U.S. Geological Survey Bulletin 2195

Rocks and Geology in the

San Francisco Bay Region

U.S. Department of the Interior

U.S. Geological Survey
1

Rocks and Geology in the

San Francisco Bay Region

Philip Stoffer

U.S. Geological Survey Bulletin 2195

U.S. Department of the Interior

U.S. Geological Survey
2 

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is 

for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

U.S. Geological Survey, Reston, Virginia: 2002

Electronic copies of this publication are available online at

http://geopubs.wr.usgs.gov/bulletin/b2195/

Additional USGS publications can be found online at

http://geology.usgs.gov/products.html

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS (1–888–275–8747)

World Wide Web: http://www.usgs.gov/

Published in the Western Region, Menlo Park, California

Manuscript approved for publication, September 30, 2002

Text and illustrations edited by James W. Hendley II

Production and design by Susan Mayfield
III

Contents

Introduction............................................................................................................................................................... 1

Geologic time....................................................................................................................................................... 2

The rock cycle..................................................................................................................................................... 3

Plate tectonics.....................................................................................................................................................4

Active versus passive continental margins....................................................................................................7

Evolution of the San Andreas Fault system.................................................................................................... 9

Generalized geologic map of the San Francisco Bay region.....................................................................12

Igneous rocks and processes.............................................................................................................................. 13

Classification of igneous rocks.......................................................................................................................14

Intrusive igneous rocks....................................................................................................................................15

 Granite........................................................................................................................................................... 15

 Granodiorite..................................................................................................................................................15

Why are there felsic intrusive igneous rocks in the San Francisco Bay region?.................................. 16

 Diorite............................................................................................................................................................ 17

 Gabbro........................................................................................................................................................... 17

Rocks from the mantle?....................................................................................................................................18

 Pyroxenite..................................................................................................................................................... 19

 Peridotite.......................................................................................................................................................19

 Chromite ore................................................................................................................................................. 19

 Anorthosite................................................................................................................................................... 20

 Jade............................................................................................................................................................... 20

Extrusive igneous rocks................................................................................................................................... 20

 Basalt.............................................................................................................................................................20

 Andesite........................................................................................................................................................ 21

 Rhyolite..........................................................................................................................................................21

Pillow basalt—how does it form?.................................................................................................................. 21

Volcanic rocks in the San Francisco Bay region.........................................................................................22

Pinnacles volcanic area—a national monument for half a volcano........................................................ 23

Hydrothermal deposits..................................................................................................................................... 23

 Travertine...................................................................................................................................................... 23

 Hydrothermally altered basalt................................................................................................................... 24

 Calc-silicate rock.........................................................................................................................................24

Springs, warm springs, and hot springs and their deposits.......................................................................24

Cinnabar (mercury ore) and the New Almaden Mining District................................................................25

Sedimentary rocks and processes......................................................................................................................26

Soil....................................................................................................................................................................... 28

Layers in sedimentary rock............................................................................................................................. 29

What strata can reveal about earth history..................................................................................................30

Sediments and sedimentary deposits............................................................................................................31

Sedimentary rocks............................................................................................................................................ 32

 Conglomerate............................................................................................................................................... 32

 Sandstone..................................................................................................................................................... 34

 Graywacke—an old sedimentary rock term with a new meaning..................................................... 36

 Mudstone and siltstone.............................................................................................................................. 37

 Marlstone......................................................................................................................................................37

 Shale.............................................................................................................................................................. 38

 Limestone and dolostone........................................................................................................................... 39

 Concretions—nature’s time capsules......................................................................................................40

 Chert...............................................................................................................................................................41

 Gypsum..........................................................................................................................................................42
IV

 Rock salt........................................................................................................................................................42

Limestone versus chert and the carbonate compensation depth (CCD).................................................43

Organic residues and rocks (petroleum and coal)...................................................................................... 44

 Asphalt...........................................................................................................................................................45

 Coal................................................................................................................................................................ 46

 Graphite.........................................................................................................................................................46

Metamorphic rocks and processes.....................................................................................................................47

Temperature and pressure related processes in metamorphism.............................................................48

Metamorphic rocks...........................................................................................................................................49

 Slate............................................................................................................................................................... 49

 Argillite.......................................................................................................................................................... 49

 Metasandstone............................................................................................................................................ 49

 Metaconglomerate......................................................................................................................................50

 Metachert..................................................................................................................................................... 50

 Quartzite........................................................................................................................................................ 51

 Marble........................................................................................................................................................... 51

 Mica schist................................................................................................................................................... 53

 Gneiss............................................................................................................................................................ 53

 Greenstone (metabasalt)............................................................................................................................54

 Serpentinite.................................................................................................................................................. 54

 Greenschist.................................................................................................................................................. 56

 Blueschist..................................................................................................................................................... 56

 Amphibolite schist....................................................................................................................................... 56

 Granulite........................................................................................................................................................57

 Eclogite.......................................................................................................................................................... 57

 Migmatite...................................................................................................................................................... 57

Additional resources..............................................................................................................................................58
Introduction 1

Introduction

The landscape of the San Francisco Bay region is host 

to a greater variety of rocks than most other regions of the 

United States. The reason for this diversity in rock types is 

related to the region’s geologic setting along the western 

margin of the North American continent. Continental margins 

experience relatively intense geological activity compared to 

most places over time.

This guide to rocks in the San Francisco Bay region is 

intentionally generalized to serve some of the needs of novice 

rock enthusiasts, teachers, students (of all ages), and geologists and other scientists. This comes at a price, however, 

because the natural world is vastly more diverse and complex 

than even the most eloquent scientist could possibly explain 

concisely. Whereas this guide provides descriptions of about 

four-dozen common rock types, observations of the variety of 

natural materials on practically any gravel bar in the region 

will demonstrate how difficult it is to assign samples to any 

rock classification scheme. Along with common rock descriptions, selected discussions are added to provide information 

Map showing locations of geology-oriented parks, visitor centers, museums, and college programs in the San Francisco Bay region.

about how geologists interpret the origin and significance of 

rocks. It is this additional information that transforms an ordinary rock sample into a knowledge portal into Earth’s history, 

the evolution of the modern landscape, and the ongoing natural processes occurring both undersea and underground. 

The discussions that follow often introduce “the cart 

before the horse.” This is because it is nearly impossible to 

present the most simplified scientific information without 

introducing appropriate terminology. This has been intentionally avoided. Figure numbers are also left out, so that 

discussion on most individual pages stand-alone. This guide 

isn’t intended to serve as a textbook, but rather as a reference 

source to supplement other information resources. If you need 

more information check out some of these San Francisco Bay 

region parks, museums, college geology departments, and visitor facilities for more geologic information. Searching on the 

World Wide Web will also lead to amazing resources. Many 

of the rocks illustrated in this guide are on display in the U.S. 

Geological Survey Library at 345 Middlefield Road in Menlo 

Park. However, the best ones lie where they occur in the field 

and hopefully you will go there to observe them!
2 Rocks and Geology in the San Francisco Bay Region 

Geologic Time

Perhaps the most fundamental concept in the study of 

the Earth and the universe is the concept of geologic time. As 

our understanding of the great antiquity of the Earth developed, names were assigned to segments of time defined in 

units ranging from thousands of years (in the relatively recent 

geologic past to historic times), to many millions or billions 

of years (in the distant geologic past). Geologists use the geologic time scale to define the ages of rocks and the fossils they 

may contain, and to define when geologic events occurred.

Geologic time is divided into four great eras. The Precambrian Era encompasses all time from the beginning of the 

known universe, the formation of the galaxy, the solar system, 

the accumulation of materials that became the Earth, and the 

organization of the early Earth into differentiated parts including early continents, oceans, and atmosphere. The oldest 

known rocks on the Earth are roughly 4 billion years old, and 

some of the earliest sedimentary rocks display evidence that 

organic life was forming and evolving already in this early 

period. The Paleozoic Era began roughly 570 million years 

ago. Paleozoic means “ancient life.” It was during this period 

that multicellular organisms flourished and diversified, particularly invertebrates in the shallow oceans, then fish, land plants, 

amphibians, and reptiles. The Mesozoic Era (meaning “middle 

life”) began roughly 245 million years ago and its start is 

defined by a mass extinction when nearly all life forms of the 

Paleozoic vanished, possibly from a variety of natural environmental calamities occurring simultaneously. During the Mesozoic dinosaurs flourished, and early mammals and flowering 

plants appeared. The Mesozoic ended in catastrophe, in part 

caused by at least one major asteroid impact. The Cenozoic 

encompasses from 65 million years ago to the present, and is 

highlighted by the evolution and diversification of mammals, 

birds, and modern plants (particularly flowering plants and 

grasses). It is near the end of this era that primate evolution 

yielded modern humans.

Eras are subdivided into periods (such as the Tertiary and 

Quaternary periods of the Cenozoic Era). Periods are in turn 

subdivided into epochs (such as the Pleistocene and Holocene 

epochs of the Quaternary Period). Time is parsed even further 

where geologic events and detail are more clearly resolved. 

Generalized geologic time scale for the San Francisco Bay region.
Introduction 3

The Rock Cycle

Geology is also the story of processes and products 

formed on or below the Earth’s surface. To simply identify the 

name of a rock is a relatively elementary task (don’t let them 

kid you... even the most experienced petrologist can have a 

hard time without sophisticated tools). However, developing 

an understanding of a rock’s occurrence, origin, and geologic 

significance and value (both economic and intellectual) is the 

greater mission within the science of geology. 

The figure below illustrates the “rock cycle,” a model 

that portrays the origin of the three basic rock groups (igneous, sedimentary, and metamorphic) and the interrelated 

nature of Earth materials and processes that transform one 

rock type into another over time. Arrows on the diagram 

below show the different paths that Earth materials may 

follow in the courses of formation and transformation. Igneous rocks form from the cooling of molten material. Sedimentary rocks form from sediments eroded and deposited 

in sedimentary environments, either on land or most often 

under water in an oceanic setting. Metamorphic rocks form 

from the chemical and physical alteration of preexisting 

rocks, usually deep within the crust of the Earth. The occurrence of different rock types on the surface of the Earth 

demonstrates that the world is a dynamic place, with materials forming, migrating, reforming, changing, and reforming 

again and again over great periods of time.

The rock cycle highlights this crucial link between geologic 

time, geologic processes, and the Earth materials produced. 

The diagram below also includes the common names of rocks. 

Names applied to rocks not only imply both textural and compositional characteristics but also are directly linked to their 

mode of origin. Discussion of each of the rock groups and rock 

types are presented later in this text. It is also important to note 

that rocks consist of chemical substances called minerals. Minerals are naturally occurring, inorganic elements or compounds 

that have an identifiable chemical composition, and typically 

have an orderly internal crystalline structure. Minerals occur as 

fragments or grains that make up rocks. Rocks are aggregates of 

minerals (usually more than one type of mineral). In most cases, 

it is difficult to identify the mineral content of rocks without the 

use of a hand lens. Identifying rocks also requires knowledge 

of rock texture characteristics, and an “open eye” approach to 

where the rock occurs relative to features on the landscape. 

For instance, if there are volcanoes around you, there is a very 

good chance that you are standing on a volcanic rock. However, 

things aren’t always that easy, especially in the San Francisco 

Bay region where so much has happened, is happening, and will 

likely continue to happen geologically.

The rock cycle illustrated.
4 Rocks and Geology in the San Francisco Bay Region 

Plate Tectonics

The rock cycle is fundamental to the greater, all-encompassing theory of plate tectonics. This body of knowledge 

explains the character, origin, and occurrence of the diverse 

rock types exposed around the world, and especially throughout the San Francisco Bay region. Before the theory of plate 

tectonics, geologists had little idea about how the geology of 

the region was created. The theory has evolved over time and 

has involved the careers of perhaps thousands of geologists 

supported by hundreds of research organizations around the 

world. Perhaps most crucial to investigation and discovery 

has been the ongoing exploration of the sea floor beneath the 

ocean basins. The combination of technological advances in 

geochemistry, geophysics, paleontology, and other geologic 

disciplines have aided in measures to support and revise the 

theory over time. 

The internal structure of the Earth has been revealed by 

the study of seismic shock waves generated by earthquakes 

and subterranean nuclear blasts. The ball of the Earth consists 

of several distinct layers. The central inner core is 758 miles 

(1221 km) thick and probably has a metallic composition similar to nickel/iron meteorites. The outer core is a molten metallic layer about 1403 miles (2259 km) thick. Above the core is 

the mantle, a layer having two zones—the lower mantle (average thickness of 1348 miles [2171 km]) and the upper mantle, 

called the asthenosphere (average thickness of 447 miles [720 

km]). The crust is the relatively light outer solid skin that 

ranges in thickness between 6 and 40 miles (10 and 64 km). 

The uppermost hundred kilometers of the mantle and the 

crust are called the lithosphere. In general, the lithosphere 

is both solid and rigid in character, and it is within this zone 

that all earthquakes occur. Beneath the lithosphere, the asthenosphere is so hot that it is near the point of melting. In this 

interval the rock behaves in a ductile manner, flowing under 

the extreme pressure rather than breaking. This zone is also the 

source of magma that may eventually migrate to the Earth’s 

surface. Rocks in this zone are easily deformed, flowing 

slowly as a result of temperature-controlled density currents. 

The mechanism for this flux is straightforward: rock heated 

from the core and mantle below expands and rises. Near the 

Earth’s surface, heat conducted slowly toward the atmosphere 

is ultimately radiated into space. This heat loss causes the 

rocks to cool, condense, and sink. The thermal exchange process has been occurring throughout Earth’s history, causing 

material to rise, cool, sink, and heat up again. 

Structure of the Earth showing the core, mantle, and crust.
Introduction 5

Due to the great pressure in the mantle, rock cannot 

melt but they can flow plastically. Rocks can only melt if 

temperatures are high and pressure is low enough (as in the 

asthenosphere and also within the crust if conditions are 

right). As a result the crust “floats” on the “fluid” mantle 

in a similar manner to the way icebergs float on the sea. 

Generally, where the crust is thin or of dense composition, 

ocean basins occur. Where the crust is thick or less dense 

in composition, continents occur and may rise above sea 

level. The rising and sinking of material within the mantle 

drags or pushes the crust of the Earth in the form of great 

plates. Where mantle material rises by convection the crust 

is pulled apart. Places where this occurs on the Earth’s surface are called divergence zones. It is along these spreading centers where new ocean crust forms. Igneous rocks 

produced in these zones have a mafic chemical composition 

(rock rich in iron and magnesium). Spreading centers define 

divergent boundaries of crustal plates. The Mid-Atlantic 

Ridge is an extensive undersea mountain range that runs 

down the center of the Atlantic Ocean and is an example of 

a divergence zone.

The new crust formed along a divergence zone is generally both thin and relatively hot. Magma derived from 

the mantle is extremely hot, typically greater than 1100oC 

(2000oF). Through time, the new crust is carried or pushed 

away from its place of origin. As the new crust cools it 

becomes denser. This new basaltic crust generally makes up 

the rocks underlying the world’s ocean basins. Sediments 

derived from continents blanket the ocean crust forming 

thick layers of sedimentary rock. The most ancient rocks 

found in the San Francisco Bay region originally formed in 

settings typically associated with divergence zones (particularly serpentinite, pillow basalt, and limestone).

As new crust is formed older crust is recycled back into 

the mantle. This occurs along convergent zones or boundaries where crustal plates collide. A convergent boundary exists 

between two crustal plates where one plate overrides another, 

usually along the margins of continents. The rocks within 

continental landmasses are generally felsic (granitic) in composition and are therefore less dense than the mafic rocks that 

underlie ocean basins. As a result, continental crust overrides 

oceanic crust. Oceanic crust sinks back into the asthenosphere 

along a subduction zone. Subduction zones consist of a great 

fault, or series of faults, defining the boundary between two 

converging crustal plates, and occur in association with ocean 

trenches. Ocean trenches occur at convergent zones along 

many plate boundaries, especially along the margin of the 

Pacific Ocean. This region is known as the “Ring of Fire” 

because where convergence zones occur there are typically 

both trenches and numerous volcanoes, many of which are 

active. A chain of volcanoes associated with a subduction zone 

is called a volcanic arc.

As the ocean crust sinks it drags rock material with it, 

including sediments saturated with seawater. Moving downward, 

this material gradually heats up and may partially melt. In the 

process, however, a natural refinement process occurs. Minerals 

that cannot withstand high temperatures (such as quartz) melt 

first, and minerals that can withstand higher temperatures (such 

as most mafic minerals) remain solid. As a result the molten 

material that forms in association with subduction zones is of 

felsic composition (mainly granite and rhyolite). Above the sinking slab of crust within subduction zones the surface is dotted 

with both active and extinct volcanoes. For instance, the modern 

Cascade Range extending from northern California to Washington is an example of an active volcanic arc. The Sierra Nevada 

was a volcanic arc that was active long ago, but is now extinct.

