basement on the east by the 
Pilarcitos fault in the north part of the map area, and the 
San Andreas fault in the south part of the map area where 
the Pilarcitos and San Andreas faults join.  In places, 
small outcrops of pre-plutonic (Paleozoic?)  rocks are also 
preserved, such as KJv in the Pigeon Point Assemblage, 
marble and hornfels (m) in the Montara Mountain 
Assemblage, and schist (sch) in the Santa Cruz 
Assemblage.  The plutonic rocks are part of a batholith 
that has been displaced northward by offset on the San 
Andreas fault system.  Estimates of total offset vary, but 
all are more than a few hundred kilometers.
	An angular unconformity at the base of the 
Tertiary strata has been preserved in all of the 
assemblages.  The exception is the Portola Valley 
Assemblage, where the base of the Tertiary strata is 
everywhere faulted.

Paleontology

Hundreds of fossil collections from the Palo Alto 30 X 60 
minute quadrangle are described in the references provided 
in this report, as well as in Brabb (1983).

Radiometric Ages

A compilation of the radiometric ages of rocks in the Palo 
Alto 30 X 60 minute quadrangle and other areas south of 
latitude 38 degrees is provided by Lindquist and 
Morganthaler (1991).  Additional data are provided by 
Sarna-Wojcicki (1976, and written comm., 1990), Sarna-
Wojcicki and others (1979), Turner (1970), Brabb and 
Hanna (1981), and Curtis (1989).

Structure

The faults of the map area are characterized by both strike-
slip and dip-slip components of displacement.  Three 
major fault systems display large late Tertiary right-lateral 
offsets-- the San Andreas, the Pilarcitos, and the San 
Gregorio fault zones.  These fault systems trend roughly 
N30W, and most of them have many fault strands in a 
broad zone as much as 10 km wide.  Offset is distributed 
on the various faults in the zones, and the locus of fault 
movement associated with a fault zone has changed 
through geologic time (see Graymer, Jones, and Brabb, 
1995, and Montgomery and Jones, 1992, for a description 
of fault zone evolution and distribution of offset on 
similar active faults in the East Bay region).  Both the 
San Andreas and San Gregorio fault zones have strands 
which display Holocene offset.  The land on either side of 
the San Andreas fault zone was displaced up to several 
meters in the map area during the 1906 earthquake.  
	Current estimates of total offset since 8 Ma are 
about 35 km for the San Andreas fault zone in the map 
area, 120 km for the Pilarcitos fault zone, and 155 km for 
the San Gregorio fault zone (Clark and others, 1984, 
McLaughlin and others, 1996, Dickinson, 1997).  
However, ongoing work by the authors and others has 
correlated the Coast Range ophiolite (Jgb, Jsv, sp)  and 
Great Valley Sequence (Ka, Ks) rocks at the base of the 
Portola Valley Assemblage with rocks in the Gualala area 
more than 150 km north of the map area, which might 
indicate even more offset on the Pilarcitos fault.  
	The three major right-lateral fault systems also 
form many of the boundaries of the assemblages.  The 
juxtaposition of rocks with different stratigraphic histories 
across these faults probably resulted from the large offsets 
on the faults.  The other assemblage-bounding faults 
probably had total offsets of tens of kilometers as well, as 
they also juxtapose rocks with different stratigraphic 
histories, although the offset is undoubtedly less than the 
three major systems, as the stratigraphic differences are 
less.
	In addition to strike-slip faults, some faults in 
the map area have a major component of reverse or thrust 
offset.  These structures run generally subparallel to the 
strike-slip faults, and reflect a component of stress 
perpendicular to the trend of the faults.  This fault-normal 
compression has generated faults that typically have 
juxtaposed older rocks above younger.  Part of the offset 
on some of these reverse and thrust faults has taken place 
during Quaternary time, as shown by faulting of QTsc 
west of Crystal Springs Reservoir. 
	Apparent offset on the large, assemblage-
bounding faults in the west-central part of the map area 
becomes progressively younger to the northeast.  The 
Zayante fault completed most of its offset by late Miocene 
time, because the base of the Santa Margarita Sandstone 
(Tsm) unconformably overlaps the fault.  The Butano 
fault appears to have completed the bulk of the offset 
along it by Pliocene or latest Miocene time, because it is 
overlapped by the Purisima Formation (Tp, Tpt), but 
truncates Tsm and Tsc.  The La Honda fault cuts the Tp, 
and so was active at least into the Pliocene, whereas the 
Pilarcitos fault cuts the QTsc so was active into the 
Pleistocene, but both show little or no evidence for 
Holocene offset.  The San Andreas fault zone is the locus 
of Holocene activity, as shown by the large offset in 
1906.  The significance of this northeastward younging 
trend is not at present understood.
	Folds in the map area can be divided into two 
categories based on axial trend and style of deformation.  
The first category includes tight folds and overturned folds 
with inclined axial planes whose axes trend obliquely to 
the major strike-slip fault zones (about N60W).  These 
folds were probably caused by the same component of 
regional stress that formed the strike-slip faults and the 
thrust and reverse faults of the second category discussed 
above.
	The second category of fold is tight, upright folds 
whose axes strike roughly parallel to the major strike-slip 
faults (about N30W).  These folds must have been formed 
by a component of regional compression perpendicular to 
the strike-slip faults (Jones and others, 1995).
	Preserved folds in the maps area for the most part 
formed in Pliocene or later time, because Pliocene strata 
(Tp), where present, are involved in the folds.  In the 
Portola Valley Assemblage, major pre-Pliocene Tertiary 
folding is not indicated, because Pliocene rocks are folded 
as much as pre-Pliocene Tertiary strata.  However, in the 
Butano Ridge and Santa Cruz Assemblages, the major 
angular unconformity at the base of the late Miocene 
strata indicates a significant period of pre-late-Miocene 
folding.  In addition, whereas all pre-late Miocene strata in 
the Butano Ridge Assemblage appear to have undergone 
the same amount of deformation, an additional major 
angular unconformity is present within the pre-late 
Miocene strata of the Santa Cruz Assemblage at the base 
of the middle Miocene Lompico Sandstone (Tlo).  In the 
Montara Mountain Assemblage a nonconformity at the 
base of the middle Miocene strata and an angular 
unconformity at the base of the Pliocene strata indicate 
two periods of pre-Pliocene Tertiary uplift and folding.  In 
the Pigeon Point Assemblage, a similar angular 
unconformity at the base of the Pliocene is present, and 
Tuv rests unconformably on Kpp.  In the Mindego Hill 
Assemblage, the base of the Pliocene unconformity is 
again present, but the middle Miocene strata there are 
folded as much as the pre-Miocene strata, indicating that 
only one period of pre-Pliocene Tertiary deformation 
occurred in that Assemblage.  In the Woodside 
Assemblage, Pliocene rocks of the Merced Formation are 
folded as much as earlier Tertiary strata, indicating that 
Tertiary deformation in that Assemblage must be Pliocene 
or younger.  The Tertiary deformation of the strata in the 
map area is, therefore, a complex amalgamation of 
independently deformed blocks that have been brought into 
proximity only in late Tertiary time.  Pre-Tertiary folding 
undoubtedly occurred, associated with subduction of the 
Franciscan complex beneath the Coast Range ophiolite 
and subsequent deformation associated with the 
unconformity at the base of the Tertiary sequence, as well 
as offset on strike-slip faults. These folds have for the 
most part been totally disrupted.  The youngest folding 
must postdate the Pliocene and Pleistocene deposition of 
QTsc and QTm, as those strata are folded in at least one 
area, and are steeply inclined throughout the county.  
Pleistocene strata and marine terraces have not been 
observed to be folded, but are tilted and uplifted in several 
places, and locally faulted.  Late Pleistocene and Holocene 
surficial deposits retain most of their original depositional 
shape and orientation, so are not tilted or folded, but late 
Pleistocene alluvium and marine terrace deposits have 
been uplifted as much as several meters in places 
throughout the county.



