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ONVIM HI3Hl-81ISO<Hfl 901039 
Flatland deposits of the San Francisco Bay Region, 

California their geology and engineering 

properties, and their importance to 

comprehensive planning

By E. J. HELLEY and K. R. LAJOIE

U.S. GEOLOGICAL SURVEY, and W. E. SPANGLE and M. L. BLAIR,

WILLIAM SPANGLE & ASSOCIATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 943

Jointly supported by the U.S. Geological Survey 

and the Department of Housing and Urban 

Development, Office of Policy Development and 

Research, as a part of a program to develop and 

apply earth-science information in support of 

land-use planning and decisionmaking

A series of closely related earth-science studies that 

define the nature and severity of earthquake 

hazards associated with geologic conditions

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1979 

Second printing 1980 

Third printing 1991

Library of Congress Catalog in Publication Data

Main entry under title:

Flatland deposits of the San Francisco Bay region, California.

(Geological Survey Professional Paper 943)

Bibliography: p. 86-88.

CONTENTS: Helley, E. J. and Lajoie, K. R. Their geology and engineering properties. Spangle, W. E. and Blair, M. L.

Their importance to comprehensive planning. 

Supt. of Docs, no.: I 19:16 

1. Sediments (Geology)-Califomia-San Francisco Bay region. 2. Natural disasters-California San Francisco Bay region.

I. Helley, Edward J. 11. Series: United States. Geological Survey. Professional Paper 943. 

QE471.2.F56 55l.3'04'097946 77-608184

For sale by the Superintendent of Documents, U.S. Government Printing Office,

Washington, B.C. 20402

Stock Number 024-001-03170-1
FOREWORD

This report is a product of the San Francisco Bay Region Environment 

and Resources Planning Study, an experimental program designed to 

facilitate the use of earth-science information in regional planning and 

decisionmaking. The study is jointly supported by the U.S. Geological 

Survey, Department of the Interior, and the Office of Policy Development 

and Research, Department of Housing and Urban Development. The Association of Bay Area Governments participates in the study and provides a 

liaison and communication link with other regional planning agencies and 

with county and local governments.

Although the study focuses on the nine-county, 7,400-square-mile 

(19,000-square-kilometers) San Francisco Bay region, it bears on an issue 

that is of national concern. This issue how best to accommodate orderly 

development and growth while conserving our natural resource base, 

insuring public health and safety, and minimizing degradation of our 

natural and man-made environment is difficult and complex. The complexity, however, can be greatly reduced if we understand the natural 

characteristics of the land, the processes that shape it, its resource potential, 

and its natural hazards. These subjects are chiefly within the domain of the 

earth sciences: geology, geophysics, hydrology, and the soil sciences. Appropriate earth-science information, if available, can be rationally applied 

in guiding growth and development, but the existence of the information 

does not assure its effective use in the day-to-day decisions that shape 

development. Planners, elected officials, and the public rarely have the 

training or experience needed to recognize the significance of basic earthscience information, and many of the conventional methods of communicating earth-science information are ill suited to their needs.

It is hoped that the study will aid the planning and decisionmaking 

community by (1) identifying important problems that are rooted in the 

earth sciences and related to growth and development in the bay region; (2) 

providing the earth-science information that is needed to solve these problems; (3) interpreting and publishing findings in forms understandable to 

and usable by nonscientists; (4) establishing new avenues of communication between scientists and users, and (5) exploring alternate ways of 

applying earth-science information in planning and decisionmaking.

Since the study was started in 1970, it has produced more than 100 

reports and maps. These cover a wide range of topics: reduction of flood and 

earthquake hazards, unstable slopes, engineering characteristics of hillside 

and lowland areas, mineral and water resources management, disposal of 

solid and liquid waste, erosion and sedimentation problems, bay-water 

circulation patterns, and others. The methods used in the study and the 

results it has produced have elicited broad interest, and a wide range of 

applications, from planners, government officials, industry, universities, 

and the general public.

This report on flatland deposits and their land-use significance examines the low-lying areas of the San Francisco Bay region where urban 

centers are numerous and the population is dense. These areas of low relief 

are also likely to be favored sites for future growth, for they are easily 

reached by existing roads and railroads, and their relatively level surface is 

adapted to a variety of intensive uses of the land. But the lowlands are not

III
IV

uniform. They include valley floors and terraces, plains, low alluvial slopes, 

marshlands, and marine terraces. These landforms result from a number of 

different geologic processes, they are underlain by deposits with different 

physical and chemical properties, and they respond differently to the impact 

of use.

This report evaluates the different flatland deposits, both in terms of 

their natural properties and in terms of the geologic processes that they 

record. It also discusses how earth-science knowledge of the deposits and the 

processes that formed them may be incorporated into planning for better use 

of the land. These two closely related topics are addressed in separate 

sections of the report. The first section identifies the different kinds of 

deposits that underlie flatlands of the bay region, describes their properties 

and the processes that formed them, and demonstrates how this knowledge 

can be used to predict the consequences of changing land use. The second 

section of the report shows how planners and decisionmakers working 

within the conventional framework of comprehensive planning can use 

interpreted earth-science information to assess alternative uses of land.

2).

R. D. Brown, Jr.

Project Director

San Francisco Bay Region Study
CONTENTS

Page

Foreword III 

Definitions of terms VII 

Abstract 1 

Introduction by William E. Spangle, Martha L. Blair, Edward J. Helley, and Kenneth R. Lajoie 1

Problems and purposes 1

Planning and decisionmaking 3

Agencies for regional land-use planning 4

Comprehensive planning agencies 5

Functional planning agencies 5

Agencies with permit authority 6

Regulatory agencies 6

Single-purpose planning and operating districts 6

Intergovernmental relations 7

Regional setting 7

Geographic setting 7

 » ____. _____ ______. ______ {

Drainage 12

Geology and engineering properties of the flatland deposits

by Edward J. Helley and Kenneth R. Lajoie 14

Geologic history 14

Geologic map of flatland areas 20

Holocene deposits (10,000 years old) 21

Bay mud 21

Beach and dune sand deposits 22

Alluvium 22

Pleistocene deposits (10,000-1.8 million years

old) 24

Alluvium 24

Marine terrace deposits 25

Geologic mapping techniques 27

Physical properties of geologic units 31

Seismic properties 33

Potential problems in flatlands regions 33

Water-related problems 33

Flooding 39

Stream channel changes 42

Salt-water intrusion 42

Subsidence 45

Page

Geology and engineering properties of the flatland deposits 

Continued

Potential problems in flatlands regions Continued 

Water-related problems Continued

Settlement 49

Shrink-swell 50

Seismic hazards 51

Seismic response 51

Seismic amplification 52

Liquefaction potential 53

Resources and potentials 61

Water resources 61

Mineral resources 

Sand and gravel

63

64 

DO 

68 

68 

68

Salines 

Shell 

Peat 

Importance of flatland geology to comprehensive planning

by William E. Spangle and Martha L. Blair 68

Description of land-capability studies 69

Importance of flatland deposits in regional studies 71

Applying information on flatland deposits 72

Defining the focus of the study 72

Determining the natural characteristics

affecting land capability 72 

Gathering, analyzing, and presenting the

information 73

Evaluating relative capability of land units 74

Use of land-capability studies 74

Planning response to flatland issues 76

Planning for the baylands 76

The urban-agricultural conflict 81

Planning for hazardous areas 81

Seismic hazards a special problem 82

Santa Clara County bayland study 82

Summary 85

References cited 86

ILLUSTRATIONS

[Plates are in pocket]

PLATES 1-3. Geologic maps of the flatland deposits of the San Francisco Bay region.

1. Northwestern part.

2. Northeastern part.

3. Southern part.

FIGURE Map showing the nine counties in the San Francisco Bay region 

Maps of San Francisco Bay region showing developed areas and flat lowlarld areas 

Diagram of the land-use planning process 

Map of California showing physiographic provinces and location of San Francisco Bay region

Page

 2

 3

 4

 8
VI CONTENTS

	Page

FIGURE 5. Map of San Francisco Bay region showing the mountainous uplands, the flat alluviated lowlands, and the bay

	and its marshes- 9

6. Diagram showing annual temperature and precipitation variations in San Francisco 10

7. Map showing precipitation in the San Francisco Bay region 11

8. Map showing area of interior California that drains through San Francisco Bay 13

9. Map of San Francisco Bay region showing major drainage divides and basins 14

10. Diagram showing geologic time in perspective 16

11. Generalized geologic map of San Francisco Bay region 17

12. Map showing shorelines of San Francisco Bay and Pacific Ocean during Holocene transgression 19

13. Diagram of Holocene processes along the bay plains 20

14. Diagram showing erosional and depositional features along the coast 21

15. Topographic map of Palo Alto area showing geomorphic features of alluvial plain 28

16. Map showing soil units in Palo Alto area 30

17. Map showing younger and older alluvium in Palo Alto area 32

18. Map showing rock units in the Palo Alto area 34

19. Block diagram showing facies relations in younger alluvial deposits 35

20. Diagram of highway test-boring log 36

21. Diagram showing ranges in dry density of geologic units 37

22. Diagram showing ranges in natural moisture content of geologic units 37

23. Diagram showing ranges in permeability of geologic units 38

24. Diagram showing penetration resistance of geologic units 38

25. Stratigraphic column showing geologic units and graphs showing P- and S-wave velocities as a function of depth

	at Ravenswood Point, San Mateo County 39

26. Block diagram showing stratigraphic relations and seismic velocities near the margin of San Francisco Bay 40

27. Block diagram showing potential water problems in the alluvial plain bordering San Francisco Bay 41

28. Map showing flood-prone areas in the San Francisco Bay basin 42

29. Diagram showing four ways that salt-water intrusion can occur 43

30. Map showing areas of salt-water intrusion in the bay region 44

31. Map showing areas of subsidence due to ground-water withdrawal, peat oxidation, and gas extraction 46

32. Map showing subsidence in Santa Clara Valley from 1934 to 1967 47

33. Graph showing amount of subsidence of bench mark BMP7 due to lowering of water level in San Jose 48

34. Graph showing amount of subsidence in delta area owing to oxidation of peat 49

35. Diagrammatic sections showing examples of settlement 50

36. Map showing settlement of the land surface on San Francisco waterfront 51

37. Photograph of house in San Francisco showing settlement 52

38. Photograph of desiccation cracks in clay-rich sediments 53

39. Block diagram showing potential seismic hazards on the alluvial plain bordering San Francisco Bay 54

40. Diagram illustrating seismic amplification in bedrock alluvium, and bay mud from nuclear test blast data 55

41. Seismograms showing seismic amplification data from Ravenswood site 55

42. Schematic cross section of southwestern San Francisco Bay and tabular description of physical properties of

	the units 56

43. Map showing estimated relative intensities of ground shaking in the south San Francisco Bay region

	from earthquakes on the San Andreas and Hayward faults 57

44. Simplified geologic map of Dunedin, New Zealand 58

45. Map showing density of damage claims resulting from the February 1974 earthquake, Dunedin, New Zealand - 59

46. Map showing liquefaction potential, southern San Francisco Bay region 60

47. Map showing high- and low-infiltration areas, San Francisco Bay region 63

48. Map showing areas of high potential for ground water recharge, southern Santa Clara County 64

49. Photograph showing soil capability classes 65

50. Diagram showing distribution of water and soil resources in the flatland deposits 66

51. Map of bay region showing mineral resources in flatland areas 67

52. Map of part of bay region showing areas with high, moderate, and low capability of urban residential

	development 75

53. Map showing reduction in area of natural tidal marsh around San Francisco Bay between 1850 and 1975 78

54. Map showing risk zones for land-use planning, Santa Clara County Baylands 84

	TABLES

	Page

TABLE 1. Factors and conditions controlling liquefaction-induced ground failure 33

2. Summary of liquefaction-potential analysis using standard penetration data 61

3. Factors and conditions controlling liquefaction 61

4. Factors affecting porosity and permeability of sedimentary deposits 62
CONTENTS VII

TABLE 5. Characteristics of flatland materials affecting capability for urban residential development 

6. Capability weighting and scoring system for urban residential development 

7. Capability rating of geologic units for urban residential development 

8. Capability rating of geologic units for selected uses 

9. Regional planning implications of capability rating for urban residential development 

10. Projected losses due to geologic problems in California, 1970-2000 

11. A scale of acceptable risks 

12. Risk zones for settlement and ground failure-- 

13. Land and building uses for various risk zones 

Page

- 72

-73

-74

-76

-77

-82

-83

-83

-85

DEFINITION OF TERMS

For the convenience of readers unfamiliar with 

some of the geologic terms used in this report, the 

following glossary has been prepared. We hope it will 

save a number of trips to the dictionary and make the 

text easier to understand.

Alluviation. The process of building up of sediments by a stream

at places where stream velocity is decreased. The coarsest

particles are the first to settle and the finest muds the last. 

Anticyclone. An atmospheric high-pressure system in which the

wind blows spirally outward in a clockwise direction in the

Northern Hemisphere. 

Aquiclude. A relatively imprevious rock layer that forms the

upper or lower boundary of an aquifer but does not transmit

enough water to supply wells or springs. 

Aquifer. A permeable layer of rock that contains enough ground

water to yield significant quantities of water to wells and

springs. 

Berm. A low bench or narrow terrace on the back shore of a beach

formed of material thrown up by storm waves. 

Colluvium. Any loose mass of soil or rock fragments that moves

downslope largely by the force of gravity. Usually it is thicker at

the base of the slope. 

Diatom. A microscopic single-celled plant that secretes siliceous

cell walls in a great variety of forms. 

Eolian. Deposits laid down by the wind, landforms eroded by the

wind, or structures such as ripple marks made by the wind. 

Facies. A rock unit distinguished from others by appearance or

composition as a result of environment of deposition. 

Geomorphic. Pertaining to the form of the surf ace features of the

earth. Specifically, geomorphology is the analysis of landforms

and their mode of origin. 

Holocene. The most recent epoch of geologic time, extending from

10,000 years ago to the present time. 

Indurated. Rock or soil compacted by pressure, cementation, and

heat.

Interfluvial. The land lying between streams. 

Isohyet. A line connecting points of equal precipitation. 

Isoseismal line. A line connecting points on the Earth's surface

at which earthquake intensity is the same.

Lithified. The consolidation of a loose sediment into a solid rock. 

Lithostatic load. The vertical load at a point in the Earth's crust

equal to the load that would be exerted by a column of the

overlying rock or soil.

Littoral drift. Material (such as sand, gravel, and shell fragments) that is moved along the coast by an ocean current.

Plate tectonics. An Earth model in which a small number (10-25) 

of large, broad, thick plates of the Earth's surface believed to 

"float" on an underlayer and move more or less independently, 

grinding against each other like ice floes in a river. The plates 

are propelled from the rear by sea-floor spreading. The continents form part of the plates and move with them like blocks of 

wood in an ice floe.

Pleistocene. An epoch of geologic time extending from 10,000 

years ago to 1.8 million years ago; it includes the last Ice Age.

Pliocene. The epoch of geologic time before the Pleistocene. Its 

age covers the span of 1.8 to 7 million years ago.

Pore pressures. The pressure exerted by the fluid that fills the 

pore spaces between the particles in a rock layer.

P wave. A type of earthquake wave that moves by alternating 

compression and expansion of material in the direction of 

movement. The P stands for primary because it is the fastest of 

the earthquake waves and so arrives before the secondary or S 

wave.

Quaternary. A period of geologic time thought to cover the past 

1.8 million years. It consists of two epochs the Pleistocene and 

Holocene.

Radiometric age. An age determined by measuring the disintegration rate of radioactive elements. The age may be calculated 

by measuring a short-life element like carbon-14, or a long-life 

element plus its decay product such as potassium-40/argon-40.

Sea-floor spreading. A theory that the oceanic crust is forming 

by upwelling and cooling of lava along the mid-oceanic ridges 

and is moving away from the ridges at the rate of 1-10 centimeters per year. This movement is thought to provide the source of 

power for the movement of large "plates" of the Earth's crust. 

(See "plate tectonics.")

Seismic impedance (acoustic impedance). The product of 

seismic S-wave velocity and bulk density.

Seismic velocity. The rate of propagation of an elastic wave. The 

velocity depends upon the type of wave and the elastic properties 

and density of the Earth material through which it travels.

Shear wave, S wave. A type of earthquake wave that moves by a 

shearing of material, so that there is movement perpendicular to 

the direction of propagation. S stands for secondary because it 

travels slower than the P or primary body waves.

Soil horizon. A layer of soil distinguished from adjacent layers 

by such characteristics as structure, color, texture, or chemical 

composition. Soil horizons are generally designated by capital 

letters, for example, A horizon or B horizon.

Soil profile. A vertical section of a soil that shows all its layers 

and the material from which it was derived.
VIII CONTENTS

Subduction. The process of one crustal block moving beneath

another by folding, faulting, or both. 

Tectonics. A study of the origin, 'relations, and evolution of

structural features of the Earth's crust, such as folding and

faulting of the rocks.

Tsunami. A sea wave produced by any large-scale, short-duration 

disturbance of the ocean floor, principally by a shallow submarine earthquake. Tsunamis are characterized by great speed and 

may cause considerable damage along an exposed coast thousand of miles from the source.
FLATLAND DEPOSITS OF THE

SAN FRANCISCO BAY REGION, CALIFORNIATHEIR GEOLOGY AND ENGINEERING PROPERTIES, AND

THEIR IMPORTANCE TO COMPREHENSIVE PLANNING

By E. J. HELLEY and K. R. LAJOIE

ABSTRACT

Geologic maps of the deposits underlying the flatlands (slope 

less than 15 percent) of the San Francisco Bay region provide 

urban and regional planners and decisionmakers with earthscience information regarding geologic hazards and resources. 

The flatlands are particularly suitable for development because 

the low relief permits simple, relatively low cost construction, but 

parts of the flatlands possess unusual geologic characteristics that 

must be taken into account if development and the use of resources 

are to be effectively managed.

To examine the flatland characteristics systematically, 13 geologic units are mapped in the area surrounding San Francisco 

Bay, in the outlying valleys, and in the coastal zone. The geologic 

units are studied and mapped by unconventional methods. These 

include the use of 19th-century maps and coastal charts, and pre1940 aerial photography; studies of environments of deposition, 

soil, and stratigraphy; and age determinations from radiometric 

methods, paleontology, and archaeology. Among the geologic 

units that can be recognized and mapped are the estuarine (bay) 

muds and marsh deposits; and alluvial-fan, channel, flood-basin, 

levee, dune, and beach deposits. The 13 geologic units shown on 

the map that accompanies the report range in age from oldest 

Pleistocene (1.8 million years) to deposits that are still forming 

today.

The engineering properties of these units are determined by 

analyzing unpublished reports by private consultants, the geologic and engineering literature, bore-hole logs for highway and 

bridge construction, and U.S. Geological Survey field studies. The 

engineering properties are shown to be systematically related to 

the age of the geologic units. These properties include bulk density, 

moisture content, penetration resistance, seismic wave velocities, 

and liquefaction potential. The depositional processes and age of 

each unit give a basis for predicting potential geologic hazards as 

well as natural resources. Most potential hazards are related to 

water or to earthquakes. Water-related hazards include flooding, 

rapid channel changes, salt-water intrusion, subsidence, ground 

settlement, and shrinking and swelling. Seismic hazards include 

ground-motion amplification and liquefaction with possible 

ground failure. The young estuarine muds and flood-basin deposits are most susceptible to seismic hazards.

Natural resources are also evaluated for each unit. The agricultural potential of the soils is probably most important because this 

vital resource is generally lost forever after urbanization. The 

flatlands are also a source for sand and gravel, clay, peat, shells, 

and salts.

The hazards and resource potential are identified for each unit 

so that regional planners will have sound information on which to 

base their decisions regarding land use.

How this information can be evaluated and applied is shown in a 

hypothetical land-capability study. Each geologic unit is rated for

relative capability for agriculture, urban residential development, 

ground-water recharge, and sand and gravel extraction. The study 

shows that bay mud has low capability for all uses considered, that 

relatively few units have high capability for agriculture or groundwater recharge, and that most units have high or moderate capability for urban residential development. The ratings bring into 

focus potential land-use conflicts that require resolution through 

evaluating economic, political, social, and other environmental 

information. Land-capability studies can assure that natural 

factors are given full consideration in the planning process and 

direct attention to areas and problems needing further investigation before land-development decisions are made.

Three examples of planning response to particular flatland 

potential problems are (1) the policies and permit procedures 

of the San Francisco Bay Conservation and Development Commission under which development proposals for construction on 

fill over bay mud are reviewed by a board including earth-science 

and engineering professionals, (2) policies and project-review 

criteria of the Association of Bay Area Governments emphasizing 

the regional importance of prime agricultural land, and (3) the 

geologic and soils report requirements of Santa Clara County, 

which are related to mapped risk zones in the baylands. These 

examples show how geologic studies can be effectively used in 

land-use planning to foster safe, economical, and environmentally 

sound land-use decisions.

INTRODUCTION

BY WILLIAM E. SPANGLE, MARTHA L. BLAIR, EDWARD J. HELLEY and 

KENNETH R. LAJOIE

PROBLEMS AND PURPOSES

The San Francisco Bay region consists of the nine 

counties surrounding San Francisco Bay (fig. 1) the 

largest estuary on the California coast. The area is 

characterized by diversity in topography, natural 

resources, and climate. It is richly endowed with 

outstanding scenic and recreational resources; a mild 

climate; large open-space areas; large areas of flat, 

easily developed land; fertile soils; good water supplies; extensive inland waterways; and fine harbors. 

These favorable natural features have attracted a 

large population which has produced a vigorous and 

diversified social and economic system. The area is 

one of the most attractive urban-suburban complexes 

in the United States.

1
FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

123°

10 20 30 MILES

I ill I

I I I I I I I

0 10 20 30 KILOMETERS

FIGURE 1. The nine counties in the San Francisco Bay Region.

The region, however, is not without its environmental problems. Many natural constraints to land 

use in the bay region were ignored or were not recognized in the period of rapid population growth since 

World War II. As a result, valuable and irreplaceable 

natural resources such as prime agricultural land 

were destroyed, houses and public facilities were 

built on landslides and active faults, overdraft of 

ground-water aquifers caused severe subsidence in 

some areas, and development took place on active 

flood plains. Recognition in the late 1960's that many 

of these problems relate to geologic factors led to the

formation of the San Francisco Bay Region Study 

jointly funded by the U.S. Geological Survey and the 

Department of Housing and Urban Development. 

This study was designed to provide some of the basic 

geologic data needed to develop guidelines for the use 

of the land and natural resources of the San Francisco Bay region.

Most of the residential, agricultural, industrial, 

and commercial development in the bay region is in 

the nearly flat lowland areas surrounding the bay 

and extending into the outlying mountainous areas 

as narrow valleys (fig. 2). In effect, these flat low-
INTRODUCTION

Developed areas 

in black

FIGURE 2. The developed areas and flat lowland areas in the San Francisco Bay region. Developed areas consist of low-, medium-, and 

high-density residential, commercial, industrial, military, and transportation facilities as of 1965. Data from Association of Bay Area 

Governments, 1966.

lands are themselves an important natural resource 

of the bay region. Most future urban development can 

be expected to occur in these flatlands. The purpose of 

this report is twofold: (1) to identify the geologic units 

and processes of these lowland areas, placing particular emphasis on the opportunities and constraints 

associated with the deposits that underlie these 

areas, and (2) to show how information concerning 

the natural characteristics of flatland areas can be 

used in regional planning to influence future landuse decisions.

PLANNING AND DECISIONMAKING

The information needed in regional planning and 

the extent to which it can affect decisions depend on 

the nature of the planning and decisionmaking process, the powers and objectives of the agencies involved in planning, and the natural features and 

processes of the planning area. These aspects of the 

San Francisco Bay region provide the basis for the 

interpretation and application of geologic information in regional planning.

Planning is the process of devising and carrying 

out a course of action to reach an identified objective. 

As an organized governmental activity, planning 

seeks to improve the decisions of public bodies and 

administrators. Comprehensive planning treats the 

long-term development of an area considering all 

major determinants of growth and change economic, political, social, and physical.

A land-use plan is a key component of a comprehensive plan providing a link between more general 

goals and policies and the pattern of land develop-
FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

ment. A land-use plan includes objectives, policies, 

and proposals for the type, pattern, and intensity of 

land use. A functional plan describes needed facilities and operations for a specific function of government such as transportation, water development, 

flood control, or emergency response; it is more specific than a comprehensive plan and usually has a 

shorter range. Any plan, when adopted by the governing body of an agency, is official public policy. 

This report is primarily concerned with land-use 

planning and decisionmaking carried out by regional agencies.

The process of developing comprehensive, landuse, and functional plans generally consists of six 

steps: identifying problems and defining goals and 

objectives, collecting and interpreting data, formulating plans, evaluating impacts, reviewing and 

adopting plans, and implementing plans. These 

steps, shown in figure 3, are interrelated. Plan formulation often indicates the need for additional information, additional information may alter the concept of the objectives and problems, and plan 

implementation may reveal the need for more additional information or modification of the plan.

The steps in the planning process consitute a 

systematic approach to informed decisionmaking 

applicable to most governmental activities. The product of the process is a logical and internally consistent plan or set of plans and programs to guide public 

and private decisions. The planning process is ongoing, producing refinements, revisions, and new plans 

and implementing programs as additional information is obtained, new issues and problems raised, or 

changes in public attitudes recognized. Public participation is essential throughout the planning process. Success in implementing a plan depends on 

widespread public support, which can be gained only 

if all major segments of the public participate in the 

planning process.

Decisions occur throughout the process, ranging 

from the decision to engage in a planning effort to the 

final approval of a plan and adoption of implementing regulations, programs, and procedures. Elected 

public officials have final responsibility for most key 

policy decisions, although persons in nonelective 

positions actually make many important day-to-day 

decisions.

The process shown is generally applicable regardless of jurisdictional level, subject area, or size of 

planning area. However, the actual content of the 

plans varies widely depending on the responsibility, 

authority, and financial position of the planning 

agency; the diversity of the planning area; scope of 

the planning effort; and availability of data. For 

example, regional councils of governments (COGs) 

are likely to emphasize developing procedures and 

criteria for use in reviewing major projects and plans. 

This is beacause the COGs' primary power derives 

from federally mandated review processes. Local 

agencies are more likely to emphasize objectives, 

policies, and criteria to provide a basis for land-use 

and development regulation traditionally a local 

responsibility.

AGENCIES FOR REGIONAL LAND-USE PLANNING

In the San Francisco Bay region, planning and 

decisionmaking authority at the regional level is

Public initiative and response

Feedback for review and revision

FIGURE 3. The land-use planning process.
INTRODUCTION

diffused among more than 20 agencies with different 

but sometimes overlaping responsibilities and jurisdictional boundaries. The agencies with duties or 

powers affecting land use fall into the following five 

basic categories:

 Comprehensive planning agencies.

 Functional planning agencies.

 Agencies with permit authority.

 Regulatory agencies.

 Single-purpose planning and operating districts.

COMPREHENSIVE PLANNING AGENCIES

The Association of Bay Area Governments 

(ABAG) is responsible for areawide comprehensive 

planning. Established in 1961 to develop plans and 

policies pertinent to region wide problems, ABAG is a 

voluntary association of local governments. Present 

voting membership includes 7 counties and 85 cities 

out of a potential voting membership of 9 counties 

and 92 cities. However, all 9 counties are included 

within the ABAG planning area.

ABAG's adopted regional plan consists of the 

Regional Plan 1970-1990 (1970), the Regional Airport 

Systems Study Final Plan (1972), Regional Open 

Space Plan, Phase 77(1972), Regional Ocean Coastline Plan (1973), plus goals and policies adopted from 

time to time by the General Assembly or Executive 

Committee (Memorandum to staff from Revan A. F. 

Tranter, Executive Director, ABAG, November 11, 

1974). An important aspect of ABAG's planning 

activity is developing procedures, standards, and 

criteria for identifying and reviewing regionally 

significant projects and plans. Geologic information 

is needed by ABAG to carry out all of its planning 

functions.

ABAG's primary power to implement its plans and 

policies derives from its designation by the Federal 

government as the areawide clearinghouse agency 

for the San Francisco Bay region. In this capacity, 

ABAG reviews local requests for Federal funds available from about 150 Federal programs, including 

among others local applications for Community Development Block Grants and Comprehensive Planning Assistance Grants authorized by the Housing 

and Community Development Act of 1974. As a 

clearinghouse agency, ABAG also reviews proposed 

Federal projects and federally required Environmental Impact Statements. ABAG's review is advisory. 

However, because the funding agency must explain 

in writing a decision to override a negative finding by 

ABAG and because many projects are usually competing for limited funds, ABAG's recommendations 

are usually heeded. ABAG may also review staterequired Environmental Impact Reports and any 

project it deems to be regionally significant. In addition, ABAG can directly participate in planning 

programs with other public agencies through joint 

memoranda of agreement. As a voluntary association with advisory functions, ABAG's powers of plan 

implementation ultimately depend on its ability, 

through well-conceived plans, development and dissemination of information, and the art of persuasion, 

to influence the decisions of other public agenciesregional, Federal, State, and local.

FUNCTIONAL PLANNING AGENCIES

The Metropolitan Transportation Commission 

(MTC) and Bay Area Comprehensive Health Planning Council (BACHPC) have specific mandates to 

engage in areawide functional planning with respect 

to transportation, feealth facilities, and waste-water 

treatment, respectively. The planning and projectreview responsibilities, and other duties these agencies may be authorized to perform, are normally 

coordinated with ABAG activities. Each agency 

reviews local plans and projects for conformance 

with its functional plan.

MTC was established to coordinate the development of regional transportation facilities. It is 

charged with preparing and adopting a Regional 

Transportation Plan including proposals for major 

highways, mass transit, transbay bridges, airports, 

and harbors. It must also develop a transportation 

improvement program and a financial program for 

carrying out the program. The adopted Regional 

Transportation Plan is to be the bay area component 

of a statewide transportation plan required by Assembly Bill 69 (1972) Metropolitan Transportation 

Commission, 1975.

MTC has stronger project review powers than the 

other functional agencies. MTC's approval is required for certain projects, including transbay 

bridges, public multi-county transit systems on exclusive rights of way, construction of State highways, and all applications from local governments or 

districts for State or Federal funds for any kind of 

transportation facility. In addition to reviewing projects, MTC administers, for the bay area, the public 

transit funds acquired from State and local sales 

taxes on gasoline. MTC needs geologic information 

in planning the location of transportation facilities 

and reviewing transportation proposals.

BACHPC is a voluntary federation of nine county 

health-planning councils. It is recognized by State
FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

and Federal authorities as the agency to undertake 

and coordinate comprehensive health planning. The 

agency was organized to prepare a regional health 

plan to evaluate proposals for construction of new 

health facilities. BACHPC's primary power is the 

review of requests for Federal and State funds. The 

comments are advisory, but since requests for funds 

usually exceed funds available, the BACHPC review 

comments can be determining. BACHPC needs to 

consider geology, particularly seismic hazards, in 

locating and designing medical facilities.