A plate tectonics model illustrating divergent, convergent, and transform plate boundaries.
6 Rocks and Geology in the San Francisco Bay Region 

Another type of convergent boundary forms when two 

continental landmasses collide. Because most continental 

rocks are too light to sink into the mantle, the rock piles 

up and forms massive mountain ranges. The best modern 

example of this is the Himalayas; the high mountain range 

formed as the Indian subcontinent collided into southern 

Asia. Before the two continents collided, a massive volume 

of sediments accumulated in a seaway between the two landmasses. Today, high in the Himalayas it is possible to find 

rocks bearing remains and traces of marine organisms that 

lived in that seaway.

All crustal plates are in motion. For instance, North 

America is cruising westward relative to Europe at a rate of 

about 4 cm per year. (Christopher Columbus’s journey would 

be 20 m farther now!) This may seem insignificant, but in the 

framework of geologic time this process becomes important. 

The figure below shows the arrangement of the continents as 

they existed at different times in Earth history (over many millions of years ago). For instance, roughly 250 million years 

The changing configuration of continental landmasses through time.

ago all of the continents were assembled in a single, great 

landmass geologists named Pangaea. This “supercontinent” 

lasted for many millions of years, and then it gradually broke 

apart. A great, developing rift system slowly gave rise to the 

Atlantic Ocean, splitting Pangaea into separate continental 

masses. This rift zone, or system, eventually became the 

spreading center associated with the Mid-Atlantic Ridge. The 

breakup of Pangaea was gradual, beginning with the opening 

of the Mid-Atlantic Ocean basin between North America and 

northern Africa. This happened between the “Triassic” (200 

million years ago) and the “mid-Cretaceous” (100 million 

years ago; see figure below). This was followed by the opening of the South Atlantic, between Africa and South America, 

and finally by the opening in the North Atlantic in the Arctic 

regions between Europe, Greenland, and North America (see 

mid-Tertiary below). The North American Plate includes both 

the more ancient North American continental landmass and the 

younger crust beneath the western Atlantic Ocean basin west 

of the Mid-Atlantic Ridge.
Introduction 7

Active Versus Passive Continental Margins

The geologic history and plate tectonics story of California doesn’t make sense without a discussion of “active” and 

“passive” continental margins. A passive continental margin 

exists on the side of continents where the continental crust has 

“fused” with the adjacent ocean crust. In these regions strong 

earthquakes are rare, volcanic activity is relatively nonexistent, 

and forces of erosion dominate the gradual evolution of the 

landscape relative to tectonic forces uplifting the landscape. 

The landscape is generally subdued (low relief) and relatively 

small changes in sea level drastically affect the shoreline configuration of the oceans. In these regions over long periods 

of time, mountain ranges are worn away by weathering and 

erosion. Through time, over hundreds of millions of years, 

river systems gradually carry sediments from inland areas to 

the shore regions. In warm, shallow marine settings along 

the continental shelf great organic reef systems also generate 

large volumes of carbonate sediment. Gradually the marginal 

regions along continents fill in with great accumulations of 

sedimentary material. 

The modern Atlantic coast of North America is a classic 

example of a passive continental margin because the North 

American continent is fused to the oceanic crust under the 

western side of the Atlantic Ocean. The Mid-Atlantic Ridge is 

a spreading center (divergence zone) that defines the eastern 

edge of the North American Plate. The eastern edge of the 

North American continent is now in the middle of the North 

American Plate. A great pile of sedimentary material now 

blankets the eastern edge of the continent, extending eastward 

from the Appalachian Mountains, beneath the coastal plain 

to the continental shelf and beyond into the deep ocean. This 

great sediment-filled basin extends from Canada to Florida and 

along the gulf coast. A similar sediment-filled basin extends 

along the North Slope of Alaska and northwestern Canada. 

Both regions are considered passive continental margins. 

In contrast, an active continental margin is a coastal 

region where tectonic forces (faulting, volcanism, and uplift) 

tend to dominate over forces of erosion. They occur where 

the edge of a continent defines a plate boundary and convergence is occurring. The western edge of the North American 

continent is an active continental margin. Along the western 

margin of the North American continent there are large mountain ranges (like the Sierra Nevada and the Coast Ranges), 

and active volcanic chains, like the Cascades volcanic arc 

of northern California, Oregon, and Washington. Relative to 

the East Coast, the West Coast experiences an abundance of 

earthquakes and volcanic eruptions. The rugged western landscape dominated by mountains, volcanoes, and canyons is a 

reflection of the relatively active tectonic activity in the region. 

Material eroded from the land may be carried downward along 

with other crustal rocks into a plate margin subduction zone. 

Some of these materials may be metamorphosed and become 

part of great blocks the material that may becomes accreted to 

the continent. Some of it may melt and migrate to eventually 

form new igneous rocks, and some may vanish into the mantle. 

Map showing the active and passive continental margins in relation to the boundaries of the North American Plate.
8 Rocks and Geology in the San Francisco Bay Region 

Plate tectonics is the driving force behind most geologic 

changes occurring on the surface as well as deep in the crust. 

The processes associated with plate tectonics through the 

expanse of geologic time are responsible for the gradual accumulation of continental landmasses through the process of 

accretion. This complex process is illustrated as follows. As subduction proceeds, the melting of crustal rocks results in a natural 

refinement of Earth material. When crust sinks into the mantle, 

some mineral materials melt at lower temperatures than others. 

The “low temperature” material that forms tends to have a felsic 

composition (rich in silica and aluminum; the primary components of quartz and feldspar minerals). When this material crystallizes to form new rock, it is typically less dense than crustal 

rock that makes up the mantle or ocean crust. In this manner, 

more and lighter felsic material becomes incorporated into the 

lithosphere. As a result, small island chains, like the Philippines and Japan, grow steadily larger through time. Throughout 

Earth history small landmasses have been constantly forming 

and being pushed around by plate motion. Some of these landmasses eventually are forced against the edges of continents. 

Because the overall density of their felsic rocks and sediments 

are relatively low, rather than being forced downward into a 

subduction zone they become attached (accreted) along the edge 

of the continent. In this manner the smaller land masses and the 

thick sequences of sedimentary deposits caught between colliding plates are squashed beneath rising mountain ranges along 

active continental margins. 

The North American continent has grown steadily larger 

by the slow process of continental accretion. This process began 

with formation of the Canadian Shield, an ancient assemblage 

of landmasses that now forms the central core of the continent 

(called a craton). The rocks of the Canadian Shield are dominantly igneous and metamorphic rocks, approximately between 

2.8 to1.8 billion years old, with some rocks preserving radiogenic dates that are much more ancient. At different times in 

Earth history, the western edge of North America was an active 

continental margin, undergoing stages of mountain building 

and continental accretion. At other times, the western edge of 

the continent was dominantly a passive margin, such as during 

much of Late Precambrian time (700 to 570 million years) and 

through much of the Paleozoic Era (570 to 225 million years). 

During this extensive period of time, the western edge of the 

North American continent was in a tropical setting, receiving 

large quantities of sediments derived from the continental interior in addition to accumulating large quantities of carbonate 

sediments (reefs and algal mud) deposited in shallow, marine 

environments. At the end of the Paleozoic, the western edge of 

the North American continent was near the Nevada/California 

border region. Through time, the western margin of the continent became an active plate margin again with subduction 

zones forming and reforming as smaller land masses and island 

arcs collided with the continent, slowly building the continental 

margin westward to its current location.

It wasn’t until the middle of the Mesozoic (the Jurassic 

Period) that rocks began to form that would eventually become 

the dominant part of what is now the San Francisco Bay region. 

Older rocks that may have existed in the region were either 

transported elsewhere or recycled back into the crust and mantle 

A simplified plate tectonic model for the western margin of North America useful for explaining the origin of 

many rock types found in the San Francisco Bay region. 
Introduction 9

along a subduction zone. The oldest rocks preserved in the 

region probably formed as igneous intrusions or volcanic flows 

on submarine volcanoes associated with a spreading center 

along a divergent plate boundary. Marine sediments were gradually deposited on top of these volcanic rocks, mostly from the 

buildup of skeletal remains of plankton and wind-blown dust. 

Later, as these rocks were gradually rafted toward the continental margin, they were blanketed with a thick sequence of 

sediments eroded from continental sources. Through time, much 

of the oceanic crust that existed in the region in the past was 

subducted back into the mantle or incorporated back into the 

continent through accretion.

Beginning roughly 200 million years ago, the opening of 

the Atlantic Ocean basin along a divergent plate boundary on 

the eastern side of the North American continent was counterbalanced by the development of a convergent boundary on the 

western side of the continent. This convergent boundary setting 

with a subduction zone operated nearly continuously for nearly 

160 million years, intermittently interrupted as small landmasses derived from distant sources rafted in and collided with 

the western margin of the continent. The figure on the previous 

page generally illustrates the geologic settings associated with 

the subduction zone complex that dominated the western margin 

of North America for this long period of time. However, starting roughly 30 million years ago, changes in plate configurations along the western edge of the North American continent 

gradually changed the entire geologic framework of California. 

Subduction gradually gave way to transform faulting, resulting 

in the formation of the San Andreas Fault system.

Evolution of the San Andreas Fault system

Transform faults result from a shearing motion in the crust. 

This results in the formation of a fault system in which one landmass moves gradually past another, or where two plates are sliding past each other. The classic North American example is the 

San Andreas Fault in California. Los Angeles and Baja California 

are attached to the Pacific Plate, which is slowly grinding northward relative to the eastern side of the fault attached to the North 

American Plate. It the future, the land that is now Los Angeles will 

gradually move northward, past San Francisco, and then onward 

towards Alaska. The figure below illustrates the changing plate 

geometry of the west coast for the past 30 million years involving the interactions of several plates—the North American Plate, 

the Pacific Plate, and the Farrallon Plate. The San Andreas Fault 

formed as the Farollon Plate disappeared beneath the western 

margin of the North American continent. (Existing remnants of 

 Formation of the San Andreas Fault system through time (from “This Dynamic Earth” (http://pubs.usgs.gov/pdf/planet.html) modified from USGS 

Professional Paper 1515).
10 Rocks and Geology in the San Francisco Bay Region 

the Farallon Plate include the smaller Cocos Plate and Juan 

de Fuca Plate). As the Farallon Plate disappeared, the North 

American Plate came in contact with the Pacific Plate. This 

collision of the Pacific Plate with the North American Plate 

changed the convergent plate boundary into a transform 

boundary, resulting in the ongoing formation of the San 

Andreas Fault system. In the early stage of formation of the 

fault system, volcanic eruptions occurred in many areas along 

the fault zone as new fracture systems propagating through 

the crust created passageways for magma migration toward 

the surface. Remnants of many of the volcanic centers occur 

throughout the bay region, particularly in the east bay hills and 

in the north bay region.

The San Andreas Fault system is a dominant geologic feature along the west coast of North America, and has been host 

to a number of great earthquakes in historic times where different portions of the fault moved. These major quakes include 

the 1857 Tejon Pass Earthquake (affected a San Andreas Fault 

segment roughly southeast of Los Angeles to Coalinga), the 

1868 Hayward Earthquake (affecting the Hayward, Calaveras, 

and San Andreas Faults between Coalinga and Hollister), and 

the 1906 San Francisco Earthquake (affecting the San Andreas 

Fault segment from near San Jose northward to where it runs 

offshore at Cape Mendicino). Numerous other significant 

earthquakes have occurred along the San Andreas Fault, but 

these quakes were perhaps the most powerful quakes that also 

affected the longest intervals of the fault system. The San 

Andreas Fault system is perhaps the best-known and most 

studied fault in North America, but it is only part of the greater 

tectonic setting of western North America. Fault systems along 

the eastern Sierra Nevada and in portions of the Great Basin 

extending as far east as central New Mexico may be part of a 

greater system of faults that really define the complex western 

boundary of the North American Plate.

Shaded relief map of California showing the San Andreas Fault system and plate motions.
Introduction 11

Evolution of western North America and cross-section of the San Francisco Bay region.
12 Rocks and Geology in the San Francisco Bay Region

 Generalized geologic map of the San Francisco Bay region.
Igneous Rocks and Processes 13

Igneous Rocks and Processes 

Igneous rocks form from the cooling and solidification 

of molten material called magma. The heat that produces 

magma has its source deep within the Earth. The heat is, in 

part, a relic of the original gravitational energy transferred 

by the collisions of asteroids and planetesimals with young 

planet Earth. Additional heat is generated from radiogenic 

decay. More heat may possibly come from tidal forces 

between the Earth, Moon, Sun, and other planets. Magma 

is derived from the melting of preexisting crustal rocks or 

from sources below the Earth’s crust. The formation and 

movement of magma in the subsurface is called plutonism 

(after “Pluto,” Greco/Roman god of the underworld). When 

magma cools in the subsurface it slowly forms rock through 

the process of crystallization. The chemical and physical 

reactions that take place in cooling magma result in the formation of interlocking mineral crystal grains (minerals such 

as quartz, feldspar, and mica). The mineral grains (crystals) 

are of a visible size, giving the rock a crystalline texture. 

Intrusive igneous rocks have a composition and texture that 

varies with the composition of the magma and physical conditions under which the rock formed. 

When magma manages to migrate upward onto the surface, 

the result is volcanism (a volcanic eruption). A volcano forms 

as molten rock and solidified volcanic debris are ejected onto 

the surface and accumulate near the eruption site. In addition 

to rock material, large quantities of water vapor and gases are 

vented from volcanoes. When magma flows on the surface the 

material is called lava. The release of pressure during eruptions 

can produce ash clouds that may travel long distances. Rocks 

that form on the surface are called extrusive igneous rocks. The 

rapid cooling of molten material on the surface hinders crystallization; as a result, extrusive igneous rocks tend not to display 

an abundance of visible mineral grains. Extrusive igneous rocks 

(of volcanic origin) take on a texture consistent with the manner 

or eruption, such as frothy or blocky lava rock, a condensed volcanic ash, or even natural glass (obsidian). 

 Intrusive igneous rocks form from magma that cools 

very slowly, deep underground. Rock is an excellent insulator of heat, so magma in a magma chamber may take many 

thousands of years to cool, allowing plenty of time for 

mineral crystal grains to form and grow, especially if the 

magma is rich in dissolved fluids that promote crystal growth 

(like carbon dioxide or water). A body of rock formed from 

magma migrating and solidifying deep in the subsurface 

is called a pluton or an igneous intrusion. Huge intrusions, 

covering areas greater than one hundred square kilometers 

are called a batholith. Batholiths typically contain many 

separate intrusions that form over a relatively long period 

of time. Other types of intrusions typically form at shallower crustal depths; these include stocks, dikes, and sills. 

A stock is smaller than a batholith and typically represents 

the subsurface passage that fed molten material to a volcano 

or field of volcanoes over time. Sills and dikes are layers of 

igneous rock that typically form along fault zones, fractures, 

or between and parallel to sedimentary layers. A laccolith is 

a blister-shaped intrusion. Stocks, sills, dikes, laccoliths and 

other intrusions are remnants of past igneous activity and are 

exposed at the surface long after erosion has stripped away 

any ancient volcanoes and other overlying rocks and sediments that may have existed in an area.

Geologic features associated with modern and ancient igneous activity. 
14 Rocks and Geology in the San Francisco Bay Region 

Classification of Igneous Rocks

Igneous rocks are classified by the geologic environment where they formed from the crystallization of molten 

material, and perhaps more importantly, by their mineral 

composition. Rocks that form underground are called intrusive igneous rocks, whereas rocks that form on the surface 

are called extrusive igneous rocks. Intrusive igneous rocks 

(like granite or gabbro) typically display visible mineral 

grains that are fairly easy to recognize with a little training. Extrusive rocks are more difficult to clearly identify 

because the mineral grains are typically invisible. However, 

the general composition of an extrusive rock is reflected by 

its color and density characteristics. General composition 

types include felsic (which is a mnemonic word derived 

from “feldspar” and “silica”), and mafic (which means rich 

in magnesium and iron, Fe). Felsic rocks tend to be light 

colored (white, pink, yellow), and are typically much less 

dense than mafic rocks that are typically dark (gray, brown, 

or black). A simplified igneous rock classification system 

is presented below.

The classification chart illustrates that rhyolite (a felsic 

extrusive rock) has the same mineral composition as granite 

(an intrusive igneous rock) and is composed dominantly of 

the minerals potassium feldspar (K-spar), quartz, and lesser 

amounts of plagioclase feldspar, mica, hornblende, and other 

minerals. Likewise, basalt (a mafic extrusive rock) has the 

same mineral composition of the mafic intrusive rock, gabbro.

This generalized composition of felsic and mafic can 

be subdivided into intermediate (between felsic and mafic 

composition), and ultramafic (rocks extremely enriched 

in magnesium and iron). Rocks of intermediate composition include diorite (intrusive) and andesite (the extrusive 

equivalent). Ultramafic rocks have special significance, in 

that they probably are derived from the mantle. They are 

relatively unstable on the Earth’s surface, and are typically 

metamorphosed. In nature and in simplistic interpretation, 

igneous rocks that make up most continental crust typically 

have a felsic composition (such as rhyolite and granite). 