Description Of Map Units

af	Artificial fill (Historic)--Loose to very well consolidated gravel, sand, silt, clay, rock fragments, organic matter, 
and man-made debris in various combinations.  Thickness is variable and may exceed 30 m in places. Some is  
compacted and quite firm, but fill made before 1965 is nearly everywhere not compacted and consists simply of 
dumped materials
alf	Artificial levee fill (Historic)--Man-made deposit of various materials and ages, forming artificial levees as much 
as 6.5 m high.  Some are compacted and quite firm, but fills made before 1965 are almost everywhere not 
compacted and consist simply of dumped materials.  The distribution of levee fill conforms to levees shown on 
the most recent U.S. Geological Survey 7.5-minute quadrangle maps
Qhasc	Artificial stream channels (Historic)--Modified stream channels, in most places where streams have been 
straightened and realigned
Qhsc	Stream channel deposits (Holocene)--Poorly to well-sorted sand, silt, silty sand, or sandy gravel with minor 
cobbles.  Cobbles are more common in the mountainous valleys.  Many stream channels are presently lined 
with concrete or rip rap.  Engineering works such as diversion dams, drop structures, energy dissipaters and 
percolation ponds also modify the original channel.  Many stream channels have been straightened, and these are 
labeled Qhasc.  This straightening is especially prevalent in the lower reaches of streams entering the estuary.  
The mapped distribution of stream channel deposits is controlled by the depiction of major creeks on the most 
recent U.S. Geological Survey 7.5-minute quadrangles.  Only those deposits related to major creeks are mapped.  
In some places these deposits are under shallow water for some or all of the year, as a result of reservoir release 
and annual variation in rainfall.
Qbs	Beach sand (Holocene) -- Unconsolidated, well-sorted sand.  Local layers of pebbles and cobbles.  Thin 
discontinuous lenses of silt relatively common in back-beach areas.  Thickness variable, in part due to seasonal 
changes in wave energy: commonly less than 10 m thick.  May interfinger with either well-sorted dune sand or, 
where adjacent to coastal cliff, poorly-sorted colluvial deposits.  Iron- and magnesium-rich heavy minerals 
locally form placers as much as 0.7 m thick.
Qhbm	Bay mud (Holocene)-- Water-saturated estuarine mud, predominantly gray, green and blue clay and silty clay 
underlying marshlands and tidal mud flats of San Francisco Bay, Pescadero, and Pacifica.  The upper surface is 
covered with cordgrass (Spartina sp.) and pickleweed (Salicornia sp.).  The mud also contains a few lenses of 
well-sorted, fine sand and silt, a few shelly layers (oysters), and peat.  The mud interfingers with and grades into 
fine-grained deposits at the distal edge of Holocene fans, and was deposited during the post-Wisconsin rise in sea-
level, about 12 ka to present (Imbrie and others, 1984).  Mud varies in thickness from zero, at landward edge, to 
as much as 40 m near north County line
Qhb	Basin deposits (Holocene)--Very fine silty clay to clay deposits occupying flat-floored basins at the distal edge of 
alluvial fans adjacent to the bay mud (Qhbm).  Also contains unconsolidated, locally organic, plastic silt and 
silty clay deposited in very flat valley floors
Qhbs	Basin deposits, salt-affected (Holocene) -- Clay to very fine silty-clay deposits similar to Qhb deposits except 
that they contain carbonate nodules and iron-stained mottles (U.S. Soil Conservation Service, 1958).  These 
deposits may have been formed by the interaction of bicarbonate-rich upland water and saline water of the San 
Francisco Bay estuary.  With minor exceptions, salt-affected basin deposits are in contact with estuary deposits.
Qhfp	Floodplain deposits (Holocene)--Medium to dark gray, dense, sandy to silty clay.  Lenses of coarser material (silt, 
sand, and pebbles) may be locally present.  Flood plain deposits usually occur between levee deposits (Qhl) and 
basin deposits (Qhb)
Qhl	Natural levee deposits (Holocene)--Loose, moderately to well-sorted sandy or clayey silt grading to sandy or silty 
clay.  These deposits are porous and permeable and provide conduits for transport of ground water.  Levee 
deposits border stream channels, usually both banks, and slope away to flatter floodplains and basins.  
Abandoned levee systems, no longer bordering stream channels, have also been mapped
Qhaf1	Younger alluvial fan deposits (Holocene) -- Brown, poorly-sorted, dense, sandy or gravelly clay.  May represent 
the modern loci of deposition for Qhaf, although small fans at mountain fronts may have a debris-flow origin.
Qhaf	Alluvial fan and fluvial deposits (Holocene)--Alluvial fan deposits are brown or tan, medium dense to dense, 
gravely sand or sandy gravel that generally grades upward to sandy or silty clay.  Near the distal fan edges, the 