AGENCIES WITH PERMIT AUTHORITY

The San Francisco Bay Conservation and Development Commission (BCDC) and the California Coastal Zone Conservation Commission (CCZCC) are the 

major agencies having permit authority. BCDC was 

created by the State Legislature to prepare a comprehensive plan for San Francisco Bay and its shores 

and to control development within its area of jurisdiction. The plan was adopted by the State Legislature, 

and BCDC became a permanent agency charged 

with carrying out the plan. The adopted plan has 

legal status and guides the review of projects. BCDC 

shares authority over land-use decisions with the 

cities and counties, which retain normal land-use 

and building-permit controls. However, with certain 

minor exceptions, a permit from BCDC is required for 

all projects within its area of jurisdiction. Thus, in 

effect, it holds veto power over any project proposal in 

conflict with the San Francisco Bay Plan. Geologic 

information was extensively used by BCDC in preparing its plan and is almost always needed for 

project review.

In 1972, California voters adopted, by initiative, 

legislation to create the California Coastal Zone 

Conservation Commission and subordinate regional 

commissions.

The CCZCC, working with six regional commissions, is charged with preparing a plan for the future 

of the California coastal zone. While the plan was 

being prepared, the commissions controlled all development, through a permit process, to insure consistency with the objectives of the established legislation and the emerging plan policies. The plan was 

presented to the Governor and legislature in December 1975 for adoption and implementation. In 1976, 

the California Coastal Act was enacted, establishing 

the policies and governmental mechanism for ensuring wise use of the State's coastal areas. Coastal 

areas of the bay region are represented by two commissions: Central (San Mateo County) and North 

Central (San Francisco, Marin, and Sonoma Counties). CCZCC used geologic information in preparing 

the coastal plan and, if maintained as a permanent 

organization, will continue to need geologic data.

REGULATORY AGENCIES

Regional agencies with a primary purpose of establishing, enforcing, and administrating regulations 

are the Bay Area Air Pollution Control District (BAAPCD) and the San Francisco Bay Regional Water 

Quality Control Board (RWQCB). The BAAPCD is 

responsible for establishing and carrying out a program to reduce air contaminants from both stationary and moving sources, and the RWQCB is responsible for formulating and adopting a water-quality 

control plan. However, the major influence of each 

agency on land-use decisions derives from its authority to set and enforce standards. For example, the 

RWQCB has issued moratoriums on sewer hook-ups 

in certain localities to prevent deterioration of water 

quality. In addition, any project that may have an 

effect on water quality must receive a permit from the 

board before it can be undertaken. Similarly, the 

BAAPCD, with jurisdiction over the nine counties, 

except for parts of Solano and Sonoma Counties, is 

required to consider the effects of land-use decisions 

on air quality. The BAAPCD has little potential need 

for geologic information, but the RWQCB can use 

such information to help determine the effect of 

proposed projects on water quality.

SINGLE-PURPOSE PLANNING 

OPERATING DISTRICTS

A number of agencies typically are given taxing 

powers and the responsibility to plan, develop, construct, operate, and maintain particular facilities or 

services. Bay Area Rapid Transit District, East Bay 

Regional Park District, Golden Gate Bridge High-
INTRODUCTION

way and Transportation District, and East Bay 

Municipal Utility District are examples of such agencies. These agencies use geologic information primarily in locating and designing regional facilities.

INTERGOVERNMENTAL RELATIONS

The diffusion of planning responsibility and regulatory powers among so many regional agencies 

works against development of a coordinated, comprehensive approach to the solution of regional land-use 

and environmental problems. This diffusion is compounded by the authority and responsibility lodged 

with governmental agencies at the other jurisdictional levels. The operations of regional agencies affect 

and are affected by the planning and decisionmaking of government agencies at Federal, State, and 

local levels.

Federal and State agencies often preempt or limit 

regional decisionmaking by imposing requirements 

for funds, criteria for programs, shared responsibility for specific functions such as transportation, and 

regulations concerning, for example, environmental 

quality or the content of local plans.

Regional and, to an increasing extent, local governmental agencies are highly dependent on Federal 

and State funds to carry out their responsibilities. 

This means that plans and programs developed at 

the regional and local level are often framed with an 

eye not only to locally expressed objectives and 

concerns but also to Federal and State programfunding requirements. Thus, individual governmental decisions become part of the network of decisions 

made by other agencies at different jurisdictional 

levels over a period of time.

Land-use decisions are made within this context of 

complex inter-governmental relations. The effective 

use of geologic information in planning by the various regional agencies often depends on complementary decisions of other regional, Federal, State and 

local agencies. All the regional agencies described 

above engage in, or have some impact on, land-use 

planning and decisionmaking. Many make use of 

geologic information in developing plans, establishing criteria, reviewing projects, locating and designing facilities, or enforcing regulations. The information on the geologic characteristics, problems, and 

potentials of flatland areas presented in this report

can assist these regional agencies to plan and act 

with greater awareness of the physical environment.

REGIONAL SETTING

GEOGRAPHIC SETTING

The San Francisco Bay region lies between lat 36° 

and 39° N. and extends across the lowest and narrowest segment of the Coast Ranges in central California 

and into the west edge of the Great Valley of California (fig. 4). This region covers an area of 7,461 square 

miles (19,320 km2 ) consisting of northwest trending 

mountain ranges, broad basins, and narrow valleys 

generally paralleling major geologic structures and 

the coastline of central California.

About 65 percent (4,920 mi2 ,12,740 km2 ) consists of 

rounded hills and rugged mountain uplands with 

many ridge crests rising above 1,000 feet (300 m) and 

a few peaks rising above 4,000 feet (1200 m) (fig. 5). 

Almost 11 percent (784 mi2 , 2,030 km2 ) consists of the 

open water and tidal marshlands of the bay itself 

that lie at or close, to sea level. The remaining 24 

percent (1,757 mi2 , 4,550 km2 ) consists of the relatively flat lowland areas (generally less than 200 feet 

(60 m) above sea level) that constitute the broad alluvial plain surrounding the bay, the broad to narrow 

valley bottoms extending from the bay plain into the 

surrounding hills, and the narrow elevated marine 

terraces cut into the mountains along the Pacific 

Coast.

CLIMATE

The San Francisco Bay region has a mediterranean climate with mild wet winters and warm dry 

summers (fig. 6). This type of climate is common on 

western continental coasts between lat 30° and 50° 

N. where coastal ocean currents moderate the effects 

of seasonal changes in temperature.

The climate along the coast is marked by moderate 

and even temperatures, heavy persistent summer 

fog, and winds from the west-northwest. In contrast, 

inland areas have a wider range of temperature and 

have less wind. Temperatures are influenced by 

elevation and local topography. Higher summer 

temperatures and lower winter temperatures occur in 

low areas isolated by mountainous terrain. This kind 

of climate is also true of areas far distant from the 

bay and its temperature-moderating waters. Precipi-
FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

TRANSVERSE RANGES

0 50 100 150 200 KILOMETERS

0

FIGURE 4. Physiographic provinces of California and location of San Francisco Bay region (California Division of Mines and Geology,

1966, p. 17).
INTRODUCTION

Bay 

Marsh

EXPLANATION

Percentage 

(mi2 ) (km2 ) of total

784

Flatlands 1757

2030

4550

Uplands 4920 12,740 

Total 7461 19,320

24

65

100

O 10 20

ill i i i

I I I I I I 1

O 10 20 30 KILOMETERS

FIGURE 5. The San Francisco Bay region showing mountainous uplands, the flat alluviated lowlands, and the bay and its tidal marshes.

heaviest on the western slopes of the Coast Ranges 

and decreases, in general, from north to south. This 

pleasant mild climate provides for a long growing 

season as well as making an attractive place for 

humans to live.

The native vegetation in the bay region north 

through Sonoma County and south through Santa 

Clara County is dominated by plants adapted to mild 

climatic conditions including a summer drought. The 

bay area is actually a meeting ground for many plant 

species from the north Pacific Coast that reach their 

southern limit in the bay region and other plants 

from southern California that reach their northern 

limit here. The native plants evolved here in their 

own ecological niches, which we refer to as biotic

tation is distinctly seasonal, most falling between 

November and March, very little between June and 

September. The seasonal distribution of precipitation is largely controlled by the location of the anticyclonic cell (high-pressure system) that is normally 

found off the California coast, particularly in the 

summer. Winter precipitation occurs when this anticyclone is absent or far south of its normal position, 

which blocks storm systems from the Gulf of Alaska. 

Almost all precipitation is in the form of rain and 

what little snow falls usually soon melts.

Average annual precipitation ranges from 10-20 

inches (25-50 cm) in the dry interior valleys to 40-60 

inches (100-150 cm) and locally to 80 inches (200 cm) 

in the high coastal mountains (fig. 7). Precipitation is
10 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

w 

u 

I 

o

Z UJ O I-

»

I x 

I- (A

gs

DC

1U

^Average monthly precipitation

Normal daily maximum f'~-^ 

temperature. ' *

ormal daily averag 

temperature

80

LJ

I

Z

70 UJDC

I

u.

0)

60

DC 

O 

UJ 

Q

50?LJ" 

DC

40 DC

UJ 

o.

5

UJ

FIGURE 6. Annual temperature and precipitation variations 

in San Francisco.

communities. These plant communities are bay and 

salt marsh, freshwater marsh, the open coast, chaparral, grasslands, broad-leaved forest, and coniferous forest. The flatland deposits underlie almost all 

but the last two communities. However, the broadleaved and coniferous forests do cover stream terrace 

deposits in the outer valleys. The native plant communities are summarized below from Metcalf (1959) 

and Smith (1968).

Community

Bay and 

salt marsh 

Characteristic vegetation

 Salt grass (Distichilis spicata) 

Cordgrass (Spartina foliosa) 

Pickle-weed (Salicornia, spp.) 

Marsh rosemary (Limonium

commune) 

Marsh Grindelia (Grindelia cuneifolia)

Sea bite (Suaeda californica) 

Freshwater 

marsh Common tule (Scirpus acutus, S.

robustus, and S. communis) 

California bulrush (Scirpus californicus)

Cat-tail (Typha latifolia) 

Sedge (Carex senta) 

Surf grass (Phyllospadix torreyi

and P. scouleri) 

Tree lupine (Lupinus arboreus) 

Sea rocket (Cakile edentula) 

Blue grass (Poa douglasi) 

Sand verbena (Abronia sp.) 

Beach grass (Ammonphila arendria)

Open coast 

Community Characteristic vegetation

Chaparral Chamise (Adenostoma fasciculatum)

Scrub oak (Quercus dumosa) 

Buckbrush (Ceanothus cuneatus)

Coffee berry (Rhamnus californica)

Leather oak (Quercus durata) 

Interior live oak (Quercus wislizeni)

Coast live oak (Quercus agrifolio) 

Manzanita (Arctostaphylos canescens)

Wild lilac (Ceanothus sp.) 

Grasslands Blue bunch grass (Festuca idahoensis) 

California oat grass (Danthonia

californica) 

Foothill sedge (Carex tumulicola)

Soft chess (Bromus hordeaceus) 

Red brome (Bromus rubens) 

Wild oats (Auena fatua) 

Coyote bush (Baccharis pilularis) 

Coast live oak (Quercus agrifolia) 

California buckeye (Aesculus

californica) 

California laurel (Umbellularia

californica)

Before discussing the broadleaf and coniferous 

communities, it is worth noting that the native trees 

of the bay area can be divided into three groups 

primarily on the basis of their leaf characteristics. 

These are the conifers, broadleaf evergreens, and 

broadleaf deciduous trees. Conifers are cone-bearing 

evergreen trees distinguished by their simple needlelike or scalelike leaves. Broadleaf evergreens are 

those trees that keep all or most of their leaves, 

usually leathery, throughout the year. Broadleaf 

deciduous trees are those that lose their leaves in 

winter. Their leaves are usually thinner and less 

leathery than the broadleaf evergreen (Williams and 

Monroe, 1967).

Community Characteristic vegetation

Broadleaf forest Sycamore (Platanus racemosa) 

Riparian 

woodlands Willow (Salix sp.)

Bigleaf Maple (Acer macrophyllum)
Community

INTRODUCTION

Characteristic vegetation Community

11

Riparian woodlands 

Continued Oregon ash (Fraximus latifolia) 

California buckeye (Aesculus

californica)

California laurel (Umbellularia 

californica)

123"

Characteristic vegetation

Ripirian woodlands 

Continued Creek dogwood (Cornus californica)

Poison oak (Rhus diversiloba) 

Hoary nettle ( Urtica holosericar)

122"

38" -

37" 

% *y . ,

ft* >&. L- '

EXPLANATION

Isohyet

Showing mean annual precipitation, 1906 - 56, in 

inches (x 2.54 = centimeters)

, , , 

0 10 20 30 40 KILOMETERS

FIGURE 7. Precipitation distribution in San Francisco Bay region. After Rantz (1971).
12 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Community Characteristic vegetation

Oak woodlands Live oak (Quercus agrifolia)

California buckeye (Aesculus

californica) 

California laurel (Umbellularia

californica)

Black oak (Quercus kelloggi) 

Blue oak (Quercus douglasi) 

Oregon oak (Quercus garryanna) 

Interior live oak (Quercus wislizeni)

Madrone (Arbutus menziesii) 

Poison oak (Rhus diversiloba) 

Manzanita (Arctostaphylos sp.) 

Coyote bush (Baccharis pilularis) 

Chamise (Adenostoma fasciculatum)

Lupines (Lupinus sp.) 

Coniferous forest Coast redwood (Sequoia sempervirens)

Douglas fir (Pseudotsuga menziesii) 

Monterey cypress (Cupressus

macrocarpa)

Monterey pine (Pinus radiata) 

Knobcone pine (Pinus attenuata)

Bishop pine (Pinus muricata) 

Wax myrtle (Myrica californica) 

Redwood sorrel (Oxalis oregona) 

Ponderosa pine (Pinus ponderosa) 

Thimble berry (Rubus parviflora) 

California sword fern (Polystichum tnunitum) 

Firs (Abies sp.) 

Spruce (Picea sp.) 

Western yew (Taxus brevifolia) 

Incense cedar (Libocedrus decurrens)

The mild climatic conditions result in relatively 

moderate rock-weathering and erosional conditions 

which give a characteristic rounded aspect to much 

of the upland landscape. Weathered rock material is 

not removed rapidly from the hilly uplands, so bedrock outcrops are few and decomposed rock debris 

mantles most hillslopes. Soil creep caused largely by 

seasonal shrinking and swelling of expansive clays 

in the mantle of weathered rock debris is a dominant 

erosional process that tends to form smooth, rounded 

hills. Landsliding, induced in large measure by

heavy winter rains, is another dominant erosional 

process and commonly forms steep, irregular slopes 

on exposed hillsides and in deep narrow canyons. 

The weathered rock debris that accumulates in gulley 

and canyon bottoms is eroded and transported to the 

alluvial lowlands mainly by winter floods. In the 

recent geologic past, when climatic conditions were 

cooler and moister, the rates and relative importance 

of various weathering and erosional processes were 

probably slightly different from those of today.

The moderate and stable climatic conditions result 

in more than 300 argicultural growing days per year 

over much of the area, but the summer drought 

prevents summer crop cultivation without irrigation. 

Local ground-water supplies are usually insufficient 

for intensive irrigation, so water impoundments and 

imports are necessary for summer cultivation.

The total argicultural, domestic, and industrial 

demands for water are far greater than local supplies. 

Consequently, water imports, primarily from the 

Sierra Nevada, are necessary for immediate use and 

recharge of ground-water aquifers in much of the

region.

DRAINAGE

The Sacramento and San Joaquin Rivers, which 

drain most of the Great Valley of interior California 

(about 27 percent of the state), coalesce in a broad 

marshy delta east of the bay region and empty into 

the Pacific Ocean through Suisun Bay, San Pablo 

Bay, and the Golden Gate (fig. 8). Almost all the fresh 

water entering the estuary is derived from this interior drainage area, which is about 50 times larger than 

the local drainage area emptying directly into the 

San Francisco Bay estuary (McCulloch and others, 

1970).

The San Francisco Bay region itself is drained by a 

network of ephemeral streams and small rivers (fig. 

9). Only the broad valleys in the central 50 percent of 

the region drain directly into the bay. About 25 

percent of the region drains northward and westward 

into the Pacific Ocean, 20 percent drains eastward 

into the Great Valley, then into the bay through the 

delta, and the remaining 5 percent drains southward 

into Monterey Bay.

The streams that drain these basins transport 

weathered rock debris from the upland areas to the 

lowlands where it is deposited to form flat alluvial 

plains and estuarine mudflats. The two large rivers 

draining the Great Valley bring debris into the bay 

region from as far away as the Sierra Nevada and 

Klamath Mountains (fig. 3). Most of the course gravel 

and sand carried by these rivers is deposited in the 

Great Valley. (Gilbert, 191 7) Much of the fine-grained
INTRODUCTION 13

silt and clay carried in suspension is transported into 

San Pablo Bay, into San Francisco Bay, and even 

directly into the Pacific Ocean through the Golden 

Gate. Estuarine currents produced by tidal action

distribute this debris throughout the bay and into 

southern San Francisco Bay where it settles to the 

bottom during slack water periods and forms the finegrained, water-saturated deposit called "bay mud."

EXPLANATION

C'\ I

Sacramento-San Joaquin River basin

San Francisco Bay region

NX

N

\

X. __ '\ Sacramento -San Joaquin delta 

X

This area historically \ 

drained northward to \ 

San Francisco Bay

S

N

FIGURE 8. Area of interior California that drains through San Francisco Bay.
14 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Most of the debris forming the sedimentary deposits, 

particularly the alluvial deposits, is derived locally, 

however, and is transported only short distances 

from the upland to immediately adjacent lowlands. 

These sediments generally consist of troken rock 

debris (gravel, sand, silt, and clay) eroded, transported, and deposited by streams to form alluvial fans on 

broad lowlands and stream terraces in narrow valleys. The mineralogic and lithologic composition of 

these sediments is variable and is governed by the 

composition of the rocks exposed in source areas, 

weathering and depositional processes, and postdepositional alteration. Much of the alluvial material 

in the narrow valleys and on the upper part of alluvial fans is merely in temporary residence, however, 

and will be eventually reworked, broken down, and 

transported to lower elevations where it will become a 

permanent part of the thick sedimentary deposits 

filling the slowly subsiding basins.

GEOLOGY AND ENGINEERING PROPERTIES 

OF THE FLATLAND DEPOSITS

By EDWARD J. HELLEV and KENNETH R. LAJOIE

GEOLOGIC HISTORY

The known geologic history of the San Francisco 

Bay region is only about one-fortieth as long as the

Sacramento River 

San Joaquin River

adalupe R 

0 5 10 15 20 MILES

EXPLANATION

'^' Drainage divide 

..-- Stream channels

Area underlain by 

flatland deposits

0 1O 20 30 KILOMETERS

FIGURE 9. Major drainage divides and basins, San Francisco Bay region.

<(....SP Llagas Creek 

ifPajaro River
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 15

earth as a whole. The oldest known rocks on earth are 

about 3.8 billion years old, whereas the oldest exposed in the bay region are only 100 million years old. 

In turn, the oldest deposits considered in detail in this 

report are only about one-half million years old, and 

the geologic history we consider in detail is less than 

one two-hundreth that of the bay region as a whole. 

The youngest deposits are still forming today. Therefore, the geologic materials exposed at the surface in 

the flat lowland region, though formed of debris 

eroded from much older rocks in the surrounding 

uplands, are extremely young in geologic terms and" 

most of the geologic processes that formed them are 

still active today and are of concern in land-use 

planning.

Figure 10 gives a perspective of geologic time from 

the origin of the earth to the present. Figure 11 shows 

the types of rocks that occur in the bay region together with their geologic age.

The following paragraphs are a brief summary of 

the history and character of the older bedrock formations of the bay region from which the deposits that 

concern us were derived. The history we infer for the 

oldest of these deposits depends largely on modern 

concepts of sea-floor spreading and plate tectonics. 

For a more complete review of plate tectonics, see 

Oxburgh, 1974.

Between 100 and 65 million years ago, during the 

closing phases of the Mesozoic Era, great quantities 

of basaltic lava flows, red siliceous ooze, and muddy 

sand and mud accumulated in irregularly interbedded layers on an ocean floor west of what is now 

California. During this same time, an equally large 

quantity of mud, sand, and gravel accumulated in 

regular beds on the floor of a deep trench along what 

was then the margin of the North American Continent and is now the site of the Great Valley and 

eastern Coast Ranges. The ocean-floor sequence was 

lithified to hard sandstone, chert, and greenstone 

(metamorphosed basalt), partly crushed, and thoroughly mixed as the sea-floor on which it rested was 

dragged beneath the edge of the continent by subduction. This sequence is now known as the Franciscan Formation. The regularly bedded sequence 

on top of the surface of underthrusting was not so 

intensely disturbed, although it too was lithified to 

relatively hard sandstone, shale, and conglomerate. 

These rocks are now known as the Great Valley 

sequence. At the time of underthrusting, masses of 

gabbro and rock that later formed serpentine, originally parts of the Earth's mantle and lower crust, 

were incorporated into the Franciscan Formation 

and along the base of the Great Valley sequence. East 

of the San Andreas fault these rocks now make up 

most of the high mountains and uplands and underlie all other deposits at depth.

Granitic rocks, which solidified from molten magma about 90 million years ago in the area now 

occupied by southern California, were subsequently 

moved northwestward along the San Andreas fault 

to a position opposite San Francisco Bay. They now 

underlie at depth all land areas and the continental 

shelf west of the fault and are exposed at the surface 

on Montara Mountain, on the Point Reyes peninsula, 

the Farallon Islands, and Bodega Head.

During most of the Cenozoic Era (the last 65 million years), the crustal blocks that are now the San 

Francisco Bay region appear to have been divided 

into smaller sinking areas, that formed basins and 

embayments, interspersed with areas of uplift that 

formed highland areas. Sand, clay, and gravel 

eroded from the highlands or brought in from the east 

were deposited beneath the sea or on alluvial plains 

above sea level in the subsided basins and were 

interbedded with accumulations of diatom ooze and 

locally of volcanic rocks. The positions of basins and 

uplifts shifted from time to time, and areas that had 

been basins of accumulation were uplifted and vice 

versa. During periods of crustal movement, the sedimentary accumulations were lithified into sandstone, shale, conglomerate, and chert and were tilted, 

folded, and faulted. In general, they did not become 

as hard as the Mesozoic rocks, the younger rocks are 

less deformed and less lithified than the older rocks.

When a former basin of accumulation was uplifted 

to form a highland, its Cenozoic cover was eroded (in 

part, at least) to fill nearby newly formed basins, and 

the Mesozoic basement was re-exposed. Remnants of 

the Cenozoic rocks are found today mainly beneath 

the floors of existing basins, but they also make up 

two important highlands of the bay region, the Santa 

Cruz Mountains west of the San Andreas fault, and 

the Berkeley Hills-Mt. Diablo area.

Volcanic rocks are found in the Santa Cruz Mountains and Berkeley Hills and make up several mountain ranges north of San Pablo Bay in Sonoma, 

Napa, and Solano Counties.

The alternation of highland and basin is still going 

on, for the Berkeley Hills was a lowland as recently 

as 6 million years ago, and is now a major highland, 

and the San Francisco Bay valley, which was probably a highland at that time, is apparently now slowly 

subsiding.

The geography of the bay region was significantly 

different 2-3 million years ago from what it is today; 

for example, Mt. Diablo and the Carquinez Straits 

did not exist (Sarna-Wojcicki, 1971). The bay as we 

know it did not exist, and the interior of California 

probably did not drain to the Pacific Ocean through 

this region until about 1 million years ago. Geologic
16 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

evidence throughout coastal California indicates 

that the terrane west of the San Andreas fault has 

been moving northwestward relative to the terrane 

east of the fault at a rate of about 1-2 inches (2.5-5.0 

cm) per year for the past several million years as a 

result of displacement along the fault. Therefore, the 

Point Reyes peninsula probably lay south of the 

present Golden Gate within the past several million 

years and actually may have blocked it within the

past 1-3 million years. Throughout this time the 

ancestral bay basin was subsiding and filling with 

sediments that today are highly folded and faulted 

(Hall, 1966). These partly indurated sediments are 

now exposed only in discontinuous outcrops around 

the margins of the lowland basins (fig. 11). During 

this time the bay, when it existed, was connected with 

the ocean through passages far different from the 

present Golden Gate.

First abundant 

fossils

Present San Francisco Bay starts to form 

as sea level rises 15,000 years ago 

\Oldest habitation of bay area 

5000 years ago

Paleozoic

£00 million years ago

Mesozoic

225 million years ago

Cenozoic

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First human beings.

deposits described in this report 

Less than 1 million years old

Origin of earth

4.5 billion years ago

FIGURE 10. Geologic time in perspective.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 17

In addition to tectonic effects, major changes in sea 

level caused by worldwide climatic fluctuations over 

the past million years or so have left their geologic 

imprint on the bay region. When the climate was 

colder than it is today, sea level was lower because 

large volumes of ocean water were stored on the

123"

continents in glaciers. During these periods there 

was no bay and the ocean shoreline was located as 

far west as the Farallon Islands. When the climate 

was relatively warm, as it is today, sea level was close 

to its present position because the glaciers partially 

melted and the water they contained returned to the

38'

EXPLANATION

Holocene estuarine 

mud

Pliocene and 

Pleistocene sediments

Sedimentary rocks of 

Eocene to Pliocene age

Sedimentary and 

ultramafic rocks

Granitic rocks r < 

J 5

10 20 3O MILES

_I

10 2O 30 4O KILOMETERS

FKHIRE 11. Generalized geologic map of San Francisco Bay region. Modified from Schlocker, 1971.
18 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

oceans. During warm periods, the bay basin was 

partially flooded, as it is today, and the ocean shoreline was located approximately in its present position. During each of these major sea-level highstands, estuarine sediments were deposited in the 

bay and wave erosion cut a nearly flat marine terrace 

into the coastal mountains. A vertical succession of 

as many as six of these terraces capped by a thin 

veneer of marine sediments occurs along the coast 

indicating general uplift of the coastal mountains 

400-600 feet (120-180 m) during the past V6-1 million 

years. The youngest marine terrace, which is the 

lowest and usually the best preserved, probably 

formed during the last major highstand of sea level 

between 120,000 and 70,000 years ago. Uplift and 

deformation of these marine terraces by faulting and 

folding clearly record the tectonic activity that has 

affected the coastal region in the fairly recent geologic past. The deformation was quite irregular 

some areas were uplifted and others depressed.

During the last Pleistocene glacial advances between about 70,000 and 10,000 years ago, sea level 

stood as much as 300-400 feet (90-120 m) below its 

present elevation (fig. 12). The streams presently 

draining into the bay were merely tributaries of a 

large river flowing through the bay region from the 

Great Valley and across the broad coastal plain 

between the narrow canyon that is now the Golden 

Gate and the Farallon Islands. Camels, bison, mammoths, sloths, and horses roamed the broad inland 

valleys whose nearly flat floors, now partly occupied 

by the bay, were covered by fresh-water marshes and 

open coniferous woodlands consisting mainly of 

Douglas-fir (Pseudostuga menziesii) and incensecedar (Libocedrus decurrens), two species tolerant of 

cooler climates.

Because the coastline lay so much farther west, it is 

reasonable to believe that Sequoia were not abundant, for today they are found only within 30 miles (49 

km) of the coastal fog belt. Perhaps the lack of oak is 

indicative of a wetter climate, at least one without a 

summer drought.

About 15,000 years ago, sea level began to rise as 

glaciers in the northern latitudes began to melt. 

Subsequent changes in sea level in the San Francisco 

Bay area are recorded by tidal-marsh deposits located at the base of Holocene estuarine sediments. 

These marsh sediments accumulated near sea level 

when it was lower than today, so their radiocarbon 

ages closely approximate the ages of sea levels in the 

past. The local record of Holocene sea-level changes 

indicates that the rising sea entered the Golden Gate 

10,000-11,000 years ago. The newly formed bay then 

spread across land areas as rapidly as 100 feet (30 m) 

per year; it reached the vicinity of the Dumbarton

Bridge about 8,000 years ago. Subsequent shoreline 

changes have been more gradual because of a decrease in the rate of sea-level rise since about 

5,000-6,000 years ago. The ocean reached its present 

level at about this time and so should have San 

Francisco Bay (fig. 12). However, the active tectonic 

nature of the bay region strongly suggests that the 

bay waters are still expanding, especially in the 

southern part of the bay. As sea level rose throughout 

this interval, the base levels of the streams in the bay 

region were raised slightly, the younger alluvial 

sediments were deposited on the flood plains around 

the growing bay, and the younger bay mud was 

deposited beneath the rising water. All these younger 

deposits exposed on the alluvial apron around the 

bay plain and the extensive valleys of the region are 

less than 5,000 years old. These deposits are also 

found buried beneath the transgressive bay muds, 

and here they are older but still less than 9,300 years 

old.

In most areas along the coast, the post-Pleistocene 

rise in sea level inundated the lower parts of the 

deeply incised valleys of the large coastal streams 

and caused them partially to fill with sediment. The 

lower course of the Russian River is a fine example.

The bedrock of the Russian River valley lies at least 

125 feet (38 m) below sea level near the river mouth 

and at least 80 feet (24 m) below sea level near 

Guerneville, about 12 miles (19 km) upstream (Higgins, 1952).

The modern depositional processes operating on 

the lowlands are mainly the result of the region's 

mediterranean climate and active tectonic setting. 

The interaction between easily eroded bedrock and 

the heavy winter rains causes large seasonal fluctuations in the sediment loads carried by streams from 

the uplands down to the bay. During times of high 

surface-water runoff, sediments are supplied from 

slopewash, landslides, and gullying, then carried by 

shifting alluvial channels to the marshlands and 

bay. Some material is deposited on the alluvial plain, 

especially when streams overtop their banks and 

spread their sediment-laden waters over the low 

plains and basins. The finest grained sediment 

reaches the bay and is moved by tidal currents and 

waves. It may be deposited as fine estuarine mud or 

be eroded and transported out of the bay through the 

Golden Gate. The modern depositional and erosional 

processes operating along the bay are summarized in 

figure 13.

Along the coast, the depositional and erosional 

processes are controlled less by the climate than by 

wind and wave action. Sediment is supplied to the 

coastline by streams and rivers but much is also 

derived by erosion of sea cliffs. Littoral drift moves
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 19

the sediment along the coast where some is lost to 

deeper water, some is blown off the beaches to form 

dunes, and the rest remains as beach material. Just 

as with the sediments on the alluvial plain surrounding San Francisco Bay, the finer grained sediments 

like clay and silt are carried farther from their source. 