The mafic rock basalt is the dominant rock type that makes 

up most ocean crust. Rocks of intermediate composition 

are derived from the mixing of continental and oceanic 

crust. Interestingly, intrusive and extrusive igneous rocks of 

nearly every composition listed below can be found in the 

greater San Francisco Bay region—a reflection of its complex geologic history.

A general classification of igneous rocks.
Igneous Rocks and Processes 15

Intrusive Igneous Rocks

Intrusive igneous rocks form from the slow crystallization 

of magma underground. Slow cooling results in the growth of 

visible interlocking grains of mineral crystals.

Granite 

Rock characteristics: Granite is a felsic rock with dominant minerals—orthoclase (a pink, potassium-rich variety 

of feldspar), quartz, and lesser amounts of mica (both silvery 

muscovite or black biotite), hornblende, and plagioclase 

(white to gray, sodium-rich varieties of feldspar). Note that 

the word “granite” is used fairly liberally. Many rocks called 

“granite” are technically something different. The word 

“granitoid” or “granitic” implies felsic to intermediate intrusive rocks of many varieties, including granite. The sample 

illustrated here is a granite pegmatite displaying a large radiating cluster of hornblende crystals and large crystal masses 

of mica, quartz, and feldspars: both pink (orthoclase) and 

white (plagioclase). A pegmatite is an exceptionally coarsegrained igneous rock with large interlocking crystals, typically forming as masses in dikes and veins, especially along 

the margins of batholiths. They represent the last and most 

hydrous magma to crystallize, and sometimes contain rare 

minerals enriched in the elements boron, lithium, uranium, 

and rare earths.

Bay region occurrence: Granitic rocks occur as mountain-sized masses and as intrusive bodies in older rocks 

(schist, gneiss, and limestone) throughout the Salinian Block, 

west of the San Andreas Fault with large areas of exposure in 

the Gabilan Range, in the Santa Cruz area, Montara Mountain, and at Point Reyes. Granitic materials also occur in 

gravels throughout the San Francisco Bay region.

Granodiorite 

Rock characteristics: Granodiorite is a felsic variety 

intrusive igneous rock containing mostly feldspar (but more 

plagioclase than orthoclase (K-spar) as in granite), quartz, 

hornblende, biotite, amphibole, and including accessory 

minerals such as magnetite, pyrite, and other minerals. It is 

typically a light-gray rock peppered with small black mafic 

crystals (biotite and hornblende). It usually has a massive, 

unstratified appearance, heavily fractured, and sometimes 

contains granitic dikes and intrusions (as in this stream 

cobble sample from Santa Cruz). 

Bay region occurrence: Granodiorite is perhaps the 

most common intrusive igneous rock found in California. 

Most of the “granite” mined in the region is really granodiorite. Granodiorite is abundant in the Salinian basement 

rocks throughout the region west of the San Andreas Fault 

with exposures through the Gabilan Range, the western Santa 

Cruz Mountains around Ben Lomond Mountain, Montara 

Mountain, and at Point Reyes. Large active “granite” quarries exist near Davenport and Aromas in Santa Cruz County. 

Granodiorite from these quarries is used extensively as railroad bed and construction aggregate.

Pegmatite from San Lorenzo Creek, Santa Cruz County.

Granite from Point Lobos, Monterey County.

A granodiorite boulder from San Lorenzo Creek, Santa Cruz County.
16 Rocks and Geology in the San Francisco Bay Region 

Why Are There Felsic Intrusive Igneous Rocks 

in the San Francisco Bay region?

Throughout the central California Coast Ranges some 

bedrock consists of igneous rocks including granodiorite and 

lesser amounts of granite, diorite, and gabbro. These igneous 

rocks occur in association with metamorphic rocks, particularly gneiss, schist, and marble. These rocks crop out in areas 

west of the San Andreas Fault, with prominent exposures at 

Point Reyes, Montara Mountain, throughout the Santa Cruz 

Mountains, the Salinas Mountains, the Gabilan Range, and 

the Monterey Peninsula region. These rocks form the “basement” (oldest crystalline rocks) of the Salinian Block (a block 

being a region underlain basically by the same kind of rocks 

and bounded by faults). These rocks are in striking contrast 

to the rocks that occur mostly east of the San Andreas Fault 

throughout the central and northern Coast Ranges (including 

the Diablo Range, San Francisco, and Marin County). The 

basement rocks in this region are called the Franciscan Assemblage, which includes a very mixed variety of sedimentary, 

igneous, and metamorphic rocks that were originally derived 

from oceanic crust, ocean sediments, and even rocks derived 

from the mantle. 

The granitic rocks in the Salinian Block baffled scientists 

for years before a clear understanding of plate tectonics and 

the nature of the San Andreas Fault system were resolved. 

The Salinian Block’s igneous and metamorphic rocks most 

resemble similar rocks from the southern Sierra Nevada batholith (a batholith is a region made up of large igneous intrusions 

of similar age and composition). Some rocks resemble other 

rocks in the Mojave Desert and in southern California. Geologists believe that through time, rocks that make up the Salinian Block were ripped away from the southern Sierra Nevada 

and were rafted northward with the other rocks making up the 

Pacific Plate side of the San Andreas Fault system. The marble 

in the Salinian Block predates the Mesozoic igneous intrusions, and may be much, much older. 

A “granite” quarry near Aromas (Santa Cruz County) yields crushed 

stone and rock that is closer in composition to granodiorite and diorite 

than true “granite.”

A granite intrusion exposed in a marble quarry in the Salinian 

Block (University of California Santa Cruz campus). 

Map showing granitic terranes in California, including the 

Salinian Block. 
Igneous Rocks and Processes 17

Diorite

Rock characteristics: Diorite is of intermediate composition. It is called “salt and pepper” rock because of its nearly 

equal proportions of light, felsic minerals (mostly plagioclase 

and quartz) and dark, mafic minerals (amphibole and biotite). 

Diorite is close in composition to granodiorite, and it requires 

fairly close inspection to differentiate the two rocks. Diorite 

typically contains no orthoclase feldspar (pink potassium 

feldspar). This sample from Santa Cruz displays a xenolith, a 

name for a piece of bedrock intruded by magma that broke off 

and was carried along before becoming incorporated into the 

rock when the magma cooled into stone. Xenoliths can range 

in size from pebbles to house-sized or even mountain-sized 

blocks depending on the size of the intrusion.

Bay region occurrence: Diorite occurs as intrusive 

bodies in the Salinian basement rocks. It occurs as mappable 

units in exposures in the Santa Cruz Mountains and Gabilan 

Range, and can be found as clasts in gravel deposits (conglomerates) throughout the region.

Gabbro

Rock characteristics: Gabbro is typically a dark gray, 

greenish gray, to black rock (mafic in composition) with 

visible crystals of plagioclase feldspar (Ca-rich varieties), 

hornblende, pyroxene, and lesser amounts of biotite mica, 

magnetite, and other minerals.

Bay region occurrence: occurs as small intrusive bodies 

in the Salinian basement (associated with granite and diorite 

Diorite from a “granite” quarry near Aromas, Santa Cruz 

County. 

Gabbro from southern Santa Cruz County.

intrusive bodies) with sizable occurrences in the region around 

Watsonville and Santa Cruz (Santa Cruz County), and in the 

Gabilan Range. Pyroxene-rich varieties of gabbro can also be 

found in association with basaltic intrusions and serpentinite 

bodies in exposures throughout the Diablo Range, particularly 

in the region around Mt. Diablo and farther south in the Clear 

Creek/New Idria region.

Paths of formation—diorite and andesite.
18 Rocks and Geology in the San Francisco Bay Region 

Igneous rocks of intermediate composition form in convergence zones. Subduction associated with convergent plate 

boundary conditions along the western margin of North America has persisted for nearly 250 million years (since North 

America split away from the African and Eurasian continents). 

Subduction ended in southern California with the formation 

of the San Andreas Fault system beginning about 20 million years ago. However, in northern California, Oregon, and 

Washington (north of the San Andreas Fault trace), subduction, 

and its associated volcanic processes, is still occurring, producing the Cascades volcanic chain. Mt. Shasta is the southernmost and largest of the Cascades volcanoes. In the past, a 

great chain of volcanoes must have existed in the region now 

occupied by the Sierra Nevada, however these volcanoes have 

long since eroded away. The chilled magma chambers that 

formed far below those ancient volcanoes are now exposed by 

ongoing erosion of the range. The rocks exposed in the Sierra 

Nevada are chemically very similar to the modern Cascades 

volcanic rocks. However, in contrast, they cooled very slowly 

far underground forming the more crystalline intrusive igneous 

rocks, diorite, granodiorite, and granite.

The modern Cascades volcanoes are a modern model for the formation of the ancient Sierra Nevada batholith. 

Rocks from the Mantle?

Plate tectonics theory suggests that hot mantle rock 

gradually rises toward the surface beneath oceanic spreading 

centers. As the hot rock rises, the release of confining pressure allows some of this mantle rock to melt. This molten 

material intrudes upward, forming new oceanic crust. Some 

of the mantle rock itself may become incorporated in the 

ocean crust. When complex faulting occurs along plate 

boundaries, sometimes pieces of mantle rock are transported upwards and are exposed at the surface. Because 

mantle rocks originally form at great depth they tend to be 

quite unstable both physically and chemically in the surface 

environment. Most exposures of rocks that were originally 

derived from the mantle have been converted to serpentinite. 

However, in some cases some of this rock has survived and 

has been recognized on the surface. Our current understanding of the composition of the mantle is that the rocks are 

of “ultramafic” composition, in that they consist of silicate 

minerals that are very enriched in iron and magnesium (and 

other metals such as calcium and chromium), more so than 

typical oceanic crust or especially continental crustal rocks. 

Mantle rocks are differentiated from crustal igneous rocks 

by their composition. They sometimes preserve their coarsecrystalline rock textures and original flow banded foliation 

that formed during the gradual upwelling of rock far below 

a spreading center. San Francisco Bay region examples of 

probable mantle rock include pyroxenite (rich in pyroxene 

minerals) and peridotite (a rock rich in the minerals, olivine, 

pyroxene, and chromite). Again, unaltered mantle rock is a 

very rare occurrence, but ultramafic rocks, both of igneous 

and metamorphic origin, are common in select areas.
Igneous Rocks and Processes 19

Pyroxenite

Rock characteristics: Pyroxenite is an ultramafic plutonic 

rock chiefly composed of pyroxene minerals, with accessory 

minerals including hornblende, olivine, spinel, biotite, and 

Ca-feldspar. Pyroxene minerals typically occur in short, stout 

masses that display prismatic near-right-angle crystal cleavage 

and are typically black to pale green in color. 

Bay region occurrence: Pyroxenite typically occurs in 

association with serpentinite, typically occurring as isolated 

outcrop masses or in resistant boulders along stream gravel 

bars. Massive ultramafic bodies in the San Francisco Bay region 

occur in the Franciscan basement east of the San Andreas Fault, 

with large bodies occurring in the southeastern foothills of the 

Santa Cruz Mountains, including on Mt. Umunhum. The Diablo 

Range has numerous large exposures of ultramafic rock from 

Mt. Diablo southward to San Benito Mountain and beyond. 

Yerba Buena Ridge on the east side of Highway 101 in southern 

San José is dominantly serpentinite, some of which preserves an 

original mantle-rock texture created by coarse crystalline rock 

flowing under extreme pressure.

Peridotite 

Rock characteristics: Peridotite has two varieties—

harzburgite (a rock that contains more olivine than pyroxene) 

and dunite (a rock consisting almost entirely of olivine). Peridotite typically displays a coarse crystalline texture and is 

both very dark and very dense. In harzburgite, the pyroxene 

is greenish, grayish to black. Some peridotite is rich in bronzite, a golden-brown, massive crystalline iron-rich variety of 

the mineral enstatite. In harzburgite and dunite, the olivine is 

brown, greenish brown to black and has a more glassy texture 

than the pyroxene. Accessory minerals include magnetite, 

chromite, and serpentine minerals. It may contain traces of 

plagioclase.

Bay region occurrence: Unaltered peridotite is a relatively rare occurrence in the San Francisco Bay region, and 

true dunite probably doesn’t occur in the region. Peridotite 

can be found in coarsely crystalline serpentinite bodies, particularly within the core of serpentinite blocks that preserve 

a “ghost” texture of the original coarse crystals grains of the 

parent rock. Weathered blocks of serpentinized peridotite 

have a brown “hob-nail” surface where the less altered and 

more resistant pyroxene stands out relative to the serpentinized olivine.

Chromite Ore

Rock characteristics: Chromite is essentially pure chromium 

oxide. It is a brownish to iron-black to brownish-black mineral of 

the spinel group. Chromite ore displays octahedral crystals and 

occurs as an accessory mineral in ultramafic igneous rocks. The 

dense crystals of chromite typically sink and accumulate at the 

bottom of an ultramafic magma chamber, forming dense, pod-like 

masses in association with pyroxenite, peridotite, and serpentinite.

Pyroxenite from Clear Creek, San Benito County.

Peridotite from alluvial gravels, Arroyo Seco Canyon, Monterey County.

Bay region occurrence: Small chromite masses have 

been mined from serpentinite areas throughout the Diablo 

Range. This sample is from the Clear Creek area of San Benito 

County. Small chromite deposits have been mined throughout 

the San Francisco Bay region, but larger chrome mining operations have been conducted further south in the Diablo Range. 

Chromium-enriched deposits (both natural and manmade sediments and fill) are blamed for high concentrations of chrome 

in ground water in some areas. Elevated chrome concentrations are toxic to both plants and animals (including humans). 

Chromite ore from Clear Creek, San Benito County.
20 Rocks and Geology in the San Francisco Bay Region 

Anorthosite

Rock characteristics: Anorthosite is dominated by plagioclase feldspar (calcium- and sodium-rich variety of feldspar 

including anorthite, labradorite, and bytownite); it is not a 

mafic or ultramafic rock, but it occurs in association with both 

rock groups. In mantle-derived rock found on the surface, plagioclase feldspar occurs in association with pyroxene, olivine, 

and other mafic minerals. Interestingly, anorthosite is perhaps 

a dominant rock-type found on the moon! 

Bay region occurrence: Anorthosite occurs in association 

with some intrusions in ultramafic rock masses in the Santa 

Cruz Mountains and in the Diablo Range. This sample came 

from a gravel bar near Lexington Reservoir in San José.

Jade

Characteristics: Jade is a semiprecious gemstone (rock) 

with a waxy or felt-like appearance (due to the microcrystalline 

texture of its mineral grains). It can be found in colors ranging 

from white, yellow, green (jade), gray to black. Jade can consist 

of two different minerals—nephrite and jadeite. Nephrite is 

a softer variety amphibole group mineral, whereas jadeite is a 

harder, denser pyroxenite group mineral. Both types occur in 

association with serpentinized ultramafic rocks that have experienced a moderate to high degree of metamorphism. 

Bay region occurrence: Nephrite jade is scarce, but the 

more common variety, jadeite, can be found in stream gravel 

bars and on beaches associated with ultramafic rocks throughout the Coast Ranges. 

Extrusive Igneous Rocks

Extrusive igneous rocks form from the cooling of molten 

material that has erupted on the surface of the Earth, as in 

a volcanic eruption. Extrusive rocks include lava flows and 

pyroclastic material, such as cinders or volcanic ash. The 

rapid cooling of molten rock hinders crystallization processes, so extrusive igneous rocks tend to have a fine-grained 

texture. In some cases, crystals that formed at depth are 

carried with the molten material to the surface. These crystals, called phenocrysts, appear to float in a matrix of finergrained rock material.

Basalt

Rock Type: mafic (rich in iron and magnesium).

Characteristics: gray to black, fine-grained, sometimes displays frozen gas bubbles (vesicular texture), and 

sometimes contains phenocrysts of feldspar (Ca-rich varieties), hornblende, pyroxene, biotite mica, sometimes quartz 

or olivine.

Bay region occurrence: Basalt occurs throughout 

the Bay Area with more ancient occurrences associated 

with the remnants of submarine volcanoes preserved in 

Anorthosite from Mt. Umunhum, Santa Clara County.

Jadeite from San Benito County.

Basalt from near Anderson Reservoir, Santa Clara County. 
Igneous Rocks and Processes 21

the Franciscan Assemblage throughout the region. Notable 

occurrences are in the Marin Headlands and near Morgan 

Hill at the spillways of Uvas and Anderson Reservoir 

dams. More recent volcanic eruptions that produced rock 

of basaltic composition are also located throughout the San 

Francisco Bay region with notable occurrences throughout 

Napa and Sonoma Counties, in the Berkeley and east bay 

hills region, and in the Stanford hills. Younger basaltic 

rocks also occur in the vicinity of Anderson and Coyote 

Reservoirs in the south bay, and at the Basalt Recreation 

Area near San Luis Reservoir.