fluvial deposits are typically brown, never reddish, medium dense sand that fines upward to sandy or silty clay
Qyf	Younger (inner) alluvial fan deposits (Holocene)--Unconsolidated fine- to coarse-grained sand, silt, and gravel, 
coarser grained at heads of fans and in narrow canyons
Qyfo	Younger (outer) alluvial fan deposits (Holocene)--Unconsolidated fine sand, silt, and clayey silt
Qcl	Colluvium (Holocene)--Loose to firm, friable, unsorted sand, silt, clay, gravel, rock debris, and organic material in 
varying proportions
Qs	Sand dune and beach deposits (Holocene)--Predominantly loose, medium- to coarse-grained, well-sorted sand 
but also includes pebbles, cobbles, and silt.  Thickness less than 6 m in most places, but in other places may 
exceed 30 m
Qal	Alluvium (Holocene)--Unconsolidated gravel, sand, silt, and  clay along streams.  Less than a few meters thick in 
most places
Qls	Landslide deposits (Pleistocene and/or Holocene) -- Poorly sorted clay, silt, sand, and gravel.  Only a few very 
large landslides have been mapped.  For a more complete map of landslide deposits, see Nilsen and others 
(1979).
Qpaf	Alluvial fan and fluvial deposits (Pleistocene)--Brown dense gravely and clayey sand or clayey gravel that fines 
upward to sandy clay.  These deposits display variable sorting and are located along most stream channels  in the 
county.  All Qpaf deposits can be related to modern stream courses.  They are distinguished from younger 
alluvial fans and fluvial deposits by higher topographic position, greater degree of dissection, and stronger soil 
profile development.  They are less permeable than Holocene deposits, and locally contain fresh water mollusks 
and extinct late Pleistocene vertebrate fossils.  They are overlain by Holocene deposits on lower parts of the 
alluvial plain, and incised by channels that are partly filled with Holocene alluvium on higher parts of the 
alluvial plain.  Maximum thickness is unknown but at least 50 m.
Qpaf1	Alluvial terrace deposits (Pleistocene)--Deposits consist of crudely - bedded, clast-supported, gravels, cobbles, 
and boulders with a sandy matrix.  Clasts are as much as 35 cm in intermediate diameter.  Coarse sand lenses 
may be locally present.  Pleistocene terrace deposits are cut into Pleistocene alluvial fan deposits (Qpaf) a few 
meters and lie up to several meters above Holocene deposits
Qpoaf	Older alluvial fan deposits (Pleistocene)--Brown dense gravely and clayey sand or clayey gravel that fines upward 
to sandy clay.  These deposits display various sorting qualities.  All Qpoaf deposits can be related to modern 
stream courses.  They are distinguished from younger alluvial fans and fluvial deposits by higher topographic 
position, greater degree of dissection, and stronger profile development.  They are less permeable than younger 
deposits, and locally contain fresh- water mollusks and extinct Pleistocene vertebrate fossils.
Qof	Coarse-grained older alluvial fan and stream terrace deposits (Pleistocene)--Poorly consolidated gravel, 
sand, and silt, coarser grained at heads of old fans and in narrow canyons
Qmt	Marine terrace deposits (Pleistocene)--Poorly consolidated and poorly indurated well- to poorly-sorted sand and 
gravel.  Thickness variable but probably less than 30 m
QTsc	Santa Clara Formation (lower Pleistocene and upper Pliocene)--Gray to red-brown poorly indurated 
conglomerate, sandstone, and mudstone in irregular and lenticular beds.  Conglomerate consists mainly of 
subangular to subrounded cobbles in a sandy matrix but locally includes pebbles and boulders.  Cobbles and 
pebbles are mainly chert, greenstone, and graywacke with some schist, serpentinite, and limestone.  On Coal 
Mine Ridge, south of Portola Valley, conglomerate contains boulders of an older conglomerate as long as one 
meter.  Gray to buff claystone and siltstone beds on Coal Mine Ridge contain carbonized wood fragments as 
large as 60 cm in diameter.  Included in Santa Clara Formation are similar coarse-grained clastic deposits near 
Burlingame.  Sarna-Wojcicki (1976) found a tuff bed in Santa Clara Formation near Woodside, and correlated it 
with a similar tuff in the Merced Formation.  Later work indicated that the tuff correlates with the 435 ka 
Rockland ash (Sarna-Wojcicki, oral comm., 1997).  Thickness of Santa Clara Formation is variable but reaches 
a maximum of about 500 m along Coal Mine Ridge
QTsl	Lake beds (upper Pliocene) -- Fine-grained sandstone, calcareous mudstone, and marl.  Locally contains 
vertebrate fossils of late Pliocene (Blancan) age.  Fossiliferous marl is best exposed near Stevens Creek 
Reservoir, where it is about 30 m thick.
QTm	Merced Formation (lower Pleistocene and upper Pliocene)--Medium-gray to yellowish gray and yellowish 
orange, medium- to very fine-grained, poorly indurated to friable sandstone, siltstone, and claystone, with some 
conglomerate lenses and a few friable beds of white volcanic ash.  In many places sandstone is silty, clayey, or 