When sediment ceases to be supplied to the coast, the 

supply of sand for beach material decreases, but the

123" 122"

V

\\ « Farallon Islands

N\ »

0 500010.00015,000 

YEARS BEFORE PRESENT

FIGURE 12. Shorelines of San Francisco Bay and Pacific Ocean during Holocene transgression.
20 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

*.

FIGURE 13. Diagram of modern depositional and erosional processes along the bay plains.

littoral drift and wind erosion continue, and the 

beaches can disappear. During winter months most 

beaches are smallest, and bedrock may be exposed 

where strong wave action during storms has removed the sand. The reverse is true during summer 

months when most beaches are largest. A summary 

of coastal processes is shown in figure 14.

GEOLOGIC MAP OF FLATLAND AREAS

The map units that appear on plates 1,2, and 3 are 

described according to geologic age from oldest to 

youngest. Most of the information used for establishing an absolute chronology and for correlating sedimentary deposits in the bay region was obtained 

from published and unpublished geologic, archeologic, and engineering reports. These diverse and readily available sources provided the data for constructing an initial working model and determined where 

subsequent dating information was needed. Many of 

the samples dated specifically for this study were 

obtained from the numerous drill cores collected as 

much as 25 years ago for engineering foundation 

studies of various proposed or existing trans-bay 

bridges. Extensive use of radiocarbon dating was 

made when appropriate sample material was already 

available or was discovered during the course of 

fieldwork.

In addition to each map unit listed by age the 

following geologic characteristics are discussed: 

physical description and lithology, thickness, age, 

distribution and stratigraphy, origin of deposit, and
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS

, UP LANDS

21

Littoral drift

Organic activity, 

platform abrasion

/ WAVE-CUT 

PLATFORM J

COASTAL PROCESSES 

WAVE EROSION 

LITTORAL DRIFT 

WIND EROSION

DEEP OCEAN

FIGURE 14. Summary of coastal processes.

topographic form and relation to modern drainage. 

Since the amount of information known about each 

characteristic varies widely, some units can be described in much greater detail than others. The data 

are tabulated here for convenience.

HOLOCENE DEPOSITS (LESS THAN 10,000 YEARS OLD)

Estuarine deposits, Bay mud (Qhbm) 

Physical description and lithology

Unconsolidated, water-saturated, dark, 

plastic clay and silty clay rich in organic material. Locally contains 

lenses and stringers of well-sorted silt 

and sand as well as beds of peat.

Locally contains fresh- and brackishwater gastropod and pelecypod shells 

and beds of peat.

See Pestrong (1972) for more detailed 

information on composition and physical properties of bay mud.

Thickness much as 120 feet (37 m) beneath the bay. Thins to less than 1 foot 

( 0.3 m) around the margins of the 

bay. Probably less than 10 feet (3 m) 

thick in small coastal lagoons and 

estuaries. 

Age

Presently forming. Oldest dated deposits in bay basin, 9,600 years old (Atwater and others, 1976).

Locally contain Holocene molluscan 

fossils. Upper part contains molluscan species introduced by man in past 

100 years. 

Distribution and stratigraphy

Underlies the waters of San Francisco 

Bay and small coastal lagoons and 

estuaries. Found generally below 8 

feet (2.5 m) above mean sea level. Interfingers with Holocene alluvial de-
22 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

posits along the margins of the bay 

and in the lowest parts of broad valleys along the Pacific Coast and also 

overlies undifferentiated young alluvial deposits and older alluvium. Forms 

flat marshlands with low levees adjacent to tidal sloughs and gently sloping mudflats exposed only during low 

tides. Much of the area of exposure 

shown is used as salt-water evaporating ponds and much is covered by a 

thin veneer of artificial fill. See Nichols and Wright (1971) for more detailed information on distribution of 

young bay mud. 

Origin of deposit

Between mean sea level and high tide 

line (+8 ft, 2.5 m). Deposited primarily 

in brackish- to salt-water marshes 

along the margins of the bay and in 

coastal lagoons. Below mean sea level 

deposited on tidally exposed mudflats 

and beneath the shallow waters of the 

bay.

Beach and dune sand deposits (Qhs) 

Physical description and lithology

Loose well-sorted fine- to mediumgrained sand. Includes some small 

deposits stabilized by vegetation. Mineral composition is variable and generally reflects local bedrock lithologies.

Locally may contain lenses of gravel or 

clay rich in organic material. Some 

dune deposits contain aboriginal artifacts and kitchen middens. 

Thickness

The thickness of beach sand varies seasonally. Heavy surf strips sand from 

beaches and deposits it in shallow 

water offshore during the winter. 

Milder surf pushes the sand back onto 

the beaches during the summer. Dune 

sand thickness varies constantly but 

may locally exceed 25 feet (8 m). 

Age

Presently forming. Large dune fields on 

Franklin Point, Ano Nuevo Point, 

Point Reyes, Toms Point, and Bodega 

Head. Probably initially formed about 

5,000 years ago when sea level attained its present elevation. 

Distribution and statigraphy

Beach sand is found intermittently 

along the Pacific Coast close to sea

level. The largest, most permanent 

deposits occur in sheltered coves and 

embankments such as Half Moon Bay, 

Bodega Bay, and Point Reyes peninsula. Dune sand occurs locally above 

beaches but is most extensive on headlands, such as Bodega Head, Point 

Reyes, Franklin Point, and Ano Nuevo Point. Beach sand overlies bedrock 

except where it overlies marine terrace 

deposits. Dune sand generally overlies 

marine terrace deposits. 

Origin of deposit

Beach sand is generally derived locally 

by wave abrasion of sea cliffs and by 

stream erosion in adjacent hills. Wave 

action winnows silt and clay from rock 

debris supplied to the beaches and 

leaves a residuum of clean, well-sorted 

sand. Dune sand is derived from 

beaches by the wind.

Human activity, such as damming 

streams and stabilizing sea cliffs and 

beaches, may have locally disrupted 

sand supply.

Topographic form and relation to modern 

drainage

Beach sand forms seasonally and 

changes configuration from broad, 

flat, gently sloping beaches to steep 

beaches with high berms. Dune sand 

forms low mounds in small deposits 

but forms parabolic dunes (U-shaped 

in the direction of the wind) and longitudinal dunes (linear ridges oriented 

parallel to the wind direction) in the 

large deposits at Point Reyes, Bodega 

Head, Franklin Point, and Ano Nuevo 

Point. 

Fine-grained alluvium (Qhaf)

Physical description and lithology

Unconsolidated, plastic, moderately to 

poorly sorted silt and clay rich in organic material. Seasonally saturated. 

Irregularly bedded.

Locally contains fresh-water gastropod 

and pelecypod shells, aboriginal artifacts, and skeletal remains. 

Thickness

Generally less than 10 feet (3 m) thick. 

Age

Presently forming where basins are not 

artificially drained or filled. Oldest 

deposits, along bay plain, are probably 5,000 to 7,000 years old.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 23

Locally contains Holocene molluscan

fossils. 

Distribution and stratigraphy

Found in poorly drained, nearly horizontal basins between active and 

abandoned stream levees at the outer 

margins of alluvial fans adjacent to 

San Francisco Bay and in the lower 

parts of broad coastal valleys. Interfingers with and grades into coarser 

grained stream deposits toward higher elevations and with finer grained 

salt-water marsh deposits toward lower elevations. Overlies older alluvial 

fan and stream terrace deposits on the 

bay margin and bedrock along the 

coast. 

Origin of deposit

Deposited from standing floodwaters 

that periodically inundate low interfluvial basin areas and locally form 

seasonal fresh-water marshes.

Presently being formed but depositional 

processes severely disrupted by modern cultural activity.

Topographic form and relation to modern 

drainage

Forms flat floodbasins between stream 

levees at or near sea level.

Presently being deposited by modern 

streams but recent cultural activity 

has disrupted formative processes. 

Potential resources and uses

Agricultural soils developed on these 

deposits provide good to fair farming 

if well drained. 

Potential geologic hazards and problems

Shrink-swell problems; periodic flooding; poorly drained, seasonal standing 

water due to high water table; potentially liquefiable where local shallow 

sand beds exist and are saturated.

Poor to fair foundation conditions because of the shrinking and swelling 

and poor drainage.

Low shear strength, high seasonal water content, and potentially liquefiable 

layers near the surface suggest strong 

ground-motion amplification during 

earthquakes and the possibility of 

ground failure.

Fine-grained salt-affected alluvium (Qhafs) 

Physical description and lithology

Unconsolidated, plastic, moderately to 

poorly sorted, silt and clay rich in

organic material. Seasonally saturated. Irregularly bedded; carbonate nodules and mottled weathering profile 

common.

Contains fresh-water gastropod and pelecypod shells; locally contains aboriginal artifacts and skeletal remains. 

Thickness

Generally less than 10 feet (3 m). 

Age

Presently forming where basins are not 

artificially drained or filled. Oldest 

deposits along bay margin, probably 

5,000 to 7,000 years old. 

Distribution and stratigraphy

Exposed along margins of south San 

Francisco Bay from latitudes of San 

Mateo and Hayward southward only. 

Interfingers with and grades into 

coarser grained stream deposits toward higher elevations and with finer 

grained salt-water marsh deposits 

toward lower elevations. 

Origin of deposit

Similar to fine-grained alluvium. 

Topographic form and relation to modern 

drainage

Similar to fine-grained alluvium. 

Medium-grained alluvium (Qham) 

Physical description and lithology

Unconsolidated, moderately sorted, 

moderately permeable fine sand, silt, 

and clayey silt with occasional thin 

beds of coarse sand. Well bedded.

Contains minor amounts of organic 

matter including fresh-water gastropod and pelecypod shells; locally contains aboriginal artifacts and skeletal 

remains. 

Thickness

Thickness ranges from 0 to about 12 feet 

(4 m). Generally thickest in levees 

along streams and where deposits interfinger with and grade into coarsegrained younger alluvial fan deposits. 

Distribution and stratigraphy

In some narrow canyons as valley fills 

and stream terraces. Interfingers with 

and grades into coarse-grained deposits along streams in broad valleys and 

on flood plains in broad valleys and on 

the outer edges of large alluvial fans 

on the broad alluvial plain marginal 

to San Francisco Bay. Interfingers 

with and grades into finer grained
24 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

basin deposits in the lower parts of 

broad coastal valleys and in the interfluvial basins'adjacent to San Francisco Bay. Overlies bedrock in narrow 

valleys; overlies older alluvial fan deposits on the alluvial plain marginal 

to the bay and older alluvial fan deposits and marine terrace deposits oh the 

lower marine terraces along the Pacific Coast. 

Origin of deposit

Origin similar to younger alluvial fan 

deposits but deposited farther from 

source, therefore finer grained than 

coarse-grained alluvium. Local bedrock characteristics may control grain 

size in narrow canyons. 

Age

Presently being formed but depositional 

processes disrupted severely by recent 

human activity.

Topographic form and relation to modern 

drainage

Forms flat flood plains in some narrow 

valleys. Forms active levees and flood 

plains in broad valleys and at the 

outer margins of large alluvial plain 

marginal to San Francisco Bay.

Presently being deposited by modern 

streams, but recent cultural activity 

has disrupted formative processes. 

Coarse-grained alluvium (Qhac)

Physical description and lithology

Unconsolidated, moderately sorted, 

permeable sand and silt with coarse 

sand and gravel becoming abundant 

toward fan heads and in narrow canyons. Well bedded.

Locally contains aboriginal artifacts

and skeletal remains. 

Thickness

Thickness ranges from less than 10 feet 

(3 m) to as much as 50 feet (15 m). 

Thickest deposits in valley bottoms 

and at the heads of alluvial fans. 

Age

Presently forming. Oldest deposits probably 5,000 to 7,000 years old. 

Distribution and stratigraphy

On active stream terraces and as valley 

fills in narrow canyons where it interfingers with and grades into colluvium along the walls and at the heads of 

the canyons. Found along streams in 

the lower parts of broad valleys and at

the heads of alluvial fans where 

streams flow from narrow bedrock 

canyons onto alluvial plains marginal 

to the bay or onto low marine terraces 

along the Pacific Coast. Interfingers 

with and grades into medium-grained 

alluvium on flood plains, in broad 

valleys, and at the outer edges of alluvial fans. Overlies bedrock at fan 

heads and in valleys; overlies late 

Pleistocene alluvial fan deposits and 

the Colma Formation on the alluvial 

plain marginal to the bay; overlies late 

Pleistocene alluvium and marine terrace deposits on the lower marine terraces along the Pacific Coast. 

Origin of deposit

Fragmented and transported material 

derived from bedrock uplands and 

older unconsolidated sediments deposited by flowing water on active stream 

levees and flood plains primarily during floods. When streams overflow 

their banks during floods, the coarsest 

debris is deposited close to stream 

channel to form natural levees. The 

factors that initiated the cycle of deposition represented by the younger sedimentary deposits are probably related 

to the climatic and base-level changes 

associated with the drastic rise in sea 

level between 15,000 and 5,000 years 

ago.

Depositional processes disrupted severely by recent human activity. 

Topographic form and relation to modern 

drainage

Forms flat flood plains and terraces in 

narrow valleys. Forms well-drained 

levees and flat flood plains in broad 

valleys. Forms well-drained small alluvial fans where small streams flow 

from narrow bedrock valleys onto 

marine terraces. Forms well-drained 

stream levees and flat flood plains 

near the heads of large alluvial fans 

on the alluvial plain marginal to San 

Francisco Bay.

PLEISTOCENE DEPOSITS (1.8MILLION-10,000YEARS OLD) 

Early Pleistocene alluvium (Qpea) 

Physical description and lithology

Moderately consolidated, deeply weathered, poorly sorted, irregularly interbedded clay, silt, sand, and gravel.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 25

Lithology is variable and dependent 

on local source rocks; for example, 

deposits in the north bay generally 

indicate the Sonoma Volcanics as a 

source. 

Thickness

Maximum thickness unknown but at 

least 150 feet (46 m) thick and probably reaches a maximum thickness of 

500 feet (150 m) under south San Francisco Bay. 

Age

About 70,000 years and older. Maximum 

age unknown but probably between 

70,000 and 1.8 million years old. 

Distribution and stratigraphy

Exposed primarily along the highest 

parts of narrow valleys outside the 

immediate San Francisco Bay valley, 

such as along Dry Creek north of 

Healdsburg, the Santa Rosa Plain 

south of the Russian River, and west of 

Napa in Redwood Valley. These deposits generally represent the highly 

dissected remnants of alluvial terrace 

deposits. The rivers and streams that 

laid them down were probably similar 

to those that deposited the late Pleistocene alluvium. The early Pleistocene 

deposits generally overlie bedrock; 

some deeply dissected channels within them are filled with younger deposits. 

Origin of deposit

Deposited from flowing water on stream 

terraces and alluvial fans. Similar in 

origin to late Pleistocene alluvium. 

Greater age is indicated by the deeper 

dissection and erosion of the soil profiles. As with the late Pleistocene alluvial deposits, these earlier ones may 

have been laid down during a lower 

stand of sea level.

Topographic form and relation to modern 

drainage

Forms very old highly dissected alluvial 

terrace deposits. Farther removed 

from the streams that deposited them 

and related, in fact, only by being in 

the same valley.

The fluvial regime that deposited these 

sediments was probably similar to the 

one that deposited the late Pleistocene 

alluvium. 

Marine and Continental deposits (Qpmc)

Physical description and lithology

In the northwest and central parts of the 

exposed area, the Colma consists of 

pale-colored, loose or friable, wellsorted, fine- to medium-grained sand 

with subordinate gravel, sandy silt, 

and clay. In the southeast part of area, 

it consists mostly of sand and silty 

clay. See Bonilla (1965) for more complete description. 

Thickness

Maximum thickness about 200 feet (60

m) (Bonilla, 1965). 

Age

Exact age unknown but may be one-half

to one million years old. 

Distribution and stratigraphy

Occurs in a narrow zone between Burlingame on the southeast, through Colma Valley to Daly City on the northwest. Overlies bedrock. Locally 

overlain by younger deposits. Probably correlates with oldest marine terrace deposits in the southern part of 

San Mateo County. 

Origin of deposit

Deposited in shallow marine and continental environment. The Colma was 

probably deposited at or near present 

sea level and has been subsequently 

uplifted and tilted to the east by tectonic forces. The site of Colma Valley was 

a shallow marine passageway connecting the ocean with an ancestral 

bay during the time the Colma Formation was deposited. The ancient site of 

Colma Valley was probably very 

much like the Golden Gate is today. 

Topographic form and relation to modern 

drainage

Forms southeasterly tilted floor of Colma Valley. Modern streams dissecting 

original flat surface have formed low 

rolling hills.

Late Pleistocene and Holocene alluvial 

deposits partially fill the lower part of 

small valleys eroded into the Colma 

Formation. 

Undifferentiated bedrock

No mapping was attempted for the older 

consolidated rock units such as the 

Franciscan assemblage or Great Valley sequence. 

MARINE TERRACE DEPOSITS (QPMT)

Physical description and lithology
26 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Weakly consolidated, slightly weathered sand and gravel. Usually consists 

of an undifferentiated lower part of 

shallow-water marine sediments and 

an upper part of alluvium and colluvium. Lower part commonly consists of 

conglomeratic sand or fine- to 

medium-grained micaceous sand overlain by horizontally bedded and crossbedded medium- to coarse-grained 

sand. Upper part generally consists of 

poorly sorted, irregularly bedded sand 

and gravelly sand.

Lower part of deposits on youngest terrace contains shallow-water molluscan shells and occasional marine 

mammal skeletal remains. 

Thickness

As much as 70 feet (20 m) thick in the 

Half Moon Bay area but generally less 

than 30 feet (10 m) thick. 

Age

Deposits on the lowest and most extensive marine terrace are probably 

70,000 to 120,000 years old. The deposits on the highest terraces may be onehalf to one million years old and may 

correlate with the Colma Formation in 

the northern part of San Mateo County.

Lower part of younger (lowest) terrace 

deposits contain late Pleistocene marine molluscan fauna. 

Distribution and stratigraphy

Occurs on flat, gently sloping, waveeroded platforms along Pacific Coast 

between sea level and 500 feet above 

sea level. Overlies bedrock. Locally 

overlain by alluvial deposits and by 

sand dunes. Some of the deposits on 

the highest terraces in the southern 

part of the coast may correlate with 

the Colma Formation in the northern 

part of San Mateo County. 

Origin of deposit

Lower marine part deposited in shallow, 

nearshore marine environment. Upper continental part deposited as alluvium and (or) colluvium by streams 

and (or) mudflows. Marine terraces 

and their deposits formed during periodic high stands of sea level. All terraces were probably formed at or near 

present sea level but have been subsequently uplifted, tilted, and folded by

tectonic forces.

Topographic form and relation to modern 

drainage

Forms flat, gently tilted and warped 

terraces along the Pacific Coast. Lateral continuity of terraces has been 

destroyed by faulting, folding, wave 

erosion (cliff retreat) and steam dissection. Terraces have generally elevated 

above sea level and are not part of the 

modern terrace erosion cycle, which 

was initiated about 5,000 years ago 

when sea level reached its present elevation. 

Late Pleistocene alluvium (Qpa)

Physical description and lithology

Weakly consolidated, slightly weathered, poorly sorted, irregular interbedded clay, silt, sand, and gravel. 

Grades progressively from coarsegrained stream deposits on abandoned terraces in bedrock canyons 

and at the heads of old alluvial fans 

into fine-grained alluvial fan and 

fresh-water marsh deposits near the 

present shore of the bay. 

Contains local accumulations of freshwater gastropods and pelecypods and 

continental vertebrate fauna including camel, bison, horse, sloth, and 

mammoth.

Thickness

Maximum thickness unknown, but at 

least 150 feet (46 m) thick. Maximum 

thickness of entire sedimentary pile 

beneath south San Francisco Bay is 

probably between 1,000 and 2,000 feet 

(300-600 m).

Age

About 10,000 years old and older. Maximum age unknown but probably between 35,000 and 70,000 years old. 

Locally contain late Pleistocene (Rancholabrean) vertebrate and invertebrate fossil faunas.

Distribution and stratigraphy

Exposed primarily along the highest 

part of the broad alluvial plain marginal to the bay. Along the northern bay 

margin and along the Pacific Coast 

occurs on abandoned flood plains 

deeply incised by channels that are 

partly filled by younger alluvial deposits. Along the southern bay margin 

overlapped by younger deposits. Inter-
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 27

fingers with and grades into older 

basin deposits near and under the bay. 

Overlies bedrock and marine terrace 

deposits along the Pacific Coast. Overlies bedrock, the Colma Formation, 

and deformed older sedimentary deposits (bedrock) on alluvial plain marginal to the bay. 

Origin of deposit

Deposited from flowing water in stream 

channels, on stream terraces, and on 

alluvial fans. Similar in origin to 

younger alluvial fan deposits; greater 

age is indicated by slight consolidation and better soil profile development than on younger deposits. The 

upper part of this unit in the bay basin 

was deposited by streams graded to 

lower stands of sea level; the bay did 

not exist during period of deposition. 

Topographic form and relation to modern 

drainage

Forms old, slightly dissected alluvial 

fans with subdued levees and stream 

channels. Forms dissected, abandoned flood plains in bedrock canyons 

and on coastal marine terraces. Incised by modern streams in canyons 

and at the heads of old alluvial fans.

The fluvial regime that deposited these 

sediments was very similar to the recent one that deposited the younger 

alluvial deposits.

Beach(?) and dune sand deposits (Merritt Sand) 

(Qps) 

Physical description and lithology

Loose, well-sorted, fine- to mediumgrained sand with subordinate silt. 

Thickness

Maximum thickness near Oakland of

about 50 feet (15 m). 

Age

Overlies peaty mud older than 40,000

years (radiocarbon-dated), therefore

oldest deposits at least 40,000 years

old but may include younger deposits.

Distribution and stratigraphy

Exposed at northeast end of south San 

Francisco Bay near southwest Oakland, in Alameda, and on Bay Farm 

Island. Recognized in borehole logs to 

extend under San Francisco Bay 

southwestward to the latitude of the 

San Mateo-Hayward Bridge. Also extends westward intermittently across

the bay and may correlate in part with 

dune deposits in San Francisco.

Origin of deposit

Probably derived chiefly by wind eroding and transporting stream sediments during lower stands of sea level 

that occurred more than 40,000 years 

ago but probably less than 100,000 

years ago. May have been modified by 

beach or shoreline processes as sea 

level rose.

Topographic form and relation to modern 

drainage

The distribution and shape suggest that 

the present form is little modified and 

is probably primary. The large pronglike dunes are oriented parallel to the 

ancient northwest wind direction.

GEOLOGIC MAPPING TECHNIQUES

One of the main purposes of the current geologic 

study of the San Francisco Bay region has been to 

differentiate the alluvial and estuarine deposits underlying the gently sloping (less than 15 percent) 

sedimentary plain between the bay and the surrounding hills. Specificially, attempts were made to relate 

deposits to the processes that were responsible for 

their origin. Shortcuts and innovative mapping techniques have been used because approximately 7,500 

square miles (19,425 km2) was mapped in only 3 

years. The alluvial units are defined by various 

geologic criteria such as depositional environment, 

geomorphic expression, soil-profile development, 

age, induration, compaction, and texture. The distribution of the units is determined primarily from 

topographic maps, published soil series maps, and 

aerial photographs. Figures 15, 16, 17, and 18 illustrate the evolution of such a geologic map for a small, 

area near Palo Alto. The late Quaternary deposits of 

the entire region are mapped on plates 1, 2, and 3. 

This mapping differs from a soil series map in that it 

considers greater thicknesses than the upper few feet 

and is largely based on the process by which the 

deposit was laid down. For example, floodbasin deposits are shown separately from alluvial fans.

On the basis of relative soil-profile development, 

the 18 alluvial soil series described in the Soil Conservation Service (1968) report on the region (fig. 16) fall 

into two distinct groups that reflect some basic differences in the deposits on which they are developed. 

The soil units with strongly developed weathering 

profiles constitute one group, and those with weakly
28 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

FIGURE 15. Alluvial deposits related to geologic processes 

in the Palo Alto area. The enhanced contour lines (derived 

from USGS 7.5-minute Mountain View and Palo Alto 

quadrangles) reveal the fluvial geomorphic features, alluvial fans, natural stream levees, and floodbasins on the 

broad alluvial plain between the bay marshlands and the 

irregular bedrock uplands. Distribution of former tidal 

marshland from Nichols and Wright (1971).
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 29

to moderately developed weathering profiles constitute the other. Soil profiles are developed by physical 

and chemical weathering processes at the surface of 

the Earth; therefore, well-developed soil profiles generally indicate that the materials on which they are 

formed have been exposed to either intense weathering conditions or moderate weathering conditions for 

a considerable length of time. The alluvial deposits 

on which the strongly developed weathering profiles 

occur were inferred to be significantly older than the 

deposits on which the weakly to moderately developed profiles occur because the weathering conditons 

in the bay region are relatively uniform. These two 

groups of deposits, therefore, simply indicate younger and older alluvial deposits, (fig. 17).

The younger deposits make up the alluvial fans 

being formed under the existing regime. The streams 

forming these young fans are graded to present sea 

level. These younger deposits and the bay mud into 

which they grade are informally referred to as Holocene deposits (fig. 18). The older alluvial deposits, 

now partly covered by the Holocene deposits, make 

up alluvial fans formed by these same streams when 

they were graded to lower stands of sea level during 

the late Pleistocene (prior to about 10,000 years ago). 

These older deposits are informally referred to as late 

Pleistocene alluvium (fig. 18).

The Holocene alluvium is differentiated further 

into depositional units called facies (fig. 18) on the 

basis of textural characteristics (that is, gravel, sand, 

silt, and clay) obtained primarily from published soil 

reports and unpublished engineering foundation 

reports, and the depositional environment (such as 

stream levees and floodbasins) determined from 

geomorphic expression as shown on topographic 

maps and aerial photographs. These facies grade 

from coarse-grained gravel and sand deposits, which 

form prominent stream levees at the highest part of 

the alluvial fans, into medium-grained sand and silt 

deposits, which form broad flood plains and subdued 

levees along the lower margins of the alluvial fans. 

These stream deposits grade into and interfinger 

with fine-grained silt and clay deposits, which form 

the flat floor of floodbasins between stream levees on 

the outer margins of the alluvial fans directly adjacent to the bay marshlands. These fine-grained basin 

deposits and some of the medium-grained levee deposits interfinger with and grade into the bay mud 

the carbonaceous silty clay that was deposited in the 

marshes and on the mudflats of San Francisco Bay 

during Holocene time (roughly the past 10,000 years).

This gradation from coarse-grained to fine-grained 

sediment in the Holocene alluvium is a natural consequence of very recent stream erosion, transportation, and deposition. The coarsest rock debris eroded 

from the bedrock uplands is deposited near the base 

of the hills where the rapidly flowing streams enter 

the broad, gently sloping alluvial plain. Only the 

finer grained debris is carried by the ever-slackening 

water to the lower parts of the alluvial fans and 

eventually into the bay itself where it is deposited as 

bay mud. The N land ward extent of the saturated, 

plastic bay mud underlying the former marshes and 

tidal mudflats of San Francisco Bay (fig. 15) was 

established from early (ca. 1850) U.S. Coast and 

Geodetic Survey hydrographic charts (Nichols and 

Wright, 1971) rather than from direct field observation, because human activity over the past 50 years 

has obscured its original distribution.

Long exposure to erosion and weathering processes has altered the original geomorphic expression and physical character of the late Pleistocene 

alluvial deposits, thus they have not been separated 

into depositional facies similar to those of the Holocene alluvial deposits in this study.

In the areas of the bay region where detailed soils 

data are not available, the alluvial deposits have 

been differentiated by using only the geomorphic and 

genetic criteria derived from aerial photographs and 

extrapolated from areas where detailed soils data 

provide the main and most reliable means of recognition. Fossils, archeological remains, and radiometric 

ages corroborate the relative ages and correlations 

based on these limited data. The upper part of the late 

Pleistocene alluvial deposits contains a Rancholabrean fossil vertebrate fauna consisting mainly of 

extinct species (for example, camel, horse, bison, 

mammoth, and ground sloth), whereas the Holocene 

alluvial deposits contain a fossil fauna completely 

modern in aspect (for example, deer and elk).

The upper part of the late Pleistocene alluvium 

contains fossil wood and fresh-water molluscan 

shells that yield radiocarbon dates of about 22,000 

years ago. Holocene alluvial deposits contain fossil 

wood, shells, and archeological remains that yield 

radiocarbon dates of about 5,000 years or less. Peat 

and shell deposits in the bay mud yield radiocarbon 

dates that range from about 9,600 years at the base of 

the unit in the lowest parts of the basin to modern age 

at the top of the unit. These ages from the bay mud 

date the latest marine transgression into the basin 

a transgression that agrees very well with the postPleistocene rise in sea level established from worldwide data (Milliman and Emery, 1968). In general, 

the partial flooding of the basin resulting from this 

rise in sea level raised the base level of the local
30 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

FIGURE 16. Soil units in Palo Alto area. Soil units are defined primarily on the basis of profile development, texture, and slope. 

