Geology of the San Francisco 

North Qu^lrangle, California

GEOLOGICAL SURVEY PROFESSIONAL PAPER 782

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GEOLOGY OF THE SAN FRANCISCO 

NORTH QUADRANGLE, CALIFORNIA
1

San Francisco Bay area aerial view east, taken November 22,1967. The Golden Gate channel is at bottom center; San Francisco

at bottom right. San Pablo Bay at left center.
Geology of the San Francisco 

North Quadrangle, California

By JULIUS SCHLOCKER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 782

The distribution and character of the bedrock and surficial deposits in 

the northern part of the City of San Francisco and southern Marin 

County, Calif., including a description of the Franciscan Formation 

in its type area and notes on engineering geology in an urban area

;s

Vr, 'S

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1974
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 73-600283

* U.S. GOVERNMENT PRINTING OFFICE: 1974 O 534-039

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

Washington, D.C. 20402 - Price $4.80 (paper cover)

Stock Number 2401-02490
CONTENTS

Page

Abstract ........................................................................................ 1

Introduction ................................................................................ 2

Purpose ................................................................................ 2

Location and topography .................................................. 3

Climate ................................................................................ 5

Early settlement ................................................................ 6

Previous work...................................................................... 7

Acknowledgments ............................................................... 8

Fieldwork ............................................................................ 8

Methods of mapping .......................................................... 8

Stratigraphy ................................................................................ 9

Bedrock ................................................................................ 9

Franciscan Formation................................................ 9

Clastic sedimentary rocks ................................ 11

Sandstone .................................................... 11

Matrix .................................................. 13

Detrital grains .................................... 14

Shale ............................................................ 18

Conglomerate .............................................. 20

Metamorphosed sedimentary rocks.......... 20

Occurrence .................................................. 23

Northeastern San Francisco.............. 23

Central highlands of San Francisco.. 24

Cliff House to Bakers Beach.............. 24

Fort Point to Potrero Hill.................. 25

Marin Peninsula ................................ 25

Angel and Alcatraz Islands and........ 26

Tiburon Peninsula .......................... 26

Origin .......................................................... 26

Environment of deposition ................ 26

Source area of detritus...................... 27

Age ................................................................ 27

Fossils .................................................. 27

Significance of potassium feldspar

content ...............................~............. 28

Greenstone ......................................... .............. 29

Megascopic features .................................. 29

Mineralogy-microscopic features ............ 32

Weathering and hydrothermal alteration 33

Occurrence .................................................. 33

Marin Peninsula ................................ 33

San Francisco ...................................... 34

Angel Island and vicinity.......................35

Origin ............................................................ 36

Radiolarian chert and shale.............................. 36

Megascopic features.................................... 37

Chemical composition ................................ 41

Microscopic features .................................. 43

Chert 43

Shale .................................................... 45

Hydrothermal alteration .......................... 45

Weathering ...................... ......................... 46

Origin ............................................................ 47

Origin of silica .................................... 47

Origin of chert...................................... 47

Origin of shale..................................... 48

Page

Stratigraphy Continued 

Bed rock Continued

Franciscan Formation Continued

Radiolarian chert and shale Continued 

O rigin Continued

Origin of color....................................... 48

Age ................................................................ 48

Occurrence .................................................. 49

Marin Peninsula .................. ............. 49

Angel Island and vicinity ................... 49

San Francisco ...................................... 49

Metamorphic rocks ............................................ 50

Metamorphic rocks on serpentine

borders ..............................................51

Parent rock types ................................ 51

Metamorphic rocks within serpentine 

bodies ........................................................ 53

Metamorphic rocks distance from known 

serpentine bodies ...... .............................. 53

Tectonic setting of metamorphic rocks.... 55

Hydrothermal alteration and weathering 55 

Sheared rocks .............................................................. 55

Occurrence ............................................_............. 56

Serpentine .................................................................... 56

Megascopic features ........................................... 56

Mineralogy and petrography..... ..................... 58

Bastite ........................................................... 58

Relict olivine and orthopyroxene.............. 58

Blue color of serpentine....... ..................... 59

Antigoritic serpentine ................................ 59

Veins in serpentine....................................... 60

Sheared serpentine .................................... 60

Chemical composition ......................................... 61

Weathering and hydrothermal alteration...... 62

Shape of serpentine bodies................................ 63

Origin .................................................................... 64

Age ........................................................................ 65

Pyroxenite ............................................................ 65

Gabbro ....................................................... .................. 65

Surficial deposits ................................................................ 66

Colma Formation ........................................................ 66

Megascopic features .......................................... 67

Composition and physical properties............... 69

Weathering .......................................................... 69

Occurrence ........................................................... 69

Origin .................................... ...... . 71

Correlation with nearby deposits....................... 73

Age relation to the ancient Sacramento River 73 

Related deposits .................................................. 74

Older beach deposits ................................................... 74

Age ........................................................................ 75

Modern beach deposits .................................. ......... 76

Source of sand on Ocean Beach......................... 77

Dune sand ..................................................................... 78

Origin and age....................................................... 80
VI CONTENTS

Page

Stratigraphy Continued

Surficial deposits Continued

Slope debris and ravine fill........................................ 80

San Francisco Bay sediments.................................... 81

Transverse sand bars.....- -... ... .- .-..-. ...... 82

Bay mud and clay................................................ 83

Engineering properties of bay mud

and clay .................................................... 84

Stratigraphic correlation, fossils and age 84 

Alluvium ...................................................................... 85

Page

Stratigraphy Continued

Surficial deposits Continued

Landslide deposits ........................................,.... 86

Characteristics of landslides.............................. 87

Artificial fill ................................................................ 88

Structure ...................................................................................... 90

Seismicity .................................................................................... 93

Engineering geology .................................................................. 94

References .................. .......... .........-.....--.....-.. .................... 100

Index................................................................................... 107

ILLUSTRATIONS

[Plates are in pocket]

FRONTISPIECE. Aerial view of San Francisco Bay area

PLATE 1. Geologic map of the San Francisco North quadrangle.

2. Composition and grain size of surficial deposits, San Francisco North quadrangle.

3. Bedrock-surface and landslide locality map of the San Francisco North quadrangle.

Page

FIGURE 1. Index maps of part of central California and the San Francisco Bay area .............................................................. 4

2-5. Photographs of:

2. Angel Island, Raccoon Strait, and Tiburon Peninsula..................................................................................... 5

3. Marin Peninsula and part of Sausalito.............................................................................................................. 5

4. Belvedere Island and Tiburon Peninsula........................................................................................................... 5

5. Mount Caroline Livermore and southwestern slope of Angel Island............................................................ 5

6. Map showing distribution of Franciscan Formation, related rocks, principal structural features of western

California .................................................................................................................................................................... 10

7-12. Photographs of:

7. Horseshoe Bay and vicinity, southern Marin Peninsula..--- ...... .................. ..................................... 11

8. Shale and thin-bedded sandstone of the Franciscan Formation showing graded and distorted bedding

and laminae. Russian Hill, San Francisco................................................................................................ 12

9. Jointed thick-bedded sandstone and interbedded 3-foot-thick section of shale and thin-bedded

sandstone, Franciscan Formation, Telegraph Hill, San Francisco ........................................................ 12

10. Sheared and shattered sandstone and shale of the Franciscan Formation. North of Golden Gate

Bridge, Marin Peninsula....................-..... -. ..................................................................................... 13

11. Sheared shale of the Franciscan Formation containing large sandstone masses. Near Sausalito,

Marin Peninsula.................................... ...... ..- ........ .. ............................................................. 13

12. Shale fragments in coarse-grained graywacke sandstone of the Franciscan Formation. Laguna

Honda Reservoir area, San Francisca........................................................................................................... 14

13. Triangular diagram showing classification of sandstone samples of the Franciscan Formation......................... 14

14. Map showing locations of sandstone samples of the Franciscan Formation and potassium feldspar content.... 16 

15-61. Photographs of:

15. Semischistose coarse-grained graywacke sandstone of the Franciscan Formation. Campbell Point,

Angel Island................................................................................................................................................... 20

16. Massive sandstone of the Franciscan Formation. Telegraph Hill, San Francisco .................................... 23

17. Thick-bedded sandstone, laminated sandstone, siltstone, and shale of the Franciscan Formation,

near Bakers Beach, San Francisco........................................................................................................... 24

18. Thick-bedded sandstone interbedded with shale and thin-bedded sandstone of the Franciscan

Formation. North of Golden Gate Bridge, Marin Peninsula.................................................................. 25

19. High cliff in greenstone in the Franciscan Formation. Lime Point, Marin Peninsula.............................. 29

20. Greenstone and radiolarian chert of the Franciscan Formation. Lime Point, Marin Peninsula............ 29

21. Close random fracturing in greenstone of the Franciscan Formation. Near Sausalito,

Marin Peninsula..........................................-............................................................................... ................ 30

22. Basalt pillows. North of Golden Gate Bridge, Marin Peninsula.................................................................... 31

23. Pillow basalt of the Franciscan Formation. Fort Baker, Marin Peninsula ................................................... 32

24. Radiolarian chert lying on basaltic greenstone, Franciscan Formation. Near Sausalito,

Marin Peninsula ....................................................... .................... ......... ......... ........................_...:.... 34

25. Internal structure of radiolarian chert bed of the Franciscan Formation. Roadcut on Sausalito

lateral, Marin County ..... ............... . .............................................................................................. 38

26. Radiolarian chert and shale of the Franciscan Formation, Twin Peaks, San Francisco.......................... 39
CONTENTS VII

FIGURES 27-61. Photographs of Continued

Page

27. Pinching and swelling in radiolarian chert beds of the Franciscan Formation. Roadcut on Sausalito

lateral, Marin Peninsula............................................................................................................................... 39

28. Chevron folds in radiolarian chert of the Franciscan Formation. Near Golden Gate Bridge,

Marin Peninsula ...............................................................................................................................................39

29. Radiolarian chert containing possible drag folding at Lime Point, Marin Peninsula.......... ..................... 40

30. Massive chert in the Franciscan Formation, Sunset Heights, San Francisco ............................................ 42

31. Quartz veins in radiolarian chert lying on altered greenstone. Twin Peaks, San Francisco.................... 42

32. Irregular band of hydrothermally altered radiolarian chert of the Franciscan Formation,

Mount Sutro, San Francisco ....................................................................................................................... 46

33. Thin soil developed on radiolarian chert and shale. North of the Golden Gate Bridge,

Marin Peninsula ........................................................................................................................................... 46

34. Franciscan Formation exposed on the north shore of Golden Gate, Marin Peninsula .............................. 49

35. Central highlands area of San Francisco showing peaks of radiolarian chert on the Francis'can

Formation....................................................................................................................................................... 50

36. Radiolarian chert and sandstone in the Franciscan Formation, 17th Street, Mount Olympus area,

San Francisco................................................................................................................................................. 50

37. Radiolarian chert and shale, Sunset Heights, San Francisco ........................................................................ 50

38. Serpentine body and metamorphic rocks, Angel Island .................................................................................. 52

39. Shoreline along Bakers Beach, the Presidio, Fort Point, and vicinity, San Francisco.............................. 56

40. Serpentine in a landslide. Fort Point, San Francisco ...................................................................................... 57

41. Serpentine nodule in sheared serpentine, Presidio, San Francisco .............................................................. 57

42. Clinochrysotile veinlets in basaltic serpentine. Fort Point, San Francisco.................................................. 61

43. Sheared serpentine containing large augens of altered and sheared gabbro-diabase. Potrero Hill,

San Francisco ............................................................................................................................................... 63

44. Serpentine and sandstone. Potrero Hill, San Francisco.................................................................................. 63

45. Colma Formation overlying greenstone of the Franciscan Formation. Sutro Reservoir, San Francisco 68

46. Rubble deposits and crossbedding in the Colma Formation. Twin Peaks, San Francisco........................ 68

47. Colma Formation. Angel Island.................. .... . ......... .... . ..... . .. ........ ........... ......... 70

48. Colma Formation filling channel in greenstone. Twin Peaks, San Francisco............................................ 71

49. Raised older beach deposits covered by sand dune. Presidio, San Francisco .............................................. 75

50. Ocean Beach ........................................................................................................................................................... 76

51. Berm development on Ocean Beach................................................................................................................... 76

52. Dune sand exposed in excavation for Brooks Hall. Civic Center, San Francisco...................................... 78

53. Sand dunes. Sunset District, San Francisco..................................................................................................... 79

54. Unstable embankment of dune sand on lee side of Sunset Heights, San Francisco.................................... 79

55. Sand dune, Aquatic Park area, San Francisco ................................................................................................ 79

56. Slope debris on weathered greenstone of the Franciscan Formation............. ............................................ 81

57. Bay mud, showing desiccation cracks, lying under artificial fill, San Francisco ........................................ 84

58. Alluvium lying on Colma Formation. Twin Peaks, San Francisco ................... .......................................... 85

59. Steeply cut slopes in serpentine, Potrero Hill, San Francisco.................................................................... 86

60. Landslide. South Bay of Golden Gate, San Francisco..................................................................................... 87

61. Landslide. One mile northwest of Lime Point, Marin Peninsula .................................................................. 88

62. Map showing shoreline of San Francisco in 1853, present shoreline, and areas formerly covered by water

that are now artificially filled present shoreline................................................................................................... 89

63. Generalized geologic map of parts of the San Francisco North and South quadrangles showing shear zones 91

64. Photographs of differential settlement of buildings in former Mission Swamp. Near Sixth and Folsom

Streets, San Francisco ............................................................................................................................................... 95

TABLES

F-?e

TABLE 1. Modal composition of sandstone samples of the Franciscan Formation ................................................................ 14

2. Potassium feldspar in sandstone of the Franciscan Formation ................................................................................ 17

3. Frequency of heavy minerals in sandstone of the Franciscan Formation................................................................ 18

4. Analyses of sandstone, shale, and a phosphate nodule from the Franciscan Formation............ ....................... 19

5. Chemical composition of sandstone samples of the Franciscan Formation from Angel Island............................ 22

6. Analyses of greenstone samples of the Franciscan Formation ................................................................................ 35

7. Analyses of radiolarian chert and shale samples of the Franciscan Formation...................................................... 43

8. Partial mineral composition and bulk density of serpentine ........................................................................................ 59

9. Optical properties of olivine and orthopyroxene and suggested composition .......................................................... 59

10. Approximate semiquantitative spectrochemical analyses of serpentine and nontronite veins in serpentine.... 62

11. Generalized description of engineering properties of map units................................................................................ 96
GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE, CALIFORNFA

By JULIUS SCHLOCKER

ABSTRACT

The San Francisco North quadrangle, about 58 square miles 

in area, includes the north half of the city of San Francisco, the 

south tip of Marin and Tiburon Peninsulas, Belvedere, Angel, 

and Alcatraz Islands, and part of Treasure Island. The topography of San Francisco proper is characterized by rolling hills, 

rounded slopes, and broad valleys, whereas other parts of the 

quadrangle are characterized by steeper and more rugged 

slopes, rounded ridges and spurs, and steep-sided V-shaped 

canyons. San Francisco Bay covers nearly half the quadrangle. It is connected to the Pacific Ocean by the Golden Gate 

channel. Water depths in the tidal channels generally range 

from 100 to more than 360 feet below mean sea level; elsewhere water depths are generally less than 30 feet.

Two distinct groups of rocks, bedrock and surficial deposits, 

differ greatly in age, lithology, and topographic expressions. 

The bedrock consists of the Franciscan Formation and associated intrusive serpentine and gabbro-diabase of Jurassic (?) 

and Cretaceous age. The Franciscan Formation is at least 

16,000 feet thick and consists mostly of graywacke sandstone, 

shale and siltstone, and mafic volcanic rocks, and lesser 

amounts of radiolarian chert, conglomerate, limestone, and 

glaucophane schist. Exposures of the Franciscan constitute 15- 

20 percent of the land area of the quadrangle, generally on 

hills. The surficial deposits, of Quaternary age, consist largely 

of unconsolidated dune sand and water-laid sand, mud, and 

clay. However, slopewash deposits, alluvium, landslide debris, 

and artificial fill are fairly extensive. Above sea level, surficial 

deposits mantle slopes and fill valleys to depths of a few feet 

to more than 100 feet. In San Francisco Bay, surficial deposits 

range in thickness from 100 to about 300 feet.

The major rock types of the Franciscan Formation in this 

quadrangle which are mappable at a scale of 1:24,000 are 

clastic sedimentary rocks, greenstone, radiolarian chert, metamorphic rocks, and sheared rocks.

The clastic sedimentary rocks of the Franciscan Formation 

consist predominantly of massive graywacke sandstone beds 

separated by sequences of shale and thin-bedded sandstone. 

Conglomerate lenses and zones, as much as 10 feet thick, are 

common on Angel and Belvedere Islands and on Tiburon 

Peninsula. Pillow lava and radiolarian chert are interbedded 

with the clastic rocks. Metamorphic equivalents of these rock 

types are present on Tiburon Peninsula and Angel Island. 

Although the clastic rocks of the Franciscan Formation consist mostly of alternating massive and thin beds, two exceptionally thick sections of predominantly thin-bedded shale and 

sandstone, each totalling more than 1,000 feet, crop out in the 

northeastern part of San Francisco.

The clastic rocks of the Franciscan Formation probably 

were deposited by turbidity currents in deep water in an 

orogenically active eugeosyncline lying west of a continental 

mass. A source area including metamorphic rocks is indicated 

by the clasts in the sandstones and conglomerates. Rocks whose

composition is similar to the sand-size detritus in Franciscan 

sandstone are exposed in the Coast Ranges, Klamath Mountains, and Sierra Nevada. The pebbles in some conglomerates 

are very similar to sandstones and shales of the Franciscan, 

indicating that the conglomerates may have been derived from 

topographically high areas created by orogeny during Franciscan time.

The term "greenstone" is used to include all volcanic and 

related intrusive rocks in the Franciscan Formation. Tl-ese 

consist mostly of fine- to medium-grained metabasalts, in Irrge 

part altered to pumpellyite-albite-chlorite rock. Most basalts 

show pillow structure; a few are pyroclastic. Pillow structure, 

the presence of marine chert and limestone between pillows, 

and the interbedding of flows and marine sediments throughout the formation suggest a long history of submarine volcanism.

Thin chert and shale beds and less common massive chert 

bodies are closely associated with the greenstones. Radiolaria 

are common in the chert, less common in the shale. The larger 

deposits of radiolarian chert are closely associated with greenstone, which is believed to be the ultimate source of the silica 

from which the chert was formed.

Metamorphic rocks include slate, phyllonite, phyllite, f neto coarse-grained schist, and granofels, commonly having relict 

textures. Many masses of metamorphic rocks occur along the 

borders of serpentine bodies or as tectonic inclusions from 

diapiric movement in the serpentine. They are believed to be 

metamorphic equivalents of Franciscan rocks and may have 

been formed under high shearing stresses or by hydrothermal 

alteration.

The map unit termed "sheared rocks" consists of a soft 

intensely sheared matrix of shale or serpentine that has me "ed 

plastically. It encloses hard blocks, as much as hundred^ of 

feet in diameter, of various rock types found elsewhere in the 

Franciscan Formation. The blocks are rounded and slicl ensided externally, but most are relatively unsheared internally. 

Blocks greater than 20 feet in diameter are generally shattered 

internally.

The map unit designated serpentine also includes a few 

small bodies of calc-silicates (rodingites), pyroxenite, and 

gabbro. Serpentine intrudes the Franciscan Formation and is 

widely associated with it. The serpentine was derived from 

peridotite, and its present position is probably due to tectonic 

movement at relatively high pressures and low temperatures. 

Shearing is widespread within the serpentine bodies.

Unconsolidated late Pleistocene and Holocene surfcial 

deposits cover about 80-85 percent of the land area of the 

quadrangle. These deposits are mostly sand, but they include 

rubbly slope debris and ravine fill, bay mud and clay, landslide deposits, and artificial fill.

The Colma Formation is the oldest of the surficial deposits. 

It includes a group of marine and dune sands that have grossly
GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

similar physical properties and that occupy approximately the 

same stratigraphic position; however, the formation probably 

includes several marine deposits related to different sea levels. 

It also contains alluvial and colluvial deposits. The Colma 

Formation generally is horizontally bedded. The formation 

lies stratigraphically below latest Pleistocene and Holocene 

deposits.

A raised beach sand, probably deposited during a warm 

interglacial epoch when sea level was higher than it is now, 

has been exposed on the Presidio Military Reservation. This 

deposit was mapped separately from the Colma Formation 

because it is less cemented than the Colma Formation and its 

grains appear to be less weathered.

Surficial deposits mapped as alluvium are very restricted in 

extent and distribution, largely because of low surface runoff 

due to low rainfall and a cover of highly permeable material 

over large areas. Most of the alluvium in the quadrangle is 

related to old drainage systems and is now moderately dissected. The alluvium is composed of medium sand mixed with 

silt and clay.

Beach sand and coarser beach deposits occur along all 

shores except along the east edge of San Francisco and at 

Sausalito. Beach deposits vary seasonally even hourly in 

thickness and extent, depending on the nature of the waves 

and the supply of sediments. Along the Pacific shore of San 

Francisco, the probable sources of the beach sand and the 

related onshore dunes are the poorly consolidated Pliocene 

and Pleistocene Merced Formation, the younger formations 

along the shore to the south, and the sands of the continental 

shelf. The sands of the continental shelf probably were 

deposited by the ancestral Sacramento-San Joaquin River, 

during the Wisconsin Glaciation, when sea level was lower.