conglomeratic.  Some of the conglomerate, especially where fossiliferous, is well cemented.  Volcanic ash is in 
beds as much as 2 m thick and consists largely of glass shards.  In type section of Merced Formation north of 
the map area, the ash was originally reported by Sarna-Wojcicki (1976) to be 1.5+0.8 m.y. old, but more recent 
work by Sarna-Wojcicki and others (1991) indicates that the formation contains both the 738+ 3 ka Bishop ash 
and the 435 ka Rockland ash (Sarna-Wojcicki, oral comm., 1997).  Merced Formation is about 1525 m thick in 
the sea cliffs north of Mussel Rock
Tp	Purisima Formation (Pliocene and upper Miocene)--Predominantly gray and greenish-gray to buff fine-grained 
sandstone, siltstone, and mudstone, but also includes some porcelaneous shale and mudstone, chert, silty 
mudstone, and volcanic ash.  West of Portola Valley, this unit consists of fine- to medium-grained silty 
sandstone.  Locally divided into:
Tptu	Tunitas Sandstone Member (Pliocene)--Greenish-gray to light-gray, pale-orange, or greenish-brown, very 
fine- to medium-grained sandstone with clay matrix.  Concretions generally less than 30 cm across are present 
locally.  Tunitas ranges in thickness from 76 m at type section to 122 m elsewhere
Tpl	Lobitos Mudstone Member (Pliocene)--Dark-gray to light-gray and shades of brown, unbedded, silty 
mudstone.  Lobitos has a maximum thickness of 140 m.
Tpsg	San Gregorio Sandstone Member (Pliocene)--Greenish-gray to light-brown fine- to coarse-grained sandstone 
containing calcareous concretions less than 30 cm across.  San Gregorio Member ranges in thickness from 45 m 
at type section to about 140 m elsewhere
Tpp	Pomponio Mudstone Member (Pliocene)--Gray to white porcelaneous shale and mudstone, in places 
rhythmically bedded with alternating layers of nonsiliceous mudstone.  This unit resembles Monterey Shale, 
Santa Cruz Mudstone, and Lambert Shale.  At its type section in Pomponio Creek the member is 700 m thick
Tpt	Tahana Member (Pliocene and upper Miocene)--Greenish-gray to white or buff, medium- to very fine-
grained sandstone and siltstone, with some silty mudstone.  Locally, such as at San Gregorio State Beach, 
sandstone is tuffaceous and weathers white.  Near Memorial Park, this member includes dark-gray porcelaneous 
mudstone.  Pebble conglomerate occurs near base from Memorial Park eastward.  Maximum thickness is 655 
m.  A tuff bed in this member west of the San Gregorio fault has been tentatively correlated with the 2.6 Ma 
Ishi Tuff (Sarna-Wojcicki and others, 1991)
Tsc	Santa Cruz Mudstone (upper Miocene)--Brown and gray to light-gray, buff, and light-yellow siliceous 
mudstone with nonsiliceous mudstone and siltstone and minor amounts of sandstone.  Santa Cruz Mudstone is 
more than 1000 m thick
Tsm	Santa Margarita Sandstone (upper Miocene)--Light-gray to grayish-orange to white, friable, very fine- to very 
coarse-grained arkosic sandstone.  Fine-grained sandstone commonly contains glauconite.  A quartz and feldspar 
pebble conglomerate crops out locally at the base of section.  Santa Margarita Sandstone is as thick as 60 m
Tms	Unnamed marine sandstone and shale (upper Miocene) -- Light-gray, grayish-orange, and white, soft, 
friable, very fine- to medium-grained, well-sorted, poorly cemented quartzose sandstone with minor interbeds of 
siliceous mudstone and semi-siliceous shale.  Contains late Miocene, shallow water marine fossils (Sorg and 
McLaughlin, 1975).
Tlad	Ladera Sandstone (upper(?) and middle Miocene)--Medium- to light-gray to yellowish-gray and buff, fine-
grained, poorly cemented sandstone and siltstone, with minor amounts of coarse-grained sandstone, yellow-
brown dolomitic claystone, and white to light-gray porcelaneous shale and porcelanite.  Fine-grained sandstone 
and siltstone comprise more than 90 percent of formation.  Coarse-grained sandstone crops out in beds less than 
a few meters thick in lower half of section; dolomitic claystone and porcelaneous shale beds are less than a meter 
thick and outcrop scattered through the upper half of the section; porcelanite crops out in thin-bedded lenses less 
than a few meters thick in the lower part of the section.  At and near base of Ladera Sandstone are medium to 
thick lenticular beds of well-cemented fossiliferous, chert-granule sandstone which interfingers with fine-grained 
sandstone.  About 450 m thick
Tm	Monterey Formation (middle Miocene)--Grayish-brown and brownish-black to very pale orange and white, 
porcelaneous shale with chert, porcelaneous mudstone, impure diatomite, calcareous claystone, and with small 
amounts of siltstone and sandstone near base.  Monterey is generally thinner-bedded than the Santa Cruz 
Mudstone but closely resembles parts of Purisima Formation, especially Pomponio Mudstone Member.  
Thickness ranges from 120 to more than 600 m
Tlo	Lompico Sandstone (middle Miocene)--Very pale orange, fine to coarse-grained, mostly well-cemented and hard 