Soil profile development is controlled by many factors including 

time, climate, parent material, slope, and biological activity. In 

this area where weathering conditions and parent material are

relatively uniform, the time factor is clearly expressed by relative soil-profile development, which can therefore be used as a 

means of differentiating alluvial deposits on the basis of relative 

age. The symbols shown on the map refer to the names of local 

soil series.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 31

	EXPLANATION

	Soil Series in the Palo Alto area* 

Alluvial soil series with weakly to moderately developed weathering profiles:

An Alviso clay

Ba Bayshore clay

Ca Campbell silty clay loam

Cc Campbell silty clay loam, clay substrate

Ch Clear Lake clay, drained

CrA Cropley clay; 0- to 2-percent slopes

CrC Cropley clay; 2- to 9-percent slopes

Csa Cropley clay loam; 0- to 2-percent slopes

GbB Garretson gravelly loam; 0- to 5-percent slopes

Pf Pacheco loams, clay substrate

Sv Sunny vale silty clay, drained

YaA Yolo loam; 0- to 2-percent slopes

YeA Yolo silty clay loam; 0- to 2-percent slopes

Ys Yolo loam; no slope given, taken from older survey

ZbA Zamora silty clay loam; 1- to 3-percent slopes

ZbC Zamora clay loam; 2- to 9-percent slopes

Zbz Zamora clay loam; 0- to 2-percent slopes

Alluvial soil series with strongly developed weathering profiles:

PoA Pleasanton loam; 0- to 2-percent slopes

PoC Pleasanton clay loam; 0- to 2-percent slopes

PpA Pleasanton gravelly loam; 0- to 2-percent slopes

PpC Pleasanton gravelly clay loam; 2- to 9-percent slopes

RaC Rincon clay loam; 0- to 2-percent slopes

RaC2 Rincon clay loam; 2- to 9-percent slopes

SdA San Ysidro loam; 0- to 2-percent slopes

SdB San Ysidro loam; 2- to 9-percent slopes

SdB2 San Ysidro loam; 2- to 9-percent slopes, eroded

Upland soil series (nonalluvial soils):

AvD Azule silty clay laom; 2- to 9-percent slopes

AvD2 Azule silty clay loam; 9- to 15-percent slopes

AvE Azule silty clay loam; 15- to 30-percent slopes

Da A Diablo clay; 0- to 2-percent slopes

DaC Diablo clay; 2- to 9-percent slopes

DaE Diablo clay; 15- to 30-percent slopes

GcE Gaviota loam; 15- to 30-percent slopes

GoD Gilroy clay loam; 5- to 15-percent slopes

GoE Gilroy clay loam; 15- to 30-percent slopes

GmE Gaviota-Los Gatos complex; 15- to 30-percent slopes

LgE Los Gatos clay loam

LoE Los Osos clay loam; 15- to 30-percent slopes

Lkg3 Los Gatos and Maymen complex; 50- to 75-percent slopes

LtD Los Trancos stony clay; 15- to 30-percent slopes

PrC Positas-Saratoga loam; 2- to 9-percent slopes

ShE Soper gravelly loam

SgC Saratoga-Positas loam; 2- to 9-percent slopes

Miscellaneous map symbols:

KfB Kitchen middens, archeological site

PkG Gravel pits

Tf Tidal mudflats 

*From U.S. Soil Conservation Service (1968)

	FIGURE 16. Continued.

streams and began the present depositional cycle 

represented by the Holocene alluvial deposits. The 

fact that the Holocene alluvial deposits around the 

margins of the bay are graded to the present sea level 

is used as one means of identifying these deposits 

and adds credence to the relative ages and correlations based on other criteria. The stratigraphic relations of the various alluvial facies are shown in figure 

19 for the plain bordering San Francisco Bay.

PHYSICAL PROPERTIES OF GEOLOGIC UNITS

Physical properties of the various geologic units 

such as texture, thickness, bulk density, induration, 

and seismic velocity are needed to assess their potential for becoming geologic hazards. These properties 

were compiled or interpreted from readily available 

sources (fig. 20) and were augmented by direct measurement where no data existed or where verification 

of existing data was needed. This information also 

was used to differentiate units where soil series were 

ambiguous or had not been mapped.

Very few data are available on the total thickness 

of the unconsolidated and semiconsolidated deposits 

filling the bay basin. A few deep drill holes and 

several seismic profiles (Hazelwood, 1974) indicate 

that these sediments are probably more than 1,970 

feet (600 m) thick in the San Jose Area and decrease 

irregularly to 200-300 feet (60-90 m) to the north near 

San Francisco.

The thickness of the younger units is fairly well 

known from numerous shallow bore holes described 

in engineering reports (fig. 20) and from shallow 

seismic surveys where bore-hole data are lacking. 

The Holocene alluvium (fig. 18) generally ranges in 

thickness from about 50 feet (15 m) near the heads of 

alluvial fans to about 10 feet (3 m) near the margins of 

the bay. The Holocene bay mud ranges in thickness 

from 0 to as much as 120 feet (36 m).

The thickness of the late Pleistocene alluvium (fig. 

18) is not precisely known because its base is not 

distinctly defined in the thick sedimentary section 

beneath the bay and the surrounding alluvial plain. 

Where the base of the late Pleistocene alluvium can 

be seen on stream terraces in narrow valleys, these 

sediments are about 10 feet (3 m) thick. They are 

probably as much as 130 feet (40 m) thick beneath the 

bay where they overlie an old estuarine mud identified by marine fossils brought up in a drill sample. 

Still older Pleistocene alluvial deposits probably 

underlie these deeply buried estuarine muds, but 

their total thickness is not known. In the southern 

bay area, these deposits may grade downward into 

the Pliocene and early Pleistocene alluvial deposits 

or may lie unconformably on them.

Published soils maps and unpublished engineering 

reports from highway departments and engineering 

consultants are the main source of data on the physical properties of the various geologic units. Each unit 

generally has a distinctive range of values for properties such as grain size, sorting, bulk density, 

compaction, induration, and moisture content. This 

relation is true partly because some of the units, in 

particular the three facies of Holocene alluvium, are 

defined and delineated by using physical properties
32 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

FIGURE 17. Distribution of younger and older alluvial deposits in 

the Palo Alto area as determined primarily from relative soilprofile development of soil series as mapped by Soil Conservation Service (fig.15). Alluvial deposits on which weak to moderate weathering profiles are developed were initially inferred to 

be younger than alluvial deposits on which strong weathering 

profiles are developed. Radiocarbon and fossil data have confirmed this relative age classification. The younger alluvial deposits contain modern vertebrate and invertebrate fossils and 

organic remains that yield radiocarbon ages of about 5,000 years 

ago and younger. The older alluvial deposits locally contain extinct late Pleistocene vertebrate fossils such as camel, sloth, bison, and mastodon, and organic remains that yield radiocarbon 

ages of about 20,000 years.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 33

such as grain size and induration.

The primary physical properties such as grain size 

and sorting are controlled by the depositional environment, whereas the secondary properties such as 

induration, compaction, and bulk density generally 

increase with age of the geologic units. This variation 

of the secondary physical properties with age is 

indicated by the increasing penetration resistance 

from the Holocene deposits to the various bedrock 

units. The very low penetration resistance of the bay 

mud is attributable to its extremely high water content, fine grain size, and loose packing, which reflect 

its youthful age. Older, deeply buried estuarine muds 

would have higher penetration resistances due to loss 

of water during compaction. Penetrometer resistance 

can be used to estimate relative densities which, with 

data on grain size, sorting, andmoisture content, can 

be used to evaluate liquefaction potential in shallow, 

unconsolidated deposits. Liquefaction of unconsolidated materials may or may not cause ground failure; 

the conditions that determine whether or not ground 

failure is likely to occur are given in table 1.

Figures 21-24 illustrate the range of some physical 

properties of the geologic map units. These properties 

were compiled and interpreted from readily available 

sources, such as bridge and highway data and private engineering reports, and were augmented by 

direct measurement where no data existed or where 

verification of existing data was needed. Numerical 

values were excluded because statistical validity 

could not be determined for so vast an area. These 

diagrams, then, are to be interpreted only as a general guide to the physical properties and should not 

be substituted for on-site investigation.

SEISMIC PROPERTIES

The relation between seismic behavior and specific 

physical properties is still not well known. Seismic 

properties of the various units are described in terms 

of their response to seismically induced energy rather 

than in terms of any inherent physical characteristic. Experience has shown, however, that lowdensity, water-saturated, soft sediments tend to 

amplify some components of seismic energy. Consequently, more shaking will occur in areas underlain 

by these sediments. Because of the amplification of 

bedrock motion, great damage may be expected (depending on the nature of the structure) where these 

soft sediments are thickest. Areas with different 

shaking characteristics can be determined from studying good geologic maps. Distinctions can commonly be made on the basis of firmness and geologic age 

(Borcherdt, Joyner, Warrick and Gibbs, 1975). Figure 

25 shows the change in P (compressional) and S

TABLE 1. Factors and conditions controlling liquefaction 

induced ground failure

Factor affecting 

liquefied material

Condition conducive 

to ground failure

Condition not conducive 

to ground failure

Lateral 

confinement Not confined Confined laterally

Along stream banks Depression such as a 

Near cliff faces floodbasin 

Flat plain

Slope Gentle to steep slopes 

Alluvial fans to 

hillslopes

Horizontal to gentle

slopes

Flat alluvial surfaces 

Flat floors of

floodbasins

Loading Nonuniform loading Uniform loading

Differential thickness Uniform thickness of 

of overlying material overlying material

(shear) wave velocities as a function of depth (also 

age) at Ravenswood Point, San Mateo County.

The stratigraphic relations and seismic velocities 

(P-wave and S-wave) of the various deposits are 

shown in a block diagram for part of south San 

Francisco Bay (fig. 26). This area was chosen because 

reliable information on depth to bedrock as well as 

reliable velocity data were available.

POTENTIAL PROBLEMS IN FLATLAND REGIONS

The geologic units shown on the regional flatlands 

map (pi. 1) have characteristics that constrain or 

favor particular uses of the land. Some potential 

problems and opportunities can be quite accurately 

identified from the map. Others can be only broadly 

inferred; definition requires additional, more specific 

information.

WATER-RELATED PROBLEMS

The flatland deposits are traversed by the many 

streams that formed them. These water courses, big 

and small, can flood, overtopping their banks and 

inundating lowlands. High ground-water conditions 

usually exist during the winter rainy season and can 

cause problems in low-lying basins and marshes 

fringing the bay that are underlain by units such as 

bay mud, fine-grained alluvium, and fine-grained 

salt-affected alluvium.

During the summer drought, ground water supplies are usually heavily used for irrigation. Overdraft of the ground water has led to both land subsidence and salt water encroachment. A general 

summary of the water-related processes is shown in 

figure 27. The diagram shows the kinds of problems 

that may be associated with the different flatland 

deposits from the foot of the bedrock uplands down to 

the tidal mudflats. Surface water obviously plays a 

significant role in distributing the sedimentary materials, which in turn can control the ground water 

characteristics.
34 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

FIGURE 18. Rock units in the Palo Alto area.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 35

FIGURE 18 EXPLANATION 

ROCK UNITS IN THE PALO ALTO AREA

Description of Map Units

Holocene deposits (less than 10,000 years old): 

Estuarine deposits (0-9,000 years old):

Qhbm Bay mud. Water-saturated estuarine mud; predominantly clay and silty clay underlying marshlands 

and tidal mudflats of San Francisco Bay. Contains 

a few lenses of well-sorted fine sand and silt and a 

few shelly and peaty layers. Interfingers with and 

grades into fine-grained and medium-grained alluvium; generally overlies early Holocene alluvium or 

late Pleistocene alluvium 0-120 ft (0-40 m) thick. 

Alluvial deposits (0-5,000 years old):

Qhaf Fine-grained alluvium. Plastic, poorly sorted carbonaceous clay and silty clay in poorly drained interfluvial basins marginal to bay marshlands. Locally 

contains thin beds of well-sorted silt, sand, and fine 

gravel; contains modern vertebrate fossils and 

fresh-water gastropod and pelecypod shells. Interfingers with and grades into bay mud and mediumgrained alluvium; overlies late Pleistocene alluvium. Generally less than 15 feet (5 m) thick.

Qhafs Salt affected fine-grained alluvium; same as Qhaf but 

containing high concentration of salt.

Qham Medium-grained alluvium. Loose, moderately 

drained, moderately sorted sand forming alluvial 

plains and stream levees. Locally contains beds of 

well-sorted clay, silt, and gravel; contains modern 

vertebrate fossils and fresh-water gastropod and 

pelecypod shells. Intermediate in character and 

lateral extent between fine-grained and coarsegrained alluvium with which it interfingers; generally overlies late Pleistocene alluvium. Generally 

less than 21 feet (7 m) thick.

Qhac Coarse-grained alluvium. Loose well-drained, moderately sorted, permeable sand and gravel forming 

stream levees and flood plains on higher parts of

alluvial fans; gravel becomes dominant toward fan 

heads. Locally contains beds of well-sorted silt, 

sand, and gravel; contains modern vertebrate 

fossils and fresh-water pelecypod and gastropod 

shells. Thickness ranges from as much as 50 feet (15 

m) at fan heads to 20 feet (6 m) where these deposits 

interfinger with and grade into medium-grained 

alluvium; overlies late Pleistocene alluvium and 

bedrock.

Pleistocene deposits (10,000-3,000,000 years old):

Qpa Late Pleistocene alluvium (10,000-70,000? years old). 

Weathered, slightly consolidated and indurated 

alluvial fan deposits consisting primarily of gravel 

and sand with some silt Less permeable than 

Holocene alluvium. Locally contains fresh-water 

pelecypod and gastropod shells and extinct late 

Pleistocene vertebrate fossils. Overlain by Holocene deposits on lower parts of alluvial plain; incised by channels that are partly filled with Holocene alluvium on higher parts of alluvial plain. 

Maximum thickness unknown but at least 150 feet 

(45 m) near margins of present bay where these 

deposits overlie deeply buried Pleistocene estuarine 

deposits.

Bedrock: 

Qpea

Br

Early Pleistocene and Pliocene alluvium. Tectonically deformed alluvial-fan deposits with local minor amounts of shallow-water marine deposits. 

Weakly to moderately indurated gravel, sand, and 

silt with subordinate amounts of lacustrine silt and 

clay; local thin tuff beds; contains late Pliocene and 

early Pleistocene vertebrate fossils. Underlies late 

Pleistocene alluvium; overlies or in fault contact 

with Franciscan Formation. Consists of the Santa 

Clara Formation in southwest bay area.

Undifferentiated Tertiary bedrock. Well-indurated 

sandstone, shale, and volcanic rocks. In map area 

underlies or is in fault contact with Pliocene and 

early Pleistocene alluvium.

FICUKK 19. Facies relations in younger alluvial deposits.
36 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

10

CO

o:

10

15

20

y> '5.

^ 

00

on 

o

_i

.s

 9\ <«|

1 --T I

60 40 20 ( 

TONS

^ 

£ 

^

=^-

P?

f

>

^"^^Graphic driving rate 

^w "" of penetrometer

- I ~~~l 1 1

400 300 200 100 0 

SECONDS PER FOOT

EXPLANAT ON g

5 E

 w- -,Tn

Size of sampler I

Blows per foot 140 poun£ 

hammer with 30" drop

Unconfined compressive^ 

strength (T/ft2 )

Shear strength (lb/ft2K 

Van.

nches), n

\\ 1 

3.4 135 \\A '% 

 /

-N 

ph

3es 

GV

in 

w - , 

+ y 

fO10 ±i

2 B-3 to 

co ro

bl. 29-8 j 

Soft dark gray silty clay >

1 9 11.4 >

Soft to stiff tan silt laslM* 

GWS. El. 15 2 Dense brown siltv sand- ""

7-31-56 some pebbles 1 47! 1.4 .

Soft brown to blue-gray I 9 11.4 '. 

sand and clayey silt ' 

some pebbles and roots 

rioTO;

'/

[Toir? >

Dense brown sand silty sand ' 57 |L4 ; 

and small gravel ^_^ ,

Stiff tan to gray clayey > 

silt some pebbles QsTQ'

Stiff and slightly compact j 

gray clayey fine sand j

Stiff tan and gray mottled I 15 1 1 -4 ;

 clayey silt >

Slightly compact sandy silt ; 

"fiber with thin sand lenses

cription Hard blue-gray clayey silt fasTO :

VS Elevation

So

+

ro in

S B-4

CO

^ El. 294

/. 

/

I

7

^

^

.

>

/t

i 

' MEAN

/ SEA LEVEL

/

/ 

/^

*

7

t 

/

/_

I

\

/

/ 

/

'/r 

/

^

7

y/

7~, /Unit weight (lb/ft3)

^f i^Percen, 

: shear' *i Moisture

30

20

10

10

20

30

40

50

60

FIGURE 20. An unpublished highway engineering boring log 

from which thickness and physical properties of alluvial deposits were partly derived. The sudden decrease in the driving rate of the penetrometer at 20 feet (6.3m) is interpreted to 

be the stratigraphic contact between the Holocene alluvium 

and the underlying late Pleistocene alluvium.

FIGURE 22. Ranges of relative natural moisture content of the ! 

geologic map units shown on plates 1-3. Moisture content is 

the amount of moisture in a given mass of natural material 

expressed as weight of water divided by weight of oven-dried 

material, usually multiplied by 100 to give a percentage. Usually moisture content decreases with geologic age; however, 

units that extend below sea level, such as beach and dune 

sand deposits (Qhs), Pleistocene beach? and dune sand deposits (Qps), and marine terrace deposits (Qpmt) may be 

saturated.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 37

Qhsc

Qhbm

Qhs

Qhafs

Qhaf

Qham

Qhac

Qpa

Qps

Qpmt

Qpmc

Br

Stream channels

Bay mud

Beach and 

dune sand

Salt-affacted 

alluvium

Fina-grained 

alluvium

Medium-grained 

alluvium

Coarsegrained 

alluvium

Pleistocene 

alluvium

Pleistocana 

sand

Marina 

terrace

Col ma 

Formation

Bedrock

I I I 

NOT APPLICABLE

Low High

DRY DENSITY

FIGURE 21. Ranges of relative dry densities of the geologic map units (pis. 1-3). Dry density is a measure of the mass of earth materials 

relative to an equal volume of water. For planning purposes geologic units that have higher densities are generally better foundation 

materials and behave with lower ground-motion amplification during earthquakes. It should be noted the dry densities increase with 

geologic age of the map unit.

Qhsc Stream channels

Qhbm

Qhs

Qhafs

Qhaf

Qham

Qhac

Qpa

Qps

Qpmt

Qpmc

Br

Bay mud

Beach and 

dune sand

Salt-affected 

fine alluvium

Fina-grained 

alluvium

Medium-grained 

alluvium

Coarsegrained 

alluvium

Pleistocene 

alluvium

Pleistocene 

sand

Marine 

terraca

Colma 

Formation

Bedrock

SATURATED BELOW SEA LEVEL

SATURATED BE LOW SEA LEVEL

SATURATED BE LOW SEA LEVEL

NOT APPLICABLE

I I I

Low High

NATURAL MOISTURE CONTENT
38 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Low Hifiti

PERMEABILITY AND DRAINAGE

FIGURE 23. Ranges of relative permeability of the geologic map grained alluvium) have low permeabilities and are poor

units. Here permeability refers to the property or capacity of transmitters of fluids. Such units will be poorly drained and

a geologic unit to transmit a fluid; it is a function of the po- pose problems of standing water especially during the rainy

rosity and compaction of the geologic units. In general, finegrained units such as Qhbm (bay mud) and Qhaf (fineseason.

Low High

PENETRATION RESISTANCE
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS

SEISMIC VELOCITY, IN METERS PER SECOND

39

1000 2000

w

40

CO£C

111

>- 80

lu

2

I

£120

UJ

Q

160

9OO

§J ,

:7.

-

_

_

O 

00to

_

~

1

, g;,

co - ~~|

O L - -

-

_

_

100

Average velocity PS

to depth O 

' 1 Interval velocity ! ' i

1 '

O

I I i 1 I i i i

EXPLANATION

Gravel

Brown sandstone 

and graywacke

IE1

Sensor location

20

40

60

80

100

120

140

16O

180

T.D. 18B

co 

SC.

UJ

ui

5

Holocene bay mud

Late Pleistocene and 

Holocane medium-to 

fine-grained elluvium

Late Pleistocene 

estuarine mud

Pleistocene alluvium

and 

estuarine muds(?)

FIGURE 25. The seismic properties of the rock 

units at Ravenswood Point, San Mateo County (Warrick, 1974). The schematic vertical

FIGURE 24. Ranges of the relative penetration resistance of the 

geologic map units. Penetration resistance is measured in the 

field by the "standard penetration test" that determines the 

number of blows required by a standard weight (140 Ibs, 63 

kg), when dropped from a standard height (30 in, 0.76 m), to 

drive a standard sampling spoon a standard penetration (12 

in, 0.3 m). Units with low penetration resistance generally 

make poor foundation materials. Note that penetration resistance tends to increase with geologic age.

Mud 

Soft clay

Clay 

Sand 

Clay 

Sand 

Clay

Soft clay 

G ravel

Clay

§ and ravel

Stiff clay 

Sand and gravel

Clay and silt

Sand

Hard clay and silt

Sand and silt

Clay and silt

Sand and clay

Brown sandstone and graywacke

Bedrock (Mesozoic?)

section shows the sediments related to depth; 

the graph shows the change in P-wave and Swave velocities in relation to depth and age.

FLOODING

Because the Holocene alluvial sediments were deposited primarily by floods along streams of the 

present drainage system, there should be a significant correlation between their distribution and the

////S//?///
40 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 41

Possible compaction of 

clay-rich beds due to 

ground-water withdrawal
42 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

areas inundated by historic floods. A comparison of 

maps showing the distribution of Holocene deposits 

(fig. 18) and flood-prone areas derived from historical 

flood data and hydrographic data (fig. 28) indicates 

that this correlation is strongest on modern flood 

plains underlain by the coarse- and medium-grained 

Holocene deposits and the low floodbasins underlain 

by fine-grained deposits at the outer edges of the 

Holocene alluvial fans. Without modern flood-control 

measures the correlation would probably have been 

greater in the alluviated areas. As expected, the tidal 

marshes underlain by Holocene estuarine deposits 

are flood prone.

The map of flood-prone areas provides a more 

accurate delineation of areas currently subject to 

flooding than the flatland deposits map. However, in 

areas where direct information pertaining to flooding is not available, an approximate delineation of 

flood-prone areas can be made if the distribution of 

young alluvial deposits is known. In addition to 

geologic maps of unconsolidated deposits, such as 

the flatland deposits map, soils maps showing alluvial materials may be used to locate flood-prone 

areas.

37°45'

37"30'

37° 15' -

0

h

10 20 MILES

10 20 30 KILOMETERS

FIC.URK 28. Flood-prone areas in the San Francisco Bay basin 

based on distribution of Holocene sediment. The sediments at 

the ground surface within the shaded areas were deposited predominantly by floodwaters over the past 5,000-7,000 years. After 

Limerines, Lee, and Lugo (1973).

STREAM CHANNEL CHANGES

Natural lateral stream migration is a slow process 

generally caused by streambank erosion and occurs 

most commonly on flat valley floors where streams 

rework their own levee and flood-plain deposits 

(principally the coarse- and medium-grained Holocene deposits). However, the channels of most 

streams have been severely modified by artificial 

channels and revetments, and lateral migration 

caused by streambank erosion has been greatly 

reduced.

Under natural conditions, sudden changes in 

stream courses may occur on gently sloping alluvial 

fans and flat valley floors when floodwaters cut 

through natural levees or when stream channels are 

filled with flood-borne debris and the stream is diverted. The numerous abandoned natural stream channel and levee systems branching from each other and 

radiating from the heads of large Holocene alluvial 

fans indicate that stream course changes were common over the past 5,000 years or so (fig. 17). Many 

streams, particularly those in the developed areas, 

have been artificially diverted into buried culverts or 

fairly straight lined ditches for flood control. Along 

many other streams natural levees have been built up 

and streambanks revetted. These and other floodcontrol practices have virtually eliminated the possibility of sudden stream course changes.

SALT-WATER INTRUSION

Salt-water intrusion or encroachment is the movement of salt water into a ground-water aquifer normally containing fresh water. Encroachment in the 

bay region is possible only along coastal regions 

where fresh-water aquifers lie below sea level and are 

in hydraulic continuity with sea water. Salt-water 

intrusion is natural in some coastal areas and bay 

margin where precipitation is low and aquifer recharge from upland areas is insufficient to prevent 

the sea water from percolating into the aquifer. Saltwater intrusion can also be caused artificially by 

overdraft of ground-water aquifers that are in hydraulic continuity with ocean water. Figure 29 illustrates four ways fresh-water aquifers may be intruded by salt water.

In the coastal area of central California, precipitation and ground-water recharge are generally sufficient to prevent salt-water intrusion. In the San 

Francisco Bay region where most ground-water 

aquifers are unconsolidated sedimentary deposits, 

salt-water intrusion is possible in alluviated lowlands surrounding the bay and in lower parts of 

narrow alluviated coastal valleys (fig. 30). In all
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 43

these areas, aquifers consisting of medium-to coarsegrained alluvium are in hydraulic continuity with 

salt water but under natural conditions have a sufficient head of fresh water to prevent intrusion.

Salt-water intrusion has affected the shallow 

aquifers (less than 100 feet (30 m) deep in the southern San Francisco Bay region. Several factors contributing to this condition include decline of pressure in 

the deep confined aquifers, lowering of the water 

table in the shallow aquifers, and subsidence, which 

has lowered stream channels and allowed tidal inflow to penetrate farther upstream and contaminate 

ground water from the surface (Page and Wire, 1969). 

Water from shallow wells in the northern Santa Clara 

Valley commonly contains 100 parts per million 

chloride but may contain as much as 700 parts per 

million (Page and Wire, 1969). The chloride content of 

water from these areas has not changed since it was 

first recorded in the late 1930's (Tolman and Poland, 

1940). Restoring ground-water levels, sealing abandoned wells, and restricting deep dredging in the bay 

which might expose fresh-water aquifers can eventuSan Francisco Bay

DEEPER0 GROUND WATER

EXPLANATION

Clay (aquiclude)

Sand and gravel (aquifer)

Salt water

1. Direct movement of salt water through natural "windows"

2. Spilling of degraded ground waters

2 m 3. Slow percolation of salt water through reservoir roofsmaller arrows indicate slower percolation rates.

4. Spilling or cascading of salty surface water or degraded 

ground water through wells

FIOURE 29. Four ways that salt-water intrusion can occur.
44 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

ally eliminate this problem (Page and Wire, 1969). 

Man-induced causes of salt-water intrusion in the 

bay region are (1) overdraft of fresh-water aquifers in

hydraulic continuity with ocean water, (2) subsidence of coastal regions owing to withdrawal of 

liquids or gas, which allows tidal ocean water to

123° 122°

38°

37°

- Natksap ;J <? %%kw-" -7 *-" * sfflv - ;;:\ '^v^'^ I . *i.~^'" l^s-^r ^"\J^^ .'""v *

EXPLANATION

Remote possibility-*JB Areas of possible salt-water intrusion into

alluvial aquifers

® Area of fresh-water (MOO ppm and <350 ppmCl) 

aquifers historically affected by intrusion

'^^, -' 

L

0 10 20 30 MILE3

I ' I I ' '

0 10 20 30 KILOMETERS

FIGURE 30. Areas of salt-water intrusion.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 45

penetrate farther up coastal streams, (3) depletion of 

discharge of coastal rivers, which allows tidal ocean 

water to penetrate farther upstream, and (4) penetrating aquicludes beneath estuaries by dredging or 

drilling, which allows salt water to enter underlying 

fresh-water aquifers.

SUBSIDENCE

Subsidence is the gradual settling or sinking of an 

area with little or no horizontal motion. Subsidence is 

common in flatland areas and creates land-use problems especially in coastal regions if land subsides 

below sea level.

Subsidence is caused by natural processes such as 

oxidation, solution, thawing, drying, wetting, compaction of subsurface materials, tectonic downwarping, or by a combination of these factors. The rate of 

natural subsidence is not known but is so low that it 

has not significantly affected man's activities in the 

bay region over the past 200 years.

Subsidence may also be caused by man's activities, 

such as removal of subsurface solids, liquids, or 

gases, or by wetting certain types of dry clay-rich 

sediments resulting in collapse of intergranular supporting structures. Subsidence caused by these processes may be relatively rapid and have significant 

impact on man's activities. This rapid subsidence is 

generally not hazardous to human life except where 

conditions are created that may trigger the failure of 

a dam or levee. However, subsidence has caused 

millions of dollars worth of property damage and 

necessitated expensive protective measures throughout the country.

Within the bay region subsidence primarily is 

restricted to the flatlands (fig. 31) and is mainly 

caused by three different processes: (1) collapse of 

clay-rich beds at depth in the center of the bay region 

owing to excessive ground water withdrawal, (2) 

oxidation of peat-rich beds near the surface in the 

delta region after lowering of the ground water table, 

and (3) withdrawal of natural gas from Tertiary 

rocks at depth in the Rio Vista area. Subsidence due 

to the first two processes is directly related to the 

character and distribution of flatland deposits.

Subsidence caused by ground-water withdrawal is 

a major problem in several parts of the bay region. 

Subsidence in Santa Clara Valley has been described 

by Poland (1971). The land adjacent to the bay subsided 2-8 feet (0.6-2.4 m) between 1912 and 1967 

because of overdraft of water from confined aquifers 

(more than 100 ft (30 m) deep) (figs. 32 and 33). Dikes 

and levees have been constructed to prevent flooding 

of about 30 square miles (78 km2 ) of salt evaporation 

ponds and 17 square miles (44 km2) of area landward

of these ponds. The public cost, as of 1971, of levee 

construction due to subsidence was about $9 million, 

and general cost of repair to water wells damaged or 

destroyed by compaction was at least $4 million.

The primary cause of the rapid subsidence in the 

Santa Clara Valley area was the compaction of finegrained, clayey sediments in the central part of the 

valley where the ground-water table was lowered 

drastically by pumping. These fine-grained materials such as clay, silt, and sandy clay, which are 

compressible and which restrict the movement of 

ground water, constitute a major part of the valley fill 

and are most abundant in the central part of the 

basin. The valley fill around the margins of the basin 

is incompressible coarser grained material such as 

sand and gravel which allows ground water infiltration. To a depth of 500 feet (153 m) the deposits at 

Campbell are about 75 percent gravel, whereas the 

deposits between Agnew and Alviso to 500 ft are 80 

percent clay. The subsidence potential due to compaction of fine-grained sediments is therefore much 

greater in the central part of the basin than around 

its margins (fig. 32).

Subsidence in the Santa Clara Valley was virtually 

halted by 1971 because the water table was raised by 

an extensive ground water recharge project. Water 

was imported and stored in percolation ponds located 

in the permeable deposits (primarily late Pleistocene 

alluvium) around the southern margins of the basin. 

Poland (1971) predicts that if ground water management can raise and maintain artesian head at least 

20 feet (7 m) above 1971 levels, man-induced subsidence will be stopped permanently in the Santa Clara 

Valley.