Dune sand, swept by prevailing westerly winds, underlies 

more than half the city of San Francisco and extends as far 

east as the area between Telegraph and Rincon Hills. It has 

been deposited more than 600 feet above sea level and attains 

a thickness of about 150 feet. The chief source of dune sand 

was the Ocean Beach area.

Slope debris derived largely from weathered rock mantles 

most slopes and fills the adjoining ravines. The slope debris 

as mapped locally includes soils developed on bedrock and 

minor amounts of alluvial, eolian, and landslide materials. The 

debris thickens downhill and locally is as much as 60 feet 

thick. The ravine-fill deposits, locally, are interbedded with 

and grade into alluvium.

Bay mud, the youngest deposit in San Francisco Bay, consists of soft unconsolidated sediment generally containing more 

than 90 percent of clay- and silt-size detritus. A similar 

deposit, bay clay, is nearly the same in texture but is more 

consolidated and contains less water. Bay mud and clay are 

found on the eastern shores of San Francisco and Sausalito, 

under surficial deposits and artificial fill adjoining the Bay, 

and under San Francisco Bay.

Landslides are among the chief agents of erosion in the 

quadrangle. Sheared rocks and serpentine are the most susceptible materials. The numerous landslides have resulted 

from combinations of hilly terrain, unconsolidated surficial 

deposits or sheared bedrock, abundant highly plastic and 

swelling clay, occasional periods of prolonged rainfall, frequent earthquakes, and disturbance and alteration of the 

terrain by man.

Artificial fill is extensive along the margins of San Francisco 

Bay, more than 3 square miles of land having been created 

by dumping artificial fill on the gently shelving tidal flats. The

average thickness of the fill in San Francisco, north of China 

Basin, is about 10 feet; south of China Basin it is about 60 

feet. The fill consists of dune sand, alluvium, debrh from the 

bay, and manmade debris. Serious engineering problems in 

areas of artificial fill have been caused by differential subsidence.

The San Francisco North quadrangle lies in tl <? western 

part of the Coast Ranges near the west edge of the North 

American crustal plate. The oldest exposed rocks in this part 

of the plate are those of the Franciscan Formation and its age 

correlative, rocks of the Great Valley sequence. The plate is 

bounded on the west by the San Andreas fault zone and farther south on San Francisco Peninsula by the Pilarcitos fault 

(both faults outside the map area), which separate the Franciscan Formation from granitic and associated metamorphic 

rocks. The Hayward fault zone is nearly parallel to and about 

20 miles east of the San Andreas fault zone in this area. East 

of the Hayward fault, the Coast Ranges consist mostly of 

Cenozoic marine and nonmarine sedimentary rocks snd marine 

sedimentary rocks of the Mesozoic Great Valley sequence. 

Movement along the San Andreas and Hayward faults is right 

lateral and has been responsible for the great eartl nuakes in 

this area.

The major faults and shear zones within the quadrangle are 

the northwest-trending Fort Point-Hunters Point ard the City 

College shear zones. The major folds are a syrcline that 

plunges northwest between Russian and Telegraph Hills, an 

anticline on Marin Peninsula whose axis lies in Pichardson 

Bay, and a broad northwest-plunging syncline on Angel Island 

and Tiburon Peninsula.

The engineering properties of the natural foundation materials in the quadrangle vary greatly over short distances; so, 

some building sites are in part on soft bay mud and in part on 

hard bedrock. Almost every site on the Franciscan Formation 

encompasses shear zones less than 1 inch to several feet wide. 

Unconsolidated surficial deposits, which generally contrast 

greatly in engineering properties from bedrock, c-»ver more 

than half the land area of the quadrangle. The location of the 

bedrock surface is therefore important in engineering work. 

As each construction site presents a unique combinaton of 

natural conditions, such as slope, hydrology, and foundation 

materials, detailed geologic investigations and slope stability 

analyses are needed in the design of specific structures.

The San Francisco North quadrangle is in a region of high 

seismic activity. Structures have been seriously damaged by 

three major earthquakes since 1865.

INTRODUCTION 

PURPOSE

San Francisco is a heavily built up city in a region 

of active faults and is subject to earthquakes and landslides. In such an area careful selection of building 

sites and foundations is imperative. This geologic 

investigation of the San Francisco North quadrangle 

was carried out to obtain basic geologic data to aid 

in construction and other civil engineering problems. 

The study also contributes to a better understanding 

of the geology of this part of the Coast Ranges in 

general and of the Franciscan Formation and surficial 

deposits in particular.
INTRODUCTION

On the geologic map (pi. 1), consolidated rocks are 

delineated by their composition, age, and origin. Such 

a classification also gives an approximate indication 

of the engineering properties of each formation. To 

make the map more useful to the civil engineer, unconsolidated deposits are shown where thick enough 

to be depicted. This report and the accompanying 

map should be a useful guide for planning foundation 

studies necessary in engineering design.

LOCATION AND TOPOGRAPHY

The San Francisco North quadrangle includes the 

north half of the city of San Francisco, part of San 

Francisco Bay, the south tip of Marin and Tiburon 

Peninsulas, Angel Island, Belvedere Island, Alcatraz 

Island, and part of Treasure Island (fig. 1).

San Francisco Bay covers approximately half of the 

quadrangle. The bay is connected to the Pacific Ocean 

by the Golden Gate channel, a deep narrow waterway 

between the San Francisco Peninsula on the south and 

Marin Peninsula on the north. The Golden Gate channel and much of the bay are drowned parts of the valleys of the Pleistocene Sacramento-San Joaquin River 

and its tributaries, and the Santa Clara Valley. Water 

depths in the quadrangle vary from shallows of less 

than 60 feet located south of Rincon Point, north of 

Treasure Island, southwest of Angel Island, in Richardson Bay, and along a narrow shoal between Alcatraz 

Island and Fort Point to deeps of more than 300 feet in 

the Golden Gate channel. The deepest point, approximately 360 feet below sea level, is in the bottom of a 

hole near the middle of the Golden Gate Bridge (Carlson and others, 1970).

The present topography of the San Francisco North 

quadrangle has resulted principally from erosion of a 

lithologically complex terrain and from deposition of 

sand dunes. Marine and estuarine processes and late 

Quaternary tectonism also have affected the topographic evolution of the quadrangle. In general, the 

city area consists of bedrock hills surrounded by broad 

valleys underlain by dune sand and other unconsolidated deposits. Hills along the north (Golden Gate) 

border of the city reach heights of approximately 375 

feet above sea level above Point Lobos, in the Presidio, 

and at Lafayette Square park; heights of approximately 325 feet are attained on Russian Hill, and 

heights of 275 feet on Telegraph Hill at the northeast 

edge of the city. Hills on the north border vary in relief 

from about 150 feet in the Richmond District on the 

west to 200-225 feet in Russian and Telegraph Hills on 

the east. Potrero Hill, in the southeast, has a relief of 

about 225 feet. The central and western parts of the 

city rise to the south border of the quadrangle to a

small group of hills including Sunset Heights, more 

than 750 feet in altitude, Mount Sutro, 908 feet in 

altitude, and South Twin Peak, 922 feet in altitude. 

The central hills of Mt. Sutro and Twin Peaks have a 

relief of about 850 feet on the east and about 500 feet 

on the west. Approximately one-third of the city area, 

the central and western parts, is more than 200 feet 

above sea level.

The topography varies from gently rounded hills 

with broad basins between to the sheer sea cliffs that 

border the Golden Gate. The rolling terrain of Golden 

Gate Park and the moderate slopes in the central part 

of the city in part are modifications by the deposition 

of tremendous quantities of dune sand.

The strong northwesterly trend of ridges and valleys 

that characterizes most of the Coast Ranges is obscured in the city itself, although it is suggested by 

such minor features as Russian and Telegraph Hills 

and the valley between them, by Hayes Valley between 

the hills at Lafayette and Alamo Square parks, and by 

Potrero Hill.

The gradient of abandoned channels filled with old 

alluvium in the Twin Peaks area appears to be much 

more gentle than that of present-day surface runoff 

channels. Evidently, Twin Peaks formed by dissection 

of a gently rolling surface.

The shoreline of San Francisco Peninsula has a 

varied topographic form. The west shore along the open 

sea is alternately low and high, marked by sandy 

beaches, such as Ocean and Bakers Beaches, and irregular bedrock cliffs such as those of the Point LobosLands End area and the Fort Point area. Except for 

Black Point, the north and east shores are low and 

mostly made land. Prior to filling, high cliffs were 

present along the shore at Telegraph Hill and at Rincon Hill.

The topography of Marin Peninsula, Belvedere 

Island, Tiburon Peninsula, and Angel Island is generally steeper and more rugged than that of San Francisco, and the shorelines are generally high bedrock 

cliffs (fig. 2). Rounded ridges and spurs and steepsided V-shaped canyons characterize these areas, 

although a few of the larger canyons widen near their 

mouths into flat-floored debris-filled valleys. Marin 

Peninsula has an average relief of 800-900 feet. The 

highest point in the quadrangle 1,125 feet above 

mean sea level is on a ridge half a mile west of Sausalito(fig.3).

The typical northwesterly structural and topographic trend of the northern California Coast Ranges 

is well shown by Belvedere Island and Tiburon Peninsula (fig. 4) and by Alcatraz Island, Richardson Bay, 

and Corinthian Island. This trend can also be found in
GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

123" 122"

0 5 10 15 20 25 MILES

FIGURE 1. Part of central California and the San Francisco Bay area showing the San Francisco North quadrangle and

major structural and physiographic features.
INTRODUCTION

FIGURE 2. Angel Island, Raccoon Strait, and Tiburon Peninsula, viewed northwest. Prominent point in middle distance 

is Stuart Point, Angel Island. White building at right 

bottom is the lighthouse at Knox Point. Both points are 

greenstone of the Franciscan Formation.

FIGURE 3. Marin Peninsula and part of Sausalito. Highest 

point in the quadrangle, with an altitude of 1,125 feet, is the 

broad peak right of center. View northwest.

FIGURE 4. Belvedere Island and Tiburon Peninsula, view 

north. The island is elongated northwestward along the 

regional structural trend. Cliffs above the shore of Belvedere 

Island expose Franciscan sandstone to right of trees at the 

shore, Franciscan greenstone to left.

the ridges of Marin Peninsula, although somewhat 

modified by the erosional features associated with a 

large transverse stream valley whose mouth is drowned 

by Rodeo Lagoon.

Angel Island is the upper part of a drowned mountain cut off from Tiburon Peninsula by Raccoon Strait, 

a former channel of the Sacramento River. The rounded 

summit, 781-foot Mount Caroline Livermore, and the 

broad radial spurs are modified by sharp ridge crests 

and by steep shorelines (fig. 5).

FIGURE 5. Mount Caroline Livermore and southwestern slope 

of Angel Island. Viewed toward the northeast. Serpentine 

and metamorphic rocks are exposed in cliffs above the shore.

CLIMATE

The influence of the Pacific Ocean keeps the temperature moderate and the air breezy, especially in the 

vicinity of the Golden Gate channel and in those parts 

of the quadrangle not separated from the ocean by 

high hills. The following climatic data were obtained 

from the U.S. Weather Bureau in July 1968: The 

average annual rainfall (1931-60) in San Francisco 

(Civic Center) is 20.78 inches, falling mostly from 

November through March. About 10 days per month 

in this period are rainy. Maximum precipitation measured at the Civic Center in 1 hour is 1.07 inches (for 

the period 1889-1950); maximum for 24 hours is 3.65 

inches. Drought conditions exist during the summer. 

Snow is exceedingly rare. The mean annual number of 

days of frost in the quadrangle is zero. Mean relative 

humidity for 4 a.m., noon, and 4 p.m. is 85, 67, and 71 

percent respectively. Annual mean temperature (1931- 

60) is 56.8°F. Highest temperature recorded is 101°F; 

lowest is 27°F. Highest monthly mean temperatures 

are 59.4°F, 62°F, and 61.4°F for August, September, 

and October, respectively. Highest wind velocity recorded at the San Francisco Civic Center is 51 miles 

per hour, for Mount Tamalpais, 120 m.p.h., and for the 

San Francisco International Airport, 62 m.p.h. (from
GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

the southwest) . During the rainy season, highest winds 

are southeast through southwest at 53 62 m.p.h. with 

gusts to 74 m.p.h. The average storm lasts 3 days.

The summer climate is greatly influenced by the 

constant presence of an anticyclonic high-pressure airmass west of the quadrangle, associated with the cold 

California Current and a zone of even colder upwelling 

water in the Pacific Ocean. Summer air temperatures 

along the ocean shore are among the coldest within the 

United States (excluding Alaska), though maximum 

temperatures 60 miles inland are among the hottest in 

the United States outside the Sonora-Mojave Desert 

(Patton, 1956, p. 113). In the summer high relative 

humidity, precipitation, and thunderstorms are exceedingly rare. Summer fog, from late afternoon through 

the night to late forenoon, is common in many parts of 

the quadrangle, especially in western San Francisco, 

Sausalito, and places reached by marine air in the 

vicinity of the Golden Gate channel. Winter fog of an 

inland origin is common also; at times it covers much 

of San Francisco Bay and connected bays and rivers 

into the Sacramento-San Joaquin Valley.

EARLY SETTLEMENT

When first discovered by Europeans in 1769, the 

area was inhabited by two Indian tribes, the Costanoan 

on San Francisco Peninsula and the Coast Miwok on 

Marin and Tiburon Peninsulas (Heizer, 1951, p. 40). 

The Indians used a variety of natural materials, such 

as seashells, rocks, and plants, for making tools and 

ornaments. They subsisted on plants and especially on 

marine life, as is shown by the contents of their refuse 

mounds ("kitchen middens") which are numerous on 

the shores of the bay and somewhat less common on 

the ocean shore (Nelson, 1909, map 1; Kroeber, 1911, 

p. 27-28). Some of the mounds are thought to have 

been started 3,500 years ago.

The first Europeans to visit the site of San Francisco were a small party of men led by Sergeant Jose 

Ortega. They were trying to reach Point Reyes from a 

base camp of the Portola expedition (1769) in San 

Pedro Valley, near the ocean 11 miles south of the 

quadrangle. The main expedition, led by Caspar de 

Portola, started in Baja California and was the first 

attempt by Spain to colonize Alta California. Portola 

planned to settle at Monterey Bay but failed to recognize it and continued northward. On November 1,1769, 

the Ortega scouting party left the sand dunes of Ocean 

Beach behind and climbed Point Lobos only to find 

their way to Point Reyes, which they had sighted 

earlier, blocked by the Golden Gate. They then turned 

eastward across the ridge and from a high hill saw San 

Francisco Bay. Point Lobos was visited again in 1773

or 1774 by Commandante Rivera with the Father 

Palou party.

In August 1775 a small boat from the ship San 

Carlos, under the command of Juan Manuel de Ayala, 

was the first vessel to enter the Golden Gate. The San 

Carlos sailed into the Golden Gate at night, searching 

for the boat, and dropped anchor off Sausalito. Using 

Hospital Cove on Angel Island as its main anchorage, 

the expedition made the first survey of San Francisco 

Bay and gave names to many of the landmarks in the 

quadrangle, such as Angel Island and Alcatraz Island.

In 1776, Captain Juan Bautista de Anza brought a 

group of colonists to the area. Captain Anza first left 

the settlers to rest in Monterey while he and a small 

party went on to San Francisco. On March 27, 1776, 

they camped at Mountain Lake and the next day 

erected a cross at Fort Point to mark Anza's choice for 

the site of the military post, or presidio. Fort Point at 

that time was bounded by high unstable cliffs of serpentine. In 1853 these cliffs were cut back in connection with the construction of Fort Scott by the United 

States, and the surface on which the cross had been 

erected was lowered.

Anza selected a mission site about 3 miles southeast 

of Fort Point, near a small creek flowing from the slopes 

of Twin Peaks into a lake. The position of this lake is 

indicated on the geologic map (pi. 1) by the area of 

artificial fill that extends westward from about 17th 

and Mission Streets.

The Presidio was established on September 17,1776, 

about half a mile southeast of Fort Point. Neglected by 

Spain and later by Mexico, it never became an important military post. The mission, however, became a 

prosperous agricultural and industrial community. 

Mexico won its independence in 1821 and acquired 

California as a province in September 1822. In 1834 

the mission in San Francisco was secularized.

Captain William A. Richardson selected the Yerba 

Buena Cove area, now artificial fill between Telegraph 

Hill and Rincon Hill, as a port site in 1835. The port 

settlement growth soon outstripped that of the Presidio 

and mission.

In 1846 the United States flag was raised in Yerba 

Buena. In 1847, when the name San Francisco was 

formally adopted, the population was approximately 

400. In May 1848 the town was abandoned by most of 

its inhabitants, who became gold seekers, but worldwide immigration brought its population to 20,000 by 

1849; the dune sand west of the cove started to acquire 

its cover of manmade structures. In 1850 more than 

500 ships were in the bay, many of them abandoned by 

their crews. Some of them in the cove became stores 

and hotels and eventually became part of the artificial 

filling of the cove, which by 1851 was largely reclaimed.
INTRODUCTION

Scott (1959) gave additional information on the city's 

expansion. The population of San Francisco in 1960 

was 750,000; its land area is 46.5 square miles.

Sausalito was long a port for whalers and other 

mariners where fresh water was obtained. In 1960 its 

population was 5,500.

PREVIOUS WORK

The first known geologic observations in the quadrangle were made in the early 1800's by voyagers on 

foreign exploring vessels visiting this remote Spanish 

port. In 1816 the Russian ship, Rurik, under the command of Otto von Kotzebue, brought Adelbert von 

Chammisso, a botanist who collected and described 

serpentine, sandstone, flinty slate (chert?), and quicksand (dune sand?) in San Francisco (VanderHoof, 

1951, p. 110). The first geologic map of any part of 

California was one of this quadrangle and vicinity on a 

scale of 1:217,000 made in 1827 by Edward Belcher 

and Alex Collie of H.M.S. Blossom, sailing under the 

command of Captain F. W. Beechey. The map, reproduced in the popular "Geologic Guidebook of the San 

Francisco Bay Counties" (VanderHoof, 1951, p. 110), 

also shows a sketch of the Needles islets, located near 

Lime Point, and the general distribution of "serpentine, 

jasper, clayslate, sandstone, and alluvial." It was 

accompanied by a two-page text (Buckland, 1839, p. 

174-176), which states that Angel Island is "of very 

confused formation" and identifies the hill west of 

Mission Dolores as serpentine.

The base for the geologic map was a hydrographic 

and topographic map prepared by the Beechey expedition and published in 1833 by the Hydrographic Office 

of The British Admiralty. It was used with minor revisions until the 1850's, when the U.S. Coast and Geodetic Survey maps were issued (Lincoln, 1969, p. 6-8).

In connection with a survey for railroad routes to the 

west Blake (1857, p. 145-162) in 1853 prepared a geologic map on a scale of 1:211,000 of the area from San 

Francisco to Richmond, including most of the San 

Francisco North quadrangle. He recognized and gave 

fair descriptions of the principal rock types of the 

Franciscan Formation (except greenstone) in chapter 

12. His geologic cross section from Point Lobos to 

Yerba Buena Island can be used today with only minor 

changes. He interpreted the serpentine in the Fort 

Point-Hunters Point shear zone as an eastwarddipping sill in the sandstone. He also described numerous artesian wells in the surficial deposits on the 

flanks of the hills in the northeastern part of presentday San Francisco and in the vicinity of Mission 

Dolores. Unfortunately, he gave no information on 

rate of flow or on declining water levels, except for the

following vague statement (p. 162): "and when the 

borings first commenced, an overflow was generally 

obtained."

The report of the 1860-64 Geological Survey of California, led by J. D. Whitney (1865, p. 76-79), includes 

observations probably made by Brewer (1930, p. 365- 

373), Whitney's principal assistant, on the geologic 

features in the San Francisco area. Unfortunately, this 

survey did not carry out its stated intention to begin 

"a map of the city and its vicinity, on a large scale, *** 

on which all the varieties of strata and of the superficial 

covering of soil and sands will be laid down." (Whitney, 

1865, p. 78).

In 1891 Professor A. C. Lawson of the University of 

California and his students began geologic mapping 

that led to the publication of the monumental San 

Francisco Folio (Lawson, 1914) with a geologic map 

scale of 1:62,500. Other papers that resulted from their 

work include those of Ransome on Angel Island (1894) 

and Point Bonita (1893) and Palache (1894) on the 

serpentine of Potrero Hill. An earlier, more detailed 

report, "Sketch of the geology of the San Francisco 

Peninsula" (Lawson, 1895), included a geologic map 

of the peninsula from San Francisco almost to Half 

Moon Bay on a l:113,000-scale shaded-relief base 

made by photographing a relief model. The field notebooks and field maps for the San Francisco Folio were 

not available to me for examination as a part of my 

study.