arkosic sandstone.  Maximum thickness about 300 m
Tpm	Page Mill Basalt (middle Miocene)--Interlayered, columnar-jointed basaltic flows and agglomerate.  Flows are 
dark greenish gray  to light gray, dense to vesicular, and finely crystalline; agglomerate is light gray to reddish 
brown.  Volcanic rocks are pyritiferous in part.  Ranges in thickness from 0 to 15 m.  The Page Mill Basalt has 
yielded a K/Ar age of 14.8+2.4 Ma (Turner, 1970, recalculated by Fox and others, 1985).
Tuv	Unnamed Sedimentary and Volcanic Rocks (Miocene and Oligocene)--Mainly dark-gray, hard mudstone in 
Aöo Nuevo area and massive, coarse-grained and pebbly, crossbedded, hard sandstone in Pescadero Point area.  
Mapped as Vaqueros(?) Formation by Hall and others (1959), but rocks do not resemble those of Vaqueros 
Sandstone in Santa Cruz Mountains.  Includes andesite breccia.  Intrusive rocks associated with the andesite have 
yielded a K/Ar age of 22.0 + 0.7 Ma (Taylor, 1990).  Contains foraminifers and mollusks of Zemorrian 
(Oligocene) and Saucesian (Miocene) age according to Clark and Brabb (1978).  About 135 m thick near 
Pescadero Point and at least 85 m thick near Aöo Nuevo
Tls	Lambert Shale and San Lorenzo Formation, Undivided (lower Miocene, Oligocene, and middle and 
upper Eocene)--Brown and dark-gray to gray, brown, and red mudstone, siltstone, and shale.  Includes some 
beds of fine- to coarse-grained sandstone.  Lambert Shale is generally more siliceous than San Lorenzo 
Formation, but the two units cannot be distinguished where out of stratigraphic sequence and without fossils
Tla	Lambert Shale (Oligocene and lower Miocene)--Dark-gray to pinkish-brown, moderately well-cemented 
mudstone, siltstone, and claystone.  Chert crops out in a few places in upper part of section, and sandstone 
bodies up to 30 m thick, glauconitic sandstone beds, and microcrystalline dolomite are present in places.  
Lambert Shale is generally more siliceous than San Lorenzo Formation and less siliceous than the Monterey 
Shale.  It resembles Santa Cruz Mudstone and parts of Purisima Formation.  Lambert Shale is about 1460 m 
thick
Tmb	Mindego Basalt and related volcanic rocks (Miocene and/or Oligocene)--Basaltic volcanic rocks, both 
extrusive and intrusive.  Extrusive rock is primarily dark-gray to orange-brown to greenish-gray flow breccia, 
but includes lesser amounts of tuffs, pillow lavas, and flows.  Extrusive rocks have a maximum thickness of 
120 m.  Intrusive rock is dark greenish gray to orange brown and medium to coarsely crystalline.  It commonly 
weathers spheroidally, and crops out as roughly tabular bodies up to 180 m thick intruding older sedimentary 
rocks.  Minor amounts of sandstone and mudstone are locally included.  The Mindego Basalt has yielded a K/Ar 
minimum age of 20.2+1.2 Ma (Turner, 1970, recalculated by Fox and others, 1985).
Tvq	Vaqueros Sandstone (lower Miocene and Oligocene)--Light-gray to buff, fine- to medium-grained, locally 
coarse-grained, arkosic sandstone interbedded with olive- and dark-gray to red and brown mudstone and shale.  
Sandstone beds are commonly 0.3 to 3 m thick and mudstone and shale beds are as much as 3 m thick.  
Vaqueros varies from a few meters to as much as 700 m in thickness
Tz	Zayante Sandstone (Oligocene) -- Thick- to very thick-bedded, yellowish-orange arkosic non-marine sandstone 
containing thin interbeds of greenish and reddish siltstone and lenses and thick interbeds of pebble and cobble 
conglomerate.  Thickness 550 m along Lompico Creek.
Tsl	San Lorenzo Formation (Oligocene and upper and middle Eocene)--Dark-gray to red and brown shale, 
mudstone, and siltstone with local interbeds of sandstone.  About 550 m thick.  Locally divided into:
Tsr	Rices Mudstone Member (Oligocene and upper Eocene)--Olive-gray to red and brown unbedded mudstone 
and siltstone with some laminated shale.  Spheroidal weathering is common, as are elongate carbonate 
concretions.  About 300 m thick
Tst	Twobar Shale Member (middle and upper Eocene)--Olive-gray to red and brown laminated shale with 
some mudstone.  Includes a few thin interbeds of very fine-grained sandstone which thicken to as much as 30 m 
near Big Basin.  About 240 m thick
Tb	Butano Sandstone (middle and lower Eocene)--Light-gray to buff, very fine- to very coarse-grained arkosic 
sandstone in thin to very thick beds interbedded with dark-gray to brown mudstone and shale.  Conglomerate, 
containing boulders of granitic and metamorphic rocks and well-rounded cobbles and pebbles of quartzite and 
porphyry, is present locally in lower part of section.  Amount of mudstone and shale varies from 10 to 40 
percent of volume of formation.  About 3000 m thick
Tbu	Upper sandstone member -- Thin-bedded to very-thick-bedded medium- gray, fine- to medium-grained arkosic 
sandstone containing thin interbeds of medium-gray siltstone.  Thickness about 215 m.
Tbm	Middle siltstone member -- Thin- to medium-bedded, nodular, olive-gray pyritic siltstone.  Thickness about 