In general the potential for subsidence caused by 

ground-water withdrawal in flatland regions is 

greatest in areas underlain by thick sequences of 

fine-grained sediment and least in areas underlain 

by a thin veneer of sediment or by coarse-grained 

deposits. On the basis of this generalization, the 

potential for subsidence in the bay region is highest 

in the low central part of the bay basin where bay 

mud and fine-grained Holocene alluvium are underlain by a thick sequence of similar deposits, and 

lowest at the high basin perimeter where coarsegrained sediments at the surface are underlain by 

thick sequences of similar deposits. Such a generalization can serve only as a very rough approximation 

of possible problem areas, which need to be investigated in more detail. Many other factors, particularly 

man's activities, influence the occurrence and rate of 

subsidence. It should be emphasized that compaction 

of fine-grained deposits makes subsidence irreversible, hence it is extremely important to avoid an 

overdraft of the ground water in the first place.
46 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Subsidence primarily caused by oxidation and 

compaction of artificially drained peat deposits as 

much as 40 feet (12m) thick, which underlie about 450

square miles (1,165 km2) of the Sacramento-San 

Joaquin delta, affects a small area in the eastern part 

of the San Francisco Bay region (Weir, 1950). Peat is

123° 122°

38° -

37*

EXPLANATION

Ground-water withdrawal Contour showing amount of .f*f 

subsidence, in feet, as of 1952

Contour interval S feet ';

Oxidation of peat plus 

ground-water withdrawal

10 20 30 MILES

Extraction of natural gas

0 10 20 30 KILOMETERS

FKHIKK 31. Areas affected by subsidence.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS

122°15' 122°00' 121°45f

47

37°30'

37° 15' 

EXPLANATION

Fault

Dashed where approximate; 

(Jotted where concealed

-0.1 

Consolidated rocks 

Undlfferentiated igneous, met. 

amorphic, and consolidated 

sedimentary rocks

Line of equal subsidence 

Interval 1,0.5, and 0.1 foot; 

dashed where poorly controlled. Compiled from leveling 

of U.S. Coast and Geodetic 

Survey in May-September, 

1934, February-March, 196 7, 

and in tervening releveling

Multiply by 0.304 to convert 

to meters

32. Land subsidence from 1934 to 1967, Santa Clara Valley, California (Poland, 1969).
48 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

20

S 20

LL 

DC

CO

Q 60

O

o

DC

tf 100

Ul

m 140

ui

Ul

Z 180

m

< 220

ocui

|

OII

t ui 

Q

Water level 

in well 

7S/1E-7R1

Level of 

bench mark 

BMP7

_L

100

98

9630

9*

92

UJ oj

Ul «

u. g

Z o

Q 5! 

b§

90

88

<S

£1

Z ±! m =

to 5

86

84

1916 1920 1930 1940 

YEAR

1950 1960 1970

FIGURE 33. Change in altitude at bench mark BMP? in San Jose and change in water-table level in nearby well (from 

Poland, 1971). Note the close correspondence with time between the drop in water level and bench mark BMP7. 

Note that the scales for water level and bench mark level are different. The water table dropped almost 200 ft 

(60m) between 1925 and 1967, which resulted in a 13 ft (4 m) subsidence of the bench mark.

an unconsolidated deposit of partially carbonized 

marsh or bog plant remains with a high water content. Peat generally accumulates under acidic conditions in quiet water where there is little bacterial 

activity. Subsidence in areas underlain by peat-rich 

sediments is caused by drainage, which results in 

shrinkage from drying, oxidation, and wind erosion 

(Weir, 1950). Compaction by farm machinery and 

burning also contributes to subsidence in these 

drained, peat-rich sediments.

The total subsidence pattern in the delta area is 

complicated because subsidence due to ground water 

withdrawal is occurring throughout much of the 

delta region and subsidence due to natural-gas extraction from bedrock units is occurring around Rio 

Vista (fig. 31). The proportion of subsidence due to 

each process is not presently known. The subsidence 

due to gas extraction is not related to the flatland 

deposits but contributes to a serious local problem.

The main problem created by subsidence in the 

delta region is flooding where low flatland areas 

have sunk below sea level. Several parts of the delta 

have flooded in recent years, and the flood potential

becomes greater as subsidence continues.

Three tracts of land adjacent to Contra Costa 

County Bacon Island, Mildred Island, and Roberts 

Island which were originally at or slightly above 

sea level were between 10 and 11 feet (3.0-3.4 m) 

below sea level (fig. 34). Between 1922 and 1946 these 

three islands subsided at rates between 0.2-0.3 feet 

(0.06 m-0.09m) per year. The most recent data on the 

subsidence in the delta were published in 1948. Unfortunately no later data are available. An extensive 

network of dikes is needed to protect the islands from 

flooding. Dike failure during high tides and periods 

of heavy runoff has resulted in flooding of extensive 

areas. In 1952 the levees on Franks Tract in eastern 

Contra Costa County failed and the island was 

flooded. This island was not drained after the flood 

and is now a state recreation area used for sport 

fishing. In the winter of 1968-69 a section of levee on 

Sherman Island failed and the island was flooded 

(Poland, 1969). This island was inundated by floodwaters again in the winter of 1970-71. The potential 

for flooding is even higher during earthquakes when 

levee failure may be extensive.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 49

SEA 

LEVEL

I -2

t* -3

rS -6

>o -7

IU >.

-I .0

UJ > -8 'o. 

5 -9

1-10

-11

\ \ I I I I i t r

1922 1926 1930 1934 

YEAR

1938 1942 1946

FIGURE 34. Progressive subsidence of Mildred, Bacon, and 

Jones Islands in the Sacramento-San Joaquin delta primarily due to peat oxidation between 1922 and 1946 

(from Weir, 1950).

Levee-maintenance problems will increase in the 

delta region as continued drainage for cultivation 

lowers the land surface farther (Poland, 1969). Total 

subsidence will be greatest in those areas where the 

proportion of peat to mineral detritus in the sediment 

is highest. In some areas where peat constituted only 

a small part of the accumulated sediment and has 

been completely oxidized, subsidence may have 

ceased or is at least greatly reduced. Where peat has 

been completely depleted, the residual mineral silt 

and sand are still rich in organic nutrients and make 

excellent agricultural soil.

SETTLEMENT

Settlement is the gradual downward movement of 

an engineered structure due to compaction of the 

unconsolidated material below the foundation. Settlement is most extreme over mud and loose finegrained sediments (clay and silt) having a high water 

content. If the load of an engineered structure is 

relatively small or is emplaced gradually, the water 

in the voids between the silt and clay grains can 

escape slowly until the grains are in sufficiently close 

contact to carry the load and resist further compaction. In this case the settlement of an engineered 

structure would be relatively slow and would eventually cease. If the load is relatively large or applied 

suddenly, the high pore water pressures produced in 

the underlying mud will reduce the shear strength of 

the mud, which may result in failure. Under these 

conditions the mud may flow laterally out from under

the load and produce bulges or mud waves around the 

sinking structure. It is likely that both these processes work simultaneously but at different rates 

under real but variable conditions. In any case the 

rate of settlement is usually most rapid immediately 

after loading and gradually decreases with time. The 

total amount of settlement that may occur is dependent on the physical properties of the sediment, its 

thickness, laterally confining conditions, and the 

size and distribution of the load (fig. 35).

The deposit that most frequently is associated with 

settlement problems in the San Francisco Bay region 

is the fine-grained Holocene estuarine mud. The 

extremely low shear strength of this loose, watersaturated material, which is a major cause of settlement, is reflected in its low shear-wave velocities, 

around 300 ft/sec (90 m/sec) (figs. 25 and 26). Lee and 

Praszker (1969) review the causes of settlement and 

discuss several case histories in the bay region.

Settlement problems are usually associated with 

structures placed on artificial fills overlying thick 

deposits (> 10ft, 3 m) of bay mud. Structures placed on 

piles founded on secure materials beneath bay mud 

may also have problems if the surrounding region 

settles and leaves the structure stranded above 

grade. This problem is particularly serious at the 

Alameda Naval Air Station where ramps have been 

built to maintain access to several buildings (case 

history 3, Edgar Becker, in Lee and Praszker, 1969).

If a structure on artificial fill settles uniformly, the 

major problems created are poor drainage and failure 

of rigid subsurface utility connections. If one part of a 

structure settles more or at a different rate than 

another part differential settlement mechanical 

components such as doors and elevators may no 

longer work, but more importantly the structure itself 

may be seriously weakened. Structures weakened by 

differential settlement may not perform well, particularly during earthquakes. Structural weakening due 

to differential settlement prior to and during earthquakes may be the main cause of the greater observed 

damage to buildings on artificially filled land over 

unconsolidated mud (Steinbrugge, 1969). Modern 

engineering practices can minimize the effects of 

settlement, but the costs of mitigating measures are 

high, and there is still the greater likelihood that an 

average modern building on bay fill will have greater 

preearthquake stresses within it then will a similar 

building on structurally firm ground (Steinbrugge, 

1969).

Some of the most severe settlement problems in the 

bay region exist in the low basin areas of San Francisco, the Embarcadero waterfront, (Lee and Praszker, 1969) (fig. 36), and the Mission basin (Steinbrugge, 1969) (fig. 37). In these areas structures were
50 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Mud wave

^ Flow *r \

Mud Compaction Mud

Compaction^

Mud

Compaction

FIGURE 35. Diagrammatic cross sections showing examples of settlement. A, Settlement of an engineered structure placed on 

artificial fill overlying mud that has low shear strength. B,

built on randomly placed fill, consisting principally 

of rubbish, overlying Holocene bay mud.

Settlement problems also occur on more recently 

placed fills overlying the bay mud at the toll plaza of 

the San Francisco-Oakland Bay Bridge, Treasure 

Island, and the Alameda Naval Air Station (Lee and 

Praszker, 1969). The phenomenon of differential 

settlement is clearly observed and felt along freeway 

segments constructed on engineered fills overlying 

bay mud, such as the Candlestick Causeway in South 

San Francisco, where the flexible asphalt road surface is deformed into a series of low-amplitude waves. 

Small offsets in runways caused by minor differential settlement are a continuous maintenance problem at airports built on fill overlying bay mud.

Differential settlement has also created problems 

in some residential communities built on artificial 

fills overlying bay mud. A recently constructed apartment complex in Larkspur along Corte Madera Creek 

was severely damaged by settlement and had to be 

abandoned (Brewer, 1973). Several homes in Foster 

City, which is built on artificial fill overlying bay 

mud along the bay shore of San Mateo County, are 

beginning to show signs of structural damage caused 

by differential settlement.

causes and effects of differential settlement. The lateral variability in the underlying mud results in differential settlement, which may damage the engineered structure.

SHRINK-SWELL

Shrink-swell is a cyclic change in volume that 

occurs in fine-grained sediments because of expansion and contraction of clay caused by wetting and 

drying (fig. 38).

The potential for shrink-swell problems in the bay 

region is highest in areas underlain by the Holocene 

bay mud and the fine-grained Holocene basin deposits, which contain high quantities of expansive clay 

derived from the uplands by erosion. The Pleistocene 

alluvial deposits may contain large quantities of 

expansive clay at the surface. (This clay was formed 

in place by weathering.) The coarse- and mediumgrained Holocene alluvial deposits contain less detrital expansive clay and are not intensely weathered, 

therefore, they have moderate or low shrink-swell 

problems.

Because of the relatively minor economic losses 

resulting from shrink-swell conditions on alluvial 

deposits, this problem is usually ignored in determining land uses; however, large economic losses have 

occurred on such soils formed on bedrock (Meehan 

and others, 1975). Shrink-swell problems can usually 

be overcome with proper foundation engineering. 

The main effect, therefore, is on the cost of construe-
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 51

Contour showing amount of settlement; in feet, 1923 - 36 

Dashed where position uncertain. 

Contour interval 0.1 foot 

0.03ml

SUTTEK

EQ__" >

FIGURE 36. Settlement of the land surface on the San Francisco waterfront. From Lee and Praszker (1969).

tion in areas underlain by clay-rich deposits with 

shrink-swell potential.

SEISMIC HAZARDS 

SEISMIC RESPONSE

Seismic response is the total reaction of a particular area to earthquake shaking. Therefore, seismic 

response includes the type and intensity of ground 

shaking, permanent changes of elevation (uplift and

subsidence), temporary changes of condition such as 

liquefaction, and ground failure, lurching, lateral 

spreading, and landsliding (fig. 39). The level of 

ground shaking at a particular site depends on distance from the epicentral region and local geologic 

conditions. In regions underlain by bedrock, ground 

shaking will be greatest at the epicenter and decrease 

with distance away from the epicenter. In regions 

underlain by thick sequences of unconsolidated sedimentary deposits, certain frequencies of ground 

shaking may be amplified above bedrock levels ow-
52 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

FIC.URE 37. Subsidence of San Francisco buildings built on filled ground that was formerly swamp. Street and sidewalk have been 

raised by additional fill; the top of the garage door on the left side of the house is now only about 2 feet (0.6m) above the sidewalk 

(California Division of Mines and Geology, 1966, p. 452).

ing to resonance and impedance contrasts. The block 

diagram (fig. 39) summarizes potential seismic hazards from the bedrock uplands downslope across the 

alluvial plain and across the tidelands into the bay 

itself. In this example, seismic energy originates at 

the San Andreas fault zone and is attenuated in the 

bedrock with distance from the fault. Seismic amplifications at the land surface on different materials 

alluvium and bay mud, for example may differ from 

the bedrock amplification in response to conditions 

described in the following section.

SEISMIC AMPLIFICATION

Medvedev (1965) showed that seismic impedance 

(defined as the product of S-wave velocity and bulk 

density, Vs x P) can be used roughly to predict relative ground response. The potential for seismic

amplification increases as the impedance contrast 

between adjacent layers increase if other parameters 

such as stratigraphic thickness are constant. Because thickness is important, downhole measurements are necessary to help predict amplification. 

Amplification of seismic waves from nuclear test 

blasts was measured at three surface sites (fig. 40), 

and amplification of horizontal ground motion generated by the San Fernando earthquake of February 

1971 was measured at a downhole site at Ravenswood Point, San Mateo County (fig. 41). The trend of 

the amount of amplification recorded at the three 

different surface sites is quite similar to the trend of 

the amplification measured at different levels at the 

downhole site.

Density, S-wave velocity, and therefore impedance 

of geologic units increase with age (fig. 42).

The Holocene bay mud has the lowest impedance 

owing to both its low density and its low S-wave
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 53

FIGURE 38. Surface cracks formed in clay-rich sediments due to 

shrinkage during desiccation. When these sediments are again 

wetted, the expansive clay (mostly montmorillonite) will absorb 

water and expand, thus causing a change in volume that will 

close these cracks and elevate the surface slightly. Movement 

caused by this cyclic change in volume may be damaging to improperly designed structures. Young unweathered coarsegrained deposits generally have low clay content and therefore 

have low shrink-swell potential. Older, deeply weathered coarsegrained deposits may contain high concentrations of clay in the 

B-horizon of soils developed on them, and therefore, may have a 

moderate to high shrink-swell potential.

velocity. The three fades of the loose Holocene alluvium (pi. 1,2, and 3) have very similar physical properties and therefore are combined into one geologic unit; 

this unit has only slightly lower impedance values 

than the weakly consolidated late Pleistocene alluvium. The variability of physical properties within and 

between the bedrock units is reflected in their relatively wide range of moderate to high impedance 

values.

The schematic cross section in figure 42 shows the 

generalized stratigraphic relations in the southwestern bay region with the high impedance contrasts 

between units represented by slightly heavier contact lines. Considering only the impedance contrasts, 

amplification of bedrock motion would be expected to 

be low to moderate where bay mud overlies late 

Pleistocene alluvium or where Holocene alluvium 

overlies Pliocene and early Pleistocene alluvium. The 

impedance data suggest that the highest levels of 

amplification would occur where thick deposits of 

bay mud directly overlie Franciscan bedrock.

Because seismic amplification is dependent frequently on and therefore controlled by other factors 

such as stratigraphic thickness, predicted amplification potential using only impedance data is not very 

precise. These crude predictions are consistent, however, with comparative low-strain ground motion 

measurements which show that the highest amplification occur on bay mud sites. It is probably significant that four of the generalized geologic units with 

distinct low-strain amplifications roughly correspond to the four groups of geologic units with similar impedance values (groups separated by heavy 

lines in figure 42).

Combining the geologic units into groups with 

similar impedance values provides a useful means of 

evaluating earthquake intensity data and low-strainlevel response data. For example, if deposits with 

similar impedance values behave differently in an 

earthquake, other variables such as stratigraphic 

thickness might be investigated. Borcherdt, Gibbs, 

and Lajoie (1975), using intensity data from the 1906 

San Francisco earthquake, amplification spectra 

from nuclear explosions, and seismic properties of 

the geologic map units, produced a map showing 

estimated relative intensities of ground shaking that 

would be produced in the south San Francisco Bay 

region by a large earthquake on either the San Andreas or the Hayward fault (fig. 43).w

Although we cannot now predict when, where, and 

how great the next earthquake will be, we can for 

planning purposes anticipate where the effects of 

ground shaking will be severe. That this can be done 

may be seen in the comparison between local geology 

and earthquake damage for the February 1974 earthquake in Dunedin, New Zealand (figs. 44 and 45). 

Areas experiencing greatest damage are commonly 

underlain by loose unconsolidated sediments, Holocene estuarine mud, and Pleistocene and Holocene 

alluvium.

LIQUEFACTION POTENTIAL

Liquefaction is defined as the transformation of a 

loose, water-saturated granular material such as 

sand from a solid state to a liquefied state as a 

consequence of increased pore-water pressure (Youd 

and others, 1973). In the solid state, the sand grains 

are touching each other and the weight of any overlying material is supported by these intergranular 

contacts. In a liquefied state, the sand grains lose 

contact, usually only momentarily, and the weight of 

any overlying material is transferred to the water, 

which is incompressible, filling the spaces between 

sand grains. In this state the sand can flow like a 

fluid but will do so only if it is not confined. This 

description of liquefaction defines liquefaction as a 

transformation process rather than the actual flow of 

liquefied material (or ground failure) that might
© © 0 

Recording sites for 

low-strain seismic 

amplification (see fig. 40)

IZ da

8COHwZTJohH

8 O

03

o

z

o > 

r

z 

5

FIGURE 39. Potential seismic hazards on the alluvial plain and near the bay.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 55

result from this flow. Therefore, a potential for liquefaction does not necessarily imply a similar potential 

for ground failure.

A major cause of liquefaction is ground shaking 

during earthquakes. Seismic shaking raises the porewater pressure repeatedly so that the sand grains are 

momentarily forced apart. The geologic materials 

most likely to liquefy during an earthquake are loose 

water-saturated well-sorted (i.e., little or no clay-size

material present) silt and sand lying within about 

100 feet below the ground surface (Youd and others, 

1975). At greater depths the pressure from the weight 

of the overlying materials is generally too high to be 

overcome by the seismically induced increased fluid 

pressures, and liquefaction rarely occurs.

If a bed of silt or sand liquefies during an earthquake and can flow toward a free face such as a cliff 

or down a gentle slope, ground failure may occur.

FIGURE 40. Seismic amplification (Gibbs and Borcherdt, 

1974). This diagram illustrates the effect of local geology on 

ground motion. Seismic waves generated in Nevada by underground nuclear test blasts were recorded at three different 

sites in the southern San Francisco Bay region. Site 1 was on 

bedrock, site 2 was on a thick sequence of alluvial deposits, 

and site 3 was on bay mud. The bedrock signal was slightly 

amplified where it traveled upward through a thick sequence 

of alluvium. This same bedrock signal was amplified even 

more where it traveled upward through a thick sequence of 

alluvium and then through a layer of bay mud. Note that the 

seismic waves with frequencies between 1 and 1.5 hertz were 

amplified the most. The seismic waves recorded at these 

three sites were so small that they could not be felt. For seismic waves that are large enough to be felt, the amplification 

ratios most likely would be smaller than those illustrated 

here, but the general relation of bay tnud amplifying more 

than alluvium would be maintained.

FREQUENCY, IN HERTZ

I

C£

o

DISTANT EARTHQUAKE

Surface of «*>wwy^^ Surface bay mud

~-S/S^/^VY%'V^^ 12 m (39 ft) £asem°f(

MIMA

bay mud

An M^1 m p|eistocene 

40m (131 ft) a,,uvium

186 m (610 ft) Bedrock

10.0 20.0 30.0 

TIME, IN SECONDS

40.0 50.0

FIGURE 41. Horizontal ground motions at Ravenswood site recorded on downhole seismometers from a distant earthquake 

(Borcherdt, Joyner, Warrick, and Gibbs, 1975). The horizontal motions of the bedrock were amplified significantly as they 

traveled upward through the 571 feet (174 m) of weakly to moderately consolidated alluvium and 39 feet (12 m) of unconsolidated estuarine mud. Note that the trend in amplification at this one site is similar to the trend observed on three different 

sites (fig. 40).
56 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

Severe and extensive ground failure occurred during 

the 1906 San Francisco earthquake in the Monterey 

Bay region where large areas along the Pajaro and 

Salinas Rivers shifted laterally due to liquefaction 

and the flow of sand and silt beds underlying the 

river banks (Youd, 1975). At several localities in this 

area, liquefied sand flowed upward through cracks to 

the ground surface and formed small volcanolike 

craters. Also during the 1906 earthquake, ground 

failure was extensive in Colma Valley in south San 

Francisco where liquefied alluvium caused lateral 

shifting of several tens of feet down the gentle slope 

along the axis of the valley (Youd, 1975).

Liquefaction-induced ground failure can also occur 

on level ground if the liquefied material is unevenly 

loaded. In this case the liquefied sand may flow out 

from under the area bearing the heaviest load. Tall 

apartment buildings in Niigata Japan, toppled over 

during an earthquake in 1964 because sand beds

beneath them liquefied and could not support the 

uneven load of the buildings.

Areas that are potentially liquefiable can be 

mapped if the physical properties of the sediments 

are known or can be reasonably inferred. Beds of 

loose water-saturated well-sorted silt and sand have 

the highest liquefaction potential, and beds of dry, 

poorly sorted, well-indurated sediment have the lowest liquefaction potential (fig. 46). A map of liquefaction potential for moderate to large earthquakes in 

the southern San Francisco Bay region (Youd and 

others, 1975) was produced using standard penetrometer data to assess relative density of the geologic units mapped by the present authors and by 

Webster (1973). The saturated clay-free coarse silt 

and sand with relative densities less than 65 percent 

(poorly consolidated) are considered to have high 

liquefaction potential even in a moderate earthquake (M4-6.5). Water-saturated clay-free sediments

WEST EAST

LOW HIGH 

IMPEDANCE CONTRAST

EXPLANATION

Bay mud

Holocene alluvium

Late Pleistocene alluvium

Early Pleistocene and Pliocene alluvium

Tertiary sandstone

Franciscan Formation

Unit

Qhbm

Qha

Qpa

QTa

Ts

Mzf

Thickness 

(m)

0-36

0-15

10-45

0-250?

0-300

......

Relative bulk 

density, p 

(g/cm3)

1.3-1.7

1.9

2.1

2.0

2.4

2.7

Penetration 

resistance1 

(blows /ft)

0

20-80

100

100- refusal

Refusal

Refusal

P-wave 

velocity, Vp 

(m/sec)

1400 2

300-600 4

1500-2100

2500

1500-3300

2800-4000

S-wave 

velocity, Vs 

(m/sec)

90-130 3

200-300

200-400 5

1200

500-1400

1400-2000

Impedance, 

VS P

117-153

380-570

420-650

2400

1200-3360

3780-5400

'Test used 140-lb hammer dropped 30 in.

2 Figures in italics are estimated values

3From Warrick (1974)

4Above water table. Below water table Vp = 1500-1700

5 R. E. Warrick (oral commun., 1974)

FIOURR 42. Schematic cross section of southwestern San Francisco Bay and description of certain physical properties of the

generalized geologic units (Lajoie and Helley, 1975, fig. 31).
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 57

with relative densities greater than 90 percent (highly consolidated) are considered to have low liquefaction potential, even in a major earthquake. Saturated

clay-free sediments with relative densities between 

65 and 90 percent have moderate liquefaction potential that depends on intensity and duration of ground

122"00'

37° 

45'

30'

15'

1O 20 MILES

10 20 KILOMETERS

FIGURE 43. Estimated relative intensities of ground shaking in 

the south San Francisco Bay region from large earthquakes 

on the San Andreas and Hayward faults (Borcherdt, Gibbs, 

and Lajoie, 1975). 1906 San Francisco earthquake scale of 

Wood (1908): (A) Very violent, (B) Violent, (C) Very strong, 

(D) Strong, (E) Weak. Note that areas of violent intensity (B)

lie immediately adjacent to the faults in the regions underlain 

by bedrock but lie at successively greater distances from the 

faults in areas underlain by alluvium and bay mud. This 

broadening of the higher intensity zones is due to seismic amplification of the bedrock waves by the unconsolidated sedimentary deposits.
58 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

170° 30'

45°55' -

Sedimentary rocks

* 

Aecelerograph

FIGURE 44. Simplified geologic map of the Dunedin area (modified from Bishop 1974) with isoseismal contours based on the number of 

fallen items in grocery stores (heavy dots). Contours at 1, 5,15, and 100 items.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 59

45" 55'

EXPLANATION

Number of damage claims 

per square kilometer

>320

1 MILE

80 -160

40-80

20-40

Q

I'KILOMETER

FIGURE 45 Density of damage claims resulting from the Dunedin where it extends beyond the lowest contour. Heavy dotted line

earthquake. Contours at 20, 40, 80, 160, and 320 claims per indicates inner zone of more intense chimney damage. Unshaded

square kilometer. Dashed line indicates limits of built-up area rectangular areas are large playing fields.
60 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

shaking and textural properties of the sediments. In 

general, granular sediments in the young alluvial 

and estuarine deposits exhibit more of the properties 

conducive to liquefaction than the older alluvial 

deposits (fig. 46). Table 2 summarizes the standard

penetration data used to determine the liquefaction 

potential of the various alluvial deposits in the southern bay region (Youd and others, 1975). Because the 

alluvial deposits over the entire bay region were 

distinguished by using the same criteria, conclusions

122°30' 122°00'

37°45' -

EXPLANATION

LIQUEFACTION POTENTIAL 

ZONE GEOLOGIC UNIT OF CLAY-FREE GRANULAR

LAYERS

Bay mud and Generally moderate, locally 

underlying high where clean granular 

sediments layers are present in bay mud '

Holocene Moderate, water table normally ^£ 

alluvium above 3 meters (10 feet)

Holocene Moderate, water table normally

below 3 meters (10 feet)

Late Pleistocene ,, . alluvium Generally low

37° 15' -

FIGURE 46. Preliminary map showing liquefaction potential for is useful for designating zones where special consideration

the southern San Francisco Bay region. The map shows general- should be given to the possibility of liquefaction, but it is not

ized liquefaction potential of granular layers in each map zone valid for assessing the liquefacation potential of a given site.

but does not delineate locations of these layers. Hence, the map From Youd, Nichols, Helley and Lajoie, 1975, fig. 50.
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 61

TABLE 2. Summary of liquefaction potential analysis using 

stanaard penetration data

[Two probable local earthquakes are considered; (a) a moderate earthquake (M=6.5) and 

(b) a large earthquake (M=8.0). Sediments likely to liquefy during a moderate earthquake are classified as having high liquefaction potential; those unlikely to liquefy 

during a large earthquake are classified as having low liquefaction potential; and 

those intermediate between these two categories are classified as having marginal 

liquefaction potential. Dr = relative density.]

Standard penetration test date (percent indicating)

Zone Sedimentary unit

65percent<Dr 

Dr <65 percent <90 percent Dr>90 percent

(high (marginal (low Number 

liquefaction liquefaction liquefaction of 

potential) potential) potential) tests

1 -Older (Pleistocene)

alluvial deposits

2 Younger (Holocene)

3 Deposits underlying 

young bay sediments 

3 Deposits within

young bay sediments

11

22

33

73

29

33

28

21

60

45

39

6

357

708

155

53

drawn from these data in the southern bay region can 

be extrapolated to other areas as a first approximation of liquefaction potential. However, because of 

local variability, these data cannot be used to determine the liquefaction potential of a particular site 

even in the southern bay area where engineering 

data were analyzed. A qualitative list of factors and 

conditions controlling liquefaction are given in table 

3. This summary provides a list of properties that 

must be investigated on any site to evaluate the 

potential for liquefaction.

RESOURCES AND POTENTIALS

WATER RESOURCES

Infiltration is the movement of surface water into 

the ground through cracks in rocks or through the 

interconnected spaces between the constituent 

grains of sedimentary deposits. The rate of infiltration into a sedimentary deposit is controlled by its 

permeability and the depth to the ground water table. 

The higher the permeability, the higher the possible 

infiltration rate. Permeability is determined by numerous factors (table 4) including grain size, grain 

shape, and sorting that combine to control the size 

and continuity of the spaces between grains. Wellsorted, coarse-grained sediments, such as alluvial 

gravel and dune sands, generally have large interconnected spaces between the particles of rock and 

therefore would be expected to have relatively high 

permeabilities and infiltration rates compared to 

finer grained sediments.

The geologic units of flatland materials can be 

roughly ranked in terms of permeability and, hence, 

infiltration rate on the basis of grain size. The infiltration rate would be expected to be high in stream 

channel deposits and sand; moderate in coarsegrained Holocene alluvium and Pleistocene deposits

TABLE 3. Factors and conditions controlling liquefaction

Factor

Condition conducive 

to liquefaction

Conditions not conducive 

to liquefaction

Grain size of 

sediment Coarse silt and fine

sand

Sorting (Variability in 

grain size) Well sorted

Uniform grain size 

Clay free (less than 

3 percent)

Cementation1 - Uncemented

Loose

Consolidation 

(Compaction)- Unconsolidated

Noncompacted

Loose

Low shear strength

Clay, coarse sand, 

gravel

Poorly sorted 

Npnuniform grain size 

High clay content 

(more than 3 percent)

Cemented 

Hard

Semiconsolidated to 

consolidated 

Moderately to high

compacted 

High shear strength

Relative 

density2 Low relative density

Less than 65 percent 

for small earthquakes 

Standard 

penetration3- Low

Geologic age4- Generally young 

Holocene 

Late Pleistocene 

Water 

saturation Saturated

Pore spaces filled 

High ground-water

table

Bay deposits, flood 

basins, lower parts 

of alluvial fans

High relative density 

More than 90 percent 

for largest earthquake

High

Generally older 

Pleistocene and older

Partly unsaturated 

to dry

Pore spaces not filled 

Low ground-water

table 

Higher parts of

alluvial fans

Pore-water 

pressureDepth 

Seismic 

activity 

- High

Greater than lithostatic load

- Within 100 feet (30 m)

of surface 

Low lithostatic load

- High seismic activity 

High probability of 

moderate to great 

earthquakes

Low

Less than lithostatic 

load

Greater than 100 feet

(30 m) depth 

Less than lithostatic 

load

Low seismic activity 

Low probability of 

moderate to large 

earthquakes

1 Particles may be cemented together by calcite, silica, iron oxides, or other materials.

z Relative density primarily reflects the degree of compaction in a sediment 100 percent relative density means a sediment is at its maximum compaction; all the pore space 

is filled. 0 percent relative density means a sediment is at its minimum compaction; it is 

in its loosest condition and pore space is at a maximum.

:t Standard penetration generally reflects degree of induration, which is a combination of compaction and cementation. Low standard penetration values indicate a 

sediment that is not compacted and is not cemented.

4 In a general way, the age of a deposit is reflected in certain physical properties 

such as induration. Older alluvial deposits are generally more highly indurated than 

younger deposits.

and low in the fine-grained Holocene deposits. Figure 

47 shows expected infiltration rates based on the 

distribution of the various sedimentary deposits.