Andesite

Rock Type: intermediate.

Characteristics: light to dark gray, sometimes with 

reddish to greenish hues; fine-grained, sometimes displays 

bubbles (vesicular texture), frequently has a brecciaed texture (tuff) and sometimes contains phenocrysts of feldspar 

(Ca and Na rich varieties), quartz, hornblende, pyroxene, 

and biotite mica. This sample shows flow banding that 

formed as the rock cooled.

Bay region occurrence: Volcanic rocks of andesite composition occur in the north bay in portions of the 

Sonoma/Clear Lake volcanic region, and in exposures 

throughout the Pinnacles National Monument area. Cobbles 

of andesite can be found in stream gravels and conglomerate beds of all ages throughout the region. Andesite outcrops 

also occur in Henry Coe State Park.

Rhyolite

Rock Type: felsic (rich in silica and aluminum).

Characteristics: light gray, typically pink to red, finegrained, sugary texture, usually has a brecciaed texture 

(tuff) and sometimes contains phenocrysts of feldspar (Narich varieties), quartz, hornblende, and biotite mica.

Bay region occurrence: Rocks of true rhyolite composition are rare in the San Francisco Bay region; exceptions 

include volcanic material from the Pinnacles National Monument area, stream cobble and conglomerate, and shippedin construction materials. 

Pillow Basalt—How Does It Form?

When flowing basaltic lava meets ocean water it quickly 

chills on the surface to become stone. However, because rock 

is an excellent insulator, the molten rock below the chilled 

crust will continue moving under the weight of the molten rock 

streaming behind it, forming tubes of lava. These lava tubes 

frequently burst, bifurcating into spreading fingers of lava 

across the seabed. In places the newly formed tubes swell into 

pod-like masses before freezing to rock. In cross section, an 

accumulation of these frozen fingers of lava look like stacked 

pillows. Ancient pillow basalt occurs in outcropping throughout 

Andesite boulder from San Luis Reservoir, eastern Merced 

County.

Rhyolite from the stream gravels in Monterey County. 

Lava flowing into the sea at Hawaii 

Volcanoes National Park. 
22 Rocks and Geology in the San Francisco Bay Region 

the San Francisco Bay region (such as these at Point Bonita in 

the Marin Headlands). At Uvas Reservoir (near Morgan Hill) 

limestone occurs between basalt pillows. At the time this pillow 

basalt formed, the water was shallow and warm enough for 

lime mud to accumulate on the seabed; such conditions might 

have occurred on top of a submarine volcano). Today, lime 

mud accumulates only in tropical to subtropical marine settings. 

These rocks must have traveled a great distance from where 

they originally formed. Gradual plate tectonic motion over millions of years can do this.

Volcanic Rocks in the San Francisco Bay Region

The regional landscape preserves the evidence of volcanism 

that occurred throughout its geologic past. The oldest rocks in 

the region record evidence of undersea volcanism (pillow basalt) 

and igneous intrusions that formed deep in the Earth. The region 

experienced significant periods of volcanism during the formation of the San Andreas Fault system in the late Tertiary (roughly 

15 to 8 million years ago). Important mineral deposits formed in 

association with some of these volcanism episodes (particularly 

the emplacement of mercury ores). The Sonoma volcanic area is 

the youngest and largest in the San Francisco Bay region and is 

still relatively hot (as shown by the existence of geysers and hot 

springs in the area). The map below shows a number of areas 

where volcanic rocks can be observed. Many other volcanic rock 

localities and hot spring areas occur throughout the region that are 

not shown on the map. Although there are no known active volcanoes in the San Francisco Bay region, great explosive volcanic 

eruptions in the Cascades and in the western Great Basin region 

blanketed the region with massive ash deposits in the not too distant past, and could potentially do so again in the future. 

Pillow basalt at Point Bonita in the Marin Headlands.

Pillow basalt with limestone in Uvas Reservoir spillway, Santa 

Clara County (the hammer handle is one foot long for scale). 

Map showing the location of volcanic rocks in the San 

Francisco Bay region. 
Igneous Rocks and Processes 23

Pinnacles Volcanic Area—A National 

Monument for Half of a Volcano 

Roughly 23 million years ago, a volcano formed near the 

southern end of what is now the Great Valley (in the region 

between Bakersfield and Los Angeles). At the climax of its formation, it was roughly 2 miles high (similar to a high Cascades 

volcano in Oregon). Fortunately for geologists, the volcano 

formed straddling the San Andreas Fault. Since the end of its 

eruption cycle, the western side of the volcano (on the Pacific 

Plate) has moved about 195 miles northwest of a smaller remMap showing the migration of the Pinnacles volcano.

View of the High Pinnacles (eroded volcanic rocks) in Pinnacles 

National Monument.

nant still attached the North American Plate on the eastern side 

of the fault. This offset volcano is one of many “match points” 

that geologists have been able to use to establish distances of 

offset and rates of fault movement through time.

Erosion over 23 million years has stripped away nearly 

the top mile of rock from the original volcano. The remnants 

of the volcanic stocks and the steep bedded layers of the volcano’s lower flanks are all that remain of the original volcano. 

The rock is chiefly of rhyolite and andesite composition, and 

has the texture of a breccia or tuff, an extrusive igneous rock 

with a rough, sometimes frothy texture and a chemical and 

mineralogical composition similar to granite (rich in silica and 

aluminum oxides). The rock formed from the accumulation of 

many volcanic eruptions through time. Some fossil wood is 

preserved in the volcanic deposits, suggesting that forests had 

time to grow back between eruptions.

Hydrothermal Deposits

Where volcanism occurs, there are associated processes 

that affect the surrounding host rock. Fluids released from the 

magma, along with fluids in the surrounding rock, migrate 

through porous spaces in the surrounding rock and especially 

along fissures, fractures, and faults. With the changing chemical conditions of these fluids as they interact with the bedrock, 

minerals are either dissolved or precipitated. Where these 

fluids reach the surface, hot springs and fumaroles (gas vents) 

occur and are typically the location of surficial mineral deposits. Fissures and fractures underground can become places 

where minerals accumulate, typically forming quartz and calcite veins, but in some places in the San Francisco Bay region 

other minerals occur, including ore minerals of mercury, 

copper, lead, and other metallic and non-metallic minerals. 

Travertine

Characteristics: A soft white, yellow, or gray rock consisting of layered microcrystalline to fine fibrous crystals of 

calcite. It occurs in fracture fillings and on surfaces of caverns 

in limestone, and around springs. Notable occurrences include 

springs at Alum Rock Park in San Jose, on Fremont Peak, and 

along streams throughout the Santa Cruz Mountains.

Travertine from Fremont Peak, San Benito County.
24 Rocks and Geology in the San Francisco Bay Region 

Hydrothermally Altered Basalt

Characteristics: yellow, orange, to reddish gray, typically 

displaying the frothy, vesicular texture of the original basalt. 

Cavities are frequently filled with rock crystal (quartz), chalcedony, common opal, and calcite crystals (both dogtooth style 

crystals and fibrous varieties), and occasionally pyrite, copper 

minerals, fluorite, and frequently cinnabar. Hydrothermally 

altered rock can be found wherever volcanic rock occurs. 

However, the mineral replacement processes make the rock 

typically much harder and denser than the original host rock. It 

is usually abundant on downstream gravel bars.

Calc-silicate rock from the Cinnabar Hills area, Santa Clara County. The Calistoga Geyser (Old Faithful Geyser of California) in northern 

Napa Valley, evidence that the Sonoma volcanic field is still relatively 

young and hot.

Hydrothermal-altered basalt from Coyote Creek, Santa Clara County.

Springs, Warm Springs, and Hot Springs and 

Their Deposits

Some precipitation seeps into the ground. As this water 

migrates downward, it reaches a point where the available 

pore space in fractures and between rock grains is nearly 

completely saturated with water. This level is called the 

water table and is basically defined by the static level that 

water will rise to in a well. The water table is not a flat surface, but rather roughly follows the general topography of the 

landscape. Where there is both permeability in the ground 

and differences in elevation of the water table, ground water 

will gradually flow in the direction from high to low. A 

spring is a passage where water flows freely from the ground. 

Springs basically define locations where the water table 

intersects the land’s surface, and sufficient water volume, 

pressure, and permeability exist to allow water to discharge. 

As ground water migrates it continuously equilibrates 

with the temperature of the rock and the chemistry of the 

materials it encounters. Depending on conditions mineral compounds will dissolve into or precipitate from ground water, 

sometimes simultaneously. In areas of “recent” (geologically 

speaking) or ongoing volcanism, heat originally derived from 

magma will be transferred into the migrating ground water. 

This heat can then be transferred to springs at the surface.

In general, shallow ground water basically is in temperature equilibrium with the annual average temperature of 

Calc-Silicate Rock

Characteristics: yellow, orange, and pale green, brown to 

gray, typically forming in fissures in limestone or serpentinite host 

rock. It typically has abundant mineral-filled cavities frequently 

containing rock crystal (quartz), calcite, and cinnabar. This is the 

host rock for most of the mercury ore (cinnabar) mined in the New 

Almaden Mining District in southwestern San Jose, with scattered 

occurrences throughout the eastern Santa Cruz Mountains and in 

the region around New Idria in the central Diablo Range.
Igneous Rocks and Processes 25

any particular location, but may vary seasonally. A “warm 

spring” is any water discharge site where the ground water 

is continuously at least several degrees higher than the 

annual or seasonal temperature average. A “hot spring” is 

a spring that has a temperature greater than human body 

temperature (98oF). Where temperatures reach the boiling 

point of water near the surface, boiling hot springs, geysers, 

and fumaroles (steam vents) may occur. In some cases, the 

water may contain fluids derived from magma (including 

water, CO2, sulfurous gases, and dissolved matter).

Perhaps the most common rocks associated with spring 

deposits are travertine and tufa. Travertine is a finely crystalline, massive deposit of calcium carbonate (calcite). Tufa 

typically consists of microcrystalline calcite sometimes 

Travertine being deposited by springs 

along Stevens Creek Canyon Road 

(Santa Clara County).

Cinnabar (Mercury Ore) And the New Almaden 

Mining District

Cinnabar (HgS), mercury ore, was mined in the hills southwest of San José from the late 1820’s until 1976. The New 

Almaden Mining District was the largest producer of mercury 

in North America. Company records indicated that over a million flasks of mercury were produced from the mining district (a 

flask holds roughly 65 pounds of liquid mercury). More than 95 

percent of the production occurred before 1900, with the most 

productive years occurring in the decade following the Civil 

War. The majority of the mercury produced was used for gold 

mining and production in the Sierra Nevada region. 

The cinnabar deposits probably formed in association with 

a period of volcanism in the region roughly 10 to 12 million 

years ago. Hydrothermal fluids (hot mineral water and gases) 

migrated along faults and fractures in the serpentinite bedrock, 

altering the bedrock while precipitating calcite and quartz (calcsilicate rock) and cinnabar. Outcrops near Guadalupe Reservoir 

(right) display typical fracture filling of the calc-silicate host 

rock. Cinnabar is typically deep red in color, but ranges to 

brown and black, and sometimes displays tiny droplets of liquid 

mercury on its surface. Be aware that cinnabar is quite toxic and 

Cinnabar 

in calcsilicate rock 

(Guadalupe 

Dam).

preserving organic 

remains. Travertine and 

tufa spring deposits are 

common in areas where 

limestone is abundant 

in the subsurface. Hot 

springs in volcanic 

areas typically precipitate silica (SiO2) with 

other mineral matter 

forming a rock called 

siliceous sinter. These 

rocks form where 

chemical and temperature changes drive 

mineral precipitation 

reactions.

especially shouldn’t be handled by children. Also, food grown 

in mercury-contaminated areas (including fish) may also contain 

hazardous levels of mercury. When in doubt, leave it alone!

San Cristobal Mine shaft entrance and ruins on Mine Hill 

in Almaden Quicksilver County Park, Santa Clara County.

Calcsilicate rock 

in altered 

serpentinite 

(Guadalupe 

Dam). 

An outcrop near Guadalupe 

Dam in Santa Clara County.
26 Rocks and Geology in the San Francisco Bay Region 

Sedimentary Rocks and Processes

The daily influences of the atmosphere and water slowly 

physically disintegrate and chemically decompose rocks 

exposed at or near the surface. These processes are collectively called weathering. Many processes contribute to converting rocks into smaller sediments. For example, rainwater 

may seep into cracks and freeze; the expansion of ice can 

create tremendous forces, enough to break rock. Likewise, 

roots penetrate into cracks and expand. Resistant fragments 

become sediments, whereas soluble compounds dissolve and 

are carried away by flowing water on the surface and underground. Erosion involves the transport of sediments by an 

“erosion agent,” such as running water, waves, wind, or glacial ice. Rocks are broken down from larger to smaller fragments: boulders, cobbles, gravel, sand, silt, and microscopic 

clay-sized particles. The finer the particle, the more easily it 

is transported. In this manner sediments become sorted into 

size fractions by waves, running water, or wind. 

Sediments are transported and deposited in a variety of depositional environments. On land, sedimentary 

environments include stream flood plains, swamps, dunes 

and desert basins. Along the coastal regions sediments 

accumulate in river deltas, lagoons, beaches, and barrier islands. Most sediment ultimately comes to rest in 

the ocean, accumulating in massive deposits that form 

the continental shelves, or continuing to the deep basins 

beyond the continental shelf margin. 

As sediments accumulate, the weight of overlying 

layers causes compaction. Dissolved minerals precipitate 

in pore spaces, cementing grains together. The process of 

lithification involves the combined effects of compaction 

and cementation, resulting in the formation of sedimentary 

rocks. Sedimentary rocks consist of pieces or fragments of 

rock (clasts), minerals precipitated from water, or a combination of the two sources. Sedimentary rocks are generally named after their primary clast sizes and their mineral 

constituents (with some exceptions).

Depositional environments where sediments may accumulate.
Sedimentary Rocks and Processes 27

Bathymetry and topography shaded relief image of the San Francisco Bay region (from: http://terraweb.wr.usgs.gov/TRS/projects/Monterey/).

The image below combines topographic and bathymetric data for the greater San Francisco Bay region and 

illustrates the extent of different sedimentary environments, as they exist today. The image is also useful for 

interpreting the kinds of sedimentary environments that 

existed in the past that are now preserved as sedimentary 

rock formations exposed in mountainous areas and road 

cuts in the region. For instance, the vast majority of the 

sediments deposited in the region today are on the continental shelf and in the deep-ocean basin beyond the continental slope. Many of the sedimentary rocks exposed in 

the Santa Cruz Mountains and the Diablo Range formed 

in similar offshore settings in the geologic past and were 

subsequently uplifted to their present locations by tectonic 

forces and then exposed by erosion. In contrast, stream and 

other terrestrial deposits only represent a small portion of 

the sedimentary environments in the region today, and this 

contrast is also reflected in the sedimentary geologic record. 

Terrestrial deposits tend to be eroded away rather that be 

preserved, but this isn’t always the case. For instance, the 

occurrences of coal mines in the Diablo Range are a testament of forested swamps in the region in the geologic past.
28 Rocks and Geology in the San Francisco Bay Region 

Soil

The breakdown of rock into soil is a slow process that 

is going on practically everywhere around us and especially 

beneath us. Most of the landscape around us doesn’t consist of 

exposed rock. In most places you would need to dig down many 

feet, even hundreds of feet, before you encountered “fresh” 

(unaltered) bedrock. This is because chemical and mechanical 

processes combine to break down rocks, leading to the formation of soil. The word soil has two meanings. In its perhaps 

more familiar sense, it is the natural medium for the growth 

of land plants. Technically, soil includes all unconsolidated 

materials above bedrock. The entire section from bedrock to the 

surface is called a soil profile and can be subdivided into soil 

horizons (O, A, B, and C). A typical soil profile consists of:

O Horizon—At the surface organic mater accumulates. 

This organic detritus, called humus, is mostly decomposing 

plant debris. This horizon (O) is where biological reworking 

of material is most intense (mixing both organic material and 

material derived from the parent rock). 

A Horizon—Below the organic-rich surface layer the 

combination of organic acids and water from precipitation 

leaches more soluble compounds from the parent rock material. 

This horizon (A) tends to have a dark stain from the organic 

acids and consists of materials that are more resistant to weathering (like quartz sand). This interval, sometimes called the 

“zone of leaching” it typically loose and friable.

B Horizon—Below the “zone of leaching” is the “zone 

of accumulation.” This is where fine material carried by water 

percolating downward settles out and accumulates. Clay-size 

grains fill in between grains, and during dry periods, minerals 

dissolved in water precipitate. The B horizon tends to be quite 

dense and impermeable. Clays in this interval may expand when 

wet, and shrink and crack when they dry out.