215 m.
Tbl	Lower conglomerate and sandstone member -- Thick to very thick interbeds of sandy pebble conglomerate 
and very-thick-bedded to massive, yellowish-gray, granular, medium- to coarse-grained arkosic sandstone. 
Thickness as much as 1500 m.
Tblc	Conglomerate -- Thick to very thick interbeds of sandy pebble conglomerate mapped locally in the lower 
member.
Tbs	Shale in Butano Sandstone (lower Eocene)--Greenish-gray, light gray, red, and reddish brown clay shale, 
mudstone, siltstone, and a few thin interbeds of light gray sandstone.  Exposed near the head of Corte Madera 
Creek.  Total thickness is unknown, but at least 200 m of this material is exposed
Tw	Whiskey Hill Formation (middle and lower Eocene)--Light-gray to buff coarse-grained arkosic sandstone, with 
light-gray to buff silty claystone, glauconitic sandstone, and tuffaceous siltstone.  Sandstone beds constitute 
about 30 percent of map unit.  Tuffaceous and silty claystone beds are expansive.  Locally, sandstone beds are 
well cemented with calcite.  At apparent base of section on north side of Jasper Ridge, just east of Searsville 
Lake, a thin greenstone-pebble conglomerate is present.  In places within this map unit, sandstone and claystone 
beds are chaotically disturbed.  This formation is as much as 900 m thick
Tws	Shale in Whiskey Hill Formation (lower Eocene)--Brown and reddish brown claystone, mudstone, siltstone 
and shale.  Locally contains lenses of sandstone up to 50 m thick.  Exposed along Highway 84, and along 
Highway 92, east of Half Moon Bay, where a small patch of red mudstone can be seen in a drainage ditch.  Total 
thickness is unknown, but at least 200 m of this material is exposed along Highway 84.
Tu	Unnamed sedimentary rocks (Eocene?) -- Mudstone, shale, and argillite with minor sandstone.
Tl	Locatelli Formation (Paleocene) -- Nodular, olive-gray to pale-yellowish-brown micaceous siltstone.  Thickness 
245-275 m.  Locally near base includes:
Tlss	Sandstone -- Massive, medium-gray, fine- to medium-grained arkosic sandstone.  Maximum thickness 25 m.
Kpp	Pigeon Point Formation (Upper Cretaceous)--Sandstone and conglomerate, interbedded with siltstone and 
mudstone and pebbly mudstone.  Sandstone is fine- to coarse-grained, arkosic, and gray to greenish gray; 
mudstone and siltstone are gray or black to buff.  Conglomerate contains well-rounded pebbles, cobbles, and 
boulders of red and gray fine-grained and porphyritic felsic volcanic rocks, granitic rocks, chert, quartzite, dark-
colored metamorphic rock, limestone, and clastic sedimentary rocks.  Pigeon Point Formation is estimated to be 
more than 2600 m thick
Ksh	Unnamed shale (Upper Cretaceous)--Dark-gray, thin-bedded, nodular shale and silty shale.  Unit is exposed only 
in the bed of San Francisquito Creek, in Menlo Park, where about 15 m of section is visible
Ka	Conglomerate of strata of Anchor Bay (Wentworth, 1968) (Cretaceous)--Massive sandstone and conglomerate 
with pebbles and cobbles of diabase, gabbro, and minor granitic rocks; contains abundant shell fragments of a 
rudistid bivalve similar to Coraliochama orcutti of Late Cretaceous (Campanian) age.
Ks	Unnamed sandstone and shale (Cretaceous(?))-- Rhythmically interbedded, indurated micaceous sandstone and 
greenish-gray argillite; age uncertain, but probably Cretaceous based on lithologic similarity to other Cretaceous 
strata in the Santa Cruz Mountains
Kgr	Granitic rocks of Montara Mountain (Cretaceous)--Very light gray to light brown, light brown, medium- to 
coarsely-crystalline foliated granitic rock, largely quartz diorite with some granite.  These rocks are highly 
fractured and deeply weathered.  Foliation is marked by an alignment of dark minerals and dark dioritic 
inclusions.  Tabular bodies of aplite and pegmatite generally parallel foliation.  Rocks from this unit have 
yielded K/Ar ages of 91.6 Ma (Curtis, Everndon, and Lipson, 1958) and 86.2+3.4 Ma (Calif. Div. Mines and 
Geol., 1965), fission track ages of 84.1+7.8 Ma and 81.7+6.3 Ma (Naeser and Ross, 1976), and most recently 
Rb/Sr and Ar/Ar ages of 93+2.5 Ma (Kistler and Champion, 1997).
Kqd	Granitic rocks of Ben Lomond Mountain (Cretaceous) -- Predominantly dark-weathering, white to light-gray, 
fine- to coarse-grained hornblende-biotite quartz diorite.  Also includes stocks and plugs of medium- to coarse-
grained, light-gray alaskite and granite, and dark, fine- to coarse-grained, hornblende-cummingtonite gabbro.  
Alaskite dikes similar to the larger alaskite body, locally intrude the quartz diorite.  The gabbro body appears in 
map view to intrude the quartz diorite as well, but contact relations have not been observed because of poor 
exposure of the gabbro.  The quartz diorite is very similar to that of Montara Mountain, but is distinguished by 
having fewer dark minerals and virtually lacking metallic opaque minerals (Ross, 1972), as well as by 
association with other types of plutonic rocks.  Rocks from this unit have yielded fission track ages of 86.9+6.6 