Depth to ground water table is another important 

factor controlling infiltration rates. If the water table 

is close to the ground surface, the infiltration rate will 

be low. Within the bay region, high water tables and 

therefore low infiltration rates are found most commonly in the tidal mudflats underlain by bay mud
62 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

TABLE 4. Factors affecting porosity and permeability of 

sedimentary deposits

Factor Porosity Permeability

Grain size: 

Fine grained 

Coarse grained 

Sorting: Well sorted 

Poorly sorted 

Compaction: 

Uncompacted 

Compacted 

- Smaller pore space

-Larger pore space

-Increased pore space

-Decreased pore space

 Pore space at 

maximum

-Pore spaced reduced

Cementation: 

Not cemented 

Cemented 

Depth of burial: Shallow 

 Pore space open

- Pore space reduced

Deep 

Low pressure, pores

open 

High pressure,

pores closed

Geologic age: Young 

Old 

At maximum 

 Decreasing

Lower 

Higher

Higher 

Lower

Increases 

Decreases

Increases 

Decreases

High

Low, but Transmissibility may 

be high

At maximum 

Reduced

and the low floodbasins at the outer margins of 

alluvial fans, which are underlain by fine-grained 

alluvial deposits. Low water tables occur along the 

higher well-drained parts of the alluviated areas 

underlain by coarse-grained alluvial deposits.

Information concerning infiltration rates is important in land-use planning for several reasons:

1. Areas with low infiltration rates are likely to 

present drainage problems that can affect 

both rural and urban use of the land.

2. Areas with very high infiltration rates are 

potential sites for aquifer recharge areas. Urbanization of such areas inhibits the replacement of ground water supplies lost through 

natural seepage or withdrawal by man.

3. The quality of ground water is affected by 

land uses within and near areas of high 

infiltration potential. For example, the location of solid-waste disposal sites in such 

areas may cause ground water contamination.

Areas favorable for ground water recharge can be 

precisely delineated by means of resistivity investigations (fig. 48). By comparing maps that show 

favorable recharge areas with other maps that show 

areas known to have high infiltration rates, unfavorable or undesirable recharge areas can be eliminated. 

It can also serve as the basis for determining where 

resistivity investigations should be undertaken. A 

modern soil survey also evaluates soil units in terms 

of permeability and, if available, can be used to

delineate areas with recharge potential and with 

probable drainage problems.

SOIL

Soil, the natural medium for growth of land plants, 

is one of the primary elements in the life support 

system of all terrestrial life and is therefore one of the 

most valuable natural resources. The highest quality 

agricultural soils are fine- to medium-grained, welldrained sediments that are high in nutrients, easily 

worked, and have moderate water-holding capacity. 

In the San Francisco Bay region, these conditions are 

found most frequently on the Holocene alluvial deposits on flat valley bottoms and on the gently sloping fans marginal to the bay. These deposits have 

been built up primarily by floods over the past 

5,000-7,000 years. Because these deposits are very 

youthful, they have not been deeply weathered nor 

significantly indurated. The soil profiles developed 

on these deposits lack well-developed clayey B horizons and are therefore permeable and easily tilled. 

Also, the lack of deep weathering means that the 

mineral nutrients have not been leached. Along 

levees and flood plains that are frequently flooded, 

fresh material brought in from the upland areas 

periodically replenishes the supply of mineral nutrients.

The most widespread ranking of land for agricultural uses is done by the U.S. Soil Conservation Service 

(1970); it has developed a standard format for expressing land capability for agriculture. Information 

developed by USGS for this report uses Soil Conservation Service soil survey data wherever available. 

In those areas where modern soil surveys have not 

been done, agricultural capability is inferred from 

the usual characteristics of the soil associated with 

each mapped geologic unit.

Figure 49 contains a brief description of the SCS 

capability classes and shows a typical topographic 

distribution of the eight classes. Generally speaking, 

class I, II, and III soils, those most suitable for 

agriculture, occur in flatland areas. Utilizing the Soil 

Conservation Service ranking system, the geologic 

units with the highest potential for agricultural use 

(class I, II, and III soils) are Holocene fine-, medium-, 

and coarse-grained alluvium, Pleistocene alluvial 

fans, and Quaternary marine terraces.

The flatlands map (pi. 1) provides a generalized 

overview of areas that may have agricultural potential. However, in areas where modern soil surveys 

have been completed, the soil surveys more accurately identify agricultural potential.

The distribution of water and soil resources in the 

flatlands bordering the bay is shown in figure 50. The
37°30' h

37"00 -

GEOLOGY AND ENGINEERING PROPERTIES

123°00' 122°30'

OF THE FLATLAND DEPOSITS

122°00' 121°30'

63

EXPLANATION

Area underlain by fine-grained, poorly sorted 

deposits with low infiltration - Includes poorly 

drained low areas with high ground-water table

Area underlain by coarser grained, moderately 

sorted to well-sorted deposits with high infiltration - Generally well drained and has low groundwater table

Area underlain by bay mud

[m Bedrock uplands

FIGURE 47. Areas of high and low rates of ground-water infiltration, San Francisco Bay region.

best soils are to be found on the active alluvial fans, 

which are the Holocene map units. These same units 

also provide the best sites for infiltration to recharge 

the ground-water reservoir. The lower areas of the 

active fans are finer grained and less permeable but 

provide fair agricultural soils when properly drained. 

The flatlands immediately adjacent to the bay are 

useful as flood basins and also provide areas in 

which to evaporate salt from the bay water.

MINERAL RESOURCES

The mineral resources of the San Francisco Bay 

region are evaluated in detail by Bailey and Harden 

(1975), and those found in the flatlands are discussed 

here only briefly. The most important and essential 

mineral resources in the flatlands are sand and 

gravel. All other mineral resources clay, salines, 

shell, and peat are of lesser economic importance.
64 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

EXPLANATION

Area investigated by resistivity 

method

Area favorable for ground-water 

recharge

i. Area highly favorable for 

ground-water recharge

01 2 3 4 SMILE h-HI i i I i

012345 KILOMETERS

FIGURE 48. Areas favorable for ground water recharge in southern Santa Clara County. After Page and Wire (1969).

SAND AND GRAVEL

Though sand and gravel basic construction 

materials are widespread in the flatlands, they are 

not unlimited resources. Indeed, minable sand and 

gravel resources are small in terms of projected needs 

(Burnett and Barneyback, 1975). About half the sand 

and gravel used in the bay area is for aggregate in 

concrete and the rest for bituminous pavement, road 

base, and fill (Burnett and Barneyback, 1975).

Deposits of sand and gravel occur in all bay area 

counties, but much of this material is unusable because it is of poor quality. Minable deposits occur in 

half the bay counties in both modern and ancient 

stream-channel, flood-plain, and alluvial-fan deposits (fig. 51). Small quantities of sand have been mined 

from the beach and dune deposits along the coast. 

Much of the originally available sand and gravel is 

not presently minable because of urbanization.

Much of the sand and gravel in the alluvial lowlands cannot be used for concrete because of certain

undesirable physical or chemical properties. The 

most stringent quality requirements apply to sand 

and gravel used for concrete aggregate. This material 

should consist of durable minerals, such as quartz 

and feldspar, and should contain minimal amounts 

of unstable minerals (dark minerals or clay, for 

example) and reactive minerals (opal, zeolites, and 

glass). Unweathered hard tough dense well-rounded 

rock granules are the most desirable for concrete 

aggregate. This type of material is commonly found 

in the stream beds and on the alluvial fans of streams 

draining terrain underlain by old volcanic rocks and 

highly indurated sedimentary rocks.

The particle size distribution is also important in 

concrete aggregate, particularly in the sand fraction 

(Price, 1966; in Burnett and Barneyback, 1975). In 

general, the largest grain size of aggregate consistent 

with practical limitations is desirable. The grain-size 

distribution of an aggregate is controlled by processing, but the primary ratio of various sizes is important in determining the feasibility of exploiting a 

particular sand and gravel deposit. Therefore, depos-
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 65

FIGURE 49. Soil capability classes. From class I to class VIII, the choices in use become fewer and the risks become greater.

From U.S. Department of Agriculture (1970).

its of relatively well sorted (low silt and clay content) 

sand and gravel consisting of tough, resistant particles are the most valuable for concrete aggregate. 

Deposits with these characteristics are abundant in 

outlying areas of the bay region (fig. 51), but many 

are a considerable distance from market centers, and 

many of those that are close are being eliminated as 

an exploitable resource by urbanization.

The deposits along the southern margin of Livermore Valley in Alameda County have supplied a 

major part of the sand and gravel used in the bay 

region. This high-quality material, which has been 

excavated to depths of 125 feet (38 m), consists of 

Holocene and, most likely, late Pleistocene alluvial 

fan deposits of Arroyo Mocho and Arroyo del Valle, 

which drain a large mountainous area underlain by 

Franciscan rocks.

The extraction of sand and gravel in large quantities can have very deleterious environmental effects. 

Large open pits frequently remain after the gravel 

and sand have been removed. The material removed 

is lost for ground-water storage, but in some areas 

such as Los Gatos, the resultant open pits are used as

percolation ponds for ground-water recharge. At 

Livermore, open pits left after sand and gravel were 

extracted have been converted to recreation lakes.

CLAY

In the early days of California, most brick and 

sewer-pipe clay used was of alluvial origin and was 

found in valley-fill and flood-plain deposits of rivers 

and streams (Turner, 1951). Local red burning clay 

for bricks and terra cotta has been producedfrom the 

Holocene and late Pleistocene deposits of Livermore 

Valley and the Niles area of Alameda County. Clays 

have been mined extensively from deposits of similar 

age in the area north of San Jose in Santa Clara 

County.

The clay resources in the alluviated flatlands have 

never been systematically mapped or evaluated. 

Most clay deposits of commercial value probably lie 

in the floodbasins of the central lower parts of the 

alluviated lowlands where the streams carry and 

deposit predominantly fine-grained silt and clay. 

Clay may also accumulate in quiet water filling cut-
66 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA
GEOLOGY AND ENGINEERING PROPERTIES OF THE FLATLAND DEPOSITS 

123° 122°

67

37°-

FIGURE 51. Resources in flatland areas of bay region.
68 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

off meander loops and other natural depressions in 

the higher parts of alluviated areas, but these deposits are smaller than those in floodbasins.

SALINES

Salines in the bay area are derived entirely from 

sea water (Ver Planck, 1951). The earliest European 

settlers in the area gathered naturally precipitated 

salt from tidal lagoons along the margins of the bay. 

When these natural deposits were depleted around 

1860, artificial evaporating ponds were constructed 

and continually expanded until most of the salt 

marsh areas in the south bay and some in northern 

San Pablo Bay were converted to salt production by 

1970 (fig. 51).

The main natural factors that make salt production by solar evaporation a profitable business are 

the broad expanse of low-lying salt marshes adj acent 

to the bay and the cloudless summer weather. The 

marshes are at or close to sea level, which minimizes 

pumping, and the fine-grained Holocene bay mud 

provides a natural water-tight bottom that reduces 

leakage (Ver Planck, 1951).

Former salt ponds are presently being converted to 

urban lands such as Foster City and Redwood 

Shores. However, the preservation of salt ponds for 

salt production, which is an important economic 

product of the bay region, and open space, such as the 

Federal wildlife preserve, will probably prevent the 

rapid conversion to other uses at least in the near 

future (Goldman, 1969).

SHELL

Deposits of oyster shell, primarily Ostrea lurida, 

occur as thin discontinuous lenses on the surface of 

and within the upper 20-30 feet (7-9 m) of Holocene 

bay mud in the southern part of the bay. Roughly 30 

million tons of these shells have been dredged from 

San Francisco Bay since 1924 (Hart, 1975; Goldman, 

1969). Most of this material was used in the manufacture of cement but some was used as a source of 

calcium in poultry and livestock feed. The dredging 

of oyster shells and mud from the bay for cement 

manufacture ceased in 1970 due to environmental 

and economic reasons, but small amounts used for 

poultry and livestock feed and soil conditioning are 

still recovered by a single operator (Hart, 1975).

The distribution and character of shell deposits 

within the bay are not well known so the reserves of 

this locally important commodity cannot be definitely determined, but Welday (in Hart, 1975, p. 64-74) 

estimates that of 60 million tons (54.4 million tonnes)

of available shell, about 40 million tons (36.3 million 

tonnes) usuable. Most of the shell extracted to date 

lies at or near the bottom of the bay and is therefore 

very young. The living populations of Ostrea lurida 

have been almost completely destroyed by pollution 

of bay waters and dredging of the oyster beds so the 

commodity is not being replenished. Consequently, 

when the present oyster beds are depleted, older and 

more deeply buried shell beds may be exploited.

PEAT

Peat reserves in the bay region are described by 

Stinson (1975). Peat, which is used exclusively as a 

soil conditioner, occurs in the eastern parts of Contra 

Costa and Solano Counties in fairly pure beds greater 

than 30 feet (10 m) thick in some places and in three 

small areas near Gilroy in southern Santa Clara 

County (fig. 50). At present peat is recovered only 

from Franks Tract, a subsided and flooded island in 

eastern Contra Costa County where it is mined by a 

floating clamshell dredge in 6-8 feet (2-3 m) of fresh 

water.

The extraction of peat from permanently flooded 

areas does not conflict with other high-priority land 

uses. Peat extraction from diked and drained lands in 

the delta region would conflict with the agricultural 

uses of these areas. Peat is obtainable from other 

parts of the Sacramento-San Joaquin delta outside 

the bay region and therefore is not a particularly 

valuable resource in the bay region. Also, various 

substitutes for peat are available for agricultural and 

horticultural uses.

IMPORTANCE OF FLATLAND GEOLOGY TO 

COMPREHENSIVE PLANNING

By WILLIAM SPANGLE and MARTHA L. BLAIR

Knowing the characteristics of flatland deposits is 

obviously important in land-use planning. Many 

costly and hazardous situations can be avoided or 

mitigated if good geologic information is available 

before land-use and development decisions are made. 

In the past, planners have relied on soil maps to 

identify the problems and resources of flat alluvial 

areas. Soil maps show deposits at the surface to a 

depth of rarely more than a few feet. Geologic mapping considers deposits at greater depth and is largely based on the processes forming the deposits. The 

delineation, description, and interpretation of flatland deposits are a new approach to geologic mapping and interpretation, designed explicitly for appli-
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 69

cation to land-use planning. In the past, geologic 

maps rarely differentiated the various alluvial deposits common to lowland areas. These differences, as 

described in the entitled sections "Potential Problems in Flatlands Regions" and "Resources and 

Potentials," have important planning implications.

Because the information presented here is new, 

actual examples of successful application to land-use 

planning do not exist. Thus, to describe the potential 

planning application of data on flatland deposits, 

two approaches are used. First, a hypothetical example of the use of flatlands information in landcapability studies is presented to illustrate how the 

interpretations of the preceding two sections can be 

integrated and related directly to land-use planning 

and decisionmaking. Second, actual planning responses to selected flatland issues are described to 

show how land-capability considerations and information describing flatland characteristics can be 

incorporated into a planning and decisionmaking 

process. In both approaches, the objective is to describe a planning framework for the effective use of 

data on flatland deposits.

A basic planning and decision making framework 

is outlined in the section "Introduction." Geologic 

and other earth-science information is important in 

each step of the planning process shown in figure 3. 

The outline below provides a general framework 

indicating how planners, working with earth scientists, can effectively introduce geologic considerations throughout the process.

1. Identify problems and define goals and objectives.

a. Obtain readily available geologic information 

for preliminary identification of natural hazards and resources.

b. Review the data in relation to current land-use 

plans and policies, projected growth trends, 

and anticipated changes.

c. Develop a tentative set of objectives and priorities, giving special consideration to hazards 

and resources.

2. Collect and interpret data.

a. Evaluate adequacy of available geologic data 

and develop a program for compiling new 

data.

b. Arrange with earth scientists to prepare basic 

and interpretive maps and texts. Map information should relate in scale and detail to 

other basic planning information.

c. Estimate the probable future demand for land, 

considering projections of population growth 

and distribution, economic activity, social 

and cultural needs, and transportation requirements.

d. Prepare land-capability maps showing the natural capability of each land unit to accommodate each potential use.

3. Formulate plans.

a. On the basis of land-capability maps, appropriate projections, and economic, social, and 

political analyses, consider feasible alternative arrangements of land uses.

b. Prepare alternative land-use policies and plans, 

incorporating as much detail as necessary to 

guide future decisions.

4. Evaluate impacts.

a. Evaluate alternative land-use policies and 

plans for environmental, economic, and social impacts.

b. Evaluate exposure to risk from natural hazards 

associated with alternative land-use policies 

and plans.

5. Review and adopt a plan.

a. Present plan alternatives for review and selection by the appropriate legislative body.

b. Schedule and hold public hearings.

c. Adopt plan with such modifications as may be 

needed to respond to information provided 

and opinions expressed at public hearings.

6. Implement the plan.

a. Prepare and seek adoption of land-use regulations and any land-acquisition and capitalimprovement programs needed to carry out 

the plan.

b. Establish guidelines and a procedure for conducting the geologic investigations needed to 

evaluate development proposals.

c. Develop the procedures and capability for reviewing soils and geology reports, environmental impact assessments, and project proposals.

d. Arrange for modification of previous steps as 

new or more detailed information becomes 

available.

DESCRIPTION OF LAND-CAPABILITY STUDIES

Land-capability studies are an important element 

of the land-use planning and decisionmaking framework outlined above. In any area, the existing natural features and processes present a range of constraints and opportunities for different uses of land. 

It is well known that the natural characteristics of 

different parcels of land vary as well as the physical 

requirements for different uses of the land. For example, farmers seek fertile soils; manufacturers in 

heavy industry want level sites with good foundation 

conditions; golf course developers look for rolling 

terrain with adequate surface and subsurface soil 

conditions.

Land-capability studies systematically record and
70 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

formalize judgments concerning the physical features of the land with regard to particular categories 

of land uses. Such studies evaluate, for a specified 

land use, the relative physical merits of the lands in a 

study area. The natural features and processes considered usually include topography, hydrology, geology, soils, vegetation, and climate.

Methods of evaluating land capability differ. A 

study may be largely descriptive, pulling together in 

narrative form information concerning the natural 

features and processes relevant to a particular land 

use. Or a study may involve a fairly sophisticated 

effort to quantify, weigh, and aggregate the earth 

science information relevant to specific uses for all 

lands within a planning area. In any case, judgment 

is needed and the studies should be carried out by 

planners with the assistance of experienced earth 

scientists. Land capability analysis involves four 

basic steps:

1. Defining the focus of the study and the land 

use or uses to be considered,

2. Determining the natural characteristics affecting capability for the use or uses selected,

3. Gathering, analyzing, and presenting information describing the natural characteristics, and

4. Evaluating relative capability of land units to 

support the selected use or uses.

Judgments concerning the importance of each 

factor as well as the range of conditions within each 

factor can be expressed numerically. The main advantage of a quantified analysis is not greater precision of results, but greater ease in combining many 

judgments into an overall rating of land capability.

Land capability studies vary in focus as well as 

method. A study may focus on identifying constraints for particular uses; another on factors favorable for particular uses; and another on both constraining and favorable factors using positive and 

negative ratings. A common variation is a risk analysis, which rates land within a study area in terms of 

relative risk from selected natural hazards. A study 

may be very detailed, dividing an area into small 

units which are evaluated for a specific use such as a 

sanitary landfill; or it may be general, dividing a 

study area into large units which are evaluated for a 

broad use category such as urban development.

Judgments about relative costs involved to overcome natural conditions adverse to particular uses 

commonly affect values assigned in a landcapability study. The Association of Bay Area Governments in a land-capability study recently completed as part of the San Francisco Bay Region Study 

(Laird and others, 1979) expressed land capability

directly in terms of the dollar costs associated with 

hazard-mitigation measures, potential property damage from natural hazards, and loss of natural 

resources. The method was tested in a pilot landcapability analysis of part of the Santa Clara Valley.

The pilot study focuses on geologic and hydrologic 

hazards and resources and makes excellent use of 

many San Francisco Bay region reports. Natural 

factors considered in evaluating land capability include earthquakes, flooding, bearing materials problems (potential for shrink and swell, settlement, 

liquefaction, and subsidence), slope stability, erosion 

and sedimentation, septic-tank limitations, and natural resources. Lands in the study area were evaluated for a range of uses: agricultural or rural, semirural residential, single-family residential, 

multi-family residential, regional commercial, downtown commercial, industrial manufacturing, and 

freeway.

The total expected cost associated with each natural constraint and resource for each land use was 

calculated. Cost information for all natural resources 

and problems for each 24.9-acre grid cell was aggregated for each land use. The resulting number indicates for each cell the expected dollar cost per acre of 

developing that cell with that land use. The range of 

total costs was divided into six capability levels and a 

land-capability map for each use was printed by 

computer.

Analyses of land capability provide only part of the 

information needed for land-use decision. Economic, 

social, political, and aesthetic considerations are also 

important. The physical capability of a parcel of land 

to support an intensive use may be poor, but other 

factors, such as location and accessibility, land cost, 

absence of alternative lands, or overriding public 

need may well indicate that the parcel should be 

intensively developed.

A study that systematically evaluates economic, 

social, and political factors, in addition to physical 

capability factors, is called a "land-suitability 

study." A land-capability study is often undertaken 

as part of a broader land-suitability study. On occasion, capability is, or should be, the determining 

factor. Areas with very low capability for sustaining 

a particular use can sometimes be eliminated from 

further consideration on that basis alone, allowing 

the planner to focus attention on more realistic 

options.

Land-capability studies are becoming increasingly 

important to land-use planning at all governmental 

levels. They assure that physical characteristics of 

the land will be given systematic consideration in the 

development of land-use plans and policies. The 

earth-science information requirements for such
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 71

studies vary with the total land area and the specific 

use to be studied. At the regional level, for example, 

fairly generalized data may be appropriate for an 

analysis of land capability for regional open space. 

On the other hand, a study undertaken, at any governmental level, to locate specific sites with good 

capability for sanitary landfill will require detailed 

information.

IMPORTANCE OF FLATLAND 

DEPOSITS IN REGIONAL STUDIES

Regional land-use planning is primarily concerned 

with those issues that are independent of local political boundaries and thus require a coordinated response. Moreover, many areawide problems, such as 

water and air pollution, are directly linked to the use 

of land. An appropriate focus of a regional land-use 

plan is to identify areas where urban growth can 

occur with minimum environmental damage, loss of 

resources, private and public cost, and risk to persons 

and property.

Federal regulations governing eligibility for funds 

under the Comprehensive Planning Assistance Program (U.S. Congress, 1974) require that all agencies 

applying for grants must have an adopted land-use 

element by August 22, 1977. For areawide agencies 

this element must include the following:

(1) Long and short term policies, and where appropriate administrative procedures and legislative proposals, with regard to 

where growth should and should not take place:

(2) The type, intensity and timing of growth;

(3) Studies, criteria, standards and implementing procedures 

necessary for effectively guiding and controlling major decisions 

as to where growth shall and shall not take place; and

(4) Policies, procedures, and mechanisms necessary for coordinating local, areawide, and State land use policies with functional 

planning and capital investment strategies, when available, and 

improvements in governmental structures, systems and procedures that will facilitate the achievement of land use objectives. 

(U.S. Department of Housing and Urban Development, 1975, p. 

36862)

The regulations also require that federally funded 

planning activities be conducted in accord with the 

National Environmental Policy Act of 1969, Public 

Law 91-190, (U.S. Congress, 1969) through the inclusion of environmental planning in the comprehensive planning process. Specifically, each agency 

shall:

(1) Identify salient elements of the natural and the man-made 

environments, their interrelationships, and major problems 

and/or opportunities they present for community development;

(2) Assess those environmental factors which will:

(i) Minimize or prevent undue damage, unwise use, or 

unwarranted pre-empting of natural resources and opportunities;

(ii) Recognize and make prudent allowance for major latent 

environmental dangers or risks (e.g., floods, mud slides, 

earthquakes, air and water pollution); and 

(iii) Foster the human benefits obtainable from use of the

natural environment by wise use of the opportunities available (e.g., use of natural drainage systems for park and 

recreational areas); (U.S. Department of Housing and Urban Development, 1975, p. 36860)

Regional or areawide planning agencies are also 

being funded by the Environmental Protection Agency (EPA) under Section 208 of the Federal Water 

Pollution Control Act of 1972 (U.S. Congress, 1972) to 

carry out waste-water management planning. Each 

plan must contain a land-use element. EPA and HUD 

have signed an interagency agreement to assure that 

land-use plans prepared under one program meet the 

requirements of the other.

Because of Federal support for areawide land-use 

planning emphasizing environmental concerns, 

land-capability studies are needed and are likely to be 

applied with increasing effectiveness. Evaluating 

land capability is important in developing policies 

with regard to "where growth should and should not 

take place" (U.S. Department of Housing and Urban 

Development, 1975). Such evaluation helps identify 

(1) areas where land preparation, building, and maintenance costs are likely to be higher than average; (2) 

areas where structures are subject to damage from 

natural processes and occupants may be subject to 

injury or loss of life; and (3) areas where certain uses 

of the land may alter or interfere with natural features or processes.

In the San Francisco Bay region, evaluation of 

capability of flatland areas to accommodate urban 

uses is particularly important, because most urban 

growth is expected to occur in these areas (Association of Bay Area Governments, 1970, p. 6). Urban 

growth is a process of converting land from rural or 

other relatively nonintensive uses to urban use. Thus 

evaluating capability of lands for agriculture and 

other open-space uses is also essential in order to 

identify areas of conflicting capability and possible 

trade-offs. As stated in ABAG's adopted regional 

plan:

It is on the Bay plain area that circles the Bay, and the larger 

valleys of Santa Clara, Napa, Sonoma, Petaluma, Livermore, and 

Ygnacio that the greatest amount of urban development has 

occurred. These valley lands, separated only by intervening 

ridges, are being steadily converted from agricultural to urban use 

to serve the needs of a growing urban population. Due to the 

difficulties of building on steeper slopes, those valley lands that 

remain unurbanized are prime targets for future urban development. The region will have to choose either to retain the prime 

agricultural lands and unique natural settings that these lands 

provide, or to allow them to be transformed by urbanization. 

(Association of Bay Area Governments, 1970, p. 4).

Regional evaluation of land capability must also 

include hillside lands. Options for location of needed 

housing or facilities are limited by existing development. In some areas, it may be necessary to choose 

between hillsides and unstable flatlands for needed
72 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

development. Information on relative capability of 

all available lands within a planning area is necessary for rational land-use decisions.

Capability studies are especially helpful in evaluating lands for uses in which natural characteristics 

are critical. For instance, urban uses can be physically accommodated on most land, at some development 

cost, but only land with specific natural characteristics can accommodate agricultural uses. Sand and 

gravel extraction, fish and wildlife habitats, sanitary 

landfills, and ground-water recharge areas are other 

examples of land uses with specific physical requirements. Early identification, at the regional level, of 

areas capable of sustaining such uses can avert 

unnecessary loss of resource potential. Recent emphasis by State and area wide agencies on identifying 

and protecting areas of critical environmental concern recognizes the relation between land use and 

natural characteristics of the land.

APPLYING INFORMATION ON FLATLAND DEPOSITS

The maps of flatland deposits and the interpretation derived from these maps provide a basis for 

ranking the flatland areas in terms of capability for 

some uses. An example of the use of the flatlands 

information in regional land-capability analysis is 

outlined in the following sections. The example indicates how the information can be used to identify 

potentials and problems that need further analysis. 

For direct use in plan formulation, the capability 

evaluation should cover the entire planning area (not 

just flatlands) and incorporate information from 

other sources. The example serves two purposes:

1. It indicates possible implications of the interpretations contained in the preceding two 

sections for regional land-use planning.

2. It illustrates a method that is useful in regional land-capability analysis.

The example is presented according to the four basic 

steps of land-capability studies already listed.

DEFINING THE FOCUS OF THE STUDY

The example uses the characteristics of flatland 

deposits to identify areas with potential land-use 

conflicts based on capability for urban residential 

development, agriculture, ground-water recharge, 

and sand and gravel extraction.

DETERMINING THE NATURAL CHARACTERISTICS 

AFFECTING LAND CAPABILITY

In the next step, those flatland characteristics 

affecting the capability of land to support each use

are selected. Capability for agriculture is based on 

soil characteristics of each geologic unit as interpreted on p. 62. Capability for ground-water recharge is 

based on permeability of the flatland materials (as 

shown in fig. 47) and capability for sand and gravel 

extraction is derived from figure 51 and the discussion on p. 64. Capability for urban residential use is 

affected by several flatland characteristics: flood 

potential, permeability, shrink-swell potential, settlement potential, possibility of liquefaction-induced 

ground failure, and seismic-wave amplification. 

Table 5 lists (1) the characteristics affecting land 

capability for urban residential development derived 

from the map of flatlands deposits, (2) the nature of 

the impact, and (3) the general relation of the characteristic to the map of flatland deposits.

Other characteristics that affect land capability 

for urban residential uses include soil-bearing capacity, erosion potential, scenic qualities, vegetation, 

slope stability, and faulting. These characteristics 

are not considered in this example because they 

cannot be directly derived from the map of flatland 

deposits.

TABLE 5. Characteristics of flatland deposits affecting 

capability for urban residential development

Characteristics Nature of impact Relation to 

flatland deposits

Flooding Potential loss of life;

damage to structures, 

personal property; 

erosion; cost of flood insurance; increase in flood 

extent or depth

Permeability Ponding; excavation

problems; cost of engineered solutions such as 

pumping, channeling, 

waterproofing

Shrink-swell Extra cost of foundation 

engineering; damages 

from shifting and cracking of improperly engineered foundations

Settlement Damage to structures, especially if differential 

settlement occurs; extra 

cost of special foundation engineering

Flood potential 

confined to 

Holocene 

alluvium and 

bay mud.

Problems severe 

in bay mud and 

fine-grained alluvium.

Problems most 

likely in bay mud 

and fine-grained 

alluvium.

Potential exists 

only in bay 

mud and delta 

area peats.

Liquefaction-induced ground 

failure Destruction or damage to 

structures during earthquakes

High potential in 

water-saturated 

sand within bay 

mud.

Seismic wave 

amplification Damage and potential des- Potential in all

truction of structures; 

loss of life from seismically induced ground 

shaking

flatland materials depending on thickness underlying 

material; most 

significant in 

bay mud.
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 73

GATHERING, ANALYZING AND 

PRESENTING THE INFORMATION

The objective of the next step is to rate the relative 

capability of selected land units for the uses being 

considered. Several tasks are usually involved. First 

the land units to be rated are defined. These may be 

census tracts, political units, grid cells, or virtually 

any configuration that can be mapped and that is 

generally consistent in size with the scale of the data 

and the purpose of the study. In the example, the land 

units evaluated are the geologic units on the map of 

flatlands deposits (pis. 1, 2, and 3).

A scoring system is then devised to express the 

degree each natural factor affects the land capability. 