C Horizon—This interval consists of slightly decayed 

parent rock material but hasn’t experienced the degree of leaching and accumulation that has occurred in the A and B horizons. 

The C horizon is characterized by large blocks to small chips of 

bedrock, most of which display some evidence of breakdown or 

alteration through the interaction with ground water.

There are many kinds of soils. The character of the soil 

depends on factors such as amount of precipitation (and evaporation), the physical and chemical characteristics of bedrock, 

plant cover, hill slope, temperature, and other factors. The 

formation of soil is significant in that most sediment is derived 

from the erosion of soil. 

A complete soil profile exposed by wave erosion along the shore of Calero Reservoir in Santa Clara County.
Sedimentary Rocks and Processes 29

Layers in Sedimentary Rock

Most sedimentary rocks display layering. However, the 

layering can be on many different scales, ranging from paperthin clay layers in a hand specimen to massive layers that crop 

out along a whole mountainside. Layering reflects many different sedimentation processes but with common characteristics. 

The parallel layers in sedimentary rocks are called strata. Strata 

generally preserve the gentle, flat-lying surface character of 

their original depositional environments. Sedimentary rocks frequently contain fossils, the remains, traces, or imprints of plants 

or animals from the geologic past. They also preserve a host of 

sedimentary structures including ripple marks, raindrop impressions, desiccation cracks, and other features that are useful tools 

for interpretation of ancient environmental settings. 

Because sedimentary rocks form near the surface, they 

may ultimately become exposed to weathering and erosion. 

However, in certain geologic settings such as in a sedimentary 

basin along the margin of a continent, they may be subjected 

to increasing burial. Because sedimentary rocks typically preserve layers (strata) that reflect their original, flat-lying mode 

of deposition, they are useful in interpreting geologic changes 

that have happened through time. For instance, they may reveal 

such processes as folding or faulting that have occurred with 

mountain uplift after the original sediments had been deposited. 

Geologists group strata into sets ranging from smallest to largest—bed, unit, member, formation, and group. A bed might 

represent sediments deposited in a single event, such as a giant 

sea storm, flood, or ash layer from a volcanic eruption, whereas 

a formation or group might represent the entire accumulation of 

sediments deposited through an entire period of geologic time 

lasting many millions of years. The most commonly referred 

designation is formations and their member subdivisions. A 

formation is a body of rock strata that consists dominantly of 

a certain rock type or combination of types, and typically is 

mappable across a wide area or region. Sedimentary formations 

may also preserve fossil plant and animal associations that exist 

together because of closely similar life patterns, habits, and climatic requirements at the time the sediments were deposited.

The law of original horizontality suggests that in any 

depositional setting, denser materials under the influence of 

gravity will settle to the bottom of a depression, with sediments 

filling in spaces to create an even surface. The law suggests 

that a water-laid stratum, at the time it was formed, must continue laterally in all directions until it thins out as a result of 

non-deposition or until it abuts against the edge of the original 

basin of deposition. The law of superposition says that lower 

areas will fill in first, with younger sedimentary deposits accumulating on top (oldest rocks on the bottom, youngest on top). 

However, these two rules wouldn’t make sense without another 

law to help us explain how we see sedimentary rock outcrops. 

Once a sedimentary stratum has formed, tectonic forces can 

uplift or distort the originally flat-lying beds. The law of crosscutting relationships basically states that rocks are older than 

the forces that change them: for instance, the rocks have to be 

older than the fault that breaks them, folds that reflect tectonic 

deformation, or igneous material that was intruded into them. 

To produce geologic maps, geologists measure the changes in 

sedimentary stratification (away from their original horizontal 

orientation) to map the structures of the earth that have resulted 

from subsequent tectonic modification of the rocks that later 

have been exposed by erosion on the land’s surface. 

Sea cliffs at Wilder Ranch State Park (Santa Cruz) display flat-lying strata consisting of mudstone that formed from mud deposited several million years ago on a seabed similar to the modern outer continental shelf. Ongoing tectonic uplift associated with the formation of the Santa Cruz 

Mountains has exposed these strata on the land’s surface. They are now being gradually destroyed by coastal erosion, with most of the sediments returning to the continental shelf or beyond to the deep-ocean basin.
30 Rocks and Geology in the San Francisco Bay Region 

What Strata Can Reveal About Earth History

Layers of shale 

exposed in a road cut 

along the Mt. Hamilton 

Road in Santa Clara 

County were probably 

originally formed from 

clay-rich sediments 

deposited in deep 

water. With time the 

mud was compacted 

and cemented into 

shale. The law of 

original horizontality 

suggests these strata 

were originally flat 

lying. Tectonic forces 

subsequently tilted the 

rocks, possibly concurrent with the uplift 

of the Coast Ranges.

A small fault offsets strata exposed along the sea cliffs near Gazos 

Creek (San Mateo County in Año Nuevo State Park). Layers “A” and 

“B” are offset about two meters (a rock hammer provides scale). The 

law of crosscutting relationships suggests that the fault is younger 

than the sedimentary layers.

Layers tectonically folded into down-warped sedimentary strata 

(a geologic structure called a syncline) are exposed at low tide 

at the James V. Fitzgerald Marine Reserve in San Mateo County. 

The law of original horizontality suggests that the sediments were 

originally deposited in horizontal layers. The law of crosscutting 

relationships suggests that the sedimentary layers are older than 

the forces that folded them.
Sedimentary Rocks and Processes 31

Sediments and Sedimentary Deposits

Erosion is the wearing away of rock, soil, and preexisting sediments by processes involving weathering, mass wasting, and the action of streams, wind, waves, glacial ice, and 

ground water. Erosion involves the movement of material 

(sediments) from one place to another. This can include clastic 

sediments (“clastic” simply means rock fragments) or material 

dissolved in water. Once sediments reach a final destination 

(a depositional environment) they can undergo lithification, 

the processes of converting sediments to sedimentary rock. 

This typically involves compaction (loss of pore space and the 

interlocking of grains) and cementation (the binding of grains 

by the precipitation of mineral cements, such as silica, calcium 

carbonate, clays, or iron oxide minerals). Common sediments and their equivalent sedimentary rock are listed below. 

Note that not all sedimentary rocks are formed from erosion 

byproducts but may form as a result of precipitation by chemical or organic reactions or a mixture of all of the above.

The coastal environment is perhaps the easiest place to 

watch the progress of geologic change, particularly in the 

San Francisco Bay region where winter storms cause a tremendous amount of erosion of the beach and the base of sea 

cliffs. This leads to landslides and slumps that carry sediment 

from the hillsides to the surf zone. Sand is carried by wave 

energy to offshore bars where it can accumulate. During the 

summer months the more quiet daily tidal cycle allows waves 

to gradually transport sand from offshore bars back onto the 

beach. Through time, the constant recycling and winnowing 

of the sediment concentrates the more durable mineral grains 

(particularly quartz sand and gravel), while the finer materials (clays) stay in suspension and get transported out to sea 

where they settle to the bottom in deeper, quieter water. In 

this manner sand tends to accumulate and concentrate in the 

coastal zone, an environment that ranges from upper beach 

dune areas to the region offshore where wave energy begins 

to influence the sea bottom (roughly at about 30 meters 

depth). With the constant supply of sediments from rivers 

and streams and the erosion of the coastline, with time, the 

amount of sandy sediment that accumulates in the coastal 

zone is tremendous.

Compounding the progress of sand accumulation in 

the coastal zone are the influences of tectonic uplift and 

subsidence. In the San Francisco Bay region many active 

earthquake faults are gradually or episodically moving the 

landscape. For instance, the sea cliffs at Thorton/Ft. Funston 

Beach (shown at right) are a great accumulation of sediments 

deposited in the coastal zone in many different sedimentary 

environments. Geologists named this thick sequence of sanddominated deposits the Merced Formation. This sequence of 

strata was deposited in a slowly subsiding basin that because 

of changes in the geometry of the San Andreas Fault system 

is now slowly rising and being exposed once again to coastal 

erosion. The Merced Formation preserves evidence of many 

cycles of sea level rise and fall through the Pleistocene 

Epoch, the geologic time period representing a series of ice 

ages that began roughly 1.8 million years ago and ended 

about 10,000 years ago. With each ice age, great continental 

Classification of chemical and organic sedimentary rocks.

Beach and sea cliffs at Thorton/Ft. Funston Beach.

Classification of clastic sediments and sedimentary rocks.

glaciers slowly formed and then melted during intervening 

warm periods. Sea level change during each glaciation’s 

cycle was in the range of 160 to 500 feet (about 50 to 150 

m). Fossiliferous marine beds alternating with terrestrial 

beach dune and shore deposits suggest that the Merced Formation preserves evidence of at least nine glaciations cycles 

in the past 600,000 years.
32 Rocks and Geology in the San Francisco Bay Region 

Sedimentary Rocks

Conglomerate

Rock characteristics: A coarse-grained clastic sedimentary rock composed of rounded to subangular fragments larger 

than 2 millimeters in diameter (pebbles, cobbles, and even 

boulders) set in a fine-grained matrix of sand or silt, and commonly cemented by calcium carbonate (calcite), iron oxides, 

silica, or hardened clay. Conglomerate is the consolidated 

equivalent of gravel. 

Bay region occurrence: Conglomerate occurs throughout 

the San Francisco Bay region with notable occurrences in the 

Pigeon Point area (San Mateo County coast), as terrace gravels 

throughout the south bay region (particularly along Highway 

101 near Anderson Reservoir) in the Santa Cruz Mountains 

with large exposures on Loma Prieta Peak, at Point Lobos on 

the Monterey Peninsula, and at the lighthouse on Point Reyes.

This sample is technically a “volcanic breccia” formed on the flank 

of an ancient submarine volcano. The volcanic cinders are cemented 

together by limey clay deposited on the sea floor. This sample is from 

Uvas Creek near Morgan Hill.

Conglomerate from the Pigeon Point Formation (Cretaceous). 

Cobbles consist of very well rounded pieces of volcanic rock (mostly 

andesite) probably worked by marine currents and deposited in the 

deep-ocean basin.

Conglomerate of undetermined age formed from well-rounded 

stream gravel consisting mostly of sandstone and chert 

derived from the Diablo Range. This sample is from a gravel 

bar along Coyote Creek.

Conglomerate containing a mix of rounded and angular fragments 

of serpentinite and sandstone, and cemented by calcite and clay. It 

probably formed fairly recently as a stream deposit near the source 

where the gravel was derived. This sample is from Clear Creek in San 

Benito County.
Sedimentary Rocks and Processes 33

Origins of conglomerates—Fast moving rivers and ocean 

currents can transport gravel long distances. Within gravel 

deposits it is sometimes possible to find materials of unique 

composition that can be correlated to a definitive source. Studies of gravel deposits in rocks of all ages in the California 

coastal ranges show that river systems originating from destinations possibly as far away as central Mexico to the Canadian 

Shield have provided sediments to coastal California in the 

past (these river systems no longer exist today due to changes 

in the landscape). For instance, certain types of quartzite and 

igneous rocks with unique mineral compositions found in 

ancient gravel deposits throughout the Santa Cruz Mountains 

can be recognized in similar gravel deposits as far away as 

Santa Barbara, the Mojave, and possibly even Wyoming! 

These gravel deposits, along with the fossils they contain, help 

geologists to understand the changes in the landscape and geologic settings through time. This information has also helped 

to prove the rate of offset motion along the San Andreas Fault 

through time. 

A modern stream gravel bar along Coyote Creek in Santa Clara County.

Origin of Cretaceous and Tertiary conglomerates in the region. The 

diagram shows movement of the gravels after deposition as a result 

of tectonic forces.

Conglomerate bearing marine fossils near the top of Loma Prieta Peak 

in Santa Cruz County.
34 Rocks and Geology in the San Francisco Bay Region 

Sandstone 

Rock characteristics: A clastic sedimentary rock composed of 80 to 90 percent sand-size grains set in a matrix of 

silt or clay and more or less firmly united by a cementing 

material (commonly silica, iron oxide, or calcium carbonate); 

the consolidated equivalent to sand. 

Bay region occurrence: Sandstone occurs throughout the 

San Francisco Bay region with notable occurrences along the 

crest of the Santa Cruz Mountains, forming the massive cliffs 

at Castle Rock State Park. Similar rock sandstone outcrops 

occur at Chitactac-Adams Heritage Park in southern Santa 

Clara County, along Highway 101 between San Juan Bautista 

and Salinas; throughout the sea cliffs of the Purisuma Formation in Santa Cruz; at Año Nuevo State Park; between Bean 

Hollow and Pescadero State Beaches; at Half Moon Bay; 

at Fort Funston Beach; and near Rodeo Beach in the Marin 

Headlands. Many of these sandstone outcrop areas contain 

beds rich in marine invertebrate fossils. 

Fossiliferous sandstone (Purisuma Formation, late Miocene) from 

New Brighton Beach near Capitola, Santa Cruz County. This piece 

preserves mollusk shells and a portion of whalebone offset by a 

tiny fault (bottom).

Sandstone from near Guadalupe Reservoir (Santa Clara County). 

Uniform sand deposits form from natural refining processes. Rivers 

transport a mix of cobbles, gravel, sand, silt, and clay. Once these 

sediments reach the ocean, wave action or marine currents constantly rework and refine sediment fractions. Gravel accumulates in 

the highest energy environments and is subjected to heavy reworking. Sand migrates and accumulates where wave energy and currents dissipate, whereas silt- and clay-sized material is transported 

far offshore to quieter, deeper water. Massive sandstone bodies 

represent ancient sedimentary environments, such as beaches or 

barrier islands, sand bars formed on the continental shelf, or sand 

accumulations in the deep ocean near the base of the continental 

slope. These deep-water deposits form when underwater landslides 

entrain large quantities of sediments down slope in the form of 

density currents called turbidity flows (muddy water is denser than 

clear water and therefore, it sinks). These create deposits called 

turbidites that consist of interbedded sandstone, mudstone, and 

shale, and sometimes, conglomerate.

Fossiliferous sandstone from the crest of the Santa Cruz Mountains. 

Although the rock is full of marine invertebrate shells, mostly late 

Tertiary gastropods, clams and barnacles, it is still considered sandstone, not a conglomerate.
Sedimentary Rocks and Processes 35

A massive outcrop of sandstone rises out of Guadalupe Reservoir 

near Almaden-Quicksilver Park in Santa Clara County (right). Marine 

fossils preserved in outcrops such as these provide reminders that 

shallow seas once covered the region long before the Santa Cruz 

Mountains formed.

Massive outcrops 

of sandstone 

occur scattered 

throughout the 

San Francisco 

Bay region, particularly along the 

crest of the Santa 

Cruz Mountains. 

Exceptional outcrops of sandstone 

in Castle Rock 

State Park (right) 

display an unusual 

weathering pattern 

called “tafoni.” 

When precipitation 

soaks into porous 

sandstone, some of 

the mineral cement 

dissolves. As the 

rock dries out, capillary action brings 

moisture along with 

dissolved minerals 

Native Americans used sandstone outcrops for many purposes. Sandstone outcrops occur along Uvas Creek at Chitactac-Adams Heritage Park 

in southern Santa Clara County (left). Many of the outcrops display an abundance of mortar holes used for grinding and preparing food and also 

display petroglyphs (carved symbols in the rock). 

to the surface. As the water evaporates, the minerals precipitate. In 

this manner, the rocks actually break down from the inside out as this 

weathering process removes the mineral cement below the surface. 

The wind and weather help to sculpt away the softer rock, leaving 

the more resistant, tightly cemented surface rock behind. Goat Rock 

(shown here) is one of many popular scenic hiking and rock climbing 

destinations at Castle Rock State Park.
36 Rocks and Geology in the San Francisco Bay Region 

Graywacke—An Old Sedimentary Rock Term With

A New Meaning

Graywacke is an old German term, now generally applied 

to a dark, firmly indurated, coarse-grained sandstone to siltstone that consists of poorly sorted angular to subangular 

grains of quartz and feldspar, with a variety of dark rock and 

mineral fragments, embedded in a compact clayey matrix. 

Where perhaps the preferred terminology for naming sedimentary rock is to focus on the host sediment (for example, “sand” 

for sandstone), the term graywacke now also implies a general 

environment of origin adjacent to an tectonically active plate 

margin where streams draining a volcanic mountain range 

dump large quantities of poorly sorted sediments into an ocean 

basin setting. Graywacke commonly exhibits graded bedding 

(thick layers containing coarse-grained sand at the base and 

finer-grained mudrock on top) and was probably deposited by 

submarine turbidity currents.

 Bay region occurrence: The term “graywacke” can 

perhaps be loosely applied to the majority of clastic sedimentary rocks that crop out in the region. Practically any brown 

or gray “dirty” sandy sedimentary rock exposure in the Santa 

Cruz Mountains or the Diablo Range could be called a graywacke. Exceptional exposures of true graywacke turbidites are 

exposed at Bean Hollow State Beach and near Rodeo Beach 

Path to formation—graywacke.