Ma (Naeser and Ross, 1976) and K/Ar ages of 71.0+0.9 Ma (Calif. Division of Mines and Geology, 1965) and 
86.9+6.6 Ma (Leo, 1967).  This unit includes, mapped locally:
ga	Granite and alaskite
hcg	Hornblende-cummingtonite gabbro
KJv	Unnamed volcanic rocks (Cretaceous or older)--Dark-gray, dense, finely-crystalline felsic volcanic rock, with 
quartz and albite phenocrysts.  Exposed only west of Pescadero.  Thickness unknown
KJf	Franciscan Complex, undivided (Cretaceous and Jurassic)--Mostly graywacke and shale (fs).  May be 
variably sheared.  Partly coeval with Pigeon Point Formation (Kpp), granitic rocks of Montara Mountain (Kgr) 
and Ben Lomond Mountain (Kqd), unnamed shale (Ksh), and unnamed volcanic rocks (KJv).  Locally divided 
into:
fs	Sandstone--Greenish-gray to buff, fine- to coarse-grained sandstone (graywacke), with interbedded siltstone and 
shale.  Siltstone and shale interbeds constitute less than 20 percent of unit, but in places form sequences as 
much as several tens of meters thick.  In many places, shearing has obscured bedding relations; rock in which 
shale has been sheared to gouge constitutes about 10 percent of unit.  Gouge is concentrated in zones that are 
commonly less than 30 m wide but in places may be as much as 150 m wide.  Total thickness of unit is 
unknown but is probably at least many hundreds of meters
fg	Greenstone--Dark-green to red altered basaltic rocks, including flows, pillow lavas, breccias, tuff breccias, tuffs, 
and minor related intrusive rocks, in unknown proportions.  Unit includes some Franciscan chert and limestone 
bodies that are too small to show on map.  Greenstone crops out in lenticular bodies varying in thickness from 
a few meters to many hundreds of meters
fc	Chert--White, green, red, and orange chert, in places interbedded with reddish-brown shale.  Chert and shale 
commonly are rhythmically banded in thin layers, but chert also crops out in very thick layers.  In San Carlos, 
chert has been altered along faults to tan- to buff-colored clay.  Chert and shale crop out in lenticular bodies as 
much as 75 m thick; chert bodies are commonly associated with Franciscan greenstone.
fl	Limestone--Light-gray, finely- to coarsely-crystalline limestone.  In places limestone is unbedded, in other places 
it is distinctly bedded between beds of black chert.  Limestone crops out in lenticular bodies up to 120 m thick, 
in most places surrounded by Franciscan greenstone
fm	Metamorphic rocks--Dusky-blue to brownish-gray blocks of metamorphic rock, commonly glaucophane schist, 
but some quartz-mica granulite.  These rocks are finely to coarsely crystalline and commonly foliated.  They 
almost always crop out as tectonic inclusions in sheared Franciscan rocks (fsr) and serpentinite (sp), and they 
reach maximum dimensions of several tens of meters though many are too small to show on map
fh	Argillite -- Dark-gray to grayish-black argillite and shale with minor beds of sandstone.
fsr	Sheared rock (melange)--Predominantly graywacke, siltstone, and shale, substantial portions of which have 
been sheared, but includes hard blocks of all other Franciscan rock types.  Total thickness of unit is unknown, 
but is probably at least several tens of meters
sp	Serpentinite (Cretaceous and/or Jurassic)--Greenish-gray to bluish-green sheared serpentinite, enclosing 
variably abundant blocks of unsheared rock.  Blocks are commonly less than 3 m in diameter, but range in size 
from several centimeters to several meters;  they consist of greenish-black serpentinite, schist, rodingite, 
ultramafic rock, and silica-carbonate rock, nearly all of which are too small to be shown on the map
Jsv	Siliceous volcanic rocks and keratophyre (Jurassic?)-- Highly altered intermediate and silicic volcanic and 
hypabyssal rocks.  Feldspars are almost all replaced by albite. Recent biostratigraphic and isotopic analyses 
yielded a Jurassic age for similar rocks in Alameda and Contra Costa Counties (Jones and Curtis, 1991)
Jgb	Gabbro (Jurassic?)--Light green-gray, dark-gray weathering, mafic intrusive rock, mostly gabbro but also includes 
some diabase locally.  The age of this unit is unknown, but the unit is probably part of the Jurassic Coast 
Range Ophiolite
db	Diabase and gabbro (Jurassic?)
gd	Gneissic granodiorite (Mesozoic or Paleozoic) -- Strongly foliated, black and white gneiss.  Foliation due to 
alignment of lenses of dark minerals in a light-colored matrix.
sch	Metasedimentary rocks (Mesozoic or Paleozoic) -- Mainly pelitic schist and quartzite.
m	Marble (Mesozoic or Paleozoic)-- White to gray finely crystalline marble and graphitic marble, in places 
distinctly bedded, in places foliated.  Near Montara Mountain, this unit also includes quartz-mica hornfels and 
crops out as rare isolated bodies as much as 75 m long in granitic rocks.  Near Ben Lomond Mountain, the unit 
locally includes schist and calc-silicate rocks.