A report by Crandall (1907), "The Geology of the 

San Francisco Peninsula," which is accompanied by a 

map on a scale of 1:126,000, is of interest because it 

includes a few geologic details not mentioned by 

Lawson.

The 1906 earthquake was a great stimulus for geologic study within the quadrangle because of the great 

havoc it wrought. Many geologists contributed to the 

voluminous report of the California State Earthquake 

Investigations Commission, edited by Lawson (1908). 

Map 17 of the atlas is a fine geologic map of the City 

of San Francisco and Yerba Buena Island on a scale of 

1:40,000. It differs from the geologic map in the San 

Francisco Folio in having a more legible base, in showing more detail in bedrock areas, and in showing areas 

of land reclaimed from San Francisco Bay.

The "Report on the Underground Water Supply of 

San Francisco County" (Bartell, 1913), popularly 

known as the "O'Shaughnessy Report," is a valuable 

source of borehole logs and well data. It contains a 

geologic map of the City of San Francisco by Lawson 

at a scale of 1:37,500, the base for which, apparently 

made by the City Engineer's office, shows streets and 

topography by contours. The geology depicted differs 

slightly from that on other geologic maps of the city.
8 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

"Hydraulic-Mining Debris in the Sierra Nevada" 

(Gilbert, 1917) contains much information on sedimentation in San Francisco Bay. A report on "Subsidence and the Foundation Problem in San Francisco" 

(American Society Civil Engineers, 1932) is a rich 

source of data on engineering, geology, and the historical development of the city.

G. D. Louderback was greatly interested in the geology of San Francisco Bay as well as in the Quaternary 

geology of the bay area. Early in his career, Louderback 

participated in a survey of the bay sediments (Sumner 

and others, 1914). Later he published a short paper on 

bay sediments (Louderback, 1939) and a geologic 

history of the bay (Louderback, 1951).

Studies in connection with constructing bridges 

across the bay have given valuable data on the geology 

of the east edge of the quadrangle. An early effort was 

reported by the Hoover-Young San Francisco Bay 

Bridge Commission (1930). Trask and Rolston (1951) 

presented data based on logs of holes bored to investigate the route of a bridge parallel to, but 300 feet north 

of, the present San Francisco-Oakland Bay Bridge and 

on holes bored to investigate a site for another bridge 

between the island of Alameda and the south edge of 

the quadrangle.

ACKNOWLEDGMENTS

The author is indebted to a great number of people 

of private and public engineering organizations and 

other organizations and to local residents who furnished logs of boreholes and other data. Ralph Wadsworth, San Francisco City Engineer, furnished, through 

the services of Sherman Duckell, Reuben Owens, and 

William Daly of his staff, many useful maps and reports and gave access to valuable data in the files of 

the Department of Public Works. Thomas Mullaney, 

Engineer, Department of Public Works, was a frequent 

guide for M. G. Bonilla and the author in tunnel excavation projects and was generous with his time and 

excavation data. E. I. Muheim, Superintendent of 

Sewers, furnished geologic cross sections of tunnels; the 

late C. E. Lee, consulting foundation and hydrologic 

engineer, furnished many logs of boreholes and other 

engineering geology data. William Moore, consulting 

engineer of the firm of Dames and Moore, Soil 

Mechanics Engineers, gave access to a large number of 

logs of boreholes made by his firm. J. G. Gratton and 

J. N. Pitcher, well drillers, furnished logs of boreholes 

and permitted observation of drilling operations and 

sampling. The California State Division of Highways 

and Division of Bay Toll Crossings furnished copies of 

logs of their boreholes.

Several Federal agencies assisted by furnishing boat 

transportation: U.S. Public Health Service; Corps of

Engineers, U.S. Army; U.S. Coast Guard; Federal 

Bureau of Prisons.

Colleagues in the Geological Survey helped in many 

phases of the work. In particular, E. B. Eckel and 

Ernest Dobrovolny conceived the project and contributed much sound advice and support. The sections 

on the Franciscan Formation benefited from many discussions with E. H. Bailey, W. P. Irwin, and D. L. 

Jones. The review of part or all of the manuscript by 

the following Geological Survey colleagues was beneficial and greatly appreciated: C. A. Kaye, W. P. Irwin, 

Richard Janda, Helen Beikman, W. R. Hansen, J. T. 

McGill, and Katherine Reed. Most of all I am grateful 

to M. G. Bonilla for his invaluable contributions.

FIELDWORK

Fieldwork was started in August 1947 by C. A. Kaye, 

who mapped approximately 2 square miles of the southern Marin Peninsula. In November, Kaye was joined 

by M. G. Bonilla, who began systematically combing 

streets and alleys in San Francisco for rock exposures. 

The project was recessed from May 1948 to November 

1948, when the author and Bonilla resumed geologic 

mapping and started a program of collecting, from private and public agencies, logs of boreholes and engineering and geologic reports of specific construction 

sites in the San Francisco Bay area. D. H. Radbruch 

started work on the project in June 1949. W. I. Konkoff assisted in geologic mapping during part of 1950 

and 1951. Fieldwork was largely completed by the close 

of 1952. The geologic map (pi. 1), however, contains 

data obtained at a few excavation sites as recently as 

1964.

Results of this study are published reports on the 

geology of the quadrangle (Schlocker and others, 

1958), on a Cretaceous ammonite found in the Franciscan Formation in San Francisco (Schlocker and 

others, 1954), on the engineering geology of Islais 

Creek Basin, located at the southeast corner of the 

San Francisco North quadrangle (Radbruch and 

Schlocker, 1958), and on rodingite on Angel Island 

(Schlocker, 1960).

METHODS OF MAPPING

Because the land surface of much of the quadrangle 

is hidden by streets, buildings, and other manmade 

structures, geologic observations were confined largely 

to undeveloped lots and to excavations opened for 

utility lines and building foundations. These observations were supplemented by data from boreholes and 

earlier foundation construction. Several thousand logs 

of boreholes drilled by private firms and Federal, State, 

county, and municipal agencies provided data that 

were invaluable in preparing the geologic map.
FRANCISCAN FORMATION

Aerial photos taken in 1946 were used extensively in 

relatively open areas such as the Presidio, the Mount 

Sutro-Twin Peaks area, Marin Peninsula, Angel Island, 

Belvedere Island, and Tiburon Peninsula. The most 

commonly used scales were 1:12,000 and 1:6,000. 

Excellent aerial photographs, at a scale of 1:2,400 

made available by the San Francisco City Engineer's 

office were used for part of the Twin Peaks-Mount 

Olympus-Corona Heights area. The topographic quadrangle map enlarged to 1:6,000 scale was used for parts 

of San Francisco covered with a dense street network.

STRATIGRAPHY

The geologic formations of the San Francisco 

North quadrangle fall into two distinct groups, bedrock and surficial deposits, that differ greatly in age, 

lithology, and topographic expression (pi. 1). The 

older group, the bedrock, comprises the sedimentary, 

igneous, and metamorphic rocks of the Franciscan Formation of Jurassic (?) and Cretaceous age, and the serpentine and gabbro associated with them. Areas of 

exposed Franciscan Formation are generally hilly.

The younger group, unconsolidated surficial deposits 

of Pleistocene and Holocene age, are predominantly 

dune sand and water-laid sand, mud, and clay, but they 

include some fairly extensive deposits of slope wash, 

alluvium, landslide debris, and artificial fill. The surficial deposits above sea level mantle and extensively 

modify the lower slopes and fill the valleys between the 

bedrock hills; their thickness varies from a few feet to 

more than 100 feet. In the bay, borings show that the 

pre-Tertiary bedrock is overlain by deposits of sand, 

clay, and mud ranging in thickness from 100 to 300 

feet. In some of the channels cut in the bay floor, 

unconsolidated material is locally absent.

BEDROCK 

FRANCISCAN FORMATION

The Franciscan Formation is a complex assemblage 

of various rock types, predominantly sedimentary, but 

also volcanic and metamorphic, exposed in southwestern Oregon and along much of western California from 

the Oregon border at least to Santa Catalina Island 

(fig. 6). Its exposures in California are generally 

bounded on the east by the Klamath Mountains and 

the Coast Range thrust fault (Bailey, 1970), which 

lies near the west border of the Great Valley, and on 

the west by the Pacific Ocean. It is exposed over 

roughly 15,000 square miles of the Coast Ranges, and 

its total extent on land and offshore may be as much 

as 75,000 square miles. The Franciscan Formation and 

its possible correlatives in Baja California, California, 

Oregon, Washington, Canada, and Alaska are typical

of rocks deposited in orogenically active borders between oceanic and continental crustal plates.

The Franciscan Formation ranges in age from Late 

Jurassic to Late Cretaceous; however, rocks deposited 

during the entire age span generally are not present in 

any one area. The rocks were folded, shattered, and 

sheared many times after they were deposited. Because of the lack of key marker beds, the scarcity of 

fossils, and structural complexities, a standard section 

has not been established, and the total thickness is not 

known. Thickness measurements of part of the Franciscan at several places show that it is probably more 

than 50,000 feet thick.

A comprehensive description of the Franciscan Formation throughout its entire distribution and a discussion of its significance to the geology of the Coast 

Ranges is contained in a report by Bailey, Irwin, and 

Jones (1964). A detailed discussion of the history of 

the study of the formation is also contained in this 

1964 report and in a report by Taliaferro (1943, p. 112- 

122).

In the San Francisco North quadrangle, the Franciscan Formation may be as much as 10,000 feet thick, 

and it consists of about 80 percent graywacke sandstone, 10 percent shale and siltstone, 6 percent mafic 

volcanic rocks, 3 percent radiolarian chert, and less 

than 1 percent conglomerate, limestone, and glaucophane schist. All these rocks have been intruded by 

ultramafic rocks, mostly serpentine. Fossils found in 

the Franciscan Formation in the San Francisco North 

quadrangle are Cretaceous in age.

The Franciscan Formation is exposed in the northeastern part of San Francisco where it is 3,650 feet 

thick and in the sea cliffs from the Cliff House to 

Bakers Beach where it is about 2,350 feet thick. In these 

places, the formation consists mostly of graywacke 

sandstone and shale. The formation also occurs as 

tectonic inclusions in a shear zone that extends from 

Hunters Point to Fort Point. An unknown thickness of 

graywacke sandstone, greenstone, and radiolarian 

chert forms the central highlands area of the city, 

which includes Sunset Heights, Mount Sutro, Twin 

Peaks, and the adjacent hills to the east.

The Franciscan Formation is estimated to be 

about 17,000 feet thick in a southwest-dipping section 

between Richardson Bay and Point Bonita (southeast 

tip of Marin Peninsula; not in quadrangle). Here the 

formation includes 8,000 feet of greenstone, more than 

6,000 feet of sandstone and shale, and about 3,000 feet 

of radiolarian chert, which is more abundant here than 

in most Franciscan terranes (fig. 7).

On Angel Island and Tiburon Peninsula, the Franciscan Formation consists of sandstone and shale, with 

a minimum thickness of 1,850 feet, interbedded green534-039 O - 74 - 2
10 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

42°

34" 

EXPLANATION

Franciscan Formation and related rocks 

KJf, rocks of predominantly 

Late Jurassic and Early Cretaceous age

Kf, rocks of predominantly 

late Early and early Late 

Cretaceous age 

f, rocks of undetermined age

f'-^&£>t

120' 118° 116'

FIGURE 6. Distribution of Franciscan Formation, related rocks, and principal structural features of western California

(modified from Bailey and others, 1964, fig. 29).

stone 350 feet thick, and minor radiolarian chert. Most 

of the Franciscan here has been metamorphosed.

Lawson (1895, p. 415) named the Franciscan Series 

and later (1914, p. 4) employed the name Franciscan 

Group. He designated San Francisco as the type area, 

stating that "the Franciscan group was named from 

San Francisco, where it occurs in extensive exposures 

***." He believed that two radiolarian cherts interbedded with the sandstone constituted "well-defined 

and easily recognizable stratigraphic horizons," and 

he used these cherts to divide the Franciscan into five 

formations. He named these formations, in ascending 

stratigraphic order, the Cahil Sandstone, the Sausalito

Chert, Marin Sandstone, Ingleside Chert, and Bonita 

Sandstone. These were later reduced to members within the Franciscan Formation. All but the Bonita were 

recognized by Lawson in this quadrangle.The present 

writer found numerous sections of lithologically indistinguishable radiolarian chert interbedded with sandstone and for the most part was unable to use them as 

stratigraphic marker beds as suggested by Lawson. 

These five names are therefore considered obsolete and 

are abandoned.

Because the Franciscan Formation in this quadrangle consists of a very complex assemblage of rocks, 

with no marker beds, the formation has been divided
CLASTIC SEDIMENTARY ROCKS 11

FIGURE 7. Horseshoe Bay and vicinity, southern Marin Peninsula. Franciscan Formation dips about 50° westward (to 

left). In the photograph, greenstone terranes appear nearly 

white; radiolarian chert, sandstone, and shale appear medium 

to dark gray. The large cut along U.S. Highway 101 exposes 

sandstone and shale on left (west) and radiolarian chert on 

right. Parking area at level of freeway was (1949) site of 75- 

foot-high hill of radiolarian chert and shale. (See figs. 10,18).

into its major rock types for purposes of discussion: 

clastic sedimentary rocks, greenstone, radiolarian 

chert, metamorphic rocks, and sheared rocks.

CLASTIC SEDIMENTARY ROCKS 

Clastic sedimentary rocks of the Franciscan Formation consist of massive sandstone beds, commonly more 

than 10 feet thick and as much as 35 feet thick, sections 

of alternating thin-bedded shale and sandstone, and 

rare thin conglomerate beds.

Sandstone of the Franciscan Formation generally 

fits the description of graywacke (in Williams and 

others, 1954, p. 297) as "aggregates of sharply angular 

fragments of every size between sand or fine gravel and 

impalpable particles." The grains are mostly angular 

and range in size from coarse sand to clay. The most 

common sandstone consists predominantly of fine to 

medium grains. The sandstone has other attributes 

characteristic of graywacke, such as poor sorting, 10 

percent or more clayey matrix, low porosity, and dark 

color.

On the geologic map (pi. 1), units consisting of massive sandstone and minor amounts of interbedded shale 

are shown as sandstone. In the northeastern part of 

San Francisco, two exceptionally thick sequences of 

shale and thin-bedded sandstone are separated from 

the massive sandstone. The shale and thin-bedded 

sandstone sequences vary considerably. In some places 

they are predominantly shale with a few 1-3-inch-thick 

beds of sandstone. In other places they are predominantly 2-3-foot-thick beds of sandstone separated by 

3-4-inch-thick beds of shale. Where exposures in the 

city are too poor to determine which combination of 

lithology and bedding predominates, the clastic rocks 

are shown as sandstone and shale, undifferentiated.

Structural and stratigraphic relations between discontinuous exposures of clastic rocks are generally 

obscure, owing to poor exposures, scarcity of fossils, 

lack of bedding in sandstone, and absence of key 

marker beds. Some separated exposures evidently are 

stratigraphically continuous, but only in a few places 

could sandstones be recognized in distinguishable 

stratigraphic positions.

Soils developed on the clastic rocks vary from 

slightly sandy clay to clayey sand, but most are sandy 

silty clay of low permeability. The soils swell greatly 

and are highly plastic when wet and shrink greatly 

when dried. Distinct soil horizons are poorly developed. Thickness of the soil (A and B horizons) above 

weathered rock (C horizon) varies from 3 to 20 feet. 

Total thickness of soil and moderately weathered rock 

is between 5 and 30 feet in most places but as much as 

70 feet in some places.

Most slopes underlain by clastic rocks of the Franciscan Formation are rounded or only moderately steep 

because of the thick cover of clayey soil. Landslides of 

surface debris are common on these slopes. Where erosion is rapid, such as along shorelines, the sandstone 

maintains natural cliffs by landsliding.

SANDSTONE

Bedding within the massive sandstone beds generally 

is absent or obscure even in good exposures. In some 

sandstone, bedding is indicated by the orientation of 

detrital mica, shale flakes, or carbonaceous matter. 

Some of the sandstone beds, as much as 25 feet thick, 

appear to be lenses that fill channels in the shale and 

thin-bedded sandstone sequences. Beds Yz-4 inches 

thick show grading from fine- to medium-grained sandstone at the base to shale at the top (fig. 8). The top 

few inches of massive sandstone beds also grade upward to shale and show channeling, ripple-type crossbedding, and, in places, distortion of beds that suggests 

preconsolidation slumping. In some localities striations, grooves, and spatulate depressions are preserved 

as casts on the underside of bedding planes.

Most of the structural attitudes shown on the geologic map (pi. 1) were observed on shale and thinbedded sandstone sequences that separate the massive 

beds.

The color of fresh sandstone, according to the rock 

color chart (Goddard and others, 1948) is dark gray 

(N3) to medium gray (N5), with rare tinges of blue 

(5B 5/1). Weathered sandstone ranges from dark 

greenish gray (5GY 4/1) and olive gray (5Y 6/2) 

when slightly altered to grayish orange (10Y7? 7/6) 

when greatly altered. Hydrothermally altered sandstone is white (N9) with streaks of yellowish orange
12 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 8. Shale and thin-bedded sandstone of the Franciscan 

Formation showing graded and distorted bedding and 

laminae. Broadway Tunnel excavation, Russian Hill, San 

Francisco.

-*»

FIGURE 9. Jointed thick-bedded sandstone and interbedded 

3-foot-thick section of shale and thin-bedded sandstone, 

Franciscan Formation. Northeast side of Telegraph Hill, San 

Francisco.

(10YE 7/7). Chloritized sandstone is light olive (10Y 

5/2) to dark greenish gray (5GY 4/1).

Closely spaced joints are common in the sandstone 

beds (fig. 9). Most joints are randomly oriented and 

have plane or curved surfaces, but many joints are in 

poorly to moderately well defined parallel sets. Some 

joint systems consist of one set parallel to the bedding 

and two or more perpendicular to the bedding.

Closely spaced joints are especially prominent in irregular shear and mylonite zones, which consist of 

thoroughly mashed shale and crushed sandstone (fig. 

10). Such zones are commonly found in exposures that 

also contain less fractured sandstone in which the joints

are from 4 to 48 inches apart. Joints in unsheared sandstone are most commonly 2-3 inches apart.

Some massive sandstone beds are adjacent to large 

bodies of dark-gray soft thoroughly sheared shale, in 

which rounded sandstone fragments, 1 inch to 25 feet 

in diameter, are embedded (fig. 11). The contact between the sheared shale bodies and the enclosing sandstone is usually sharp. Tabular shale bodies in the sandstone (1-6 in. thick) also have intruded fractures at all 

angles to the bedding; these condition indicate that 

shale apparently becomes very plastic under shear 

stresses. The mineralogy of some of these shale segregations is given in a later section.

Grain size of sandstone in the Franciscan Formation 

in this area ranges from coarse sand to clay; however, 

fine to medium sand sizes predominate. The predominant grain-size range in any one bed is fairly narrow 

and is a parameter that can readily be assigned in the 

field.

Nearly all the sand-size grains are angular. In a few 

sandstone beds, as much as 3 percent of the grains are 

subangular to round; in some tuffaceous sandstone, as 

much as 25 percent of the volcanic rock grains and 

about 3 percent of the quartz and feldspar grains are 

subround to round. In one unusual sandstone found in 

the excavation for the foundation of the Masonic 

Temple at California and Taylor Streets, about 25 percent of the grains were subround to round. The DouuilJezceras-bearing sandstone west of James D. Phelan 

Beach contains 1-2 percent well-rounded grains and 

about 5 percent of grains that are well rounded on one 

side and angular on the other side. Evidently, these 

grains were once well rounded and subsequently were 

broken.

In 50 percent of the sandstone specimens that were 

examined for grain sphericity, about 90 percent of the 

grains were equidimensional and 10 percent were elongate. In the other specimens, 50 65 percent of the 

grains were equidimensional and the remaining grains 

were elongate. The long axes of elongate grains generally lie in the plane of the bedding.

Dark-gray to black angular gravel-size fragments of 

shale, commonly oriented in the plane of the bedding, 

occur in some sandstone, especially coarse-grained 

sandstone (fig. 12). The largest fragments, as much as 

4 by 4^/2 inches in size, were found in the Douvilleiceras-bearing sandstone. In a few places, large abundant paper-thin flakes of coaly material, 1 or 2 square 

inches in area, parallel the bedding.

MATRIX

The amount of matrix (detritus smaller than 0.02 

mm) in sandstone beds of the Franciscan Formation
CLASTIC SEDIMENTARY ROCKS 13

FIGURE 10. Sheared and shattered sandstone and shale of the Franciscan Formation. Continuity of beds, seen near bottom of 

cut, is uncommon in the Franciscan Formation. Sandstone and shale at right edge of cut are especially strongly sheared. 