Graywacke at Bean Hollow State Beach. 

in the Marin Headlands. Graywacke is abundant in Franciscan 

Assemblage rocks practically wherever they occur. Throughout the Diablo Range these rocks have experienced varying 

grades of metamorphism, ranging from relatively soft mudstone to dense metasandstone.
Sedimentary Rocks and Processes 37

Mudstone and Siltstone 

Rock characteristics: Sedimentary rock composed of 

indurated mud and having the texture and composition of 

shale, but lacking its fine lamination or fissility (ability to 

break apart into tiny flakes). The difference between the two 

is that siltstone is greater than 50 percent silt, and mudstone is 

greater than 50 percent clay. Both rock types tend to be brittle. 

Siltstone tends to be flaggy, splitting into thick layers and displaying current structures. Both rock types preserve marine 

fossils (mostly microscopic plankton), including occasionally 

fish scales, bone, and invertebrate shells and their traces. Mud 

and silt tend to be deposited in deeper water conditions below 

wave base on the continental shelf or in bays. 

Bay region occurrence: The Santa Cruz Mudstone, a Miocene rock formation, forms sea cliffs throughout northern Santa 

Cruz County. The Monterey Formation, another Miocene-age 

rock formation, crops out in massive cliff exposures throughout 

the Salinas Range between Carmel and Arroyo Seco Canyon. 

Mudstone and siltstone of “graywacke” composition is perhaps 

the most abundant rock type in the region, cropping out throughout the Santa Cruz Mountains west of the San Andreas Fault and 

along the flanks of the Diablo Range (accessible exposures occur 

along the Mt. Hamilton Road in Santa Clara County). 

Siltstone from Arroyo Seco Canyon, Monterey County. Note the flaggy 

splitting character of the rock.

Marlstone 

Rock characteristics: Marlstone is an indurated rock 

consisting of a mix of clay, mud, sand, and an abundance of 

calcareous material, mostly shell material. It sometimes contains a green grainy mineral called glauconite, a mineral that 

typically replaces marine invertebrate fecal material. These 

rocks form sediments typically deposited in shallow marine 

conditions.

Bay region occurrence: Fossiliferous marlstone is not 

uncommon throughout the Santa Cruz Mountains. 

Santa Cruz Mudstone (from near Davenport). This sample displays a 

weathering pattern of banded, brown rings and spots called liesgang 

rings. It forms as is iron oxide precipitated by ground water influenced 

by alternate periods of wetting and drying.

A block of marlstone from Stevens Creek, Santa Cruz Mountains.

A marlstone shell hash from the Purisuma Formation from a beach at 

Capitola, Santa Cruz County.
38 Rocks and Geology in the San Francisco Bay Region 

Shale 

Rock characteristics: A fine-grained clastic sedimentary 

rock formed by the compaction of clay, mud or silt. Shale differs 

from mudstone in that it has finely laminated bedding structure 

along which the rock splits readily (a property of shale called 

fissility). This characteristic of fissility is greatest in shale that 

formed from mud enriched in clay and depleted in silt or coarser 

materials. In fresh exposures, shale is a well-indurated sedimentary rock, but it tends to quickly break down into small flakes 

and clay mud when exposed to surface weathering. Shale formed 

from mud deposited in the ocean, bays, or swamps tends to be 

greenish to bluish gray to black (rich in organic matter) or red to 

brown when formed from mud in a terrestrial environment, such 

as a floodplain soil. 

Bay region occurrence: Shale tends to break down so 

quickly that landscape underlain by shale tends to be well forested or grass covered. It occurs in association with sandstone 

Interbedded shale and 

sandstone (turbidites) 

exposed at Bean Hollow 

State Beach along the 

San Mateo Coast.

in most areas; however, massive exposures of shale occur along 

the eastern flank and foothills of the Diablo Range. This thick 

belt of sedimentary rock is called the Great Valley Sequence, and 

is perhaps easiest to see exposed along the shores of San Luis 

Reservoir along Highway 152 between Pacheco Pass and Los 

Baños. This great belt of sedimentary rock is many thousands of 

feet thick and crops out in the foothills west of Interstate 5 from 

Tracy to Bakersfield and beyond. It represents mud that accumulated along the western edge of the continent long before the 

Coast Ranges were uplifted. Shale occurs interbedded with sandstone and conglomerate in sedimentary deposits called turbidites. 

These represent undersea landslide deposits that accumulated 

in the deep ocean setting at the base of the continental slope. 

Also throughout the region, the combination of shearing action 

by fault motion and exposure to ground water have converted 

pulverized rock into a flaky mudrock that has the appearance and 

composition of shale but lacks its original bedding structure. In 

some cases the original rock may have been shale. 

Shale of the Great Valley 

Sequence crops out in 

badlands in the Panoche 

Hills, south of Los Baños. 
Sedimentary Rocks and Processes 39

Limestone and Dolostone 

Rock characteristics: Limestone is a sedimentary rock 

consisting chiefly of the mineral calcite (CaCO3, calcium 

carbonate), with or without magnesium carbonate. Common 

impurities include chert and clay. Dolostone is a sedimentary 

rock that is enriched in the magnesium-carbonate mineral, 

dolomite, CaMg(CO3)2. A carbonate rock is considered dolostone if it is greater than 50 percent dolomite. Limestone fizzes 

vigorously when exposed to hydrochloric acid; dolomite 

fizzes poorly by comparison. The two rocks frequently occur 

together. Limestone is typically white, pale gray to black (due 

to high organic content). Dolostone is light gray to pink.

An unusual limestone consisting dominantly of fossil 

oyster shells. This example is from the southern Santa Cruz 

Mountains.

Bay region occurrence: Limestone and dolomite are 

common in areas west of the San Andreas Fault with large 

mining operations near Stevens Creek near Cupertino, and 

an abandoned quarry near Rockaway Beach along the San 

Mateo Coast. Fremont Peak south of Hollister is a massive 

outcropping of limestone and dolostone. Smaller masses of 

these rocks crop out throughout the Santa Cruz Mountains 

with notable exposures near Uvas, Calero, and Lexington 

reservoirs in the south bay. Limestone nodules (called concretions) occur in the Cretaceous Great Valley Sequence, 

with exposures around Anderson Reservoir and throughout 

the eastern flank of the Diablo Range.

Limestone (tan to bluish-gray) with chert (gray) from the Cinnabar Hills near Calero Reservoir 

in the south bay. Limestone was mined in the area before the reservoir was constructed.

A rock consisting of alternating layers of dolostone (pink) and 

chert (red), from hillside exposures south of Morgan Hill.
40 Rocks and Geology in the San Francisco Bay Region 

Concretions—Nature’s Time Capsules

Concretions are small to large rounded nodules that 

form in sedimentary rocks, typically in shale or sandstone. 

Concretions form slowly as minerals precipitate from ground 

water around a “nucleation site.” A nucleation site can be a 

shell or bone, a piece of organic matter, or even a mineral 

grain. Minerals such as calcite, dolomite, or limonite precipitate and grow pervasively in the microscopic voids in 

between sedimentary grains. Over hundreds or even millions 

of years concretions can continue to grow. Sometimes fossils 

inside are well preserved. 

A limestone septarian concretion from the Great Valley Sequence 

exposed along the shore of Anderson Reservoir. Concretions grow 

slowly through rock-water reactions in shallow sediments. As they 

mature, they may desiccate; the desiccation fissures then become 

sites for mineral precipitation, such as calcite or barite.

Examples of concretions and the fossils they preserve can be seen 

in abundance in the sea cliffs at Año Nuevo State Park. These exposures are part of the Purisuma Formation of late Miocene to Early 

Pliocene age (roughly 8 to 5 million years old). These sedimentary 

rocks formed from sediments deposited on the shallow continental 

shelf. They were uplifted and exposed to coastal erosion by the 

greater forces lifting the Santa Cruz Mountains and the coast.

A concretion exposed in the sea cliff at Año Nuevo displays an abundance of well-preserved fossils inside.

Concretions eroding from the sea cliffs at Wilder Ranch near Santa 

Cruz. These concretions formed in the marine sediments of the Santa 

Cruz Mudstone.
Sedimentary Rocks and Processes 41

Chert 

Rock characteristics: Chert occurs in practically any color 

of the visible spectrum. In the region its occurrence is typically 

red (jasper) or gray, green, brown, or black. Chert’s texture is 

compact, very fine-grained, and has a blocky to conchoidal 

fracture. In outcrop, chert occurs in layers with thin shale partings, reflecting how these rocks originally formed from ocean 

mud (biogenic ooze from the buildup of the remains of siliceous plankton) deposited on the deep ocean floor far from any 

sources of terrigenous sediments. The thin shale partings probably represent periods when dust from distant continents periodically was blown into the deep ocean setting, or alternatively, 

these shale partings represent cyclic periods when organic 

productivity declined in the ocean setting. Not all chert displays 

layering. Silica (SiO2), the primary mineral component in chert, 

is relatively soluble under certain conditions; it also melts at 

fairly low temperatures relative to other rocks, Therefore, it is 

easily altered to different forms, either through hydrothermal 

activity or other metamorphic processes.

Bay region occurrence: Perhaps the most famous exposures of chert beds are in the Marin Headlands north of the 

Golden Gate Bridge. Layered chert can be found in Franciscan 

Assemblage rocks throughout the San Francisco Bay region, 

with notable exposures on the road up Mt. Diablo, along Mt. 

Umunhum Road in southwest San José, and the spillway area 

at Anderson Reservoir near Morgan Hill. Morgan Hill “poppy 

jasper” has been mined as an ornamental gemstone.

A beach cobble 

of chert derived 

from the Monterey 

Formation displaying 

alternating layers 

of black phosphatic 

chert and white porcellanite (Waddell 

Creek Beach, Santa 

Cruz County).

 Folded marine chert layers exposed along the road on Mt. Diablo.

Folded white chert from the Diablo Range (Coyote 

Creek, Santa Clara County).
42 Rocks and Geology in the San Francisco Bay Region 

Gypsum 

Rock characteristics: Gypsum (hydrous calcium sulfate, 

CaSO4-2H2O) occurs as clear, white, yellow to gray, thin to 

thick-bedded layers interbedded with shale or other sedimentary rocks. In some cases, it occurs in anhydrous form (called 

anhydrite, plain CaSO4). The rock forms from the evaporative 

concentration of salty water, either seawater or briny ground 

water. In the geologic past, gypsum precipitated in isolated 

bays in arid regions along the coast. A perhaps more common 

crystalline variety, a mineral called selenite, is common in 

weathered shale slopes where it crystallizes from evaporating 

ground water in fractures and along bedding planes.

Bay region occurrence: The occurrence of gypsum 

(and selenite) is limited to only the most arid locations in the 

region. Traces of it occur on shale slopes, particularly in landslide deposits, along hillsides in the east bay, but it is common 

to abundant throughout the lower shale-covered hillsides along 

the east side of the Diablo Range.

Rock Salt

Rock characteristics: Rock salt is a rock made up of the 

mineral halite, along with other impurities. Purified halite is 

common table salt (sodium chloride, NaCl). Rock salt forms 

from the evaporative concentration of seawater. 

Bay region occurrence: Natural salt can be found in the 

upper tidal zone where wave splash is trapped in depressions 

and evaporates. Small patches of salt crystals will temporarily 

build up during hot, dry periods along the coast, but vanish 

with the first good storm. Rock salt is manufactured artificially by pumping seawater through a series of salt evaporator 

ponds in the San Francisco Bay area. Salt crystallizes to a 

pink, massive crystalline form when supersaturated conditions 

are achieved; the pink coloration is primarily due to algae 

trapped within the salt. 

A 40-cm-long, doubleterminated selenite crystal from 

the Panoche Hills region.

Salt evaporator ponds in northern 

San Pablo Bay (enhanced satellite 

image). 

Selenite (gypsum) crystals litter the surface below the location where crystals formed along a sand-filled fracture. 

Exposures like this are common in the hills along the east 

side of the Diablo Range.

Rock salt (halite crystals).
Sedimentary Rocks and Processes 43

 Paths to formation—limestone and chert.

Limestone versus Chert and the 

Carbonate Compensation Depth (CCD) 

Chert and limestone occur in scattered locations within 

the Franciscan Assemblage rocks throughout the region, 

sometimes occurring separately, sometimes interbedded. 

These two rock types support an interesting story about the 

original ancient ocean crust and sediments that were eventually accreted onto the continent (now exposed throughout 

the region). On the basis of where calcareous sediments and 

siliceous sediments are forming today (eventually becoming 

limestone and chert, respectively), geologists can interpret 

the kinds of locations where these rocks formed in the past. 

Critical to this interpretation is the concept of the carbonatecompensation depth (CCD). The CCD is the depth at which 

calcium-carbonate concentrations in sediment decreases most 

rapidly and is defined as the depth at which the rate of dissolution of solid calcium carbonate equals its rate of supply. 

Just as cold soda holds dissolved carbon dioxide better than 

warm soda, the same is true of calcium carbonate dissolved in 

seawater. In shallow, warm water environments, calcium carbonate (calcite) is a byproduct of biological respiration. Some 

of this calcite is incorporated into the skeletons of organisms 

(particularly calcareous algae). If the amount of calcareous 

material accumulating on the sea floor is faster that the rate it 

dissolves, then lime mud (ooze) will build up, possibly becoming limestone through time. Inversely, if the water is too cold 

and deep, calcium carbonate will not be preserved in sediments, rather the remains of plankton that produce siliceous 

skeletons will accumulate to produce siliceous ooze that possibly may become chert through time. Changing water depths or 

ocean temperatures may result in alternating layers of calcareous and siliceous sediments. 

The occurrence of massive limestone and chert deposits in 

the region suggests that rock materials were transported long 

distances from their place of origin to where they occur today. 

Limestone deposits in the region may represent carbonate sediments that built up on or around ancient volcanoes or uplifts, 

possibly associated with a divergent plate boundary (spreading 

center), whereas chert-dominated rocks represent cooler ocean 

crust that had migrated away from the spreading center areas 

into deeper water settings, yet far enough away from shore not 

to receive sediments from land. This is another method used to 

interpret the regional geologic history. 
44 Rocks and Geology in the San Francisco Bay Region 

Organic Residues and Rocks 

(Petroleum and Coal)

When plants and animals die, their organic residues are 

in most cases consumed or decay under attack of microorganisms. What remains is gradually oxidized by exposure 

to free oxygen in the air or dissolved in water. In some 

sedimentary environments, however, organic residues can 

become isolated from organic decay or oxidation, such 

as by sinking into a swamp or by being rapidly buried by 

marine sediments. With time and increasing burial (and 

associated increasing geothermal temperature with depth), 

organic residues undergo chemical alteration along with the 

surrounding sediments. Organic residues may be converted 

to petroleum or coal. In general, ancient marine organic 

matter is the primary source of petroleum, whereas coal 

formed from ancient land plants.

Petroleum is a general term for all naturally occurring hydrocarbons, whether gaseous, liquid, or solid. 

When exposed to the geothermal heat over a long period 

of time, organic matter gradually loses its volatile compounds (including H2O, CO2, and natural gas consisting 

of light hydrocarbons like methane, propane, and others). 

Some of the organic matter will convert to liquid oil and 

can migrate and accumulate in an “oil pool” if geologic 

conditions are right. Petroleum geologists search for oil 

Simplified diagram showing the relationship of increasing depth (increasing heat and pressure over 

time) to the production of different kinds of organic residues and rocks.

in places where they know organic-rich rocks have been 

exposed to the right amount of heat for the right amount 

of time to “generate oil.” Without enough time and heat 

oil won’t form. Too much heat over time can overcook the 

oil, and it will break down to form natural gas and asphalt. 

Geologists also look for special conditions where fractures and pore spaces in the rock will allow petroleum to 

migrate and accumulate in a “trap.” Because petroleum is 

less dense than water, it will migrate upward toward the 

surface. However, if an impermeable layer impedes this 

upward migration, an oil pool will accumulate in porous 

rocks beneath a cap rock seal (such as a shale layer overlying a folded, porous sandstone layer).

Natural asphalt formed in oil-bearing rocks by the 

evaporation of volatiles (removal of gases and light-weight 

hydrocarbons). Asphalt can form deep in the subsurface 

where geothermal heat can degrade oil, and it can form 

near the surface where oil seeps onto the surface or where 

oxidation reactions and microorganisms attack shallow oil 

pools. Natural asphaltic sand (or tar sand) can be found in 

outcrops in many places along the coast in San Mateo and 

Santa Cruz Counties. The knowledge of oil seeps and tar 

sand deposits in the San Francisco Bay region led to some 

of the earliest petroleum exploration and economic production of petroleum in California. Several dozen exploratory 

wells were drilled in the region, and small oil fields were 
Sedimentary Rocks and Processes 45

developed and tar recovery efforts were conducted in 

Santa Clara County near Gilroy (the old town of Sargent), 

in Moody Gulch (west of Cupertino), and in San Mateo 

County in the region around Pescadero and Half Moon Bay. 