In the example, a simple scoring system is used: 3, 

indicates high capability; 2, moderate capability; 1, 

low capability; and 0, no capability for the use. More 

complex scoring systems can be used to define high, 

moderate, and low categories more precisely. For 

example, categories of flooding might be: Areas subject to flooding on the average of once every 1-10 

years, once every 10-100 years, once every 100-500 

years, or areas not subject to flooding. Regardless of 

how precisely the categories are described, the assignment of a numerical score is a judgment based on 

knowledge of the factor.

A weight is also assigned to each natural factor 

representing the importance relative to the other 

factors as a determinant of capability. The judgments made in assigning the rating and weights are 

crucial to the conclusions and should always be 

explicitly stated. In the example, a simple weighting 

scale of 1 to 5 was chosen.

The score is multiplied by the weight to get a 

weighted capability rating. The ratings for all natural factors for each unit are totalled to produce an 

aggregated weighted rating. The aggregated weighted ratings permit a comparison of relative capability 

of each unit for each use. Tables 6 and 7 summarize 

these operations. The scores shown in table 6 are 

based on the following judgments:

(1) High permeability and low potential for 

flooding, shrink-swell, settlement, liquefaction, and seismic wave amplification are 

favorable for urban residential development.

(2) High potential for flooding is a severe constraint for urban residential development 

and, accordingly, is given a score of zero.

The judgments made in assigning weight are less 

evident. They include the following considerations:

(1) Flooding affects capability for urban residential development more than the other five

TABLE 6. Capability weighting and scoring system for urban 

residential development

Factor Degree of 

potential

Score Weight Weighted 

capability

Flooding -High 0

Moderate 1

Low 3

Permeability 

Shrink-swellSettlementLiquefaction-

-High 

Moderate 

Low

-High 

Moderate 

Low

-High 

Moderate 

Low

-High 

Moderate 

Low

Seismic-wave 

amplification High 1

Moderate 2

Low 3

0

5

15

6

4

2

1

2

3

3

6

9

3

6

9

4

8

12

factors. This judgment is based on the high 

cost, both monetary and environmental, of 

providing flood protection; the relatively 

small area involved; the importance of 

flood-prone areas for other uses; and the 

high social and economic loss caused by 

floods in residential areas.

(2) Shrink-swell is judged the least important of 

the factors because the potential for damage can be readily and fairly inexpensively 

identified and mitigated with appropriate 

site investigation and foundation engineering.

(3) Seismic-wave amplification can cause severe 

damage to residential structures but, because of the infrequency of major earthquakes, uncertainties in delineating areas 

of high potential, and ability to design 

residential structures to withstand ground 

shaking, this factor is judged slightly less 

important than flooding.

(4) Settlement and liquefaction are considered 

moderately important. Damage to residences caused by settlement can usually be 

averted with appropriate site investigations and properly engineered fills. Ground 

failure from liquefaction can cause severe 

damage to residential structures, but it 

occurs infrequently and only in parts of the 

areas with high or moderate liquefaction 

potential.

(5) Permeability is important primarily if septictank systems are to be used in a residential 

development. Problems resulting from low
74 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

permeability can generally be overcome by 

installing adequate drainage and sewer 

systems.

EVALUATING RELATIVE CAPABILITY 

OF LAND UNITS

The capability ratings in table 7 indicate that 

Holocene deposits generally have lower capability 

for urban development than do Pleistocene deposits, 

and among the Holocene deposits, bay mud appears 

to be in a category by itself with very low capability. 

The numbers employed in the analysis provide only a 

means of aggregating the impact of various physical 

characteristics of each unit on urban development. 

The results thus need to be expressed in generalized 

terms. Possible aggregated ratings range from 13 to 

54. Dividing the range into three nearly equal ranges 

representing high, moderate, and low capability 

gives the following results:

13-26 low capability: bay mud (Qhbm) 

27-40 moderate capability: fine-grained alluvium, fine-grained-salt affected alluvium, 

medium-grained alluvium, and beach and 

dune sand deposits

41-54 high capability: coarse-grained alluvium, 

late Pleistocene alluvium, early Pleistocene alluvium, Pleistocene sand, Quaternary marine terrace deposits, and Colma 

Formation

These ratings can then be mapped as shown in figure 

52 to show the general distribution of areas with 

high, moderate, and low capability for residential 

uses based on the factors considered in the example. 

This map and ratings can then be compared with

those for the other uses being considered to identify 

areas of potential conflicts.

Developing a capability rating of the land units for 

agriculture, ground-water recharge, and sand and 

gravel extraction is, in this case, very simple. The 

ratings expressed in terms of high, moderate, and low 

capability are derived directly from the sectidn, on 

"Resources and Potentials." These are summarized, 

along with the ratings for urban residential development, in table 8.

USE OF LAND-CAPABILITY STUDIES

At this point it is appropriate to see what has been 

learned from this preliminary exercise as a start in 

directing subsequent more complete and detailed 

investigations. The following observations can be 

made:

(1) Bay mud has low capability for all the uses 

considered. Special attention should be 

given to identifying positive attributes for 

other uses. Fish and wildlife habitats, oyster shell deposits, salt production, and 

water-related recreational activities should 

be identified and evaluated.

(2) High capability for all uses exists in areas 

underlain by coarse-grained alluvium and 

moderate or high in areas underlain by 

medium-grained alluvium and late Pleistocene alluvium. The need to determine appropriate land uses for these areas is obvious. Since relatively few areas have high 

potential for agriculture and ground-water 

recharge, these uses should be given priority consideration in land-use determinaTABLE 7. Capability rating of geologic units for urban residential development

[Qhsc, which is shown on the map, is not rated because it occurs only in active stream channels and is not suitable for urban development ]

Capability rating Low Moderate High

Geologic units

Characteristic

Flooding

Permeability

Shrink-swell

Settlement __________ 

Liquefaction2

Seismic wave 

amplification

Aggregated weighted 

capability

Qhbm

0 

2 

1 

3 

3

4

13

Qhaf

0 

2 

1 

9

6 

12 

30

Qhafs

0 

2 

1 

9 

6

12 

30

Qham

0 

4

2 

9 

6

12 

38

Qhs

'5 

6 

3 

9 

3

12 

38

Qhac

5 

6

3 

9 

9

12 

44

Qpa

5 

6 

2 

9 

9

12 

53

Qpea

15 

6

3 

9 

9

12 

54

Qps

15 

6 

3 

9 

6

12 

51

Qpmt

15 

6 

2 

9 

9

12 

53

Qpmc

15 

6 

3 

9 

9

12 

54

'Qhs, where it occurs as beach sand, is subject to daily tidal flooding; where it occurs as dunes, it may not be flooded at all.

"Liquefaction potential exists only where well-sorted granular material at shallow depth is saturated with water and is confined within impermeable layers.

CAUTION: Taking these numbers literally may be hazardous to your planning area!
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 75

122"

37° 30'

%^£&3hHW^

01234 KILOMETERS

FIGURE 52. Part of bay region showing areas with high, moderate, and low capability for urban residential development.
76 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

TABLE 8. Capability rating of geologic units for selected uses

Land uses

Land 

units

Agriculture1

Qhbm Low

Qhaf Moderate

Qhafs Low

Qham High

Qhac High Qhs Low

Qpa Moderate Qpca Low

Ops Low

Qpmt Moderate

EJpmc Low Qnsc Low

Ground-water 

recharge2

Low

Low

Low

Moderate

High 

High

Moderate

Moderate

Moderate

Moderate

Moderate

High

Sand and 

gravel 

extraction-'1

Low

Low

Low

High

High 

Moderate

High

Low

Low

Low

Low

High

Urban 

residential 

development

Low

Moderate

Moderate

Moderate

High 

Moderate

High

High

High

High

High

Not applicable

Rating from section on "Soil."

' Rating from figure 47 and section on "Water Resources." 

1 Rating from figure 51 and section on "Sand and Gravel."

tions. Suitability factors are likely to be 

important in these areas.

(3) Many of the units have high capability for 

urban development. Natural characteristics are less constraining for this use than 

for the other uses considered. The ratings 

indicate where constraints to urban development may occur and where potential for 

urban growth conflicts with potential for 

priority uses.

Capability ratings are only part of the information 

needed to formulate land-use plans. Final decisions 

must rest upon other environmental, social, economic, and cultural information. Combining all relevant 

information can be accomplished through a suitability study, as noted earlier. The method is the same as 

for a capability study, but the information requirements are broader. For most urban uses, capability 

analysis is appropriately carried out as an integral 

part of a broader suitability study. Even for uses 

where natural characteristics may be paramount, 

such as agriculture, sanitary landfills, or other openspace uses, the final decision on land use usually 

depends, at least in part, on suitability factors. Accessibility, distance from markets, and ecological 

constraints, for example, may be critical. A capability study is, however, valuable in defining options and 

limitations early in the planning process and can be 

effective in narrowing the scope of a subsequent 

suitability analysis. For example, identifying lands 

with high capability for agricultural uses early in the 

planning process focuses the analysis of suitability 

factors on those lands with high capability. Sequential analysis can save the time and expense of acquiring needless data concerning agricultural suitability 

for lands clearly incapable of supporting such uses. 

In evaluating land for urban uses, suitability factors can be usefully incorporated into the original 

analysis. This is because most land can physically 

accommodate most uses. The question is, usually, at 

what cost and risk. The answer to these questions 

involves suitability factors.

The ratings based on characteristics of flatland 

deposits can focus further investigation by identifying problems and potential to be considered throughout the planning process. Table 9 sets forth, by way of 

example, the planning implications of the capability 

ratings for urban residential development.

Basically, as noted earlier, a capability rating for 

urban uses provides an overview of potential public 

and private costs; risks to property and persons; and, 

combined with capability information for priority 

uses, some possible adverse impacts.

PLANNING RESPONSE TO FLATLAND ISSUES 

PLANNING FOR THE BAYLANDS

The capability ratings for urban residential development provide a generalized view of differences in 

the ease and cost of construction and risks of building 

on different flatland deposits. Characteristics of 

flatland deposits rarely cause land to be totally 

incapable of supporting residential uses. They may, 

however, strongly affect the cost of land preparation, 

construction, and maintenance. Shrink-swell and 

settlement problems, poor permeability, and inadequate drainage can generally be overcome using wellestablished engineering techniques, but these techniques can be expensive, and they do not always 

avert continuing maintenance problems.

In the San Francisco Bay region, such problems 

are particularly severe on bay mud, as shown in the 

capability example and described in the section "Potential Problems in Flatlands Regions." Most land 

areas underlain by bay mud are subject to tidal 

action and must be (or have been) diked and filled 

prior to development. In many places, the demand for 

flat, buildable land, adjacent to existing urban areas 

has been strong enough to make diking and filling 

worth the cost. Commercial, industrial, and residential developments as well as major public facilities 

including two international airports and a naval air 

station have been built on fill over bay mud. It is 

possible to develop the land-fill on top of the bay mud, 

but the cost will be high.

Foster City is a privately developed new town 

started in 1963 on a diked and filled island in San 

Francisco Bay just south of the San Francisco International Airport (also built on filled land). Buildings 

require foundations of reinforced concrete with continuous footings or piles. In the mid-1960's founda-
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 77

TABLE 9. Regional planning implications of capability rating for urban residential development

Steps in Planning 

Process

Informational 

Requirements

Land Use 

Evaluation

Developmental 

Factors

Data collection 

and interpretation . Other natural data and economic, social, political, esthetic, 

and other relevant information 

should he collected.

Additional detailed information 

concerning characteristics of 

flatland deposits, particularly 

potential for agriculture, 

should be collected.

Plan formulation_ ________ Areas can be considered for ur- Evaluation of other natural and

ban development by process of 

elimination.

Project review A-95 review (Assoc. of Bay Area Governments 

or permit process (Bay Conservation and Development 

Commission) _________ Unless other factors indicate

problems, additional information concerning flatland deposits may not be needed.

Environmental Impact Statement and Environmental

Impact Report ________ Environmental problems not related to nature of flatland deposits likely to be more 

important.

economic, social, political, esthetic factors is needed to 

make initial judgments concerning land use. Agricultural 

uses should be considered.

Site investigations of soil and geology may be needed in reviewing projects.

Environmental assessment 

should consider constraints 

posed by flatland deposits.

Information detailing potential 

for open space or other nonintensive uses of such areas 

should be collected.

Areas can be considered for open 

space by process of elimination; presence of wildlife habitats, sensitive ecological 

systems, potential for waterrelated recreation may be important. Urban development 

should be recommended only if 

great public benefit can be 

shown.

Site investigations of soil and 

geologic conditions needed. 

Developer must provide evidence that site can be safely 

developed.

Detailed environmental assessment of flatland constraints 

should be provided and considered.

tion construction alone cost an average of 

$1.00-$1.50 per square foot of floor space more than it 

would have on firmer ground; the cost would be 

higher today (Ronald Campbell, oral commun., July 

20, 1975). An additional cost of approximately $100 

per house was required to provide utility service in 

the mid-1960's; the additional cost today (1975) is 

about $300 (Lee Ham, oral commun., July 25, 1975).

Land-preparation costs are also high. The land 

must be diked, drained, and filled to be usable. At 

Redwood Shores, a major community development 

on bay fill just south of Foster City, it costs an 

average of $8,000 per acre to dewater, demulch, and 

recondition the already-diked land prior to filling. 

The filling operation costs about $6,000 per acre per 

foot of fill to place, spread, and compact (Gene Mascarelli, oral commun., July 25, 1975).

Costs such as these can be readily justified by a 

land developer, because of the high market value of 

the prepared land. In Redwood Shores, filled residential land commands a price between $73,000 and 

$93,000 per acre. The costs of land preparation and 

construction are assumed by the private sector of the 

economy. But public costs may also be high, particularly for maintenance of roads, storm and sewer

pipelines, and other public improvements. Thus construction on bay mud involves a public as well as 

private economic commitment. This fiscal impact is 

an appropriate concern of land-use planning.

As discussed in the section on "Potential Problems 

in Flatlands Regions," risks from flooding, liquefaction, and seismic wave amplification are also high on 

lands underlain by bay mud. The ecologicalenvironmental impact from filling the bay for urban 

developments is also great. Yet in spite of these 

constraints, urban development on bay mud has 

taken and continues to take place. The San Francisco 

Bay Conservation and Development Commission 

Plan states:

As the Bay Area's population increases, pressure to fill the Bay 

for many purposes will increase. New flat land will be sought for 

many urban uses because most, if not all, of the flat land in 

communities bordering the Bay is already in use for residences, 

businesses, industries, airports, roadways, etc. Past diking and 

filling of tidelands and marshlands has already reduced the size of 

the Bay from about 680 square miles in area to little more than 400. 

Although some of this diked land remains, at least temporarily, as 

salt ponds or managed wetlands, it has nevertheless been removed 

from the tides of the Bay. (San Francisco Bay Conservation and 

Development Commission, 1969, p. 2).

The reduction in size of the bay by diking and
78 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

filling (fig. 53) became a major public issue during the 

1960's, because of the environmental, ecological, and 

climatological impacts of continued bay filling as 

well as growing concern over the safety of structures 

on bay fill particularly during an earthquake. 

Public efforts to stop indiscriminate filling of the

bay led to State adoption of the McAteer-Petrie Act of 

1965, creating the BCDC, (Bay Conservation and 

Development Commission). BCDC was authorized to 

prepare a comprehensive plan for the conservation of 

the bay and development of its shorelands and to 

recommend appropriate procedures and institutions

122°30'

37-30'

FIOURE 53. Reduction in area of natural tidal marsh around San Francisco Bay between 1850 and 1975. Location of 1850 shoreline from

Nichols and Wright (1971).
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 79

to carry out the plan. The San Francisco Bay Plan 

was completed in 1969 and adopted by the State 

Legislature (San Francisco Bay Conservation and 

Development Commission, 1969). BCDC became a 

permanent organization with responsibility to revise 

and implement the plan. The findings, policies, and 

regulatory procedures that are set forth in the San 

Francisco Bay Plan demonstrate how earth-science 

information was used to address problems of regional concern.

Although no formal capability analysis was undertaken in preparing the San Francisco Bay Plan, 

specific information concerning known natural constraints and opportunities for various land uses was 

sought and applied in the planning process. Twentythree separate technical reports on the physical, 

ecological, social, and economic aspects of the bay 

and its use were published during the planning 

period, including several related to the nature of 

flatland deposits. The plan addresses the issues of 

the environmental impact of bay fill and the safety of 

fills (San Francisco Bay Conservation and Development Commission, 1969, p. 1).

Major undesirable effects of fill were recognized 

(San Francisco Bay Conservation and Development 

Commission, 1969, p. 1-2) as: (1) loss or degradation 

of an important fish and wildlife habitat, (2) decrease 

in the ability of the bay to assimilate pollutants, (3) 

reduction of the moderating effect of the bay upon 

local climate and an increase in the danger of air 

pollution, and (4) diminished scenic beauty of the 

bay.

Despite these harmful impacts, some bay filling 

may be desirable or necessary if the benefits outweigh the disadvantages. A permit is required from 

BCDC for any filling or dredging of the bay and 

lands subject to tidal action. Permits may be issued 

for fills if one of the following four conditions is met:

(1) The filling is in accord with the Bay Plan policies as to the 

Bay-related purposes for which filling may be needed (i.e., ports, 

water-related industry, and water-related recreation) and is shown 

on the Bay Plan maps as likely to be needed; or (2) The filling is in 

accord with Bay Plan policies as to purposes for which some fill 

may be needed if there is no other alternative (i.e., airports, roads, 

and utility routes); or (3) The filling is in accord with the Bay Plan 

policies as to minor fills for improving shoreline appearance or 

public access; or (4) The filling would provide on privately-owned 

property for new public access to the Bay and for improvement of 

shoreline appearance in addition to what would be provided by 

the other Bay Plan policies and the filling would be for Bayoriented commercial recreation and Bay-oriented public assembly 

purposes. (San Francisco Bay Conservation and Development 

Commission, 1969, p. 36).

The question of safety of the fill must also be 

addressed before BCDC can issue a permit for filling.

The following section, adapted from San Francisco 

Bay Conservation and Development Commission 

(1969, p. 15,17), lists the findings and policies of the 

BCDC plan relevant to safety of fills.

Finding. Virtually all fills in San Francisco Bay 

are placed on top of bay mud, which presents many 

engineering problems. The construction of a sound 

fill depends in part on the stability of the base upon 

which it is placed. Safety of a fill also depends on the 

manner in which the filling is done and the materials 

used for the fill. Construction of a fill or building that 

will be safe enough for the intended use requires (1) 

recognition and investigation of all potential 

hazards including (a) settling of a fill or a building 

over a long period of time, and (b) ground failure 

caused by the manner of constructing the fill or by 

shaking during a major earthquake and (2) construction of the fill or building in a manner specifically designed to minimize these hazards. While the 

construction of buildings on fills overlying bay deposits involves a greater number of potential hazards 

than construction on rock or on dense hard soil 

deposits, adequate design measures can be taken to 

reduce the hazards to acceptable levels.

Policy. The bay agency should appoint a Fill 

Review Board consisting of geologists, civil 

engineers specializing in soils engineering, structural engineers, and architects competent to and adequately empowered to (a) establish and revise safety 

criteria for bay fills and structure thereon, (b) review 

all except minor projects for the adequacy of their 

specific safety provisions and make recommendations concerning these provisions, (c) prescribe an 

inspection system to assure placement of fill according to approved designs, and (d) gather and make 

available performance data developed from specific 

projects. These activities would complement the functions of local building departments and local planning departments, none of which is presently staffed 

to provide soils inspections.

Even if the bay plan indicates that a fill may be 

permissible, no fill or building should be constructed 

if hazards cannot be overcome adequately for the 

intended use in accordance with the criteria prescribed by the Fill Review Board.

To provide vitally needed information on the effects of earthquakes on all kinds of soils, installation 

of strong-motion seismographs should be required on 

all future major land fills. In addition, the Bay agency should encourage installation of strong-motion 

seismographs in other developments on problem 

soils, and in other areas recommended by the U.S. 

Geological Survey, for purposes of data comparison 

and evaluation.

Finding. Flood damage to fills and shore-line
80 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

areas can result from a combination of heavy rainfall, high tides, and winds blowing onshore. To prevent such damage, buildings near the shoreline 

should be above the highest expected flood mark (9 

feet above sea level is generally set as the safe mark 

except in the southern part of the south bay, where 

the higher tides require almost a foot more elevation) 

or should be protected by dikes of an adequate height.

Policy. To prevent damage from flooding, buildings on fill or near the shoreline should have adequate flood protection as determined by competent 

engineers. As a general rule, buildings near the 

shoreline should be at least 9 feet above mean sea 

level (standard USGS datum) or should be protected 

by dikes of an equivalent height and by any necessary pumping facilities. In the south half of the south 

bay, this height should be at least 10 feet. Exceptions 

to the general height rule may be made for development specifically designed to tolerate periodic flooding.

Finding. Excessive pumping from underground 

fresh-water reservoirs has caused extensive subsidence of the ground surface in the San Jose area and 

as far north as Dumbarton Bridge (Poland, 1971, 

shows subsidence from 1934 to 1967). Indications are 

that if heavy ground-water pumping is continued 

indefinitely in the south Bay area, land in the Alviso 

area (which has already subsided about 7 ft since 

1912) could subside up to 7 ft more; if this occurs, 

extensive dikes may be needed to prevent inundation 

of low-lying areas by the high tides.

Policy. To minimize the potential hazard to bayside development from subsidence due to groundwater withdrawal, all proposed developments at the 

lower end of the south bay should be sufficiently high 

above mean sea level or sufficiently protected by 

dikes to allow for the effects of additional subsidence, 

utilizing the latest information available from the 

U.S. Geological Survey.

A permit from BCDC is also required for development of the bay shoreline the area extending inland 

from the bay a maximum of 1,000 feet (305 m). The 

control over shoreline development is considered 

necessary to reduce pressures for bay filling and 

provide public access to the bay. Water-related uses 

such as ports, recreation, and wildlife preserves are 

given priority.

The BCDC plan and permit procedures provide a 

framework for the continuing use of geologic information in land-use planning and decisionmaking. 

The kind of information summarized in this report 

can be used directly by BCDC in revising its plan and 

in a State-sponsored study of Suisun Marsh. BCDC 

uses information on flatland deposits in its permit 

process to evaluate the safety of proposed fills. This

general information of geologic characteristics is 

used to help determine what site-specific information 

should be required from the applicant.

The Fill Review Board, as presently constituted, 

has the expertise to review and evaluate soils and 

geologic reports provided by an applicant. This activity is a critical factor in the agency's effective use of 

geologic information. Significant improvement in 

the seismic engineering of fills and design of structures has resulted from the board's insistence on a 

thorough evaluation of geologic hazards at a project 

site (San Francisco Bay Conservation and Development Commission, 1974, p. 8).

The availability of geologic information presented 

in a form understandable to nongeologists is considered very important to BCDC. Such information 

expands the understanding of both decisionmakers 

and project applicants and can improve the quality of 

project proposals and commission decisions.

THE URBAN-AGRICULTURAL CONFLICT

The generally high capability of many flatland 

areas for both agricultural and urban uses is a major 

land-use planning concern. In California, the process 

of urban growth is, for the most part, the process of 

converting land from agricultural to urban uses. 

Nearly 4.5 million acres of the State's most productive farmland have been converted to nonagricultural uses since 1947 (Dean, 1975, p. 18). The California 

OPR (Office of Planning and Research) (California 

Office of Planning and Research, 1974) states that 

over the last two decades, 15,000-20,000 acres of 

highly productive land per year have been converted 

to nonagricultural uses. Although OPR reports that 

from 1960-72 California had a net gain of 56,000 

acres of agricultural land per year, reversing the 

trend of the 1950's, the gain is in irrigated land, 

which commonly requires intensive use of energy 

and chemical fertilizers to produce high yields.

Planning future land uses to meet the land requirements of both agricultural and urban uses must be 

based on the best information available concerning 

land capability for both categories of use. As described in the section on "Land Capability Studies," 

information on flatland deposits is important in 

evaluating agricultural and urban land capability. 

The map of flatland deposits (pi. 1, 2, and 3) provides 

a generalized view of agricultural capability (see 

section on "Soil"). Information such as this can be 

used in combination with Soil Conservation Service 

soil ratings, if available, to identify lands meriting 

further investigation for agricultural potential.
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 81

Planning for the use of agricultural land requires a 

broad, even a global, perspective of future productive 

capacity and food needs. Within a national and State 

policy framework, regional planning can be particularly important in reconciling urban and agricultural 

demands for land. The Association of Bay Area 

Governments (1970) recommends a "city-centered" 

pattern of development accommodating new growth 

through in-filling and extension of existing urban 

areas and development of new communities. A major 

justification for this growth pattern is to preserve 

resources, particularly agricultural lands.

The need to preserve agricultural lands is more 

explicity expressed in ABAG's Open Space Plan, 

Phase II (Association of Bay Area Governments, 

1972). This plan includes lands with agricultural 

capability in the category, "open space for managed 

resource production" (Association of Bay Area Governments, 1972). ABAG's draft report, Areas of Critical Environmental Concern (Association of Bay 

Area Governments, 1975), spells out more specifically the "regional interest" in agricultural lands. The 

report sets forth the following policies concerning 

agricultural lands:

(1) Preserve agricultural lands which serve the following 

functions:

(a) Production of a unique or specialty crop, a high percentage of which is grown in the region (e.g., wine 

grapes, brussels sprouts);

(b) Production of crops and commodities which, in order to 

realize their productive value, must be produced in 

locations proximate to urban areas (e.g., dairy products, cut flowers).

(2) Protect all agricultural lands through the support of public 

service policies which prevent the premature conversion 

of such lands to urban uses. (Association of Bay Area 

Governments, 1975, p. 11).

Specific criteria, based on evidence of the capability of land for various agricultural uses, were developed by ABAG to identify "critical areas." Lands 

meeting these criteria are regionally significant 

areas of critical environmental concern. Local plans 

and projects are to be reviewed by ABAG for consistency with the policies and criteria. ABAG also 

expects local projects and plans to be reviewed for 

consistency with policies and criteria regarding 

areas of critical environmental concern as part of 

required environmental impact statements or reports. As stated by ABAG:

An Environmental Impact Statement or Report, if it is to be 

considered adequate by ABAG, should follow a two-step process:

1. It should assess first of all whether a proposed plan or 

project falls within the purview of the Critical Area policies;

2. It should make a finding as to whether such activity is 

consistent with the policies and criteria contained in this 

document. (Association of Bay Area Governments, 1975, 

83 p.)

In addition to project review, ABAG recommends 

preserving agricultural lands through local installment purchase and saleback or zoning (Association 

of Bay Area Governments, 1972). Installment purchase and saleback has not been used to any extent, 

but local zoning has been effectively used. For example, most of the productive agricultural land of the 

Napa Valley has been zoned "Agricultural Preserve" 

(AP) since 1968 (Napa County, 1968). Minimum lot 

size for new parcels within the 25,000-acre "preserve" 

is 20 acres. This zoning, permitting extensive agriculture and processing in addition to grazing and cultivation, clearly establishes agriculture as the primary 

use of the valley.

The AP zone was challenged in the courts by the 

Napa Valley United Farmers but was upheld by the 

Superior Court of the County of Napa, in a decision 

on February 17, 1971, as a proper exercise of police 

powers which benefited, not only the public at large, 

but also those whose land was regulated (Overview 

Corp., 1973, p. 20).

PLANNING FOR HAZARDOUS AREAS

Information on flatlands deposits can help identify 

areas posing risk from natural hazards. The degree of 

risk depends on the severity, pervasiveness, and 

frequency of hazardous events together with the 

land-use intensity, building type, and occupancy of 

the hazardous areas. Decisions regarding risk involve balancing the public and private benefits from 

reducing risk against the costs. The threshold point 

beyond which no further public action is considered 

necessary or worthwhile to reduce risk is often referred to as the "acceptable level of risk". The California Council on Intergovernmental Relations defines 

risk levels as follows:

Acceptable Risk: The level of risk below which no specific action 

by local government is deemed necessary, other than making the 

risk known.

Unacceptable Risk: Level of risk above which specific action by 

government is deemed necessary to protect life and property.

Avoidable Risk: Risk not necessary to take because the individual or public goals can be achieved at the same or less total 'cost' by 

other means without taking the risk. (California Council of Intergovernmental Relations, 1973, p. IV-26).

Individual judgments concerning acceptable, unacceptable, and avoidable risks are highly subjective 

judgments at the community level and are made 

through the political process. These judgments can 

be improved if information relating the possible 

losses from the hazards to the cost of reducing them is 

provided and widely disseminated.

The Urban Geology Master Plan (California Division of Mines and Geology, 1973) attempts to place 

risk from natural hazards in perspective by project-
82 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

ing losses from major hazards from 1970-2000 assuming the present level of risk mitigation. The study 

estimates both the costs of hazard mitigation and the 

benefits that could be achieved if all feasible methods 

of hazard mitigation were employed. This provides 

the framework for assigning a benefit/cost ratio for 

risk reduction efforts. Table 10 summarizes the results for hazards present in many flatland areas.

The benefit/cost approach can help a public agency assign priorities for the use of funds for risk 

reduction. A thorough understanding of the nature of 

the hazard is necessary to evaluate risk and risk 

reduction measures.

SEISMIC HAZARDS A SPECIAL PROBLEM

A major cause of structural damage from earthquakes is ground shaking. The severity of damage 

from ground shaking depends on both the nature and 

design of the structure and on the geologic materials 

that underlie it. A structure resting on bedrock may 

be relatively little damaged by ground motion, while 

another of similar design and construction but located on thick, water-saturated unconsolidated deposits 

may be heavily damaged.

In seismically active areas, the need to plan urban 

facilities to minimize risk is extremely important. In 

recent years, new information on the causes and 

nature of earthquakes and their effects has led to 

efforts to mitigate those effects where possible. In 

California, a seismic safety element is now a required 

part of the general plan for all cities and counties. 

Development along active faults is controlled under 

provisions of the Alquist-Priolo Special Studies 

Zones Act (California State Legislature Public Resources Code, Sec. 2621, 2625,1972 as amended 1975), 

and plans for disaster preparedness are now being 

developed for all seismically active areas of the State. 

Consequently, many planning agencies need information of the effects of earthquakes, and if this 

information is available, its use is assured. Seismic 

risk maps are one means of predicting earthquake 

effects in a form useful to land-use planners. Such 

risk maps divide a planning area into zones according to the relative degree of potential damage to 

structures and are commonly related to an earthquake of stated magnitude (called the "design earthquake" .

Knowledge of the properties and thickness of flatland deposits is critical in evaluating the potential 

risk from ground shaking, whether this risk results 

from seismic wave amplification or from liquefaction 

potential. The planner, working closely with a seismic specialist, can then relate land uses, building, 

and occupancy types to the risk zones. Guidance in 

this task is provided by the California Joint Committee for Seismic Safety (January 1974). Table 11 describes acceptable risk in terms of the kinds of structures and their occupancy.