Old deposit of slope debris and ravine fill at top of cut. West side of U.S. Highway 101, three-quarters of a mile north of 

Golden Gate Bridge.

varies irregularly over short distances but generally 

constitutes 15-20 percent of the rock. Dikelike segregations of matrix-rich sandstone, generally less than 1 

inch thick, are common. The amount of matrix in such

FIGURE 11. Sheared shale of the Franciscan Formation containing large hard sandstone masses. Exposure is near top of 

blanket (melange) of sheared Franciscan rocks, approximately 1,000 feet thick, that covers many squares miles 

northwest of the San Francisco North quadrangle. Note 

berms and pipes draining water from nearly horizontal holes 

in slope used to prevent and control landslides along California highways. West side of U.S. Highway 101, 1 mile west 

of Sausalito Point, Marin Peninsula.
14 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 12. Shale fragments (black) in coarse-grained graywacke sandstone of the Franciscan Formation. Section 

parallel to bedding. Laguna Honda Reservoir area, San 

Francisco.

segregations in sandstone beds on Marin Peninsula is 

so great that the rock is more properly called sandy 

mudstone.

Microscopic examination shows the matrix to be aggregates of silt- and clay-size crystals with equigranular, acicular, and micaceous habits. In some places, 

the matrix consists of shale clasts squeezed around 

harder grains. Constituents that can be easily identified are quartz, plagioclase, muscovite, biotite, and lignitic material. Exceedingly fine-grained micaceous 

minerals with moderate to high birefringence are very 

abundant.

To investigate the clay minerals in the matrix, the 

rock was crushed; the <2-micron fraction of the material passing a 65-mesh (0.210 mm) screen was assumed 

to be largely matrix and was used to prepare oriented 

aggregates for X-ray diffractometer analyses. This 

material from unweathered sandstone collected from 

widely separated localities consists mostly of mica and 

chlorite which are fairly well ordered, though some 

contain a low percentage of expandable layers. Mica is 

both the dioctahedral (muscovite) and trioctahedral 

(biotite, phlogopite) type; kaolinite is absent.

The matrix of the sandstone from Alcatraz and 

Yerba Buena Islands, Quarry Point on Angel Island, 

Telegraph and Russian Hills in San Francisco, and 

from north of Lime Point contains fibrous aggregates 

of acicular pumpellyite crystals. Some of the adjoining 

volcanic, feldspar, and quartz clasts also contain pumpellyite. The pumpellyite in the matrix and adjoining 

clasts appears to be metamorphic. Some isolated clasts 

of pumpellyite in radial fibrous sheaves with or without 

quartz may be detrital.

DETRITAL GRAINS

The lithologic composition of the sandstone beds 

varies from arkose to volcanic graywacke, but most 

beds are arkosic and lithic graywacke (fig. 13). The 

plotted positions of sandstone samples shown in figure 

13 are estimates based on examination of thin sections 

and cut surfaces. Results of point-count analyses of 

thin sections of some of the sandstone samples are 

given in table 1.

With one exception, data are insufficient to suggest a 

correlation between sandstone composition and stratigraphic position or geographic location. The mineral 

and chemical composition of sandstone beds west of 

Corona Heights and east of Twin Peaks and their 

megascopic appearance suggest that this belt of sandstone is largely volcanic graywacke. In several other 

places throughout the quadrangle, greenstone grades 

into volcanic graywacke.

STABLE GRAINS 

(Quartz, chert, and quartzite)

QUARTZGRAYWACKE

FELDSPATHIC GRAYWACKE

FELDSPARS

FIGURE 13. Classification of sandstone samples of the Franciscan Formation. Location of samples is shown in figure 14.

TABLE 1. Modal composition of sandstone samples 

of the Franciscan Formation

Approximate volume percent determined by pointcount analyses of thin sections

ti

tC

d

a

S

91

45 .. .......

40 .......

46 ..........

39.. .......

No. rniplc 

re

1649

1228

1899

1934

1979

£ 

tj

s

32.5

55.7

24.4

50.8

a1

14.9

18.4

35.0

20.2

a

12.5

17.6

16.2

15.1

rocks olcanic 

>

14.9

1.6

10.4

3.0

"rt

3.6

1.5

4.1

16.4

6.8

s

0

10.5

1.6

4.3

2.7

3.6

and aartzite metachert

a

7.6

2.0

4.4

9.4

1 JS3

g o.S;S

o i

0.2 3.3

- 1.5

- 1.2

.8 .6

- .5

'Grains smaller than about 0.03 mm, but matrix also includes all sizes of 

mica crystals and carbonaceous material.
CLASTIC SEDIMENTARY ROCKS 15

Mineral grains are mostly quartz and feldspar. About 

one-half to three-fourths of the quartz grains found in 

the sandstone of the Franciscan Formation show slight 

strain effects at extinction under crossed nicols. A small 

number of quartz grains show strong strain effects and 

brecciation. Most of these grains were classed as quart - 

zites in the grain composition studies. The slight strain 

effects shown by most of the quartz grains are much 

less intense than those shown by quartz in semischistose sandstone of Angel Island but are of slightly 

greater intensity than those shown by quartz crystals 

in veinlets cutting the sandstone.

Most of the feldspar grains are plagioclase. They 

range in composition from albite (An4 ) to sodic labradorite (Ansi). Andesine, An33 An39, is the plagioclase 

in two-thirds of the sandstone. In some sandstone, 

andesine is accompanied by oligoclase, An12-An20 . Even 

in sandstone containing large amounts of basic volcanic 

rock fragments, the predominating feldspar detritus is 

andesine. Less than 1 percent of the plagioclase shows 

zoning.

A remarkable aspect of the sandstone is the scarcity 

of potassium feldspar grains (fig. 14; table 2). An exception is the sandstone of Point Lobos, which contains 

5-10 percent potassium feldspar. Sandstone from a few 

other localities also contains small but persistent 

amounts of potassium feldspar; these are unsheared, 

unmetamorphosed sandstone beds of Quarry Point on 

Angel Island, the Douvilleiceras-bearing sandstone and 

adjoining sandstone in the cliffs of South Bay and 

Bakers Beach, and the sandstone of Alcatraz Island.

Heavy mineral determinations of nine sandstone 

samples were made, using procedures outlined by Hutton (1950, p. 639). The approximate proportions of 

heavy minerals in the sandstone (table 3) are weightedaverage frequencies (Hutton, 1950, p. 650) from estimates made by counting grains in the 20-74- and 74- 

149-micron-size ranges identified under the petrographic microscope. The weighted-average frequencies 

also include less accurate estimates for coarser sizes, 

for which only representative grains were subjected to 

petrographic and X-ray diffraction analysis.

Biotite, chlorite, vermiculite, and, to a lesser extent, 

nontronite predominate in the heavy fraction of almost 

all sandstone. X-ray diffraction analysis shows that 

these minerals are present together in composite grains. 

Individual flakes of chlorite and muscovite are fairly 

abundant. The epidote group minerals, epidote, clinozoisite, and zoisite, the related mineral pumpellyite, 

and sphene and garnet are present in conspicuous proportions.

The heavy-mineral analyses show some slight, inconsistent differences between sandstones containing potassium feldspars and those that do not. Because potassium feldspar in a sandstone suggests that granitic 

rocks were components of the source area, minerals 

characteristic of a granitic source, such as zircon and 

tourmaline, would also be expected to be more abundant in the potassium feldspar-bearing sandstone, but 

the results do not show this. Sandstone sample 1931 

from the Golden Gate Bridge area in Marin County 

contained rare grains of the chromian spinel, picotite, 

as did a sample of semichistose sandstone near the west 

shore of Angel Island 1,000 feet south of Point lone, 

suggesting that ultramafic rocks were in the source 

area.

All the sandstones contain sedimentary, volcanic, 

granitic, and metamorphic rock grains. Shale is conspicuous in many sandstone samples, and silty, clayey, 

and carbonaceous varieties can be seen in a single thin 

section. Chert grains, in which no Radiolaria have 

been found, are common in all sandstones. Graywacke 

sandstone grains are sparse; quartz sandstone or orthoquartzite grains are somewhat more common. Detrital 

carbonaceous matter, probably representing plant remains, is also common.

Altered volcanic rocks are found in almost all sandstone. Many sandstones contain grains of chlorite or 

vermiculite-chlorite-nontronite aggregates that appear 

to be altered glassy basalts. Some of these grains contain microlites of albite-oligoclase and rarely labradorite. Grains of greenstone with spherulitic, hyaloophitic, 

intersertal, and pilotaxitic textures are common, as are 

what appear to be grains of acidic felsites. Acidic to 

intermediate volcanic grains are abundant in coarsegrained sandstone at California and Taylor Streets.

Small amounts of lithic grains with fine-grained 

granitic textures are found in most sandstones. Most of 

these grains are quartz-plagioclase in composition, 

but grains of quartz-plagioclase-orthoclase, quartzperthite, and quartz-microcline rocks are present in 

small amounts in a few sandstone beds. For example, 

the Douvilleicems-bearing sandstone on the cliffs of the 

South Bay part of Golden Gate channel is distinguished 

by the presence of such lithic granitic grains as well as 

by a more-than-average amount of mica quartzite and 

graphite quartzite. Thin sections of sandstone from the 

Presidio, Nob Hill, and southern Marin Peninsula contain a small percentage of serpentine grains.

Grains of fine-textured metamorphic rocks, such as 

quartzite or what appears to be metachert, are common in almost all sandstones. These consist largely of 

quartz, generally with suture texture, with one or more 

of the following minerals: Epidote-group minerals, 

particularly epidote and clinozoisite, rarely zoisite; 

amphiboles, generally hornblende pleochroic in brown 

hues, pumpellyite in radial-fibrous sheaves; micas, commonly muscovite, rarely brown biotite; and graphite.
16 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

37° 45'

2 MILES

«1 T 36 A 9I B 100

None Trace 1/2 to 3 More than 

percent 3 percent 

Sample localities

Showing potassium feldspar content of sandstone. Map numbers refer to table H

Bedrock Quaternary 

outcrops deposits

FIGURE 14. Locations of sandstone samples of the Franciscan Formation and annotations as to potassium feldspar content.
CLASTIC SEDIMENTARY ROCKS 17

TABLE 2. Potassium feldspar in sandstone of the 

Franciscan Formation

[Grain size: VF=very fine, F=fine, M=medium, C=coarse, and VC=very

coarse. Samples are fresh except as noted. Massive sandstone samples are 

from beds more than 3 ft thick, and thin-bedded samples are from beds 

less than 4 in. thick. Bedding indications not noted where exposures are 

poor. Tr.=trace]

° - -s

* | E g 

o rt v '* g'Z Descriptive remarks -g

' § si "5 8.5 e *

81 § a-8-g g iJ <5 W < E u PL,

Angel Island

S

oon f* j

4 S28 M Semischistose, slightly to moderately

5 S27 M-C Semischistose ........................................................................

6 S33 C Semischistose, slightly to moderately

9 S74 F M ................................................................................... 4

10 B40 M Massive ................................................. ................ 2

11 B34 M-C Semischistose ........................................................................

15 S47 F ............ do ........................... .... ............ ...

16 B56 M C ............ do ..............................................................

17 S13 C ............ do .............................................................. ... .........

Tiburon Peninsula and Belvedere Island

oc 7^Q C1

Marin Peninsula

26 594 F-M .. . ... . ... ..... .... ............

28 602 M ....................................................................................................

29 2026 M Massive ....................................................................................

...... 2027 M ............ do ................................................................ Tr.

...... 2028 M ............ do ................................................................ Tr.

...... 2029 M ............ do ................................................................ Tr.

...... 2031 F Thin-bedded, laminated ......................................................

30 612 VC ....................................................................................................

621 M ....................................................................................................

31 619 F M ....................................................................................................

32 627 M-C ....................................................................................................

34 2041 F-M Graded bedding, thin bedded, small-scale

36 2035 M Massive .................................................................... Tr.

...... 2036 F-M Thin bedded, small-scale channeling........... .....................

...... 2038 M ............ do ................................................................................

37 1931 C ............ do ................................................................................

2033 M C Thin bedded, small-scale channeling................ %

...... 2034 M ............ do ................................................................ V2

39 1979 M C Massive .................................................................... Tr.

Alcatraz Island

40 1899 M Massive .................................................................... 1

Pyritic orthoquartzite, mica schist, amphibolite, and 

epidote-albite rocks are also present in some sandstones. 

Chemical composition. Franciscan sandstone contains less SiO2 and more A12O3 , FeO, MgO, and Na2O 

than Clarke's (1924, p. 30) average sandstone (table 

4, column 6a). Taliaferro (1943, p. 136) showed the

TABLE 2. Potassium feldspar in sandstone of the 

Franciscan Formation Continued

 70* o . 2 

^ fc E " §-H v '£ .5 Descriptive remarks +»

' § ti "S 8.5 c p.

gtc g ft-S'S %

o d § P-* 2 - »

h5 0 W < ti M fn

San Francisco

42 1250 M ............ do ................................................................................

...... 1363 F Thin bedded, small-scale channeling..... ...........................

45 1195 M Massive ....................................................................................

1228 M ............ do ................................................................................

...... 1230 M ............ do ................................................................................

46 1934 VC ....................................................................................................

1935 M ....................................................................................................

48 38 M Laminated ............................ ....................................... .........

49 834 M Massive ....................................................................................

50 2082 F Laminated ..............................................................................

51 148 M Moderately weathered ..........................................................

52 358 M ....................................................................................................

53 2005 F Very coarse grained muscovite flakes... ...........................

54 8 VC Semischistose, slightly to moderately 

weathered. ..........................................................................

55 1272 F-M ....................................................................................................

56 1263 M ....................................................................................................

57 1345 M Massive .......................................................................... .........

58 117 VC Semischistose ........................................................................

118 C ............ do ................................................................................

59 1276 F M ....................................................................................................

60 150 M Massive ....................................................................................

61 341 C .......... v ............................................................................... ........

62 110 C Massive .................................................................... Tr.

63 131 M ....................................................................................................

65 923 F ................................................................................... Tr(?).

66 1275 M Massive ....................................................................................

1275 F Thin bedded ............................................................................

67 112 M Massive .................................................................... %

68 147 M ....................................................................................................

70 348 M Moderately weathered ..........................................................

71 36 M .......................................................................................... .........

FTO 1 AQ"P ^/"IT1

7 A 1 Hfi"P T71

78 1556 C ....................................................................................................

...... 1442 C ............ do ................................................................................

ci "\aacL T71 tui rTj*( rt\

83 1466 M ............ do ................................................................ Tr(?).

84 510 M

91 1566 M C .................................................................................... 2

...... 1566 F M - ................................................................................. 2

97 2155 7 M Massive .................................................................... 7

98 2155 6 M C -Massive, hydrothermally altered ...................... 5 

99 2155 5 M C Massive .................................................................... 7

100 2155 8 M ............ do ................................................................ 7

chemical composition of Franciscan sandstone is close 

to that of granodiorite, but microscopic observation 

shows that in this area sandstones contain substartial 

amounts of other rock types. The composition of a 

representative graywacke from the sandstone belt vest 

of Corona Heights and east of Twin Peaks (tabte 4,
18 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

TABLE 3. Frequency of heavy minerals in sandstone of the Franciscan Formation

[Modified from Evans and others (1934, p. 39 ff.) ]

Frequency

Q J.

8 ....................... 

8- ................... 

7+ ................... 

7 ....................... 

7- ................... 

6+ ...................

Map No. (fig. 14)... ..............................................................

Sample No. ............................................ ....... ......... ...............

Apatite ......................................... ................ ..........................

Biotite-chlorite-nontronite-vermiculite composite .......

Chlorite ................................. ............. .......... ...........................

Calcite ...................................................................................

Epidote ..................................... .......... ........ ......... ...................

Garnet ...................................................................................

Hypersthene-enstatite .......................................................

Magnetite-ilmenite .............................................................

Monazite ...............................................................................

Muscovite ...............................................................................

Picotite ...................................................................................

Pyrite .....................................................................................

Rutile ....................................................................................

Saussuritic rock ..................................................................

Sphene ...................................................................................

Tourmaline ...........................................................................

Tremolite-actinolite ............................................................

Zircon ....................................................................................

Zoisite ....................................................................................

Percent of heavy minerals (by weight)................,......

Approximate 

percent 

......... 90 100

75- 89 

60- 74 

45- 59 

35- 44 

28- 34 

.......... 23- 27

30

621

1 

1 

8

3

3

3

1

4

3

1 

4

1 

1 

4.6

37

1931

1 

1 

8

4

1

2

1

1 

2

2

1 

1

3

1 ___

3

1 

3

1

4

1.5

45

1195

1

5

1 

5

2

3

5

3

1

1

5

2

1 

7+

4

1 

2

1

1 

1.2

frequency Approximate 

percent 

6 ia 99

6- 

5 

4 

3 

2 

1 ........ 1-

45

1363

1 

1 

8

5

1 

1 

1 

5

j _

1 

5

1 

1 

1.5

88

374

2

4

4

4

1

1

1 

1 

2

2

1

7+

1

1 

2

3

1.2

14-17 

7-13 

4- 6 

2- 3 

1- 2

Less than }

91

1649

1

7

6+

3

3

5

1 

2

1

1

1

4

1

1

2

4

1.6

6

40

1899

1

6 

1 

1

5

5

1

1 

2

3

4

5

6 

5

1 

1

3

4

10.3

W

1938A

1 

6

2

5

2

1 

1

1 

3

5

1 

7

1 

1 

1 

1 

1 

4

1.5

74

180

2

7+

5

5

2

2

1

1

3

4

6+

3

1 

2

1 

3.1

JEast shore Yerba Buena Island, 0.8 mile east of San Francisco North quadrangle.

column 5) substantiates its identity as a volcanic graywacke.

Specific gravity. The specific gravity of fresh sandstone ranges from 2.62 to 2.69; most samples are 2.68. 

Fresh sandstone from the Laguna Honda area, with the 

lowest specific gravity, contains a more-than-usual 

amount of shale detritus. Specific gravity of semischistose sandstone on Angel Island, Belvedere Island, and 

Tiburon Peninsula ranges from 2.68 to 2.83. Sandstones containing jadeite generally have a specific gravity greater than 2.72. Specific gravity of fresh laminated 

siltstone and sandstone from the excavation for the 

Broadway Tunnel in Russian Hill is 2.69-2.70, though 

the value for fresh laminated siltstone and sandstone 

1 mile north of the Golden Gate Bridge on Marin Peninsula is 2.64.

Weathering and alteration. Weathering reduces the 

sandstone to a sandy silty clay containing detrital 

quartz and the clay minerals montmorillonite, vermiculite, and mixed layered minerals of expansive clay 

minerals with mica and chlorite. The high vermiculite 

and montmorillonite content causes the clayey soil to 

swell and become highly plastic when wet and shrink 

and crack when dry.

Hydrothermal alteration locally produces 1'aolinitegroup clay minerals and iron oxides by replacement of 

quartz, feldspar, and other silica-bearing minerals. 

Joints in altered sandstone are commonly coated or 

filled with a brown waxy clay mineral that varies in 

thickness from a film to nearly one-fourth inch and 

composed randomly mixed layered mica, chlorite, vermiculite, and montmorillonite. Within or near the edge 

of serpentine bodies, some sandstone blocks retain the 

textural and structural appearance of sandstone, but 

their grains have been completely replaced, mostly by 

chlorite and randomly mixed layered mica-chlcnte, and 

minor talc, calcite, and pyrite.

SHALE

Shale beds are massive, banded or laminated, silty 

and clayey layers. Fissility is poor even in the laminated variety, and the shale breaks along irregular 

slickensided shear surfaces that are commonlj7 parallel 

to the bedding. The color of fresh shale is dark gray 

(N2), and the colors of altered shale are similar to 

those of altered sandstone described elsewhere. Jointing is generally more pronounced and more closely
CLASTIC SEDIMENTARY ROCKS 19

TABLE 4. Analyses of sandstone, shale, and a phosphate nodule from the Franciscan Formation

[Chemical analyses (rapid rock methods) by Paul Elmore, Samuel Botts, I. H. Barlow, and Gillison Chloe. Semiquantitative spectrochemical analyses by 

H. W. Worthing. Looked for but not found: As, Au, Bi, Cd, Cs, Dy, Er, Eu, Gd, Ge, Hf, Hg, Ho, In, Ir, Lu, Nb, Os, Pd, Pr, Pt, Rb, Re, Rh, Ru, Sb, Ta, 

Tb, Te, Th, Tl, Tm. U, W, Zn]

Sandstone

Si02 ..........................................

AbOs ..........................................

FeaOa ........................................

FeO ............................................

MgO ..........................................

CaO ..........................................

Na2O ........................................

K20 ............................................

H2O+ ..........................................

H2O- ..........................................

TiOa ..........................................

P2Os ..........................................

MnO ..........................................

C02 ............................................

Total ..............................

S (aqua regia soluble)...........

Specific gravity (powder) ...

1

.. 70.8

.. 14.0

.6

.. 2.5

1.7

1.5

3.7

.. 2.2

.. 2.2

.22

.42

.10

.05

.07

.. 100

0.06

2.70

2

68.9

12.7

1.5

2.8

2.5

1.9

2.7

2.1

2.8

.68

.68

.13

.06

.28

100

0.03

2.68

3

Chemical

67.0

14.1

.9

4.0

2.8

1.3

3.4

1.6

2.8

.59

.60

.12

.08

.14

99

0.22

2.66

4

analyses

67.1

14.9

1.0

2.9

1.6

2

3.1

2.3

2.6

.78

.51

.12

.06

.17

99

0.08

2.69

5 6 6a

Shale

7 7a

Phosphate

noiule

8

(weight percent)

60.9

16.4

1.4

4.4

3.1

3.9

4.2

.58

3.7

.50

.65

.15

.10

.10

100

0.02

2.73

Semiquantitative spectrochemical analyses

Ag ..............................................