Acknowledgments

We are grateful to the following U.S. Geological Survey paleontologists who have examined our fossils and provided ages 
necessary to establish the stratigraphic sequence and structure:  David Bukry (Cretaceous and Tertiary nannoplankton), Kristin 
McDougall (Tertiary foraminifers), William Sliter (deceased - Cretaceous and Eocene foraminifers), John Barron (Tertiary 
diatoms), Charles Powell II (Tertiary mollusks), and Bonita Murchey (Mesozoic radiolaria).
	We are also very grateful to managers and staff of Chevron, EXXON, UNOCAL, ARCO, and Shell Petroleum 
Companies who have provided reports, maps, picked slides, and residues for about 25,000 microfossil localities in the San 
Francisco Bay Region.
	We are grateful to Tracey Felger, who made the original scan and a preliminary edit of author materials; to Judy 
Mariant, who did additional editing and tagging of lines; and to Dominique Garnier, who digitized many of the bedding 
attitudes.  Carl Wentworth kindly provided advice on digitizing and editing procedures.  Ed Helley provided materials and 
advice on Quaternary units.

Sources Of Data

Quaternary deposits in Santa Cruz County nearly entirely from DuprÚ (1975). Quaternary deposits in the west half of San 
Mateo County are mostly from Lajoie and others (1974), and in the east half are modified from  Helley and Graymer 
(1997), Pampeyan (1994), Pampeyan (1993), and  Lajoie and others (1974).  Lines for San Andreas fault in San Mateo 
County are from Brown (1972)

1.	Pampeyan (1993) and Helley and others (1994).  See also Helley and Lajoie (1979), and Pampeyan (1970a).  Faults 
inferred from geophysical investigations by Brabb and Hanna (1981) and Carle and others (1990).

2.	Pampeyan (1993 and 1970a), some data from Dibblee (1966), Brown (1972), Dickinson (1970), Rodine (1973), and 
Fleck (1967).  See also Brabb and others (1991), and Beaulieu (1970).

3.	Dibblee (1966).

4	Cummings (1960) and Dibblee (1966).  See also Cummings and others (1962).

5.  	McLaughlin (1969);  some field checking by E.E. Brabb, 1969, and E.H. Pampeyan, 1971.

6.  	Rogers and Armstrong (1973), some data from Dibblee (1966).

7.  	Unpublished geologic mapping by S.A. Brooks, K.F. Oles, and Eugene Borax, Union Oil Company of California, 
1956, scale 1:24,000;  some data from Classen (1959).  Additional field work by T.W. Dibblee, Jr.,1947-49, and by 
E.E. Brabb, 1968-69.

8.  	New mapping by authors.

9.  	Thomas (1951) and field reconnaissance by E.E. Brabb, 1969, and E.H. Pampeyan, 1963.

10.  	Schlocker and others (1965), and Brown (1972).

11.  	Esser (1958), and Mack (1959), modified by field checking by E.E. Brabb, 1968-69.

12.  	Pampeyan (1970a and 1993).  Some data from Dibblee (1966).

13.  	Helley and Graymer (1997).

14.  	Touring (1959).  See also Cummings and others (1962).  Bedrock geology modified from unpublished maps, scale 
1:24,000, by F.J. Noble, S.A. Brooks, Eugene Borax, K.F. Oles, R.S. Fiske, H.L. Fothergill, R.N. Hacker, and 
J.H. van Amringe, Union Oil Company of California, 1951-57.  Additional field work by T.W. Dibblee, Jr., 1947-
49, and E.E. Brabb, 1958, 1968-69.

15.  	Dibblee (1966), and Johnson and Ellen (1968).

16.  	Dibblee (1966).  Some data from Cummings (1960) and Cummings and others (1962).

17.  	Unpublished geologic mapping, scale 1:24,000, by S.A. Brooks, and R.S. Fiske, Union Oil Company of 
California, 1956.  Additional field work by T.W. Dibblee, Jr., 1947-49, J.C. Clark and E.E. Brabb, 1969-70, and 
T.R. Simoni, 1971.  Some San Gregorio fault lines from Weber and Lajoie (1980) and Roberta Smith (written 
comm., 1993).  See also Hall and others (1959). 

18. 	Brabb (1960).  See also Cummings and others (1962) and Brabb (1964).

19.  	Cummings (1960).  See also Cummings and others (1962).

20.  	Clark (1970), Clark and Brabb (1978), and Brabb, Clark, and Throckmorton (1977).  More recently Clark (1981).

21.	Miller-Hoare (1980).

22.	Sorg and McLaughlin (1975).  Some data from Dibblee (1966), Cotton and Associates (1978), W. McCormick 
(written comm., 1991), Cotton and Associates (written comm., 1991), and Helley and others (1994).

23.	Helley and others (1994).  See also Helley and Lajoie (1979).  Faults inferred from geophysical investigations by 
Brabb and Hanna (1981) and Carle and others (1990).

24.	Rogers (1972), supplemented with data from Sorg and McLaughlin (1975), Cotton and Associates (1977), Helley 
(written comm., 1991), and Brabb and Dibblee (1979).

25.	Dibblee (1966).

26.	Unpublished geologic mapping, scale 1:24,000, by S.A. Brooks and Richard Fiske, Union Oil Company of 
California, 1956.

27.	Brabb (1960) and Cummings and others (1962).  See also report by McJunken (1984).

28.	Cotton and Associates (1978).  Some data from Dibblee (1966), D. Sorg and R. McLaughlin (written comm., 
1991), and Cotton and Associates (written comm., 1991).

29.	Brabb and Dibblee (1979), Burchfiel (1958), and Hector (1976).

30.	Clark (1981), Leo (1961, 1967).  Ben Lomond fault extended from Stanley and McCaffrey (1983).

References Cited

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western California:  California Division of Mines and Geology Bulletin 183, 177 p.
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Andreas fault:  Stanford University, Ph.D. Dissertation, 202 p., map scale 1:24,000.
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