At the regional level seismic risk studies are useful 

in formulating regional growth policies. Careful 

study of high-risk areas will indicate the cost of 

hazard mitigation and may reveal areas that are well 

suited for low-intensity recreational and open-space 

uses.

Many areas underlain by unstable bay muds, for 

example are marshes that provide important wildlife 

habitats. Areas with open-space potential together 

with risk to urban uses can be important elements of 

regional open-space plans and preservation efforts.

SANTA CLARA COUNTY BAYLANDS STUDY

Santa Clara County prepared a subarea plan for 

the baylands within its jurisdiction which takes into 

account the hazards seismic and nonseismic 

associated with flatland deposits. Consultants' studies of geologic and structural engineering problems 

were used to identify the natural hazards of the 

planning area and to describe their implications for 

specific land uses. The resulting report divided the 

planning area into risk zones (the basis of potential 

for settlement and ground failure, under both seismic 

and nonseismic conditions. Table 12 lists the risk 

zones and the nature of the hazard in each. Figure 54 

is a map of the risk zones. Table 13 relates land and 

building uses to the risk zones.

TABLE 10. Projected losses due to geologic problems in California, 1970-2000

[Adapted from California Division of Mines and Geology, 1973, p. 4]

Geologic problem

Earthquake shaking

Loss of mineral resources _____ 

Flooding

Erosion

Expansive soils

Subsidence

Projected losses 

1970-2000

$21 billion

17 billion 

6.5 billion

600 million

150 million

26 million

Possible loss 

reduction

$10.5 billion

15 billion 

3.4 billion

400 million

149 million

13 million

Cost of loss 

reduction

$2 billion

90 million 

2.7 billion

250 million

7.5 million

9 million

Benefit/cost 

ratio

5

167 

1.3

1.5

20

1.5
IMPORTANCE OF FLATLAND GEOLOGY TO COMPREHENSIVE PLANNING 83

TABLE 11. A scale of acceptable risks

Level of acceptable risk Kinds of structures

Extra project 

cost probably

required to

reduce risk to

an acceptable

level

Extremely low1 Structures whose continued func- No set pertiqning is critical, or whose 

failure might be catastrophic: 

nuclear reactors, large dams, 

power intertie systems, plants 

manufacturing or storing explosives or toxic materials.

centage 

(whatever 

is required for 

maximum attainable 

safety).

Slightly higher

than under

level I 1 Structures whose use is critically 5-25 perneeded after a disaster: im- centofproportant utility centers; ject cost-, 

hospitals; fire, police, and 

emergency communication 

facilities; fire stations; and 

critical transportation elements 

such as bridges and overpasses; also smaller dams.

Lowest possible 

risk to occupants of the

structure11 Structures of high occupancy, 

or whose use after a disaster 

would be particularly convenient: schools, churches, 

theaters, large hotels, and 

high-rise buildings housing 

large numbers of people, other 

places normally attracting 

large concentrations of people, 

civic buildings such as fire 

stations, secondary utility 

structures, extremely large 

commercial enterprises, most 

roads, alternative or noncritical bridges and overpasses.

An "ordinary"

level of risk to

occupants of

the structure:V5 -The vast majority of structures: 

most commercial and industrial buildings small hotels 

and apartment buildings, and 

and single family residences.

5-15 percent of 

project 

cost4 .

1-2 percent 

of project 

cost, in 

most 

cases(2- 10 percent of 

project 

cost in a 

minority 

of cases). 4

cost

th

Caliornia pra. oeover, ae aona cos presumes a e srucures

in this acceptable-risk category are to embody sufficient safety to remain functional after

an earthquake.

1 Failure of a single structure would affect primarily only the occupants.

IJi evciiuiig Liijuiy u

remain functional.

" "Ordinary risk": Resist minor earthquakes without damage; resist moderate earthquakes without structural damage, but with some non-structural damage; resist major 

earthquakes of the intensity or severity of the strongest experienced in California, 

without collapse, but with some structural as well as nonstructural damage. In most 

structures, it is expected that structural damage, even in a major earthquake, could be 

limited to repairable damage (Danehy, 1969).

TABLE 12. Risk zones for settlement and ground failure

[Established by subsurface conditions in the baylands of Santa Clara County. Adapted 

from Woodward-Clyde and Assoc. and McClure and Messinger (1970), p. 10.]

Risk zone Surface effect Subsurface cause

A Little risk of settlement or 

ground failure

B DL Significant settlement

Cs Moderate to substantial 

settlement and(or) differential settlement

DD Substantial settlement

and(or) differential settle 

ment

DSL Failure of ground surface

DLS Failure of ground surface

Liquefaction of confined 

granular layer in alluvium (seismic loading).

Consolidated of bay mud 

or soft clay (static unloading).

Consolidation of uncon- 

 trolled dump fill or sanitary land fill (static 

loading).

Liquefaction of granular 

surface layer (seismic 

loading).

Lateral spreading toward 

free face (seismic 

loading).

The plan adopts these uses with the stipulation 

that any developer in the baylands provide data from 

test boring and sample testing in depth to demonstrate that a proposed development site is not in a 

higher risk zone than shown. Establishing an Advisory Review Board was recommended to advise public agencies on the adequacy of engineering investigations, design, and construction methods in the 

baylands.

On the basis of the plan, the county adopted an 

ordinance requiring a soils report for all major subdivisions unless specifically exempted. Geologic reports and site investigations are required for all 

subdivisions on or adjacent to potentially hazardous 

areas as depicted on official county hazard maps. 

The map of risk zones for land-use planning (fig. 54) 

is one of the official hazard maps. Geologic reports 

are normally required for development in risk zones 

C and D and may be required in risk zones A and B.

A key feature of the successful integration of geologic considerations into plan development and administration in the Santa Clara County Baylands 

program is the use of the appropriate expertise at 

each stage of the planning process. In the baylands 

study, an engineering geologist gathered available 

geologic data, and together with a structural 

engineer, divided the study area into zones on the 

basis of the expected surface effects of geologic conditions. They then worked cooperatively to determine 

the range of land uses and building types that could 

be accommodated in each zone with reasonable safety on the basis of:

(1) The possible types of geologic risks such as settlement, liquefaction, lateral spreading, ground shaking, tsunamis and fault rup-
Mountain View Slough

Stevens Creek

EXPLANATION

Based on possibility

of deep liquefaction

Based on possibility of

lateral spreading

Based on probability

Based on presence of

uncontrolled dump fill Risk zone D

Based on possibility of

shallow liquefaction

Limit of study area

NOTE:

The delineation of risk zones is 

based on limited subsurface information. Risk zone designations 

could change as more detailed subsurface information becomes available.

FIGURE 54. Risk zones for land-use planning, Santa Clara County baylands. From Woodward-Clyde and Associates and McClure and Messinger (197
SUMMARY 85

TABLE 13. Land and building uses for various risk zones

[Adapted from Santa Clara County Planning Policy Committee, 1972, p. 22]

Land and building uses Risk zones 

(see fig. 54)

Group A buildings: 

Hospital and nursing homes 

Auditoriums and theaters Schools 

Transportation and airportsPublic and private offices 

Major Utility 

Other building uses 

C D

Group B buildings: 

Residential-multiple units 

Residential, 1 and 2 family - 

Small Commercial - 

Small public - 

Small schools, one story - Utilities -

Group C buildings: 

"Industrial park" commercial 

Light and heavy industry - 

Small public, if mandatory 

Airport maintenance 

Group D buildings: 

Water-oriented industry 

Wharves and docks 

Warehouses -

Group D open space:

Agriculture, marinas, public and private open

spaces, marshlands and saltponds, and small

appurtenant buildings

A B

X - 

X

X -

X -

X -

X -

X -

X X - - 

X X - - 

X X - - 

XX

X X - - 

X X - -

XXX 

XXX 

XXX 

XXX

XXX 

XXX 

XXX

X X X X

ture; (2) The ability to identify land areas where these types of 

geologic risks are possible; (3) The ability to assign either qualitative or quantitive limits on the possible effects of these risks; (4) The 

types of land and building uses and their socio-economic importance; (5!The behavior of buildings and other improvements under 

static and seismic conditions; (6) The ability of "normal" practices 

of investigation, design, construction, inspection, and enforcement to develop recommendations and procedures to provide 

adequate levels of protection; (7) The ability to develop and implement special investigation, design, construction, inspection, and 

enforcement procedures where normal procedures are considered 

inadequate. (Santa Clara County Planning Policy Committee, 

1972, p. 27).

Using the interpretations and recommendations of 

the engineering geologist and structural engineer, 

plus other relevant information, the planner developed land-use policies and recommendations and 

defined procedures for obtaining more detailed geologic investigations, if needed, at the time of an 

application for a zoning change, land division, grading permit or building permit. After adoption of the 

policies and procedures, a geologist was added to the 

County staff to assist planners and engineers in 

administering the policies and review procedures.

The relation between planning phase and the use 

of geologic expertise in the Santa Clara County 

Baylands study provides a model applicable to other

planning areas with known or suspected geologic 

hazards. However, the sequence as described assumes the availability of basic geologic data. The 

process of incorporating geologic considerations into 

land-use planning starts with basic geologic maps or 

more specialized maps such as the map of flatland 

deposits and continues through successive refinements of the basic geologic data and interpretations 

to relate geologic conditions to particular land uses 

and structural types.

SUMMARY

Geologic information about flatland areas is of 

particular importance in land-use planning because 

most urban development occurs in relatively flat 

terrain. Geologic studies can help identify potential 

problems such as flooding, stream-channel changes, 

salt-water intrusion, subsidence, settlement, shrink 

and swell, and various earthquake-induced hazards. 

In addition, geologic studies can locate many natural 

resources including ground-water recharge and storage areas, agricultural soils, and deposits of sand, 

salt, gravel, and clay. A planning jurisdiction can 

then direct future growth to reduce risk from natural 

hazards, avoid excessive development costs, and 

preserve essential resources.

The maps of flatland deposits included with this 

report (pis. 1, 2, and 3) illustrate an innovative approach to geologic mapping. Unconsolidated alluvial 

deposits occurring in areas with slopes of less than 15 

percent are differentiated largely on the basis of 

composition and age. These characteristics are important in determining the engineering properties of 

the deposits, particularly the potential for seismic 

wave amplification and liquefaction. With the aid of 

existing soils data, topographic maps, aerial photographs, and limited fieldwork, geologists completed 

the mapping of flatland deposits of the entire 7,281 

square mile (19,316 km2 ) San Francisco Bay region in 

only three years.

The maps were prepared specifically for use in 

land-use planning. Each map unit is described in 

terms of its geologic characteristics, engineering 

properties, resource potential, and any special problems related to the safety and cost of land development.

In the past, planners have used modern soils series 

maps prepared primarily by the U.S. Soil Conservation Service to identify the problems and resources 

discussed here. Where available, these maps are an 

excellent source of information, particularly to evaluate land capability for agriculture, erosion potential,
86 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

and tendencies of the soil to corrode or expand. 

However, the approach used here adds a new dimension to the usual soils data by considering greater 

depth of deposits and the processes by which they 

formed. This added dimension is especially important in geologically active areas such as the west 

coast, but it can also be useful elsewhere in the 

country. The geologic approach to mapping flatland 

surficial deposits is essential in seismically active 

areas in order to evaluate ground shaking and the 

possibility of ground failure. In addition, geologic 

information is needed to assess potential for groundwater storage and recharge, and problems of settlement and subsidence. These are major concerns 

throughout the country.

Since formative processes are emphasized, geologic description of alluvial deposits explicitly includes 

geologic-hydrologic relations. These relations are 

important in understanding the direction and nature 

of long-term changes in the natural environment 

that can affect the use of land.

The map of flatland deposits, at a scale of 

1:125,000, provides a regional overview of flatland 

characteristics. Areawide information at this scale 

provides an excellent framework for developing a 

data-collection program at the regional and local 

levels. It allows a planning agency to focus on those 

areas with identified problems or resource potential, 

and those areas where land uses are most likely to 

change. The public and private benefits of specific 

identification of resource and hazard areas before 

land development occurs are substantial because it is 

almost always easier and less expensive to address 

site problems before development than to try to 

correct them afterward.

As discussed in the sections on planning approaches to flatland problems and resources, geologic information is important throughout the planning 

process. Information on flatland geology, along with 

data on hillside geology and other earth-science and 

environmental information, is needed to:

1. Evaluate the relative physical capability of 

land to accommodate proposed uses;

2. Develop land-use policies and plans at the 

regional and local levels responsive to the 

physical problems and potentials of the land;

3. Define appropriate soils and geologic investigations needed for project design and review;

4. Establish procedures to review soils and geologic reports, environmental impact assessments, and other information submitted for 

project review;

5. Prepare and administer land-use and development regulations implementing adopted policies and plans.

A qualified earth scientist, on or available to, the 

planning staff to assist in interpreting and applying 

geologic data is essential. Wisely used, geologic information can lead to safer, less costly, and more environmentally sensitive use of our valuable, but limited, land resources.

REFERENCES CITED

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plan for the San Francisco Bay region: Berkeley, California, 

52 p.

___ 1970, Regional Plan 1970:1900: Berkeley, California, 32 p.

___ 1972, Regional open space plan, phase II: Berkeley, California, 15 p.

___ 1975, Areas of critical environmental concern, report to the

Regional Planning Committee: Berkeley, California, 83 p.

Atwater, B. F., Hedel, C. W., Helley, E. J., 1976, Late Quaternary 

depositional history, Holocene sea-level changes, and vertical 

crusted movement, southern San Francisco Bay, California: 

U.S. Geol. Survey. Prof. Paper 1014, 15 p.

Bailey, E. H., and Harden, D. R., 1975, Mineral Resources of the 

San Francisco Bay region, California Present availability 

and planning for the future: U.S. Geol. Survey open-file report, 

182 p.

Bishop, D. G., 1974, The Dunedin earthquake, 9 April 1974, Part 2 

local effects: New Zealand Natl. Socl. Earthquake Eng. Bull., 

v. 7, no. 3, p. 123-129.

Bonilla, M. G., 1965, Geologic map of the San Francisco South 

quadrangle, California: U.S. Geol. Survey open-file map, scale 

1:24,000.

Borcherdt, R. D., Gibbs, J. F., and Lajoie, K. R., 1975, Prediction of 

maximum earthquake intensities in the southern San Francisco Bay region, California for large earthquakes on the San 

Andreas and Hayward faults: U.S. Geol. Survey Misc. Field 

Studies Map MF-709, Scale 1:125,000.

Borcherdt, R. D., Joyner, W. B., Warrick, R. E., and Gibbs, J. F., 

1975, Response of local geologic units to ground shaking, in 

Borcherdt, R. D., editor, Studies for seismic zonation of the San 

Francisco Bay region: U.S. Geol. Survey Prof. Paper 941-A, p. 

A52-A67.

Brewer, James, 1973, Bay land sinks, tenants moved: San Francisco Chronicle, p. 1., Sept. 14 issue.

Burnett, J. L., and Barneyback, R. S., Jr., 1975, Sand and gravel, in 

Bailey, E. H., and Harden, D. R., Mineral resources of the San 

Francisco Bay region, California Present availability and 

planning for the future: U.S. Geol. Survey open-file report, 

182 p.

California Council on Intergovernmental Relations, 1973, General 

plan guidelines: Sacramento, CA., (noncontinuous paging).

California Division of Mines and Geology, 1966, Geology of northern California: California Div. Mines and Geology Bull. 190, 

507 p.

___ 1973, Urban geology master plan for California: California 

Div. Mines and Geology Bull. 198, 112 p.

California Office of Planning and Research, 1974, Prime agricultural lands report: Sacramento, Calif., not paged.

California State Legislature, 1965, McAteer-Petris act, government code, title 7.2, sections 66600-66660, 12 p.

___ 1972, Alquist-Priolo special studies zones act, as amended 

1975: Public resources code, sec. 2621-2625.
REFERENCES CITED 87

Danehy, E. A., editor, 1969, Urban environmental geology in the

San Francisco Bay region: Assoc. Engineering Geologists, San

Francisco Section, Spec. Pub., 162 p. 

Dean, J. B., 1975, A panacea that wasn't: Cry California: San

Francisco, California, California Tomorrow, v. 10, no. 3, p.

18-23. 

Gibbs, J. F., and Borcherdt, R. D., 1974, Effects of local geology on

ground motion in the San Francisco Bay region [abs.J: Geol.

Soc. America Abs. with Programs, v. 6, no. 3, p. 284. 

Gilbert, G. K., 1971, Hydraulic-mining debris in the Sierra Nevada:

U.S. Geol. Survey Prof. Paper 105, 154 p. 

Goldman, H. B., 1969, Geology of San Francisco Bay, in Geologic

and engineering aspects of San Francisco Bay fill: California

Div. Mines and Geology Spec. Rept. 97, p. 11-27.

Hall, N. T., 1966, Late Cenozoic stratigraphy between Mussel Rock 

and Fleishhacker Zoo: Mineral Information Service, v. 19, no. 

11, p. 522-525.

Hart, E. W., 1975, Limestone and shells, in Bailey, E.H., and 

Horden, D.R., Mineral resources of the San Francisco Bay 

region, California present availability and planning for the 

future: U.S. Geol. Survey open-file report, 182 p.

Hazelwood, R. M., 1974, Preliminary report of seismic refraction 

survey along the east side of the San Francisco Bay, Alameda 

County, California: U.S. Geol. Survey open-file report, 10 p.

Higgins, C. G., 1952, The lower course of the Russian River, 

California: California Univ. Pubs, in Geol. Sci., v. 29, no. 5, 

83 p.

Joint Committee on Seismic Safety, 1974, Meeting the earthquake 

challenge, part 1, a comprehensive approach to seismic safety: 

Sacramento, California, California State Legislature, 32 p.

Laird, R.T., Perkins, J.B., Bainbridge, D.A., Baker, J.B., Boyd, 

R.T., Huntsman, Daniel, Zucker, M.B., and Staub, Paul, 1979, 

Quantitative land capability analysis: U.S. Geol. Survey Professional Paper 945, 115 p.

Lajoie, K. R., and Helley, E. J., 1975, Differentiation of 

Sedimentary Deposits for Purposes of Seismic Zonation, in 

Borcherdt, R. D., ed., Studies for seismic zonation of the San 

Francisco Bay region: U.S. Geol. Survey Prof. Paper 941-A, p. 

A39-A51.

Lee, C. H., and Praszker, Michael, 1969, Bay mud developments 

and related structural foundations, in Geologic and engineering aspects of San Francisco Bay fill: California Div. Mines 

and Geology Spec. Rept. 97, p. 41-87.

Limerinos, J. T., Lee, K. W., and Lugo, P. E., 1973, Flood-prone 

areas in the San Francisco Bay region, California: U.S. Geol. 

Survey open-file report, p. 37-73, scale 1:125,000.

McCulloch, D. S., Peterson, D. H., Carlson, P. R., and Conomos, T.

J., 1970, A preliminary study of the effects of water circulation

in the San Francisco Bay estuary: U.S. Geol. Survey Circ.

637-A, B, 35 p. 

Medvedev, S. V., 1965, Engineering seismology: U.S. Geol. Survey

rept., 256 p.; available from the Nat. Tech. Inf. Service, U.S.

Dept. Commerce, Springfield, Va., NTIS no. TT65-50011.

Meehan, R. L., Dukes, M. T., and Shires, P. O., 1975, A case history

of expansive claystone damage: Am. Soc. Civil Engineers

Proc., Jour. Geotech. Engineering Div., v. 101, no. Gt9, p.

933-948. 

Metcalf, Woodridge, 1959, Native trees of the San Francisco Bay

region, California: Berkeley, University of California Press,

Natural History Guides, no. 4, 70 p. 

Metropolitan Transportation Commission, 1975, Metropolitan

Transportation Commission, 5 years of progress 1970-1975:

Berkeley, Calif., 16 p. 

Milliman, S. D., and Emery, K. O., 1968, Sea levels during the past

35,000 years: Science, v. 162, p. 1121.

Napa County, 1968, Ordinance, no. 274 amending zoning ordinance no. 186: Napa, Calif.

Nichols, D. R., and Wright, N. A., 1971, Preliminary map of 

historic margins of marshlands, San Francisco Bay, California: U.S. Geol. Survey open-file map, scale 1:125,000.

Overview Corp., 1973, How to implement open space plans for the 

San Francisco Bay area: Berkeley, Calif., 172 p.

Oxburgh, E. R., 1974, The plain man's guide to plate tectonics: 

Geologists' Assoc. Proc. (Colchester, England), v. 85, pt. 3, p. 

299-358.

Page, J. M., and Wire, J. C., 1969, How geology serves the water 

needs of an instant metropolis northern Santa Clara Valley, 

in Urban environmental geology in the San Francisco Bay 

region: Assoc. Eng. Geologists, San Francisco Sec., Spec. Pub., 

p. 105-124.

Pestrong, Raymond, 1972, San Francisco Bay tidelands: California Geology, v. 25, no. 2, p. 27-40.

Poland, J. F., 1969, Land subsidence in the western United States: 

Office of Emergency Preparedness, Geologic Hazards and 

Problems Section, Proc., p. 77-96.

___1971, Land subsidence in the Santa Clara Valley, Alameda, 

San Mateo and Santa Clara Counties, California: U.S. Geol. 

Survey Misc. Field Studies Map, MF-336, scale 1:125,000.

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and properties of concrete and concrete-making materials: Soc. 

Testing and Materials, Spec. Tech. Pub., 169-A, p. 404-414.

Rantz, S. E., 1971, Precipitation depth-duration-frequency relations for the San Francisco Bay region, California, in Geological Survey research 1971: U.S. Geol. Survey Prof. Paper 750-C, 

p. C237-C241.

San Francisco Bay Conservation and Development Commission, 

1969, San Francisco Bay plan: San Francisco, California, 41 p., 

and maps.

___1974, Safety of fills: guidelines for bay plan revision, Bay 

Plan Evaluation Project: San Francisco, California, 23 p.

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soils and geology reports: San Jose, California, lip.

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plan for the baylands of Santa Clara County, Santa Clara 

County Planning Dept.: San Jose, California, 76 p.

Sarna-Wojcicki, A. M., 1971, Correlation of late Cenozoic pyroclastic deposits in the central Coast Ranges of California: California Univ. Berkeley, Ph.D. thesis, 170 p.

Schlocker, Julius, 1971, Generalized geologic map of the San 

Francisco Bay region: U.S. Geol. Survey open-file map, scale 

1:500,000.

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on filled lands in the San Francisco Bay, in Geologic and 

engineering aspects of San Francisco Bay fill: California Div. 

Mines and Geology Spec. Rept. 97, p. 103-115.

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Mineral resources of the San Francisco Bay region, 

California present availability and planning for the future: 

U.S. Geol. Survey open-file report, 182 p.

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infiltration and ground-surface recession in Santa Clara Valley, Santa Clara County, Calif.: Am. Geophys. Union Trans. 

pt. 1, p. 23-35.

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the San Francisco Bay counties, in Geologic guidebook of the 

San Francisco Bay counties: California Div. Mines and Geology Bull., 154, p. 235-238.
88 FLATLAND DEPOSITS OF THE SAN FRANCISCO BAY REGION, CALIFORNIA

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law 91-190, 5 p. 

___ 1972, Federal water pollution control act amendments of

1972, section 208, public law 92-500, 89 p. 

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part 600: Federal Register, v. 40, no. 164, p. 36856-36865.

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part of San Francisco Bay where a high-water table may 

adversely affect land use: U.S. Geol. Survey Misc. Field Studies

Map MF-530. 

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Francisco, in The California earthquake of April 18, 1906,

Report of the State Earthquake Commission: Carnegie Inst.

Washington Pub. 87, p. 220-245. 

Woodward-Clyde and Associates, and McClure and Messinger,

1970, Geology and structural engineering, parts I and II,

prepared for Planning Policy Committee Santa Clara County,

Oakland, Calif., 112 p. (pub. rept.) 

Youd, T. L., Nichols, D. R., Helley, E. J., and Lajoie, K. R., 1973,

Liquefaction potential of unconsolidated sediments in the

southern San Francisco Bay region, California: U.S. Geol.

Survey open-file report, 23 p. 

___ 1975, Liquefaction potential, in Studies for seismic zonation

of the San Francisco Bay region: U.S. Geol. Survey Prof. Paper

941-A, p. A68-A74.
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

PROFESSIONAL PAPER 943 

PLATE 1

123 45' 

38"52'30"

500

123COO'

122°37'30"

1650000 FEET R.12W. I «85 38°52'30"

;Vrvv..^

w^'-Tr^W'-ix,'-

t-J . v»- .>- - i

Stewards Points

t T - *ermo*| B<i!/

\ Horseshoe Ppinit^i^V,

\ -^.1 x Timber CW«\ .

^K^A£/C:&& ,

CORRELATION OF MAP UNITS

ESTUARINE MARINE UNDIFFERENTIATED 

DEPOSITS DEPOSITS DEPOSITS

 ham I Qhaf 1 Qhaft ' Qhbm j

\ 'M iC 1

QUATERNARY \

6 \ \ \ -' 

\ Uhjv ofXXaP 

Marine Biology Keseaiih Station'^

J COUNTY PfKK'

" ' < '

1.8 million years 

before present

TERTIARY 

PRE-TERTIARY

DESCRIPTION OF MAP UNITS

STREAM CHANNELS - Open alluvial channels with movable beds, mapped where scale

permits 

COARSE-GRAINED ALLUVIUM - Unconsolidated, moderately sorted permeable sand

and silt with coarse sand and gravel; more abundant toward fan heads 

MEDIUM-GRAINED ALLUVIUM - Unconsolidated moderately sorted moderately sorted

permeable fine sand, silt, and clayey silt with a few thin beds of coarse sand

FINE-GRAINED ALLUVIUM - Unconsolidated plastic moderately to poorly sorted carbonaceous silt and clay

FINE-GRAINED SALT - AFFECTED ALLUVIUM - Unconsolidated plastic moderately to

poorly sorted carbonaceous silt and clay. Irregularly bedded with carbonate nodules

BAY MUD - Unconsolidated water-saturated dark plastic carbonaceous clay and silty clay

BEACH AND DUNE SAND DEPOSITS - Loose well-sorted fine- to medium-grained sand

a LATE PLEISTOCENE ALLUVIUM - Weakly consolidated slightly weathered poorly sorted 

irregularly interbedded clay, silt, sand, and gravel

PLEISTOCENE BEACH AND DUNE SAND DEPOSITS (MERRIT SAND) - Loose wellsorted fine'ttf medium sand 

EARLY PLEISTOCENE ALLUVIUM - Moderately consolidated deeply weathered poorly

sorted irregularly interbedded clay, silt, sand, and gravel

MARINE TERRACE DEPOSITS - Weakly consolidated slightly weathered sand and gravel

Qpmc COLMA FORMATION - Pale, loose or friable well-sorted fine- to medium-grained sandstone with subordinate gravel, sandstone, siltstone, and claystone

UNDIVIDED QUATERNARY DEPOSITS - Not recognizable as discrete map units because

original form obliterated by urbanization 

UNDIVIDED BEDROCK - Older than Pleistocene

HOLLAND _KELS&«ILLE_ _LOWE ",- a£*» '

VfaLEY OIMIA

^v;^^

«"\r i " 

VPITTSBURG- /IO VIST

T 1953 A 1953 J

Index map of the San Francisco Bay region, 

showing the ten Bay Area counties and the 

names of topographic map sheets

HAVWARD UVERMORE- _ ALTAMDN

. 1947 59 1953*1 . CE 1937

.OMOND. -iUOS GATOS1939 ^*HJS-« .»

COUNTY KEY 

A. MARIN 

B SONOMA 

C. NAPA 

O. SOLANO 

E. CONTRA COSTA 

F. ALAMEDA 

G. SANTA CLARA 

H. SANTA CRUZ 

I. SAN MATED 

J. SAN FRANCISCO

Faults shown on these maps depict offsets of young alluvial deposits. This 

interrelation is based on the geologic mapping techniques described in the 

text. Faults are mapped on the basis of anomalies in alluvial deposits that are 

inconsistent with surficial processes, fault-related geomorphic features, 

seismic activity, or surface creep. In some instances faulting older than the 

deposits may have provided a basin of deposition and hence give an apparent 

offset of the young alluvial deposits. Fault lines are dashed where evidence is 

not clear or urbanization has obliterated fault features

SI ATE pA&C-'Jt 

53 \Nv-j if-

«50 ' 1 550 000 FEE1

37 C 52'30"

123M5

37°52'30"

Base from U.S. Geological Survey, San

Francisco Bay Region, Sheet 1, 1970

50,000-foot grid based on California coordinate system, zone 2

5000-meter Universal Transverse Mercator grid ticks, zone 10

1231° 00'

500

SCALE 1:125000 

4 10 MILES

' Interior - Geological Survey, Reston, VA. - 1977 - G772SO |530000rlE- 122°37'30"

Geology by D. B. Burke, E. J. Helley, K. R. LaJoie, 

J. C. Tinsley, and G. E. Weber, 1972 - 74

2 1 10 KILOMETERS

CONTOUR INTERVAL 200 FEET 

DATUM IS MEAN SEA LEVEL

GEOLOGIC MAP OF THE FLATLAND DEPOSITS OF THE NORTHWESTERN PART OF THE SAN FRANCISCO BAY REGION

This map was printed from negatives prepared by photographic 

color separation of the author's hand-colored original.
38°52'30

UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

122°37'30"

n. / vj

'- <,\i- X x>

PROFESSIONAL PAPER 943 

PLATE 2

121/30'

38°52'30"

12 N

2 100 000 FEET I R. 3 E.

3 W. 11950000 FEET 1575 R.21W. I 585 122fOO' R. 1 W. | 590

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Geology by D. B. Burke, E. J. Helley, K. R. LaJoie, 

J. C. Tinsley, and G. E. Weber, 1972 - 74

1O KILOMETERS

CONTOUR INTERVAL 200 FEET 

DATUM IS MEAN SEA LEVEL

GEOLOGIC MAP OF THE FLATLAND DEPOSITS OF THE NORTHEASTERN PART OF THE SAN FRANCISCO BAY REGION

This map was printed from negatives prepared by photographic 

color separation of the author's hand-colored original.
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

1 OO 0 '37''an"

37'52'SO'' ,- __U!3r:;E: . I^000°O.^T J340

PROFESSIONAL PAPER 943 

PLATE 3

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Geology by D. B. Burke, E. J. Helley, K. R. LaJoie, 

J. C. Tinsley, and G. E. Weber, 1972 - 74

tO KILOMETERS

CONTOUR INTERVAL 200 FEET 

DATUM IS MEAN SEA LEVEL

GEOLOGIC MAP OF THE FLATLAND DEPOSITS OF THE SOUTHERN PART OF THE SAN FRANCISCO BAY REGION

This map was printed from negatives prepared by photographic 

color separation of the author's hand-colored original.