B ................................................

Ba ..............................................

Be ..............................................

Ce ..............................................

Co ..............................................

Cr ..............................................

Cu ..............................................

Ga ..............................................

La ..............................................

Nd ..............................................

Ni ..............................................

Pb ..............................................

Se ..............................................

Sn ..............................................

Sr ..............................................

Sm ............................................

V ................................................

Y

Yb ...............................................

Zr ..............................................

.. 0.000015

.003

.03

.00015

0

.0015

.007

.0015

.0007

0

.. 0

.003

.00015

.0007

0

.015

0

.003

.0015

.00015

.007

0.000015

.007

.03

.00015

0

.003

.007

.0015

.0007

0

0

.007

.0003

.0007

0

.007

0

.007

.0015

.00015

.007

0.000015

.007

.03

.00015

0

.003

.007

.003

.0007

0

0

.007

.0007

.0007

0

.007

0

.007

.0015

.00015

.007

0

.007

.03

.00015

0

.0015

.003

.0015

.0007

0

0

.003

.0003

.0007

0

.015

0

.003

.0015

.00015

.007

0

.007

.007

0

0

.003

.003

.0015

.0007

0

0

.007

.00015

.0007

0

.007

0

.007

.0015

.00015

.007

67.3

15.5

.4

3.8

1.9

.61

4.2

3.2

1.8

.18

.60

.15

.08

<.05

100

0.01

2.66

(weight

0

.003

.15

0

0

.003

.003

.0007

.0007

0

0

.007

0

.0007

0

.015

0

.003

.0015

.00015

.007

78.7

4.8

1.1

.3

1.2

5.5

.45

1.3

1.3

.31

.25

.08

Trace

5.04

100

percent)

63.2 58.4

16.1 15.5

.7 4

4.9 2.5

3.1 2.5

1.1 3.1

2.4 1.3

2.5 3.3

3.7 3.7

.49 1.3

.68 .65

.20 .17

.09 Trace

<.05 2.64

99 99

0.22 ..................

2.70 ..................

0.000015 ..................

.007 ..................

.07 ..................

.00015 ..................

0 ..................

.003 ..................

.007 ..................

.007 ..................

.0007 ..................

0 ..................

0 ..................

.015 ..................

.007 ..................

.0007 ..................

.0015 ..................

.007 ..................

0 ..................

.007 ..................

.0015 ..................

.00015 ..................

.015 ..................

43.4

9.2

.7

11.2

2.6

15.3

.52

.54

4.6

.52

.32

9.8

.12

.34

99

0.18

2.83

0.00103

.OOT

.07

0

.015

.0073

.0007

.015

.0007

.007

.015

.003

.015

.0007

.003

.015

.015

.0015

.015

.0015

.003

1. Graywacke, northeast base of Telegraph Hill, 200 ft west of Kearny St., 

50 ft south of Francisco St., San Francisco (sample No. SF-2148).

2. Lithic graywacke, west of James D. Phelan Beach State Park, fresh rock 

near sea level below Douvilleiceras-bearing graywacke (sample No. 

SF-373).

3. Graywacke, Marin Peninsula, 5,050 ft northwest of Lime Point lighthouse, north end of roadcut west side of U.S. Highway 101 (sample 

No. SF-2114).

4. Graywacke, south base of Mount Sutro, east end of Laguna Honda 

(sample No. SF-2141).

5. Volcanic graywacke, 1,000 ft south of top of Buena Vista Park, San 

Francisco (sample No. SF-2140).

spaced in shales of the Franciscan Formation than in 

sandstones.

The shales are made up mostly of micaceous minerals in the silt sizes (2-62 microns). However, the 

shales range from those containing no material greater 

than silt size to those containing substantial amounts. 

Most contain more than 15 percent silt-size quartz and 

feldspar. Carbonaceous material is in sand- to claysize plates and fibers that are nearly opaque, dark 

brown in transmitted light, and dark gray to black in 

reflected light. Pyrite is often found in tiny crystals 

disseminated in large carbonaceous particles and layers.

The mineral composition of shale is similar to that of 

the sandstone matrix. Predominating constituents are 

mica, clay minerals, quartz, feldspar, and carbonaceous 

material. About half the shale consists of quartz and

6. Arkosic graywacke, San Francisco South quadrangle, 4,100 ft east cf peak 

1314, San Bruno Mountain, Macco-P.C.A. quarry (sample No. SF2122).

6a. Average sandstone, Clarke (1924, p. 30).

7. Shale near east end of Broadway Tunnel, Russian Hill, San Francisco

(sample No. SF-1140). 

7a. Average shale, Clarke (1924, p. 30).

8. Phosphatic nodule in shale, southeast side of Laguna Honda, San Francisco (sample No. SF-101).

feldspar. The micaceous cleavage surfaces and the carbonaceous plates and fibers are roughly parallel. The 

mica crystals are generally not alined in a and b crystallographic directions. In a slaty shale obtained from the 

excavation for the Broadway Tunnel in Russian Hill, 

San Francisco, the mica plates appear to be thicker 

than those in nonslaty shale.

Microscopic examination shows that the micaceous 

minerals in the shale contain abundant unidentified 

inclusions of colorless generally equidimensional crystals, about ^-1 micron in diameter, of moderate 

refractive index and birefringence. The mica mry be 

partly or wholly authigenic. X-ray diffraction analyses 

of the smaller-than-2-micron fraction of pulverized and 

deflocculated fresh unsheared shale show that mica 

predominates, though chlorite is abundant. Randomly
20 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

mixed layers of mica, chlorite, vermiculite, or montmorillonite are present in some shales. Kaolinite is generally absent or minor.

Soft, sheared, and plastically deformed shale within 

shear zones consists predominantly of clay minerals 

and smaller amounts of pyrite-bearing carbonaceous 

streaks, nonclay minerals, and sheared rock pieces. 

Clay minerals are mostly chlorite or randomly mixed 

layered chlorite-montmorillonite and randomly mixed 

layered mica-montmorillonite. Some sheared shales are 

mostly montmorillonite and randomly mixed layered 

montmorillonite-mica-chlorite.

The chemical composition of shale obtained from the 

excavation of the Broadway Tunnel is given in table 4. 

As compared with the average shale, most of the differences are similar to the differences between Franciscan 

graywacke-type sandstone and Clarke's (1924, p. 30) 

average sandstone.

Weathered shale consists of mica, montmorillonite 

and (or) vermiculite, and randomly mixed layered 

mica-montmorillonite. Hydrothermally altered shale 

contains these minerals and abundant kaolinite-group 

clay minerals.

CONGLOMERATE

Conglomerate lenses and pebbly zones, 1 or 2 feet 

thick and interbedded with sandstone, are rare in San 

Francisco and on Marin Peninsula but are common on 

Angel Island, Belvedere Island, and Tiburon Peninsula, where some reach a thickness of about 10 feet. At 

the north end of Simpton Point on Angel Island, conglomerate beds in sandstone are well exposed. The 

thickest is a 12-foot bed composed by volume of about 

one-third well-rounded large clasts and two-thirds 

matrix of coarse-grained graywacke. The average diameter of the large clasts is approximately 2 y2 inches and 

the maximum is 12 inches. Sandstone beds above and 

below the conglomerate bed contain isolated wellrounded clasts 2-3 inches in diameter.

Conglomerate at approximately the same stratigraphic position is also exposed about 2,400 feet east 

of Stuart Point. Here it is about 10 feet thick and consists of about 20 percent well-rounded large clasts and 

80 percent coarse-grained graywacke matrix. The average diameter of the large clasts is 1 l/2 inches, and the 

maximum is 4 inches.

Along the beach on Angel Island, halfway between 

the north end of the serpentine exposure and the point 

west of Hospital Cove, a pebbly sandstone which is 

probably at the same stratigraphic position as the conglomerates was described by Ransome (1894, p. 197) 

as follows: "The pebbles are usually disposed as bands 

in the sandstone, and not as separate sharply defined 

beds. They are generally rather flattened and of all

sizes up to 10 inches (25 centimeters) in diameter, and 

4 inches (10 centimeters) thick."

Pebbles in the conglomerate at Simptor Point, 

Angel Island, consist of graywacke, 30 percert; shaly 

sandstone and shale, 20 percent; red, brown, and black 

chert, 15 percent; black orthoquartzite, 15 percent; 

and felsite, porphyry, and intermediate volcanic rocks, 

20 percent. The pebbles in the conglomerate exposed 

east of Stuart Point are largely black or dark-gray 

chert and orthoquartzite and a small amount of graywacke.

Phosphate nodules were found in a shear zone in 

Franciscan clastic rocks near the east end of Laguna 

Honda. Nodules are 1-12 inches in diameter. T^Q. small 

nodules are massive. The largest one found consisted 

of concentric layers, 2-5 mm thick, of brown phosphatic material around an angular chert fragment (2 

by 5 by 5 in.) and angular pieces of gray limestone. 

Both phosphatic material and limestone contained 

abundant Radiolaria. X-ray diffraction analysis showed 

the phosphatic layers to consist mostly of carbonate 

fluorapatite and smaller amounts of chlorite, calcite, 

quartz, and mica. The chemical composition of a small 

massive nodule is given in table 4, column 8.

METAMORPHOSED SEDIMENTARY ROCKS

Although their clastic, sedimentary nature ir unmistakable, most of the sandstone, shale, and conglomerate of Angel and Belvedere Islands and Tiburon 

Peninsula are semischists of low metamorphic grade 

with a foliation more or less parallel to the bedding 

(fig. 15). They have been metamorphosed under conditions of the zeolite to glaucophane schist fac; ?s. The 

semischists are associated with sedimentary rocks that 

are not sheared and that otherwise appear to be 

unmetamorphosed; however, the boundaries between 

the two types are vague, and to show them separately

FIGURE 15. Semichistose coarse-grained graywacke sandstone 

of the Franciscan Formation. Campbell Point, Ange1 , Island.
CLASTIC SEDIMENTARY ROCKS 21

on the geologic map (pi. 1) was impractical. Sedimentary rocks in the vinicity of Quarry Point on Angel 

Island and on Corinthian Island are unfoliated and 

relatively unmetamorphosed, though weak metamorphism has formed pumpellyite in part of the matrix 

and in some of the quartz, feldspar, and volcanic clasts. 

Metamorphic pumpellyite is also found in the Franciscan sandstone of Alcatraz and Yerba Buena Islands 

and scattered localities in San Francisco and Marin 

Peninsula.

The semischists show several degrees of metamorphism. Those most strongly affected that retain sedimentary textures are tough and have a bluish or 

greenish cast. Foliation in the sandstone is shown by 

light-colored hard detrital grains of quartz and feldspar, which are commonly elongated and lens and 

spindle shaped. Foliation is also shown by darker material, representing easily deformed and chemically 

unstable detritus, and by carbonaceous and micaceous 

matrix, which appears as crenulated wisps wrapped 

around the larger harder grains. The shales are generally slaty. Semischistose conglomerates show characteristics of the semischistose sandstones with sheared 

lens-shaped pebbles.

Variations in degree of metamorphism are best shown 

in thin section. In the least affected sandstones, some 

large detrital grains of quartz and feldspar may have 

undulose extinction, internal cracks, or a fine-grained 

mosaic texture and a sutured lens-shaped outline. All 

the matrix and unstable detritus such as shale, greenstone, and micaceous minerals show marked effects. 

The unstable detritus is flattened and squeezed into 

streamlined clots, thin layers, veinlets, and complex 

plications around and between quartz and feldspar 

grains. Most of this material appears to be recrystallized largely to stilpnomelane, muscovite, and chlorite. 

In the least metamorphosed rocks it may be difficult 

to distinguish recrystallized micaceous material from 

mechanically distorted detrital micaceous material. 

Sparse biotite is believed to be distorted detrital material, but the microscopic evidence is inconclusive. Magnetite and other iron oxides, as well as ilmenite, leucoxene, and sphene, are commonly found in the micaceous layers. In interlaminated shale and fine-grained 

sandstone, the laminations are crenulated into tiny 

folds in which the shale is squeezed into the sandstone 

at the fold axes. Although much relict quartz and feldspar is present in the sandstone laminae, the shale is 

recrystallized largely to stilpnomelane, muscovite, 

quartz, and albite (?), and the rock has a phyllitic sheen.

In some weakly metamorphosed sandstones, some 

of the matrix and unstable detrital grains are converted 

to fibrous bundles of exceedingly small amphibole 

crystals of the riebeckite-glaucophane series, partly

replaced by chlorite and containing laths of lawsonite. 

In places, fibrous amphibole has been recrystallized to 

large euhedral crystals. Bluish-gray slaty shale consists 

mostly of fine-grained amphibole needles of the glaucophane-riebeckite series, small amounts of platy stilpnomelane, and albite veinlets.

In sandstones that have been subjected to nore 

severe metamorphism, most of the detrital quartz 

grains are recrystallized and elongated into a mosaic 

of small crystals. The dark micaceous aggregates, representing the matrix and unstable detrital grains, are 

replaced by varied mineral assemblages. Lawsoritejadeite aggregates are common, and lawsonite-qua rtzchlorite aggregates are also present. Stilpnomelane 

commonly replaces biotite and chlorite. Clots of stilpnomelane-muscovite studded with lawsonite laths are 

common in sandstones that contain much unmetmorphosed plagioclase. Leucoxene-magnetite aggregates 

are commonly associated with stilpnomelane. One semischistose sandstone shows jadeite replacing stilpnomelane in aggregates of stilpnomelane and lawsorite. 

Some metasiltstones near Knox Point consist of lawsonite, stilpnomelane, albite, and quartz. In other sandstones that are closer to serpentine borders where magnesium metasomatism was affective, tremolite instead 

of lawsonite is present.

A conglomerate exposed at Blunt Point on Angel 

Island contains bluish sheared pebbles enclosed in a 

pale-blue-green matrix with a micaceous sheen. Isolated bluish pebbles are also found in semischistose 

sandstone along the main road on the west-facing slope 

southwest of Hospital Cove. The conglomerate contains a large variety of low-grade metamorphic mineral 

assemblages that vary with the composition and text lire 

of the pebbles and their matrix. Lawsonite, in isolated 

laths and in clots of randomly oriented laths, is widely 

distributed. Bundles of fine-grained amphibole of the 

glaucophane-riebeckite series and muscovite plates are 

common in the matrix. Chlorite, jadeite, hornblende 

with glaucophane rims, and intergrowths of muscovitestilpnomelane, zoisite-lawsonite, lawsonite-stilpmmelane are found in some of the pebbles, whereas metaquartzite pebbles have little or none of these minerals. 

The pebbles and the matrix appear to have been metamorphosed after the conglomerate was deposited, for 

both appear to be of the same metamorphic grade and 

metamorphic mineral crystals traverse the boundary 

between pebbles and matrix.

Jadeite occurs in sandstone on Angel Island as 

porphyroblasts and as tiny stumpy prisms intergrcwn 

with lawsonite. The sandstone also contains quartz, 

muscovite, biotite, chlorite and, locally, glaucophane 

and veins and clots of aragonite (Coleman and Lee, 

1962, p. 581). Detrital plagioclase is generally absent.
22 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

Jadeite-bearing rock is abundant at or near intrusive 

greenstone. Point-count analyses of thin sections of 

sandstones between Campbell Point and Simpton Point 

and west of Quarry Point show 34-56 percent jadeite. 

Jadeite is also found in some of the semischistose 

sandstone near Knox Point. Bloxam (1956, p. 489) 

reported that "Franciscan graywackes are locally jadeitized***" on the shore midway between the point 

west of Hospital Cove and a greenstone exposure to 

the south.

Chemical analyses for three sandstones, including a 

jadeite-bearing semischist, a jadeite-free semischist, 

and a jadeite-free unmetamorphosed sandstone, are 

given in table 5. The analyses show that the three sandstones are remarkably similar in composition and indiTABLE 5. Chemical composition of sandstone samples of the 

Franciscan Formation from Angel Island

[Rapid-rock analyses by Paul Elmore, I. H. Barlow, S. D. Botts, 

M. S. Mack, and Gillison Chloe. N.d.=not determined]

SiO2 ......................................

A1203 ....................................

Fe2O3 ....................................

FeO ......................................

MgO ......................................

CaO ......................................

Na2O ....................................

K2O ........................................

H20+ ......................................

H20- ......................................

TiO2 ......................................

P2O5 ......................................

MnO ......................................

C02 ........................................

Total ........................

S (aqua regia soluble) ...... 

Specific gravity (powder)

l

68.2

13.0

1.6

2.7

2.4

2.0

2.2

2.2

3.2

.50

.62

.10

.08

1.0

100

N.d. 

2.75

2

69

11.7

1.0

4.2

3.8

1.3

2.0

2.3

3.3

.30

.70

.17

.09

.05

100

0.02 

2.72

3

72.9

11.3

1.1

2.8

2.7

.60

3.8

.90

2.2

.39

.56

.12

.20

.15

100

N.a.

2.62

1. Semischistose; estimated 20 percent jadeite by volume. Collected at shoreline 950 ft southeast of Campbell Point at the boundary line of state 

park (sample No. 80-RGC-58).

2. Semischistose, jadeite free; contains detrital quartz and plagioclase, 

authigenic muscovite, stilpnomelane, chlorite, lawsonite, and pumpellyite. Collected at shoreline 750 ft southeast of Campbell Point (sample 

No. S-39-A).

3. Massive, unsheared, unmetamorphosed, jadeite free. Collected 4,000 ft N. 

25° W. of Blunt Point lighthouse (sample No. 80-RGC-58-1).

cate that jadeitization and metamorphism took place 

under isochemical conditions and that sodium metasomatism was not involved.

Jadeite porphyroblasts consist of single anhedral to 

subhedral equant crystals as much as 1.5 mm in diameter or of slightly larger clots of several crystals. The 

average diameter of most of the porphyroblasts is larger 

than what is believed to have been the diameter of the 

largest detrital grains. Jadeite porphyroblasts are distributed fairly evenly and randomly in the sandstone.

Several stages in the development of jadeite porphyroblasts may be seen in thin section. In an early 

stage, the jadeite appears in straight-sided masses 

(suggesting relict plagioclase) of a fine-grained aggregate with considerable amounts of finely divided 

quartz. In a later stage, it is less clouded with quartz

inclusions but occurs in single crystals or in c^ts of a 

small number of crystals showing prominent cleavage 

fractures. At a still later stage, it is mostly transparent 

and free of inclusions, except for large lawsonite laths 

in some, and appears as large crystals or clots with 

irregular borders.

Detrital plagioclase is sparse or completely lacking 

in semischistose sandstone rich in jadeite, whereas 

jadeite porphyroblasts are sparse or absent in semischistose sandstone having approximately tl:^ usual 

complement of detrital plagioclase. This relationship 

strongly indicates that plagioclase is involved in the 

formation of jadeite porphyroblasts; however, the 

change is evidently not a direct, simple replacement of 

the albite component of plagioclase by jadeite and 

quartz, as suggested by de Roever (1955, p. 289) for 

jadeitized sandstones from the Celebes. Coleman 

(1965, p. C27) showed that the Angel Island jadeite 

contains acmite, diopside, and hedenbergite components. Plagioclase can furnish only part of the elements 

needed to form the jadeite of Angel Island; thus other 

detrital minerals must have been involved in its 

formation.

The chemical composition, physical properties, and 

paragenesis of the jadeite in the metagraywacve sandstones of Angel Island were given by Coleman (1965, 

p. 25-34). Coleman found that the jadeite cortains 86 

percent jadeite molecule (NaAlSi2O6 ) and that its unit 

cell volume is larger than pure jadeite. He suggested 

that this is evidence that the formation of impure 

jadeite in the metagraywackes of Angel Island requires 

less pressure than is required to form pure jadeite. 

Additional evidence of less pressure is the preservation 

of clastic textures and other sedimentary fertures in 

the jadeitized sandstone (Bloxam, 1956, p. 49£).

The metamorphic grade under which the semischists 

formed though it varied in intensity from place to 

place was that of the greenschist facies, comparable 

with Mutton's (1940, p. 28, 60-61) subzones Chlorite 

1 and 2 of the chlorite zone of the Otago area of New 

Zealand. Blake, Irwin, and Coleman (1967, p. C3-C5) 

described metagraywackes in northern California and 

southwestern Oregon similar to those of the S<m Francisco North quadrangle and placed them in their textural zones 1 and 2. Some of the rocks shown on the 

geologic map (pi. 1) as metamorphic rocks are also 

low-grade schists and in addition may have been graywacke-type sediments. These metamorphic rocks, however, differ from the semischists in having b^en subjected to more intense metamorphism, though nowhere 

more intense than that of the greenschist or albiteepidote-amphibolite facies. These schists are generally 

coarser in texture than the semischists, are wholly 

crystalloblastic, and are without obvious sedimentary
CLASTIC SEDIMENTARY ROCKS 23

features, such as detrital grains. They contain welldeveloped amphibole crystals of glaucophane-riebeckite and tremolite-actinolite, in addition to lawsonite, 

muscovite, albite, quartz, and other minerals. They 

correspond to metagraywackes of textural zone 3 of 

Blake, Irwin, and Coleman (1967). These schists are 

unevenly disposed through the semischists, generally 

at or near intrusive greenstone or serpentine, but many 

of them are too small to be shown on the geologic map 

(pLl).