More significant resources were discovered and developed 

in the Great Valley along the eastern flank of the Diablo 

Range. Most of California’s oil and gas has been produced 

in southern California in the region between Bakersfield, 

Los Angeles, and along the coast and offshore around Santa 

Barbara. There is currently no known significant petroleum 

production in the San Francisco Bay region.

Asphalt

Asphalt is a dark brown to black viscous liquid or 

low-melting-temperature solid bitumen that consists 

entirely of hydrocarbons. Asphalt is a residue formed by 

the removal of lighter volatile hydrocarbons (gases and 

fuel-grade oils) from crude oil through evaporation or 

distillation. It can form in several ways. Bacterial decay 

of natural oil releases hydrogen from hydrocarbons, concentrating carbon residues. The same effect can happen 

from the natural overheating of oil at great depth—oil 

breaks down forming natural gas and asphalt or even pure 

hydrogen gas and carbon (as graphite) under extreme pressure and temperature conditions. However, most asphalt 

(or tar) found naturally on the surface forms mostly from 

the bacterial degradation of oil migrating to the surface or 

near-surface environment.

This pumping jack in a bank parking lot in Los Gatos is connected to one of the last remaining oil wells in the region; 

however, it is not actively producing oil.

Natural asphalt can be found coating surfaces of boulders and outcrops along the base of sea cliffs at Wilder Ranch State Park in Santa 

Cruz County.

Natural tar sand (sand impregnated with asphalt) can be found in sea 

cliffs and in gravel deposits by the shore and along streams in many 

places along the coast in San Mateo and Santa Cruz counties, such 

as this cobble of tar sand washed up on the beach in Half Moon Bay. 

Fossils and sedimentary structures help differentiate natural asphalt 

rocks from manmade materials that have been dumped or reworked 

by erosion, but the natural and manmade materials are essentially 

identical in character.
46 Rocks and Geology in the San Francisco Bay Region 

Coal

Coal is 50 percent by weight and more than 70 percent 

by volume carbonaceous material, including inherent moisture, formed from compaction and induration of variously 

altered plant remains similar to peat. Differences in degree 

of metamorphism (rank) and in the range of impurity (grade) 

are characteristics of coal and are used to describe and classify it. With burial, dead plant material may become peat. 

With increasing time and burial plant material will progressively lose its volatile components (hydrogen, oxygen, and 

others). With increasing metamorphism peat will become 

lignite, subbituminous coal, bituminous coal, and anthracite. 

Lignite is very low grade, whereas anthracite is very highgrade coal that is nearly pure carbon. 

Deposits of subbituminous grades of coal and lignite 

occur in many places throughout the region in Tertiary- and 

Quaternary-age sedimentary rocks. Low-rank and -grade 

coal was discovered and mined in Alameda and Contra 

Costa Counties in the Mt. Diablo Mining District where 

over 3,500,000 tons of coal was produced. The majority of 

production occurred at the Black Diamond Mine near the 

now abandoned mining town of Tesla. Smaller but economic 

quantities of coal were mined in the Corral Hollow Mining 

District in Alameda County and at Stone Canyon in Monterey County. Nearly all California coal production occurred 

before 1900. Coal occurs in small, intermittent seams in 

many places in early Tertiary sedimentary rocks throughout 

the Coast Ranges, but mostly along the eastern flank of the 

Diablo Range. Small coal deposits can be found in the hills 

around Portola Valley. Lignite can be found in the sea cliffs 

at Thorton and Funston Beaches in San Mateo County and 

in the Miocene and younger sediments throughout the region 

southeast of Hollister.

Subbituminous coal from the Black 

Diamond Mine located near the abandoned 

mining town of Tesla in Alameda County.

Schist bearing graphite from Santa Cruz County.

Graphite

Graphite is a naturally-occurring crystalline carbon 

mineral that is opaque, gray to black, soft, and greasy to the 

touch, and may occur in fine grained masses, platy crystalline form, or more commonly, disseminated as crystalline 

flakes amongst other mineral grains. Graphite is a common 

mineral component in metamorphic rocks that were originally derived from sedimentary rocks containing organic 

matter (possibly as either coal or asphaltic residue), but can 

also form by igneous processes. Graphite can be found disseminated in schist and marble in Salinian basement rocks in 

Santa Cruz County. Small deposits were mined in Sonoma 

County and elsewhere in California, particularly in the Sierra 

Nevada foothills and in region around Los Angeles. The easiest way to determine if a rock contains graphite is that your 

hands still get a dirty gray color after handling rock samples 

that have been washed. Pencil “lead” is actually graphite (but 

modern pencil lead is synthetic). 
Metamorphic Rocks and Processes 47

Metamorphic Rocks and Processes

Processes involving the folding or faulting of rock are 

called tectonism. Tectonic forces move rocks from one 

geologic setting to another. This movement, through time, 

results in both physical and chemical changes to the rock. As 

rocks sink further into the crust, the combinations of heat, 

pressure, and the interaction of hot mineralizing fluids alter 

their original chemical composition and physical structures. 

Minerals stable at low temperatures recrystallize into different minerals stable at higher temperatures and pressures. 

This process, called metamorphism, results in the formation 

of metamorphic rocks. Igneous, sedimentary, and preexisting 

metamorphic rocks may all be subjected to metamorphism. 

With increased heating all types of rock will eventually melt, 

forming magma, which too can migrate to form new igneous 

rock. Metamorphism is gradational, ranging from low grades 

to high grades for both temperature and pressure parameters. 

For low-grade metamorphism, the original host rock is fairly 

easy to determine, but determination grows progressively difficult with increasing metamorphism.

This simplified diagram illustrates the relationships of metamorphic rocks with their parent materials.

A metamorphic rock is any rock derived from preexisting rocks by mineralogical, chemical, and(or) structural 

changes, essentially in a solid state, in response to marked 

changes in temperature, pressure, shearing stress, and 

chemical environment at depth in the Earth’s crust. The 

diagram below illustrates the progression of earth materials from their origin through increasing metamorphic 

grades. Regional metamorphism implies that metamorphic 

processes affect a great volume of rock throughout a widespread area in the Earth’s crust (such as in the formation of 

a mountain range during the collision of continental landmasses). Local metamorphism is what the name implies, 

metamorphic processes affecting a smaller volume of rock, 

such as within a fault zone or near an igneous intrusion. 

The alteration of rock by heat and fluids derived from an 

igneous intrusion is called contact metamorphism.
48 Rocks and Geology in the San Francisco Bay Region 

Temperature and Pressure Related Processes 

in Metamorphism

Both pressure and temperature go up with increasing 

depth into the Earth. In most places, this mutual relationship 

between increasing pressure and temperature conditions 

with depth is called the average geothermal gradient that 

is approximately 72°F/mile (25°C/kilometer). Conversely, 

as Earth materials are subjected to increasing pressure and 

depth through time, they gradually equilibrate to the physical and chemical conditions of their physical environment. 

However, conditions are not uniform beneath the surface. 

For instance, in subduction zones where cool surface rocks 

are sinking into the crust and upper mantle, the pressure 

increases with depth at a normal rate, but temperature does 

not. Where magma is rising, such as at a spreading center or 

under a volcanic arc, the temperature gradient is higher than 

the average geothermal gradient. Through time and changing geologic conditions, new minerals (and rocks) form at 

the expense of preexisting materials. Some of these minerals 

require specific conditions to form. The pressure/temperature range in which a group of minerals form within the 

Earth is called a metamorphic facies. 

The diagram below illustrates the range of temperature 

and pressure (also average depth) that different metamorphic facies develop below the surface. It is important to note 

that “average geothermal gradient” scarcely applies to the 

complex geologic environments found along the margin of 

continental plates (especially in California). Other factors 

are significant relating to metamorphism. In regions where 

mountain ranges are gradually developing over a long period 

of time, rocks throughout a broad region can be subjected to 

deep burial before being gradually uplifted and exposed at 

the surface by erosion. Regional metamorphism is a term that 

applies to metamorphism throughout a broad region. This is 

opposed to settings where metamorphism occurs in localized 

areas, such as contact metamorphism that occurs around the 

margins of an igneous intrusion. Also, hydrothermal alteration from migrating fluids in the subsurface can result in local 

metamorphism. When the temperature of the rock increases 

to the melting point of minerals in the rock, either higher temperature metamorphic minerals will form, or the “low temperature” minerals will melt and the molten material (magma) 

may migrate (forming dikes and intrusions) if differential 

pressure conditions are right. High-grade metamorphic rocks 

that contain many small intrusions are called migmatite.

Pressure, temperature, and depth relationships in the formation of metamorphic rocks. 
Metamorphic Rocks and Processes 49

Metamorphic Rocks

Slate 

Rock characteristics: A compact, fine-grained metamorphic rock that possesses cleavage and hence can be split 

into slabs and thin plates. Most slate was formed from shale. 

Slaty cleavage is a splitting character of fine-grained rock 

along planes parallel to the orientation of fine, platy mineral 

grains formed in the process of low-grade metamorphism. 

Minerals in slate, such as chlorite and cericite (a white 

mica), have a platy mineralogical structure similar to micas 

(biotite and muscovite). During metamorphism, as these 

mineral crystals form from preexisting clay minerals they 

align themselves in a direction perpendicular to the principle 

direction of force crushing the rock (a term geologists call 

the direction of “crustal shortening” during regional metamorphism, typically during a mountain-building period). 

Whereas shale splits along bedding planes, slate splits along 

cleavage planes formed along parallel layers of the platy 

metamorphic minerals. 

Bay region occurrence: Slate occurs amongst metamorphosed graywacke, argillite, and sandstone in Franciscan Assemblage rocks throughout the region. Exposures 

occur along the eastern flank of the southern Santa Cruz 

Mountains, particularly around Lexington Reservoir near 

San José. It also occurs throughout the metamorphic rocks 

exposed in the core of the Diablo Range, with accessible 

exposures along the Mt. Hamilton Road and throughout the 

vicinity of Henry Coe State Park. 

Argillite

Rock characteristics: Argillite is a compact rock, 

derived form mudstone or shale, but shows a higher degree 

of metamorphism than either of those rocks. For instance, 

under a hand lens, tiny flakes of mica may be seen along 

fracture surfaces. Argillite lacks the fissility of shale or the 

cleavage of slate. 

Bay region occurrence: Argillite occurs throughout the 

Franciscan Assemblage rocks exposed throughout the core 

of the Diablo Range and the southeastern Santa Cruz Mountains (same as slate).

Metasandstone

Rock characteristics: Metasandstone is sandstone that 

shows evidence of having been exposed to metamorphism. 

Bay region occurrence: Metasandstone occurs 

throughout the Franciscan Assemblage rocks exposed 

throughout the core of the Diablo Range with massive exposures in eastern Alum Rock Park near San José and throughout the Henry Coe State Park region. Exposures occur 

along the eastern flank of the Santa Cruz Mountains, with 

exposures around Lexington Reservoir and in Uvas Canyon 

County Park in southwest Santa Clara County. 

Slate from near Lexington Reservoir, Santa Clara County.

Argillite from Clear Creek in San Benito County. 

Metasandstone from San Benito County.
50 Rocks and Geology in the San Francisco Bay Region 

Metaconglomerate

Rock characteristics: Metaconglomerate is conglomerate that displays evidence of metamorphism. Typically, 

gravel clasts in the rock display traces of “stretching” 

or crushing, and the matrix material shows evidence of 

recrystallization.

Bay region occurrence: Metaconglomerate occurs 

in association with metasandstone. Examples of metaconglomerate can be seen throughout the core of the Diablo 

Range with exceptional exposures throughout the range 

southeast of Hollister. Metaconglomerate occurs between 

Gazos Creek and Pigeon Point along the San Mateo Coast, 

and occurs in scattered outcrops throughout Henry Coe 

State Park.

Metachert

Rock characteristics: Metachert is a dense, very fine 

grained (crypto-crystalline) quartz-rich rock. It occurs in 

practically any color, though dominantly brown, red (jasper), 

orange, or white. Green, gray, blue, to black metacherts are 

not uncommon. Pure silica is clear to white; it is the impurities (mostly clays and iron oxide) that give chert its color. 

With progressive metamorphism, the original sedimentary 

bedding of marine cherts vanishes. Because chert is dominantly quartz, it flows at moderately low temperatures and 

pressures. In addition, silica dissolved in ground water will 

precipitate to form chert even at great depths in the crust, 

typically replacing other minerals. 

Bay region occurrence: Metachert is abundant throughout the region occurring in many cases as resistant blocks or 

rocky ridges rising above the surrounding soil-covered hillsides (geologists call these features knockers). Chert is abundant in Franciscan Assemblage rocks wherever they occur. 

Examples of where metachert knockers occur in abundance 

throughout Santa Clara County such as in the Cinnabar Hills 

area around Calero Reservoir; in Santa Teresa County Park in 

south San José; throughout Henry Coe State Park; in Joseph 

Grant County Park along the Mt. Hamilton Road. It also 

occurs throughout northern Marin County. Because metachert 

is hard, durable, and abundant, it can be found as cobbles and 

pebbles in stream gravel bars throughout the region.

Metaconglomerate from San Benito County. 

Chert breccia from Coyote Creek, Santa Clara County.

Metachert from the southern Santa Cruz Mountains 

displays a wide range of colors.
Metamorphic Rocks and Processes 51

Quartzite

Rock characteristics: Quartzite is a dense, extremely 

hard quartz-rich rock that displays a frosty texture, conchoidal fracture, and is typically white, but it is commonly 

gray, pink, red, or black. Quartzite occurs in all metamorphic grades. If the rock is clearly derived originally from 

sandstone, the correct term to apply is metasandstone. This 

is because quartzite can form from any rhyolite, chert, siltstone, aplite-granite, or any quartz-rich rock. Although most 

quartzite is probably derived from ancient quartz sandstone, 

it is difficult to say explicitly what the original parent rock 

was that formed quartzite.

Bay region occurrence: Metasandstone occurs in 

the Franciscan Assemblage throughout the region; some 

of it could be called quartzite. Quartzite occurs as gravel-, 

cobble-, to boulder-sized blocks, most of which are derived 

from Cretaceous and Tertiary-age conglomerates. These are 

commonly found reworked in stream gravels throughout the 

region. These rocks tend to be well rounded and their surfaces 

display “percussion marks”—curved, conchoidal fractures 

that formed as the rocks crashed into other rocks as they were 

transported along rushing streams. Many have broken surfaces formed from the shattering effect of faulting when they 

were deeply buried at different times in their past. Because 

quartzite cobbles are highly durable, they may survive many 

cycles of erosion and deposition. Quartzite cobbles derived 

from conglomerate beds are perhaps the oldest rock material 

in the region, and may share a common origin with quartzite 

cobbles found throughout western North America from the 

Canadian Shield to Mexico. Some of these reworked quartzite 

cobbles may be billions of years old. 

Marble

Rock characteristics: Marble is essentially pure calcite 

but may contain traces of dolomite, graphite, pyrite, quartz, 

mica, and other minerals. In most occurrences, marble is considered to have formed from limestone; however, any calciterich rock (sedimentary or igneous) can become marble with 

either regional or contact metamorphism.

A reworked quartzite cobble from San Lorenzo Creek, 

Santa Cruz County.

Stevens Creek near Cupertino and along Lime Kiln Creek 

near Lexington Reservoir. Marble and limestone masses 

occur throughout the Cinnabar Hills in southern San José 

with historic mining conducted in the vicinity of what is 

now Calero Reservoir. This marble and limestone occurs as 

small to hill-sized blocks in a “mélange” (mix) of serpentinite, graywacke sandstone, argillite, metasandstone, and 

greenstone (metabasalt). 

Marble from the University of California Santa Cruz campus.

Bay region occurrence: Marble occurs 

throughout the Salinian basement west of the main 

trace of the San Andreas Fault. Large masses of 

high-grade marble occur in the southwestern Santa 

Cruz Mountains throughout the Ben Lomond 

Mountain area and throughout the Gabilan Range, 

with large outcrop areas on Fremont Peak. Pits 

from historic mining operations occur throughout 

the University of California Santa Cruz campus. In 

these high-grade occurrences, the marble occurs in association with mica schist and gneiss. Low-grade marble and 

limestone occur as scattered masses in Franciscan Assemblage rocks throughout the region, with large quarries and 

pits located near Rockaway Beach on the San Mateo coast. 

Active mining operations of limestone and low-grade marble 

are in progress in the eastern Santa Cruz Mountains along 
52 Rocks and Geology in the San Francisco Bay Region 

Marble’s role in the making of California—In 1846, after 

being told by Mexican authorities to leave California, John 

C. Fremont lead his followers to the highest peak in the 

Gavilan Range (between Monterey and San Benito Counties). There, on what is now called Fremont Peak, his men 

erected a small earthen fort and raised the first American 

flag to fly over California. Soon afterward, the Mexican 

General Jose Castro prepared to dislodge the Americans. 