Such an erratic distribution of metamorphic intensities is to be expected in an area where rocks that are 

heated to temperatures in the lower part of the metamorphic range by burial during orogeny are subject to 

local shearing or to reaction with chemically active 

waters (Turner, in Williams and others, 1954, p. 209). 

Evidently, the presence of the greenstone body 

influenced the formation of jadeite for the sandstone 

richest in jadeite lies near or at the border of the greenstone. Semischistose sandstones elsewhere generally 

lack jadeite. Conceivably, the border zone sustained 

high pressure during orogeny and also provided access 

for fluids needed to jadeitize the sandstone. The sodium 

content (table 5) of jadeite-bearing and jadeite-free 

sandstones from Angel Island indicates that sodium 

metasomatism was not significant in the formation of 

jadeite. Bloxam (1956, p. 493-494) found similar evidence at Valley Ford, Calif.

Inasmuch as the semischists have a greater bulk 

density than unmetamorphosed rocks, pressure probably also was a factor in jadeitization and metamorphism. High pressure is suggested also by veins and 

clots of aragonite in the jadeitized sandstone, for calcite is converted to aragonite under pressures greater 

than 4,000 bars (Coleman and Lee, 1962). Nonschistose sandstone contains calcite rather than aragonite.

Blake, Irwin, and Coleman (1969) suggested that 

the lawsonite- and jadeite-bearing metagraywackes 

formed at high-pressure low-temperature conditions 

below a regional low-angle thrust fault. The presence 

on Angel Island and Tiburon Peninsula of metagraywackes similar to those of textural zones 2 and 3 of 

Blake, Irwin, and Coleman (1967) suggests that the 

metagraywackes were not far below the postulated 

thrust fault.

Hard dark-gray siliceous beds are abundant in sandstone near some greenstone bodies. The siliceous beds 

are silicified shale beds and quartz veins following 

bedding planes. They are %-l l/2 inches thick and are 

separated by three-fourth of an inch of sandstone in 

some places and 3-5 feet of sandstone in other places. 

Where they are abundant and closely spaced, they are 

cut by abundant white quartz veinlets as much as 

one-half inch thick.

Weathering of metamorphosed sedimentary rocks 

produces sandy silty soils as much as 20 feet thick and 

weathered rock as deep as 70 feet. The chief clay mineral of the soil and weathered rock is vermiculite, which 

causes the soil to become plastic and lose considerable 

shearing strength when wet.

OCCURRENCE 

NORTHEASTERN SAN FRANCISCO

Clastic rocks of the Franciscan Formation are the 

predominating bedrock in northeastern San Francisco. 

Russian, Nob, and Telegraph Hills consist of two 

sequences of massive sandstone and of two thick 

sequences of shale and thin-bedded sandstone. The 

observed thickness of the four units is 3,650 feet. The 

basal 400 feet of the youngest sequence, a massive 

sandstone, is exposed on the north side of Russian Hill. 

It overlies a shale and thin-bedded sandstone sequence, 

1,350 feet thick, exposed on the south side of Russian 

Hill and on the west side of Telegraph Hill. The shale 

and sandstone sequence in turn overlies a lower massive sandstone 700 feet thick, exposed on the north side 

of Nob Hill and over most of Telegraph Hill (fig. 16). 

The oldest sequence is shale and thin-bedded sandstone of which the upper 1,200 feet is exposed. It constitutes most of Nob Hill.

Bedrock on hills such as Rincon Hill and the hills 

west of Van Ness Avenue is so poorly exposed, primarily because of intense urban development, that recognition of the predominant lithology was impossible. 

Consequently, the bedrock in those areas has been 

mapped as undifferentiated sandstone and shale of 

the Franciscan Formation.

FIGURE 16. Massive sandstone of the Franciscan Formation. 

Old quarry face on the northeast side of Telegraph Hill, San 

Francisco.
24 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

CENTRAL HIGHLANDS OF SAN FRANCISCO

The sandstones exposed in the central highlands of 

San Francisco also appear to be in two separate stratigraphic positions. The older sandstone is found at the 

south base of Mount Sutro and south of Laguna 

Honda. Radiolarian chert, which forms Mount Sutro, 

separates it from younger sandstone exposed in the 

hills south of Mission Dolores. The younger sandstone 

sequence may be the same sandstone as that exposed 

to the northwest between Buena Vista Park and Mount 

Olympus. In other parts of this area, thin sandstone 

lenses are found interbedded with radiolarian chert 

and greenstone.

The sandstone at Laguna Honda evidently lies 

nearly horizontal, judging from the attitude of its contact with overlying radiolarian chert; if so, no more 

than 200 feet of sandstone is exposed here. The sandstone in the hills south of Mission Dolores dips northeastward and is at least 1,000 feet thick. In the area 

between Buena Vista Park and Mount Olympus, the 

sandstone is only about 400 feet thick.

CLIFF HOUSE TO BAKERS BEACH

Clastic rocks of the Franciscan Formation form the 

100- to 200-foot-high cliffs along the shore from Cliff 

House to Bakers Beach and also occur at the north 

border of Bakers Beach (fig. 17).

In the cliffs from Cliff House to the shear zone west 

of Lands End, massive sandstone is interbedded with 

100-foot-thick sequences of shale and thin-bedded 

sandstone. The estimated total thickness is 750 feet. 

Because of its high potassium feldspar content, the 

sandstone here is believed to be correlative with sandstone exposed at San Bruno Mountain, 6 miles to the 

southeast. Mapping by Bonilla (1961) in the San 

Francisco South quadrangle disclosed the northwesttrending City College fault, which separates the 

potassium feldspar-bearing sandstone at San Bruno 

Mountain from the potassium feldspar-free sandstone 

northeast of San Bruno Mountain. The shear zone at 

Lands End is thought to be an extension of the City 

College fault and similarly is the east boundary of 

potassium feldspar-bearing sandstone. East of the

FIGURE 17. Thick-bedded sandstone, laminated sandstone, siltstone, and shale of the Franciscan Formation. Note a small offset 

in closeup view (right). Near the north end of Bakers Beach, San Francisco.
CLASTIC SEDIMENTARY ROCKS 25

shear zone, the sandstone exposed from the east side 

of the landslide east of Lands End to Bakers Beach is 

approximately 2,400 feet thick, if the section is a continuous homocline, and dips 30°. This thickness 

excludes the serpentine at Phelan Beach. In a few 

places near Bakers Beach, sedimentary features, such 

as small-scale channels and graded bedding, suggest 

that part or all of this sequence is overturned.

FORT POINT TO POTRERO HILL

Sedimentary rocks mostly sandstone and volcanic and metamorphic rocks occur as tectonic inclusions in a northwest-trending zone of sheared rocks 

between Fort Point and Potrero Hill. The largest exposed sandstone body is the northeast spur of Potrero 

Hill. Sandstone predominates in the sheared-rock zone 

between the U.S. Mint and the Presidio, although most 

of this area is shown by Lawson (1914) as a continuous 

body of serpentine.

MARIN PENINSULA

On Marin Peninsula sandstone occurs in at least 

three separate sections. The oldest sequence, Lawson's 

(1914) Cahil Sandstone, now abandoned, is about 

1,700 feet thick and is exposed along the northeast 

shore in a narrow belt that widens on the ridges to the 

northwest of Sausalito. It is overlain by greenstone and 

radiolarian chert beds 1,000-3,000 feet thick, which in 

turn are overlain by younger sandstone, about 1,000 

feet thick, exposed from near Lime Point nearly continuously for more than 2 miles along its strike. The 

roadcut on the west side of U.S. Highway 101, about 

three-fourths of a mile north of the Golden Gate 

Bridge, contains an excellent exposure of the lower 

part of this sandstone (fig. 18). The youngest sandstone sequence crops out about one-third mile west of 

the second sandstone. It is largely concealed by slope 

debris and ravine fill, but it is nearly continuous for 

about one-third mile from the large exposure shown on

FIGURE 18. Thick-bedded sandstone interbedded with shale and thin-bedded sandstone of the Franciscan Formation. West side 

of U.S. Highway 101, three-fourths of a mile north of Golden Gate Bridge, Marin Peninsula.

534-039 O - 74 - 3
26 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

the geologic map (pi. 1) northwest toward the buildings in Fort Baker. Thin belts of sandstone are also 

exposed to the west and southwest.

ANGEL AND ALCATEAZ ISLANDS AND TIBUEON PENINSULA

Sandstone is exposed over about two-thirds of Angel 

Island and makes up the crest of Mount Caroline 

Livermore (fig. 5). Almost all the clastic rocks here, 

however, are semischistose. (See page 20.) The southwestward-dipping section from Campbell Point to Hospital Cove is approximately 1,300 feet thick. The 

northward-dipping section from the south shore, west 

of Blunt Point, to the top of Mount Caroline Livermore is estimated to be at least 1,850 feet thick. An 

intervening greenstone body, 350 feet thick, was 

excluded from this total. Ransome (1894, p. 198) 

believed that 2,000 feet is a conservative estimate for 

the exposed thickness of the sandstone on Angel Island.

In the 1860's, sandstone of the Franciscan Formation was quarried on Angel Island and used extensively 

as a building material. The rock is described as being 

"used chiefly for purposes where beauty and strength 

are not absolutely required, for this rock possesses 

neither qualification in a high degree, although it 

answers very well for ordinary uses in the mild and 

equable climate of San Francisco." (Whitney, 1865, 

p. 77).

Conglomerate is interbedded with sandstone at the 

north end of Simpton Point on Angel Island. It also is 

exposed about 2,400 feet east of Stuart Point. A pebbly 

sandstone crops out along the west shore, north of the 

serpentine body. All these occurrences probably have 

the same stratigraphic position. (See p. 20.)

Alcatraz Island, except for a few patches of surficial 

material, is entirely sandstone and a few minor shale 

beds. The exposed thickness of rocks is approximately 

500 feet.

On Tiburon Peninsula exposed sequences of sandstone and shale are 5-30 feet thick. Much of the southeastern part of Belvedere Island, as well as a narrow 

belt along the southwest shore, is sandstone that contains small amounts of shale. The exposed thickness of 

clastic rocks on Belvedere Island is estimated to be 

1,000 feet. Most sandstone and shale on Tiburon Peninsula and Belvedere and Corinthian Islands are semischistose.

ORIGIN 

ENVIRONMENT OF DEPOSITION

A marine environment of deposition for the Franciscan Formation is indicated by fossils and the interbedded greenstones. Though sparse, marine Mollusca 

are found in clastic rocks, and Radiolaria, today exclusively marine, are found in chert and in some limestone.

Radiolarian chert is also found interbedded with greenstone flows which generally show pillow structure, a 

characteristic of subaqueous eruption.

Formations equivalent in age to the Franciscan, such 

as the Knoxville Formation of Late Jurassic age and 

the Lower and Upper Cretaceous beds of the Great 

Valley sequence, represent slope and shelf sedimentation in the less orogenically active parts of the continental crustal plate that existed in Franciscan time 

(Irwin, 1957, p. 2292). The juxtaposition of the eugeosynclinal facies and the shelf and slope facies in the 

modern Coast Ranges is a result of movements of the 

oceanic and continental crustal plates.

Turbidity currents are thought to be capable of carrying sand to deep-sea basins (Kuenen, 1950, p. 360, 

367; Shepard, 1951, p. 53-65). Kopstein (1954, p. 63) 

suggested also that the high velocities measured on 

submarine turbidity currents render them capable of 

carrying "large pieces of gravel." The materials for the 

sparse conglomerate beds and for the massive nongraded or poorly graded graywacke sandstone beds and 

the interbedded shale and thin-bedded sandstone 

sequences were probably transported by turbidity currents. A mass of unconsolidated detritus of various 

sizes from clay to sand or possibly gravel, lying on a 

nearshore submarine slope, could be set in motion by 

an earthquake or by storm waves (Heezen and Ewing, 

1955, p. 2505-2514). The ensuing slump or mudflow 

could become a turbidity current of sufficient energy 

to transport most of the material en masse (Kuenen, 

1951, p. 31). Thin graded beds of sand and mud would 

be deposited by weaker turbidity currents. Many of the 

lithologic features of the Franciscan clastic sedimentary rocks, such as graded bedding, small-scale channeling, ripple marks, and cross-bedding, can also be 

produced by turbidity currents. Thus preponderant 

evidence suggests a deepwater turbidity current origin 

for the Franciscan Formation.

As compared with Kopstein's (1954, p. 92-93) tabulation of evidence for deepwater accumulation of graywacke in Harlech Dome, Wales, further evidence for 

deepwater accumulation in the Franciscan includes 

the following: (1) Shale layers between graded beds; 

(2) extremely rare fossils; (3) abundant small-scale 

current bedding; (4) current-ripple marks; and (5) 

graded bedding, load casts, flow markings, slump structures, and associated features in some sections. Furthermore, Sanders and Swinchatt (1957, p. 1791) 

believed that the deepwater hypothesis is correct for 

the origin of the radiolarian cherts of the Franciscan 

Formation.

Local large masses of greenstone in the Franciscan 

Formation have been compared with seamounts 

(Bailey and others, 1964, p. 43), and limestone asso-
CLASTIC SEDIMENTARY ROCKS 27

dated with greenstone may represent corals and other 

shallow-water calcareous organisms that grew on the 

tops of seamounts (E. H. Bailey, oral commun., 1966). 

Gradation of greenstone agglomerate and tuff into 

tuffaceous sandstone and nontuffaceous sandstone 

could have been brought about by reworking and mixing of detritus from land and from pyroclastic rocks 

erupted during Franciscan time.

SOURCE AREA OF DETRITUS

Clasts in the conglomerates and sandstones provide 

a few clues to the kinds of rocks present in their source 

area. The coarse clasts consist of (1) sandstone, shale, 

and volcanic rocks similar to those of the Franciscan 

Formation and (2) cherts, quartzites, volcanic, or dike 

rocks unlike those of the Franciscan Formation. The 

sandstones of the quadrangle also contain clasts of 

granitic rocks, metamorphic rocks, and serpentine as 

well as clasts of the same rock types as the conglomerates. Heavy mineral grains in the sandstone are predominantly mica, chlorite, vermiculite, epidote, clinozoisite, pumpellyite, sphene, and garnet, which are 

common in low-grade metamorphic rocks and altered 

volcanic rocks. The heavy mineral fraction of the sandstones contains only small amounts of zircon, rutile, 

and tourmaline, which are minerals characteristic of 

granitic and related metamorphic rocks. Chromian 

spinel found in sandstone near the Golden Gate Bridge 

and on Angel Island was probably derived from ultramafic rocks or from an older sedimentary rock containing ultramafic-rock detritus.

Abundant clasts of Franciscan-type sandstone, shale, 

and volcanic rocks in the sandstones and conglomerates suggest "self digestion" of topographically high 

areas above or below sea level created by local orogeny 

during the time of accumulation of the Franciscan Formation. The clasts may also have been derived, however, from pre-Franciscan sedimentary and volcanic 

rocks. In addition, sandstone and conglomerate clasts 

suggest a source area containing metamorphic and nonFranciscan volcanic rocks. Rocks similar to the clasts 

are exposed in the Coast Ranges, Klamath Mountains, 

and Sierra Nevada (fig. 6). Bailey, Irwin, and Jones 

(1964, p. 39-41) are in agreement with these conclusions.

Taliaferro (1943, p. 136-139, 141) suggested that 

granodiorite was a prominent rock in the landmass 

from which the Franciscan Formation was derived. 

Granodiorite or other granitic rock types, however, 

were not seen in the Angel Island conglomerates and 

are rare in other conglomerates in the quadrangle. If 

detrital grains had been derived from a granodiorite terrane, their composition would be similar to the constituents of the granodiorite and would be present in 

similar proportions. The freshness of the plagioclase 

grains and their angularity indicate that mechanical 

weathering predominated over chemical weathering. 

The composition of detrital grains is more similar to 

metamorphic and volcanic rocks.

The average granodiorite, according to Johannsen 

(1932, p. 321), contains 22 percent potassium feldspar. 

The granodiorite or quartz diorite of Montara Mountain (14 miles south of the mapped area), which would 

be typical of a possible local source rock, contains as 

much as 10 percent potassium feldspar. The complete 

absence of detrital potassium feldspar in most of the 

Franciscan sandstones in the map area precludes 

granodiorite as a prominent rock type in the source 

area. And as potassium feldspar is generally more 

stable than plagioclase feldspar, which is present in the 

sandstone, the absence of potassium feldspar cannot be 

attributed to differential alteration of the sand grains.

The mapped area is too small and the data are too 

sparse to determine the direction of the source area 

from textural coarsening and thickening of conglomerate beds. A few linear structures suggest a westerly 

current flow, such as casts of striations and grooves on 

the bottom of sandstone beds in the section nearest 

the Golden Gate Bridge in Marin County.

AGE 

FOSSILS

Fossils have been found at only five localities in 

clastic rocks of the Franciscan Formation in this quadrangle. The only ones useful for age determination are 

Cretaceous in age.

At an unnamed point along the shore of the South 

Bay of the Golden Gate, 800 feet west of James D. 

Phelan Beach State Park (pi. 1), the ammonite Douvilleiceras sp. was found in beds called Marin Sandstone by Lawson. This ammonite is thought to be of 

late Early Cretaceous (Albian) age (Schlocker and 

others, 1954, p. 2373). At this same locality, the writer 

found a gastropod resembling the holotype of Paladmete perforata and an undetermined pelecypod, but 

neither was useful for age designation.

On the beach north of the Needles on Marin Peninsula near the Golden Gate Bridge, Mantelliceras sp. 

was found in float probably derived from nearby sandstone beds. This ammonite is thought to be of Early or 

early Late Cretaceous age "a little later than that given 

for the specimen of Douvilleiceras***" (Hertlein, 1956, 

p. 1987.).

Pelecypod casts from an unrecorded locality on Alcatraz Island were described by Gabb (1869, p. 193), by 

Stewart (1930, p. 106), and by Anderson (1938, p.
28 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

121). According to Stewart, Inoceramus ellioti Gabb is 

probably Cretaceous in age. Anderson assigned an 

Early Cretaceous (?) age to Lucina alcatrazis.

Bailey, Irwin, and Jones (1964, p. 115-123) discussed fossils in the Franciscan Formation in localities 

outside the quadrangle.

Gray limestone lenses found in sheared sandstone 

and shale north of Sausalito contain Radiolaria preserved in calcite and pyrite. Similar Radiolaria are 

abundant in phosphate nodules found in sheared sandstone and shale near the southeast end of Laguna 

Honda Reservoir. The Radiolaria are similar in appearance to some of those in the chert and shale described 

by Hinde (1894, p. 235-240) and by Riedel and 

Schlocker (1956, p. 357-360). The Radiolaria indicate 

a Jurassic or Cretaceous age.

In a discussion of the phosphate nodules just mentioned, Dickert (1966, p. 292) attributes to W. R. Evitt 

the identification of Marthasterites tribrachiatus [recte 

Discoaster tribrachiatus Bramlette and Sullivan], a 

calcareous nannofossil known from the California Eocene. Evitt reexamined the thin section studied by 

Dickert and reported (W. R. Evitt, oral commun., 

1966) that the objects referred to are not Marthasterites or triradiate discoasters but are probably radiolarian spines with conspicuously triradiate cross 

sections. Similar fossils are common in Franciscan 

cherts north of the Golden Gate in association with 

abundant Radiolaria. According to Evitt, no age significance should be attached to these triradiate objects; 

certainly they should not be the basis for suggesting 

that Tertiary fossils have been identified from Franciscan sedimentary rocks.

Lawson (1914) thought that the oldest formation of 

his Franciscan Group was the Cahil Sandstone; he 

thought this sandstone was separated from the overlying Marin Sandstone by the Sausalito Chert. Lawson's Cahil Sandstone contains the foraminiferal Calera 

Limestone Member, whose type locality is along the 

Pacific Ocean shoreline 9 l/2 miles south of the map 

area. Foraminifera from the Calera Limestone are considered to be early Late Cretaceous in age (Kupper, 

1956, p. 41). Inasmuch as Lawson referred to the DouitfMeiceros-bearing sandstone of late Early Cretaceous 

age as his Marin Sandstone, paleontologic data suggest 

that his Cahil Sandstone is younger than his Marin 

Sandstone.

SIGNIFICANCE OF POTASSIUM FELDSPAR CONTENT

Bailey and Irwin (1959) and Bailey, Irwin, and 

Jones (1964, p. 138-141) determined the potassium 

feldspar content of the thick shelf and slope facies 

(miogeosynclinal) in the Coast Ranges (Great Valley

sequence) along the west border of the Sacramento 

Valley. This facies represents almost continual deposition from Late Jurassic to Late Cretaceous time. The 

same workers determined the potassium feldspar content of Mesozoic miogeosynclinal sandstone in other 

parts of the Coast Ranges. They found that the potassium feldspar content increased systematically with 

decreasing age of the rocks. The median potassium 

feldspar content of Upper Jurassic rocks was found to 

be less than 0.5 percent, Lower Cretaceous rocks 1.1 

percent, and Upper Cretaceous rocks 13 percent. 

Bailey, Irwin, and Jones (1964) suggested that all the 

sediments were derived from the same source the 

ancestral Sierra Nevada and the Klamath Mountains 

of northwestern California, which were created during 

the Nevadan orogeny. As the granitic rocks emplaced 

during the orogeny became increasingly exposed and 

eroded, the potassium feldspar content of the sediments derived from them increased. Thus the potassium feldspar content should roughly indicate the age 

of the sediments.

Contradictions unfortunately arise, however, when 

this dating technique is applied to the Franciscan Formation in the mapped area, with the assumption that 

the rocks have all been derived from the same source 

area. Thus, the complete absence of potassium feldspar 

in most sandstones (fig. 14, table 2) would indicate a 

Jurassic age; yet, the fossils are of late Early or early 

Late Cretaceous age. Bailey, Irwin, and Jones (1964, 

p. 141) suggested, therefore, that the assumption of a 

similar source for all the rocks is invalid and that the 

absence of potassium feldspar is not necessarily diagnostic of age.

A notable exception to the general absence of potassium feldspar in Franciscan sandstone is the sandstone 

in the Point Lobos area, which averages 5-10 percent. 

On the basis of its high potassium feldspar content and 

its location southwest of a shear zone thought to be 

part of the City College fault (see section "Cliff House 

to Bakers Beach"), this sandstone may be part of the 

Great Valley sequence sandstone of San Bruno Mountain (Bailey and others, 1964, pi. 1).

The high (about 20 percent) potassium feldspar content of the sandstone of San Bruno Mountain together 

with such features as well-developed bedding, abundant 

flow casts and load casts, and general absence of shearing and interbedding greenstone and chert suggest that 

this sandstone is part of the miogeosynclinal (Great 

Valley sequence) facies. Lawson (1914, p. 17) also 

noted that this sandstone "differs from the usual sandstone of the Franciscan."

Thus, until more is known of the structure of the 

Franciscan Formation and better indications of its age 

are found, the age of the Franciscan in the quadrangle
GREENSTONE 29

must be considered to range from probable Jurassic to 

Early and Late Cretaceous. The probable Jurassic age 

is suggested by the Jurassic or Cretaceous age of the 

Radiolaria and by the presence of a great thickness of 

Franciscan Formation lying structurally below the 

sandstones containing the Cretaceous fossils.

GREENSTONE

Greenstone is a term often used for dark igneous 

rocks of indeterminate composition and origin having 

green alteration products. The greenstone of the San 

Francisco area is mostly fine- and medium-grained 

basalt that has been subjected to little or no alteration 

and basalt that has been subjected to a variety of local 

and regional conditions of alteration and metamorphism. The most common types are unmetamorphosed 

moderately fresh basalt and rocks of basaltic chemical 

composition that consist mostly of pumpellyite, primary 

pyroxene, albite, and chlorite. Some of these are rich 

in albite and could be called spilite. The metamorphosed greenstone of Angel Island, Belvedere Island, 

and Tiburon Peninsula also contains epidote, lawsonite, 

glaucophane, hydrogarnet, vesuvianite, and pyroxene. 

The field identity of many of these rocks is obscured by 

their dense, aphanitic texture, by close fracturing, and 

by a variety of chemical and mineralogical alterations. 

The bulk of the greenstones are probably flows and 

commonly show pillow structure. Pyroclastic rocks are 

present in smaller volume. Except for thin intrusive 

rocks in radiolarian chert, intrusive greenstone could 

not be distinguished from flows.

A long history of volcanism during Franciscan time 

is indicated by the large masses of greenstone that 

occur throughout thick sections of clastic rocks and 

radiolarian chert.

Like other rocks of the Franciscan Formation, greenstone forms the hilly parts of the quadrangle. The widespread alteration of greenstone to rock containing 

substantial proportions of clay minerals and the almost 

universal randomly oriented close fracturing of greenstone are important influences on its natural slopes. 

With a few notable exceptions, greenstone slopes are 

smooth and subdued. Small landslides of weathered 

greenstone debris are common.

On Marin Peninsula several large valleys, such as the 

one draining into Horseshoe Bay and the one draining 

into the Golden Gate channel west of Lime Point, are 

bordered by greenstone and presumably have been cut 

into it (fig. 7). Fresh greenstone is very resistant to 

erosion in such places and forms the steep sea cliffs and 

stacks at Stuart, Knox, and Blunt Points on Angel 

Island, Point Diablo along the Golden Gate channel, 

Marin Peninsula, and Lands End (figs. 19, 20).

FIGURE 19. High cliff of greenstone in the Franciscan Formation. Lime Point, Marin Peninsula, viewed north. The cliff 

rises 400 feet above the water in the Golden Gate. In the 

middle distance, greenstone underlies white to light-gray 

slopes; radiolarian chert or graywacke sandstone and shale 

underlie dark slopes. West (left) side of Golden Gate Bridge 

tower rests on greenstone; east side rests on radiolarian chert 

and shale.

FIGURE 20. Greenstone and radiolarian chert of the Franciscan Formation. Point Diablo, north shore of Golden Gate, 

Marin Peninsula, on the west edge of the quadrangle. The 

top of the cliff is 625 feet above sea level. Conspicuously 

jointed rock at the extremity of the point is greenstone 

(KJg). It is separated by an east-west shear zone from radiolarian chert (KJc) that makes up the ridge and the lightcolored base of the cliff. Greenstone also makes up the 

central part of the cliff.

MEGASCOPIC FEATURES

Fresh greenstones is dark gray or greenish gray. 

Moderately altered greenstone is grayish green and 

grayish olive. The greenstone most commonly exposed 

is weathered or hydrothermally altered rock of moderate brown, dark yellowish-brown, or moderate reddish-brown colors. Hydrothermally altered greenstone
30 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

is also grayish orange and light yellow green at and 

near radiolarian chert contacts.

Iron and manganese oxidation and hydration products are commonly found as thin films of bluish black, 

brownish black, yellow, orange, or brown on joints in 

altered greenstone. Brown and yellow-green waxy 

scaly deposits of nontronite, %-4 mm thick, are also 

commonly found in joints. Moderately fresh greenstone may have the deceptive appearance of being 

highly altered, owing to the presence of thin earthy 

coatings of nontronite and iron oxides on joints. Most 

of the freshest pillow basalt near Horseshoe Bay has a 

unique greenish-gray vitreous or pitchy luster that is 

caused by a thin film of slightly sheared chlorite on 

slickensides, joints, or small faults. Fresh fractures in 

this rock usually reveal a medium- to fine-grained holocrystalline rock.

On Angel and Belvedere Islands and on Tiburon 

Peninsula, bright blue joint fillings of crocidolite, as 

much as one-eighth inch thick, are conspicuous in 

exposures of reddish-brown altered metamorphosed 

greenstone.

Closely spaced fractures are the most conspicuous 

structural feature of greenstone (fig. 21). Most of the 

greenstone is so thoroughly fractured that it shatters 

easily into pieces %-% inch across. Only rare exposures yield coherent pieces more than 2 inches across. 

Examples of the latter rock are the aphanitic and 

porphyritic greenstone on Anza Street on the northwest side of Lone Mountain in San Francisco and the 

fresh aphanitic pillow greenstone exposed in the west 

roadcut on U.S. Highway 101, 400 feet northwest of 

the Waldo Tunnel in Marin County. Fracturing may 

have resulted from violent steam explosions during

FIGURE 21. Close random fracturing in greenstone of the 

Franciscan Formation. South side of road to Sausalito, 0.1 

mile east of U.S. Highway 101, 0.8 mile north of Lime Point, 

Marin Peninsula. The wood stake lying on the cut is 1 foot 

long.

submarine eruptions, rapid cooling-shrinkage of the hot 

lava and pyroclastic deposits, or severe and repeated 

stresses during several erogenic episodes.

Tectonic breccias occur in 5-40-foot-wide zones 

bordering some faults. In these zones the rock has been 

fractured, sheared, and crushed and now consists of 

rock pieces, %-l inch in diameter, in a matrix of 

crushed rock of sand and smaller sizes. It is difficult to 

distinguish such breccias from agglomerates and volcanic breccias. Because of small-scale random fracturing, massive and pillow greenstone at many exposures 

shows rough surfaces that consist of irregularly shaped 

l/s~l/2 -inch projections, which give the rock the appearance of a tectonic or volcanic breccia.

In some exposures of massive greenstones, in contrast with randomly oriented fracturing, fractures at 

intervals of %-l inch are open, parallel, and straight 

sided. Subparallel, irregular, and discontinuous vesicles 

also occur in such exposures. Both the fractures and 

vesicles have drusy linings of calcite, doubly terminated 

quartz, zeolites, and hematite. These fractures and 

vesicles apparently formed during and shortly after 

solidification of the lava.

Pillow structures occur in basalts, in altered rocks of 

basaltic composition, and in spilites. Pillow structures 

are rounded bodies 6 inches to 10 feet across in greatest dimension. About half the observed pillows are 8-10 

inches thick, 12-18 inches wide, and 18-24 inches long, 

although some exposures consist predominantly of 

pillows several times as large. Exposures consisting 

predominantly of large pillows also contain some 

smaller more spherical pillows, 6-10 inches in diameter. 

Small pillows tend to be spheroidal rather than ellipsoidal, although spheroidal pillows 4 feet in diameter have 

been seen.

The surfaces of adjacent pillows are closely conformable in shape to one another, like mold and cast, but 

are separated by gaps of clayey material %-12 inches 

thick. Pillows are generally shaped like a stretched-out 

bun and usually have a convex top and a flat or concave 

bottom (fig. 22). The bottom of some pillows projects 

downward between adjacent underlying pillows. The 

middle of the upper surface of some pillows is depressed; such pillows commonly are overlain by pillows 

with convex bottoms that conform in shape to the pillows below. Commonly, a pillow may consist of a thick 

central part and a thinner appendage.

In the Fort Baker area of Marin County, the bottom 

surfaces, top surfaces, and longest axes of the pillows 

are roughly parallel to the bedding of the enclosing 

rocks (fig. 23).

The outer surface of a fresh pillow commonly consists of a rind of slickensided chloritic material about 

one-fourth inch thick, evidently derived from glass
GREENSTONE 31

FIGURE 22. Basalt pillows. The basalt here consists of pumpellyite, pyroxene, albite, and chlorite. West side of U.S. Highway 101,

1% miles north of Golden Gate Bridge, Marin Peninsula.

formed by rapid cooling. Shallow chlorite-lined joints 

about %-2 inches apart thoroughly dissect the surface and probably also formed during cooling. Other 

pillows have a rough, knobby surface. Some pillows 

show a slight increase in grain size from rim to center, 

although many fresh pillows are wholly aphanitic or 

consist wholly or partly of fine- to medium-grained 

varioles. Many pillows are cut by strong radial joints 

and veins. The radial joint pattern is considerably 

modified in many pillows by a swarm of randomly 

oriented onionskinlike fractures that cut the pillow 

into pieces not more than one-half inch thick (fig. 24). 

Although most pillows are not vesicular, the outer 

3-4 inches of a few are spotted with sparse vesicles or 

amygdules as large as three-eighths inch in diameter. 

Still others have vesicular cores and solid rims.

Pillow structure is readily discerned in fresh and 

moderately altered greenstone, but it can also be 

recognized even in highly altered greenstone by strong 

curving joints that follow the surface of the pillows. 

Intense shearing and brecciation, however, destroy 

pillow structure.

Most pillows are separated from each other by soft 

sheared chloritic or nontronitic clay, which may be 

easily dug out with the fingers. Rarely, the material 

between the pillows is massive red or green chert or

green, gray, or brown limestone. The separation of the 

pillows averages 1 inch or less; a greater spacing is not 

uncommon, but a spacing as great as 1 foot is rare.

Massive greenstone is second to pillow greenstone 

in abundance. Although its altered and fractured condition does not permit positive identification of flow 

surfaces or flow jointing or banding, much of this 

greenstone is believed to have originated as submarine 

lava flows, for it is closely associated with pillow 

greenstone, radiolarian chert, and marine sandstone. 

The better exposed contacts show that the association 

of massive greenstone with these rocks is probably 

depositional, although many contacts are abrupt faults 

perpendicular to the bedding. Tabular bodies of massive and pillow greenstone, 2-5 feet thick, intrude 

radiolarian chert and shale. At some places they connect with irregular intrusive bodies 10-15 feet in 

diameter. In a few localities massive greenstone is 

moderately vesicular or amygdaloidal.

Pyroclastic greenstones are minor in bulk, but they 

occur in small volume in nearly every greenstone 

terrane. One of the largest exposures of pyroclastic 

greenstone is on Marin Peninsula facing Golden Gate 

channel at the west end of a small elbow beach, 3,500 

feet west of Lime Point (fig. 34). The rocks are bedded 

tuffs, lapilli tuff, agglomerate, and pillow lava, all dip-
32 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 23. Pillow basalt of the Franciscan Formation, Fort 

Baker, Marin Peninsula. Dip down to the right (southwest) 

is suggested by the orientation of the pillows.

ping 35°-45° southwest. The lowermost bed, about 

130 feet thick, is mostly grayish-orange laminated tuff 

containing a few lenses and nodules of red and green 

chert and a few agglomeratic beds made up of 1-4- 

inch ejecta. The overlying bed, about 40 feet thick, is 

predominantly agglomerate, but it contains a few thin 

flows of pillow lava and a few beds of lapilli tuff. The 

topmost part of the section, about 80 feet thick, is 

mostly pillow lava with a minor amount of lapilli tuff. 

Reddish-brown tuff, lapilli tuff, and agglomerate beds, 

closely associated with pillow lava, can also be seen 

at Stuart Point, Knox Point, and northwest of Simp ton 

Point on Angel Island, on Sunset Heights (near 16th 

Avenue and Moraga Street) in San Francisco, and on 

Twin Peaks at the south edge of the quadrangle.

In the volcanic breccias and agglomerates, pieces as 

much as 1 inch in diameter are most abundant; pieces 

1-4 inches in diameter are common, but larger pieces 

are uncommon. Greenstones at the shore between 

Campbell and Simpton Points on Angel Island contain rounded blocks, as large as 1 foot in diameter,

that appear to be ejecta that may be transitional 

between volcanic breccia and pillow lava.

Tuff beds are sparse. They are of fine to medium 

grain size. Some of them may have been mapped inadvertently with sandstone and shale of the Franciscan 

Formation, which they resemble closely and into which 

many of them grade.

Pipelike bodies of greenstone consisting of unsorted 

breccia pieces as much as 8 inches in diameter may 

be volcanic vent fillings.

MINERALOGY-MICROSCOPIC FEATURES

The greenstones of San Francisco are aphanitic to 

fine-grained altered basalts or altered rocks of basaltic 

chemical composition. The most common textures in 

basalt are intergranular, intersertal, or subophitic. 

Ophitic and hyaloophitic textures are less common. 

One thin section may show several textures. Glomeroporphyritic clots of pyroxene ophitically enclosing 

plagioclase laths are common in some pillow basalts. 

Other pillow basalts contain intergrowths of plagioclase and pyroxene laths that are arranged radially as 

varioles in the outer half of the pillow. Plagioclase 

laths in basalt reach lengths of 2 mm and vary from 

andesine to labradorite (An35-An62 ). Pyroxene is 

mostly augite and pigeonitic augite; pigeonite and 

titaniferous augite are less common. One rock contains aegerine microphenocrysts, whereas the groundmass pyroxene is plumose augite. Magnetite and 

ilmenite are abundant in basalt. Olivine, now replaced 

by chlorite, is found in a few basalts. Pyrite is common in small amounts in the freshest basalts; apatite 

is rare. A medium-grained basalt northwest of Horseshoe Bay contains about 5 percent quartz; its major 

constituents are labradorite and augite.

The basalts are all altered to varying degrees. Nontronite, chlorite, and leucoxene are common alteration products. Plagioclase cores in some basalts are 

altered to epidote and micaceous minerals. Veinlets 

of secondary minerals are abundant and include nontronite, chlorite, calcite, aragonite, quartz, stilpnomelane, hematite, leucoxene, zeolites, pumpellyite, and 

albite. The occurrence of veinlets of pumpellyite, albite, 

and laumontite indicates a gradational metamorphism 

to the zeolite facies (Turner and Verhoogen, 1960, p. 

532). Tiny amygdules, about 1 mm in diameter, are 

found sparsely disseminated in most basalts. Most of 

them are chlorite and (or) nontronite. Larger amygdules are abundant in some rocks and consist of 

prehnite, calcite, and mordenite, enclosed and veined 

by chlorite and nontronite.

The altered rocks of basaltic composition (zeolite 

facies) contain relict basalt textures preserved mostly
GREENSTONE 33

by primary pyroxene and by albite (An0-Ane ), 

chlorite, and pumpellyite. Some rocks contain abundant altered olivine phenocrysts; others may be spilites (Turner and Verhoogen, 1960, p. 258). Pyroxene 

is generally pigeonitic augite or titaniferous augite, 

though it is absent in some rocks. Nontronite, stilpnomelane, quartz, calcite, epidote, sphene, amphiboles, 

zeolites, iron oxides, and leucoxene are also present. 

Calcic plagioclase is generally sparse.

Greenstones on Rincon Hill, Russian Hill, Lone 

Mountain, and a few other locations are porphyritic 

with plagioclase (albite, An6, to andesine, An43 ) 

phenocrysts as much as 5 mm long set in a dense, 

cryptocrystalline groundmass. Albite is also in the 

groundmass as microlites or as clear rims on cloudy 

plagioclase microlites.

Another common type of greenstone is typified by 

the dense pillow lavas exposed in cuts along U.S. 

Highway 101, 1,000 feet north of the twin vehicular 

tunnels in Marin County; this rock consists largely 

of varioles, a few microphenocrysts, and abundant 

veinlets. Most of the varioles are radially arranged 

laths of pumpellyite, diopsidic augite, and albite, small 

amounts of opaque ores, and sphene. Pyroxene and 

olivine microphenocrysts are replaced by chlorite and 

pumpellyite; calcite, quartz, albite, and pumpellyite 

veins cut the rock. Veinlets of quartz with borders of 

pumpellyite needles are very common.

WEATHERING AND HYDROTHERMAL ALTERATION

Most exposed greenstone is weathered or hydrothermally altered to brown and orange rock. Fresh 

gray and green greenstone is rarely seen in natural 

exposures except at steep shores that are being 

actively washed by waves. At such places on Marin 

Peninsula and on Angel Island, the weathered zone 

is a band 10-40 feet thick. The base of the weathered 

band is a 1-2-foot-thick transitional zone that is 

parallel to the configuration of the ground surface at 

the top of the exposure. Hydrothermally altered 

greenstone masses generally do not conform to the 

configuration and may be much thicker than the zone 

of weathered greenstone. In many places weathered 

greenstone is hard to distinguish from hydrothermally 

altered greenstone. The rocks in many natural exposures, moreover, have been affected by both processes.

Hydrothermal alteration in greenstone is most 

intense in and adjacent to faults and at contacts with 

radiolarian chert. The zone of hydrothermal alteration 

along faults may be as much as 50 feet wide. Intense 

hydrothermal alteration along many contacts with 

radiolarian chert has converted the greenstone, in an 

irregular zone 5-20 feet wide, to a soft clayey and

sandy material of a distinctive grayish-orange color 

(fig. 24). Because of its light color and softness, this 

material resembles an acid tuff; however, hand-lens 

examination reveals a relict intergranular texture. 

Wide, gradational, and irregular contacts with fresher 

massive or pillow lavas also suggest altered greenstone.

The mineralogy of the altered greenstones is variable. In weathering, the chief alteration product is 

generally nontronite, an iron-bearing member of the 

swelling-clay group montmorillonite. An early effect 

of weathering is the conversion of chlorite to vermiculite and nontronite. At a further stage of weathering, 

feldspars, pyroxene, and other minerals are converted 

to nontronite.

Hydrothermally altered greenstone contains one or 

more of the following minerals in the clay-size fraction (smaller than 2 microns): Vermiculite, halloysite, hydrous mica, chlorite, and nontronite. Random 

mixed-layering of mica, chlorite, and vermiculite is 

common. In many localities halloysite or hydrous mica 

is the predominating mineral in the greenstone at contacts with radiolarian chert. The formation of halloysite, a common mineral in hydrothermally altered 

greenstone, is favored by an acidic environment during 

alteration (Keller, 1956, p. 2701). Radiolarian chert 

at contacts with greenstone is often replaced almost 

entirely by oxidized manganese minerals in a zone a 

few inches thick. Joints in the adjacent chert and in 

highly altered greenstone are lined with films of conspicuous black manganese oxide minerals.

The chemical composition of greenstones is given 

in table 6. The composition of the basalt (analysis 1) 

is similar to that of the pumpellyite-pyroxene-albitechlorite rocks (analyses 2, 3, 4), and compositions of 

both rock types generally fall within the composition 

range for basalts as given by Kuno, Yamasaki, lida, 

and Nagashima (1957, p. 213), Turner and Verhoogen 

(1960, p. 208, 220), Poldervaart (1955, p. 134), and 

MacDonald and Katsura (1961, p. 362).