Outnumbered five to one, Fremont retreated to Sutter’s Fort. 

Although the affair was a military loss, it bolstered American sentiment to gain California.

It was possibly 

the mines, such 

as this one, in 

the marble that 

attracted John 

C. Fremont and 

his followers 

to what is now 

Fremont Peak. 

At the time, 

these mines produced cerrusite 

(PbCO3), a grayish-white ore 

that was the only 

known source of 

lead for gun shot 

in the region. 

Historic lime-kiln processing works located on Bay Street on the University of 

California Santa Cruz campus.

Marble, and its sedimentary parent, limestone, continues to be mined in the region, primarily for road aggregate 

and, to a lesser degree, as lime for cement. Cement production requires significant amounts of energy to bake the rock. 

The primary mineral, calcite (CaCO3), is heated, driving off 

carbon dioxide, leaving lime (CaO). Early lime production 

used large quantities of charcoal derived from the forests in 

the Santa Cruz Mountains. By 1910, nearly all the forested 

areas were stripped from the Santa Cruz Mountains before 

environmental outcry and dwindling wood supply basically 

Fremont Peak, 

San Benito County.

An active limestone mine along Stevens Creek in Santa Clara County.

ended the industry. Large active quarries, such as this one along Stevens 

Creek, are now heavily regulated. 

They produce only a fraction of the 

lime now consumed in construction 

in the region. 

Marble was mined in the Santa 

Cruz Mountains as early as the 

1850’s; production peaked in the 

early 1920’s. In this time period, 

the Santa Cruz Marble was the only 

large-scale source of lime for cement 

available in the entire Pacific basin. 

Cement from Santa Cruz was used 

to rebuild San Francisco after the 

1906 earthquake, and lime from these 

mines were used in the construction 

of the Panama Canal. Marble and 

limestone are still mined in the in the 

Santa Cruz Mountains. 
Metamorphic Rocks and Processes 53

Mica Schist

Rock characteristics: Mica schist is a medium- to highgrade foliated metamorphic rock that consists dominantly of 

mica (biotite, muscovite, or other flaky sheet mica minerals); 

other common minerals that occur with mica include: quartz, 

tourmaline, garnet, pyrite, kyanite, plagioclase, epidote, and 

many others. “Foliated” means that the crystal grains in a 

metamorphic rock are aligned, producing a layered texture 

(not sedimentary bedding). Mica must make up greater than 

50 percent of a metamorphic rock for it to be considered mica 

schist. In bright sunlight, the abundance of mica gives the 

rock a bright, glittery appearance. Mica is a sheet silicate 

mineral that easily peels into flakes when picked at with a 

knife blade. Because mica is a very soft mineral it usually 

preserves evidence of having been folded under great pressure, and because mica schist forms under high pressure and 

temperatures, it is frequently intruded by small quartz-rich 

igneous veins (called aplite dikes). 

Bay region occurrence: Mica schist occurs in Salinian 

basement rocks west of the San Andreas Fault, with perhaps 

its best-known abundance occurrence in pockets scattered 

across Ben Lomond Mountain in the Santa Cruz area. It 

occurs with marble and gneiss in abandoned quarries on the 

University of California Santa Cruz campus and along the 

San Lorenzo River Gorge. Mica schist occurs as remnant 

host rock (xenoliths) in larger granite bodies throughout the 

Salinian basement exposed in the Monterey Peninsula region, 

A cut slab showing folded bands of mica schist with crosscutting quartz-rich dikes (lower right). High-grade metamorphic rock cut with small light-colored dikes is called a 

migmatite.

Layered gneiss from along San Lorenzo Creek in Santa 

Cruz County.

Schist with kyanite crystals from along San Lorenzo Creek 

in Santa Cruz County.

the Gabilan Range, and elsewhere in the region. It occurs as 

clasts in conglomerate beds and can be found as isolated cobbles in stream gravels (originally the mica schist was derived 

from the southern Sierra Foothills and then incorporated into 

reworked sediments).

Gneiss

Rock characteristics: Gneiss is a banded (foliated) rock 

formed by regional metamorphism. It is rich in quartz and 

feldspar that occurs in bands of mineral grains, alternating 

with dark mica, hornblende, amphibole or other characteristic 

minerals. Gneiss forms from the metamorphism of granitic 

rocks, sedimentary rocks, or other metamorphic rocks. 

Bay region occurrence: Gneiss occurs in the Salinian 

basement rocks west of the San Andreas Fault with massive 

exposures in the Ben Lomond Mountain area around Santa 

Cruz, in the Gabilan Range, and in conglomerate beds and 

stream gravels downstream of these areas.
54 Rocks and Geology in the San Francisco Bay Region 

Greenstone (Metabasalt)

Rock characteristics: Greenstone is a term applied to 

any compact dark-green altered or low-grade metamorphosed 

basic igneous rock that owes it green color to the minerals 

chlorite or epidote. Ancient pillow basalts frequently preserve their original volcanic flow structures and gas vesicles 

but have been chemically altered to greenstone. 

Bay region occurrence: Greenstone occurs in Franciscan Assemblage rocks throughout the region. Greenstone 

is easily seen at Point Bonita in the Marin Headlands. Near 

Morgan Hill, greenstone crops out at the boat-dock parking lot at Anderson Reservoir and in the spillway at Uvas 

Reservoir. The durable character and relative abundance of 

metabasalt makes it a common rock in most stream gravel 

bars throughout the region.

Greenstone from Coyote Creek, Santa Clara County.

A serpentinite sample dominated by the green, frothy-, 

fibrous- or earthy-textured serpentine mineral antigorite. 

The veins are filled with a type of asbestos called chrysotile 

(also a serpentine mineral). The “hob nail” texture of the 

rock reflects the crystalline texture of the original mantle 

rock (peridotite) before it was converted to serpentinite. 

Serpentinite

Rock characteristics: Serpentinite (the rock) is made 

up of a variety of serpentine minerals, including the most 

common variety, antigorite (typically light-to-dark green, 

tough, compact, and often in pleated masses), lizardite (typically occurs as a white to green, scaly textured mass), and 

sometimes contains thin veins of chysotile (a yellow-green, 

fibrous variety of asbestos). Serpentinite forms from the 

metamorphism of mantle-derived ultramafic rocks (peridotite) and oceanic crust in a water-rich environment. When 

ultramafic rocks are exposed to the wet, low temperature, 

low pressure near the Earth’s surface, they may gradually 

convert to serpentinite. 

Bay region occurrence: Serpentinite is abundant 

throughout the eastern foothills of the Santa Cruz Mountains, east of the main trace of the San Andreas Fault. 

Exceptional exposures occur in the sea cliffs just west of 

the Golden Gate Bridge (in Golden Gate National Recreation Area), and along Highway 280 throughout the 

vicinity of Crystal Springs Reservoir. In the south bay, 

serpentinite exposures crop out at Guadalupe Reservoir, 

throughout Almaden-Quicksilver County Park, and on hillsides throughout Santa Teresa Hills and the Cinnabar Hills 

(including Calero County Park). Exceptional exposures 

are located at Chesbro Reservoir dam near Morgan Hill. 

The Edenvale Hills that line the eastern side of Santa Clara 

Valley are dominantly serpentinite, with exposures throughout the Metcalfe Motorcycle Park and the Field Sports Park 

areas. Serpentinte crops out in many places throughout the 

Diablo Range. The New Idria-Clear Creek area in Fresno 

and San Benito Counties is an area rich in mineralized serpentinite-dominated ultramafic rocks.

A serpentinite sample dominated by lizardite, a mineral that 

gives the rock a soft, soapy feel.
Metamorphic Rocks and Processes 55

Hazards associated with serpentinite—In fresh outcrops 

or road cuts, serpentinite appears as a chaotic mass of mixed 

blue, green, and gray pod-shaped rocks within a surrounding 

scaly or pulverized-appearing rock matrix. This appearance 

is caused by the expansion of the rock when unconfined at 

the Earth’s surface and its ability to absorb abundant water 

as it weathers to soil (which it does relatively quickly). 

These characteristics help make hilly areas underlain with 

serpentinite bedrock prone to landsliding and failure during 

earthquake shaking. The natural abundance of asbestos in serpentinite may be a health threat in some areas where wind or 

human activities generate dust clouds. Naturally high concentrations of heavy metals, particularly chromium and mercury, 

can also present a chemical hazard in ground water or surface 

runoff, particularly downstream of mining or landfill operations involving serpentinite bedrock or earth fill.

Serpentinite and other ultramafic rocks in the San Francisco Bay region reflect many different degrees of metamorphism. 

A landslide formed in weathered serpentinite bedrock along the 

Limekiln Trail near Lexington Reservoir, Santa Clara County.
56 Rocks and Geology in the San Francisco Bay Region 

Greenschist

Rock characteristics: Greenschist is a schistose metamorphic rock whose green color is due to the presence of 

the minerals chlorite, epidote, or actinolite. This low-grade 

regional metamorphic rock forms in variable-pressure and 

low-temperature metamorphic environments (between 300 

to 500oC or 570 to 930oF). 

Bay region occurrence: Greenschist occurs in scattered 

pods in Franciscan Assemblage rocks, particularly in serpentinite belts with exposures throughout the central Diablo 

Range, throughout the north bay, and in the Cinnabar Hills 

region between New Almaden and Morgan Hill.

Blueschist

Rock characteristics: Blueschist is a metamorphic 

rock type (or metamorphic environment) that forms at high 

pressures and low temperatures, such as in a subduction 

zone where cool crustal rocks sink into the mantle. The 

dominant mineral in blueschist in the region is glaucophane, 

a blue-to-black platy amphibole mineral. Blueschist rocks 

on the surface suggest that the rocks survived a complex 

geologic transport history—ocean crust that had survived 

being deeply buried in a subduction zone before being thrust 

upward to the surface.

Bay region occurrence: Blueschist occurs in scattered 

pods or belts in Franciscan Assemblage rocks, typically in 

association with serpentinite. Scattered exposures occur 

throughout the Diablo Range and in the southeastern Santa 

Cruz Mountains in the region between Lexington Reservoir 

and Morgan Hill. 

Amphibolite Schist

Rock characteristics: Amphibolite schist forms in 

high-temperature, high-pressure regional metamorphic 

environments. The rock is dominated by small to large 

crystals of various amphibole minerals (greenish, gray to 

black mafic minerals with blade-like crystals). Hornblende 

is a common mineral in the amphibole group. Plagioclase, 

mica, quartz, magnetite, garnet, and other minerals can 

occur in amphibolite schist.

Bay region occurrence: Amphibolite schist occurs 

in scattered pod-like masses in Franciscan Assemblage 

rocks in association with blueschist and other metamorphic 

rocks. Occurrences include small areas in the Cinnabar 

Hills between New Almaden and Morgan Hill and scattered 

throughout the Diablo Range.

Blueschist from near Lexington Reservoir, Santa Clara 

County. This sample of glaucophane schist is naturally polished by the scour of recent fault motion along the base of 

a landslide. 

Amphibolite schist from Arroyo Seco, Monterey County.

Greenschist from Coyote Creek, Santa Clara County.
Metamorphic Rocks and Processes 57

Granulite 

Rock characteristics: Granulite is a high-grade 

regional metamorphic rock type (or metamorphic environment) that forms at high temperatures and moderate 

to high pressures in the middle to lower crust. The rock 

typically contains red garnet (pyrope), is rich in quartz, 

and may contain a variety of other high temperature/high 

pressure metamorphic minerals. It typically has an evengrained, visibly crystalline texture (granoblastic) with porphyroblasts of garnet (crystals that grow at the expense of 

the surrounding rock).

Bay region occurrence: Granulite occurs in Salinian 

basement associated with the older schists and gneisses 

that predate the granodiorite and diorite intrusions. Granulite occurs in the Ben Lomond Mountain area southward 

into the Gabilan Range (including around Fremont Peak). 

It also occurs in the Sierra de Salinia (Salinian Range) in 

Monterey County in association with migmatites.

Eclogite 

Rock characteristics: Eclogite is a high-grade 

regional metamorphism rock type (or metamorphic environment) that forms at very high pressures and moderate to high temperatures in a lower crust setting (such 

as deep within a subduction zone. The rock is typically 

mafic in composition (rich in pyroxene), garnet (pyrope), 

but may also contain quartz, pyrite, glaucophane, kyanite, 

and other minerals. Eclogite has a granoblastic texture 

(a crystalline granular texture), with porphyroblasts of 

garnet and pyrite. 

Bay region occurrence: “Eclogite-like” rocks occur 

in the region in association with blue schist and amphibolite schist. Examples occur in northern Marin County near 

Mt. Tamalpias and on Angel Island, but true eclogites 

occur further north in portions of the northern Coastal 

Ranges. Ultramafic rocks derived from slivers the mantle 

material found in the region formed in similar pressure 

and temperature conditions but are not considered eclogites. Eclogites are associated with metamorphic materials 

formed in a subduction zone.

 Migmatite 

Rock characteristics: Migmatites represent middle to 

lower crustal metamorphic environments where rocks were 

hot enough that they begin to melt and the molten material 

began to separate from the higher temperature minerals 

that have not yet melted. The low-temperature minerals 

become molten (mostly quartz) and begin to flow, filling 

fractures in the surrounding host rock. The result is that the 

dikes and intrusions typically have a light color in relation 

to the surrounding host rock. Typically, many small dikes 

and intrusions crisscross each other. Migmatites represent 

a boundary condition between high-grade metamorphism 

Granulite displaying garnets, Santa Cruz County.

Eclogite displaying garnet-rich folded layers.

Migmatite (schist with granitic-granulite dikes and intrusions) 

exposed along Arroyo Seco Creek in Monterey County. 

and the melting associated with the formation of batholiths 

and large intrusions.

Bay region occurrence: Migmatite can be seen in many 

areas throughout the Salinian basement, with exceptional 

exposures in Arroyo Seco Canyon in Monterey County.
58 Rocks and Geology in the San Francisco Bay Region

Additional Resources

Excellent general field guides to rocks and minerals include:

National Audubon Society, 1995, Field Guide to Rocks and Minerals: 

New York, Alfred A. Knopf, Inc.

Motanna, A., and others, 1978, Simon and Schuster’s Guide to Rocks 

and Minerals: New York, Simon & Schuster, Inc.

Many definitions provided in this text were modified after:

Jackson, Julia A., (ed.), 1997. AGI Glossary of Geology, 4th Edition. 

Alexandria, VA: American Geological Institute. 769 p.

Many excellent resources are available on the World Wide Web. 

Here is a small selection of starting points for getting more geologic 

information about the San Francisco Bay region:

USGS on-line geologic maps and publications about the San 

Francisco Bay Area include:

San Francisco Bay Region Geology website. This website prepared 

by the USGS Western Earth Surface Processes team provides 

access to dozens of regional geologic maps and reports. It is 

online at http://sfgeo.wr.usgs.gov.

Stoffer, P. & Gordon, L., 2001, Geology and Natural History of the San 

Francisco Bay Area. 2001 Fall Field Conference Guidebook, 

National Association of Geoscience Teachers, Far Western 

Section: U.S. Geological Survey Bulletin 2188, online at http:

//geopubs.wr.usgs.gov/bulletin/b2188/.

Access USGS—San Francisco Bay and Delta is a website is a portal to 

many data sets, reports, publications, digital maps of many kinds 

including subjects relating to marine studies, regional weather, 

earthquake information, regional landscape and bathymetry, geologic mapping, water resources, biology, hazards, wetlands and 

more. It can be found online at: http://sfbay.wr.usgs.gov. 

Many other resources can be found in the USGS Library at 345 

Middlefield Road in Menlo Park, CA. The USGS Library catalog and the National Geologic Map Database are available 

free online at http://library.usgs.gov.

Many classic books exist about rocks, minerals, and fossils in 

California, but most are now out of print. Publications used to 

research this report include:

California Department of Natural Resources, 1943, Geologic 

Formations and Economic Development of the Oil and Gas 

Fields of California: California Division of Mines and Geology 

Bulletin 118, 773 p.

California Department of Natural Resources, 1951. Geologic 

Guidebook of the San Francisco Bay Counties: California 

Division of Mines and Geology Bulletin 154, 392 p.

California Division of Mines and Geology, 1966, Minerals of California, 

Centennial Volume (1866-1966): California Division of Mines 

and Geology Bulletin 189, 559 p.

California Division of Mines and Geology, 1966. Mineral Resources 

of California: California Division of Mines and Geology Bulletin 

191, 450 p.

Bailey, E.H. and Everhart, D.L., 1964, Geology and Quicksilver Deposits 

of the New Almaden District, Santa Clara County, California: 

U.S. Geological Survey Professional Paper 360, 201 p.