OCCURRENCE 

MARIN PENINSULA

On Marin Peninsula, the greatest thickness of 

greenstone in the quadrangle, totaling 4,800 feet, is 

exposed at several stratigraphic positions in the thick 

southwestward-dipping section of the Franciscan 

Formation. Here the greenstone consists predominantly of pillow lavas but also includes massive 

greenstone and pyroclastic rocks. The lowermost mass 

exposed along the shore of San Francisco Bay west 

of Yellow Bluff is at least 1,700 feet thick; it may be 

as much as 2,700 feet thick if it underlies the Quaternary deposits in the valley north of Horseshoe Bay.
34 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 24. Radiolarian chert, KJc, overlying basaltic greenstone, KJg, all within the Franciscan Formation. The contact is 

offset by a small fault. Light-colored greenstone at the contact is almost completely altered to clay minerals. North roadcut 

on lateral road to Sausalito, 400 feet east of U.S. Highway 101, 1.2 miles north of Lime Point, Marin Peninsula.

Radiolarian chert north of Horseshoe Bay is interbedded with this greenstone. The chert increases in 

thickness northward, and at a point 1 % miles northwest of Yellow Bluff, the greenstone lenses out (pi. 

1). Such large variations in thickness over short distances and close association with radiolarian chert 

are characteristic of greenstone. Unaltered basalt 

containing augite and labradorite is especially abundant, though erratically distributed, near Lime Point 

and west of Horseshoe Bay.

SAN FRANCISCO

The largest mass of greenstone in San Francisco, 

approximately 2,400 feet thick, is exposed from Twin 

Peaks southward into the adjoining quadrangle. Two 

greenstone bodies, 700-1,000 feet thick, are found in 

the hills between Portola Drive and Valencia Street. 

Unaltered basalt containing augite and labradorite is 

also erratically distributed in this area.

A large mass of greenstone may underlie the cover

of dune sand at Buena Vista Park hill. Small exposures 

of greenstone are found widely scattered in other 

parts of San Francisco.

A small body of porphyritic dacite breccia near 

Lands End and one of medium-grained hornblende 

quartz keratophyre along Market Street east of Twin 

Peaks are included with the greenstones, but nothing 

is known of their field relations with other rock types.

In contrast with the great volume of greenstone 

interbedded with the sedimentary rocks elsewhere, 

greenstone is almost completely absent from the 

clastic rocks exposed between Rincon Hill and the 

Presidio. Volcanism, therefore, was largely inactive 

when these rocks were deposited; these rocks probably are not the same age as the Franciscan Formation a short distance to the north on Marin Peninsula 

and a short distance to the south at Twin Peaks and 

vicinity, unless as seems unlikely volcanic centers 

were so localized that erupted material did not reach 

this area.
GREENSTONE 35

TABLE 6. Analyses of greenstone samples of the Franciscan Formation

[Chemical analyses (rapid rock methods) by Paul Elmore, Samuel Botts, I. H. Barlow, and Gillison Chloe. Semiquantitative spectrochemieal analyses by 

H. W. Worthing. Looked for but not found: As, Au, Be, Bi, Cd, Ce, Cs, Dy, Er, Eu, Gd, Ge, Hf, He, Ho, In, Ir, La, Lu, Nb, Nd, Os, Pd, Fr, Ft, Rb, 

Re, Eh, Eu, Sb. Sn, Sm, Ta, Tb, Te, Th, Tl, Tm, U, W, Zn]

3 4a

Chemical analysis (weight percent)

SiO2 .............................

A1203 ............................

FeA ................................

FeO ..................................

MgO

PaH

Na2O ..............................

K2O

H2O+ .........-.....-..---.

H2O- ..............................

TiO2 ................................

P2OC ................................

MnO ................................

CO2 ..................................

Total ....................

S (aqua regia soluble).. 

Specific gravity 

(powder) . . .

Ae

B

Ba

Co .........................

Cr .............

Cu ......................... ........

Ga ....................................

Li ....................................

Mo

Ni ......................... ........

Pb ....................................

Sc . ........... ..... ......

Sr ......................... ........

V

Y ..

Yb .

Zr ............ .... ........

47.8 

12.5 

4.8 

11.7 

4.8

8.3 

3.0 

.30 

2.3 

.36

3.3 

.35 

.26

<.05

100

0.22 

3.04

0.00003 

.003 

.015 

.007 

.0015

.007 

.0007 

.03 

.0007 

.015

0

.0015 

.015 

.03 

.003

.0003 

.015

49.3 

17.3 

2.6 

5.5 

3.7

10.8 

3.8 

.25 

3.8 

.22

1.6 

.22 

.14

<.05

99

0.01

2.97

Semiquantitative

0 

.003 

.003 

.007 

.03

.003 

.0007 

0 

0 

.015

.00015 

.0015 

.003 

.015 

.0015

.00015 

.007

42.6 

15.2 

5.7 

4.4 

4.8

15.8 

3.1 

.02 

3.8

.4

.76 

.08 

.18

2.7

100

0.03 

3.02

spectrochemieal

0.000015 

.007 

.003 

.007 

.03

.003 

.0007 

0 

.0003 

.03

0 

.0015 

.03 

.015 

.0007

0 

.003

45.6 

16.3 

6.1 

4.7 

5.1

13.3 

3.3 

.02 

4.1

.4

.81 

.09 

.19

100

analyses

44.9 

15.9 

3.9 

6.6 

7.9

8.5 

2.7 

.76 

5.0 

1.2

.95 

.08 

.22 

1.2

100

0.01

2.86

(weight percent)

0

0.003

...... .15

.003

.007

.007

.0007

0

0

.015

.0015

.0007

.003

.003

.0003

0

.0015

46.2 

16.4 

4.0 

6.8 

8.1

7.2 

2.8 

.78 

5.2 

1.2

.98 

.08 

.23

100

43.3 

14.4 

4.0 

5.6 

10.9

5.2 

.14 

2.0 

7.4 

1.7

.62 

.04 

.20 

1.2

100

0.02 

2.76

0 

.003 

.015 

.003 

.015

.007 

.0007 

0 

0 

.015

0 

.0015 

.015 

.007 

.0007

0 

.0015

1. Basalt, 0.4 mile north of Lime Point, Marin Peninsula (sample No. 

60-804).

2. Pump ellyite-pyr oxen e-albite-ch lor ite-quartz rock. Spilite(?) northwest

slope of Lone Mountain, San Francisco (sample No. SF-97). 

Dumpellyite-pyroxene-albite-chlorite rock. Core of pillow 1.55 miles north

Sa. Analysis No. 3 recast by subtracting CaCOs corresponding to COa

content.

4. Rim of same pillow as rock of analysis No. 3 (sample No. 60-806). 

4a. Analysis No. 4 recast by subtracting CaCOs corresponding to CO2

. ----. . . . . .

slope of Lone Mountain, San Francisco (sample No. SF-97). 4a. Analysis No. 4 recast by subtracting CaCOs corresponding to COa

3. Pumpellyite-pyroxene-albite-chlorite rock. Core of pillow 1.55 miles north content.

of Lime Point, Marin Peninsula; west cut along U.S. Highway 101 5. Matrix between pillows of locality of analyses No. 3 and 4 (sample No.

(sample No. 60-805). 60-808).

ANGEL ISLAND AND VICINITY

The exposed thickness of greenstone on Belvedere 

Island is approximately 1,800 feet. Greenstone at 

Stuart and Knox Points on Angel Island may be a 

southeastern extension of this body, which may have 

been separated from the greenstone on Marin Peninsula by folding and erosion. The arcuate exposure of 

greenstone in the central part of Angel Island ranges 

in thickness from 100 feet at its western exposure to 

a maximum of 600 feet southwest of Mount Caroline 

Livermore and about 400 feet near Simpton Point, 

its eastern exposure. This eastern exposure may be a 

northeastern extension of the greenstone of Stuart and 

Knox Points, it may be stratigraphically higher, or 

it may be partly intrusive.

Ransome (1894, p. 201) believed that the central 

Angel Island greenstone body intrudes sandstone of 

the Franciscan Formation, but the presence of pyroclastic and pillow-form phases at several widely 

scattered places suggests that it is partly or wholly 

volcanic or a shallow plutonic body intrusive into soft 

sediments on the sea floor. Unfortunately, the only 

well-exposed contact between this body and sandstone 

is near Campbell Point, where faulting obscures their 

relations. Intrusion, admittedly, is suggested by the 

swarms of ^-1%-inch-thick calcite-quartz veins in 

the sandstone and the increase in size and abundance 

of these veins as the greenstone body is approached. 

Ransome (1894, p. 201) also suggested that "the 

irregular shape of the*** (greenstone body) and the 

accompanying contact metamorphism, prove it to be
36 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

a true sill and not an interbedded flow." He evidently 

believed it was a sill rather than a dike because of 

the general conformity of the greenstone-sandstone 

contact with the sandstone bedding. The rocks 

ascribed by Ransome (1894, p. 211) to contact metamorphism are glaucophane-bearing metagreenstones 

that are found sporadically in small isolated masses 

along greenstone borders, but also within the greenstone body. Jadeitization of Franciscan sandstone also 

seems to be strongest at and near the greenstone 

border. The author believes that similar metamorphic 

processes affected the entire greenstone body and 

other rock masses far from its borders after emplacement. The extensive calcite-quartz veining may also 

be related to a later action.

Pillow lavas and volcanic breccia are seen at several places in the sill-like body. These features are 

more likely to develop under extrusive conditions than 

under intrusive conditions. East of the large valley 

between Campbell and Simpton Points, the greenstone is a breccia containing reworked, rounded pieces 

as much as 10 inches in diameter. The largest part of 

the greenstone body, however, does not show pillow 

or breccia development and may indeed be intrusive, 

but evidence for its intrusive nature is inconclusive.

The central Angel Island greenstone body is mostly 

metagreenstone, but it also contains dense variolitic 

greenstone. Chemical composition of the central Angel 

Island greenstone body is given by Ransome (1894, 

p. 231) and Bloxam (1960, p. 564). They showed that 

greenstones and metagreenstones of this body are 

generally similar in composition and that they are 

similar in composition range. (See table 6.) This 

similarity suggests that the formation of metagreenstones from greenstones was mainly isochemical.

The greenstone in central Angel Island, and most 

other greenstones in these areas, are either similar in 

texture and mineralogy to the dense variolitic greenstone (see greenstone discussion on "MineralogyMicroscopic Features") or they are slightly more 

metamorphosed or hydrothermally altered.

The metagreenstones generally consist of phenocrysts or large feathery plates of pyroxene as much 

as 3 mm in diameter in a matrix of small feathery 

pyroxene plates, lawsonite, blue amphibole of the 

glaucophane-riebeckite series, stilpnomelane, opaque 

ores, and sphene. Pyroxene is augite, titanaugite, 

diopsidic augite, and diopside-jadeite. In some rocks 

it is partly or almost completely replaced by blue 

amphibole of the glaucophane-riebeckite series, chlorite, lawsonite, pumpellyite, iron oxides, stilpnomelane, 

nontronite, clinozoisite, zoisite, muscovite, albite, calcite, and quartz. These secondary minerals are also

found in the matrix. Skeletal pyrite is commonly intergrown with pyroxene.

ORIGIN

Greenstone of the Franciscan Formation is a product of volcanism. It was emplaced as lava flows, tuffs, 

agglomerates, and associated dikes, sills, and plugs. 

Volcanism was active repeatedly during Franciscan 

time. Most if not all the volcanic rocks were erupted 

on the sea floor, for pillow structure is widespread, 

marine chert and limestone are found in the space 

between some pillows, and the greenstone flows and 

pyroclastics are interbedded with marine sediments. 

Pillow structure is also developed by shallow intrusion of flows into wet unconsolidated sediments; small 

sills and dikes, 2-10 feet thick and showing pillow 

structure, intrude radiolarian chert at several localities. Such intrusives may have been parts of flows 

that sank into the soft siliceous gel that later hardened into chert.

Tuffaceous greenstone grades into graywacke at several localities where ash was deposited concomitantly 

with graywacke. Volcanic graywackes are largely reworked ash and lapilli.

Coarse lapilli and ejecta greater than about 4 inches 

in diameter are evidence of local vents. Indeed, some 

masses of brecciated greenstone may be plugs or necks 

of Franciscan volcanoes.

Metamorphosed and altered basalt is present in 

almost every greenstone body shown on the geologic 

map (pi. 1). Its erratic distribution, however, indicates great local variations in conditions of alteration 

and metamorphism, including (1) deuteric or hydrothermal alteration from the action of volcanic emanations (Williams and others, 1955, p. 59), (2) the 

reaction of hot volcanic rocks with hot sea water, and 

(3) low-grade metamorphism of the zeolite, greenschist, and glaucophane schist facies (Turner and 

Verhoogen, 1960, p. 531-544). The common greenstone mineral assemblage of primary pyroxene and 

pumpellyite, albite, and chlorite would probably be 

created if basalt were subjected to zeolite-facies 

metamorphism (Turner and Verhoogen, 1960, p. 532). 

Much of the greenstone of Angel and Belvedere 

Islands and Tiburon Peninsula probably was metamorphosed under the slightly greater pressures and 

temperatures of the greenschist and glaucophane 

schist facies, as indicated by lawsonite, glaucophane, 

epidote, and metamorphic pyroxene.

RADIOLARIAN CHERT AND SHALE

The radiolarian chert and shale of the Franciscan 

Formation consist predominantly of thin alternating
RADIOLARIAN CHERT AND SHALE 37

beds of chert and siliceous shale. In places these sedimentary rocks make up sections more than 1,000 feet 

thick. Radiolarian remains are common in the chert 

and less common in the shale. Massive generally isolated shale-free bodies of chert 4-25 feet thick, as well 

as thin-bedded shale-free cherts, are also included in 

this map unit. Interbedded chert and shale predominate, however, and in this report the term "chert" is 

occasionally used for all three types. The larger 

deposits of radiolarian chert are closely associated 

with greenstone. Like other map units in the Franciscan Formation in this quadrangle, this unit is 

primarily lithologic and has limited stratigraphic significance, for it occurs at several stratigraphic positions 

in the Franciscan Formation. Moreover, rocks at 

various stratigraphic positions are similar in appearance; no distinct characteristics distinguish one radiolarian chert section from another.

Davis (1918b, p. 239-408) and Bailey, Irwin, and 

Jones (1964, p. 55-68) include descriptions of deposits 

outside the San Francisco North quadrangle.

The radiolarian chert and shale unit generally forms 

topographically high areas because the chert resists 

weathering and hydrothermal alteration. The contact 

of radiolarian chert and shale with other rock types 

usually is easily recognized, for this unit commonly 

stands 3-10 feet higher than the adjoining rock. AH 

the peaks in the central highland area of San Francisco are underlain by radiolarian chert and shale. 

Most of the higher ridges of Marin Peninsula, including the highest point in the quadrangle, are also underlain by this rock.

MEGASCOPIC FEATURES

Color of the radiolarian chert and shale varies with 

the content and state of oxidation of iron and manganese. The most common colors of fresh thin-bedded 

radiolarian chert are dusky red to dark reddish brown. 

Grayish-green chert is found in small volume. Generally, the associated shale has the same color as the 

chert, except where it is altered.

Under hydrothermal reducing action, the red and 

brown chert becomes grayish green, dark greenish 

gray, or grayish yellow green. Hydrothermal activity 

and weathering may also remove iron and manganese, 

giving the chert and shale a white, very light gray, 

grayish-orange, or light-bluish-gray color. In many 

places hydrothermal activity and weathering were 

limited and variable from place to place, and only 

part of the rock was affected. Mottled cherts are 

formed in this manner; they commonly are green, 

gray, and white adjacent to joints and along the bedding surfaces. The joints themselves and the shale

between the chert beds are often stained with yellow 

and brown iron oxides or black or bluish-black manganese oxides.

Some single chert beds in a chert and shale 

sequence contain distinctly colored blebs of red, 

orange, yellow, and brown chert elongated parallel 

to the bedding with irregular but sharp and tightly 

bonded borders (fig. 25). Other red chert beds contain pale-gray and pale-green irregular color bands 

that are generally parallel to the bedding and 

probably are formed by hydrothermal alteration or 

weathering.

Near some greenstone bodies, radiolarian chert that 

is enriched in manganese is colored very dusky red; 

the associated shale is dusky brown or almost black. 

Chert colors, however, are difficult to distinguish 

because of the masking effects of numerous fractures 

lined with black manganese oxides. Bright red, bright 

orange, bright yellow, and brown mottling of chert 

and shale beds may be heat effects caused by contact 

with molten basalt.

Massive chert interbedded with thin-bedded chert 

is generally paler in color than the associated thinbedded chert and shale. Gray, white, pale green, and 

yellowish orange are common colors of massive chert.

Metacherts of Angel Island and Tiburon and Belvedere Peninsulas are commonly medium dark gray, 

brownish gray, and bluish gray.

Radiolarian chert consists typically of alternating 

thin beds of chert and shale. Chert beds are generally 

1-5 inches thick. Some beds, however, are much 

thicker. Rare single chert beds, 1-25 feet thick, are 

interbedded with chert and shale of normal thickness. 

Shale beds range from mere films to beds about threefourths inch thick (fig. 26); they seldom exceed 1 or 

2 inches in thickness. Very rare ones exceed 1 foot.

The chert is aphanitic and hard. Fresh unfractured 

chert is difficult to break and, on breaking, yields sharp 

edges and conchoidal to hackly surfaces. Fractures, 

however, are almost universal in chert of the Franciscan Formation. In most places the fractures are 

cemented, and the rock as a whole is fairly tough; 

nevertheless, it readily breaks to coherent pieces 

generally 1-3 inches in diameter. In a few places 

adjoining faults, the rock is severely fractured and 

breaks down to small splinters. Individual chert beds 

generally lack fissility, for fractures or surfaces of 

weakness parallel to the bedding are uncommon.

Clastic micaceous silt-size and smaller particles 

occur on bedding surfaces of the shale. These particles 

give the shale its poor to moderate fissility. The shale 

is fairly well indurated, although somewhat brittle; 

however, some of it can be scratched with the finger-
38 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 25. Internal structure of radiolarian chert bed of the 

Franciscan Formation, 6^2 inches thick. Isolated blebs that 

appear to be light gray on the photograph are really moderate 

yellow (5Y 7/6) in a moderate-reddish-brown (lOfi 4/4) to 

dusky-red (5R 3/4) matrix. Structural features are elongated

nail and all of it with a steel blade, in contrast with 

the chert, which cannot be scratched with a steel 

blade. Most shale remains hard when soaked in water 

for long periods of time. Where shale between chert 

layers is 1 inch or more thick, it often contains thin 

distinct layers rich in Radiolaria; these layers are 

somewhat harder than the average. Most chert beds 

are sharply bounded by interbedded shale, although 

some highly ferruginous chert grades into shale.

Pinching, swelling, and lensing of chert beds are 

very common and take many forms. An individual 

chert bed may have bulges two to three times its 

minimum thickness a few inches to 1 foot apart (fig. 

27). The underlying and overlying chert beds are

parallel to bedding. Structure suggests this rock formed from 

silica gel masses that differed in amount, particle size, and 

state of oxidation and hydration of iron and manganese oxides. 

From quarry on Sausalito lateral, one-half mile northwest of 

Yellow Bluff, Marin County.

generally pinched at the point of the bulge, but the 

thickness of the interlayered shale beds is affected only 

slightly. As a chert bed lenses out and terminates, 

adjacent chert beds thicken or bend towards each 

other, whereas the two shale beds enclosing a wedgedout chert bed merge into one shale bed only slightly 

thicker than either of the individual shale beds. Or, 

the lensing ends of two chert beds may overlap; so, 

the thickness of adjacent chert beds remains unchanged. Each shale bed is generally less than 25 feet 

long in horizontal exposure. Bedding irregularities are 

also described and illustrated by Davis (1918b, p. 248- 

252), Taliaferro and Hudson (1943, p. 227-229), and 

Bailey, Irwin, and Jones (1964, p. 55-68).
RADIOLARIAN CHERT AND SHALE 39

FIGURE 26. Radiolarian chert and shale of the Franciscan Formation. Note pinching, swelling, and wedging out of individual chert beds. Twin Peaks, San Francisco.

FIGURE 27. Pinching and swelling in radiolarian chert beds of 

the Franciscan Formation. Raveled shale bed (undercut 

zone in shadow between the man's hands) is unusually thick. 

On lateral road to Sausalito from U.S. Highway 101, threefourths mile north of Lime Point, Marin Peninsula.

Small tight rounded folds and sharp chevron folds 

are common in radiolarian chert and shale, although 

many exposures show only nonfolded bedding or a 

gentle waviness of bedding (fig. 28). Bedding surfaces 

of chert are generally curved rather than flat because 

of the small-scale tight folding and because of pinching and swelling. Attitude measurements on small 

exposures of radiolarian chert beds, therefore, must be 

used with caution in deciphering the broad structural 

features of the Franciscan Formation. Where exposures are extensive, as on Twin Peaks and in Fort 

Baker on Marin Peninsula, local contortions as well 

as consistent attitudes can be recognized.

FIGURE 28. Chevron folds in radiolarian chert of the Franciscan Formation 2 miles northwest of Golden Gate Bridge, 

Marin Peninsula.

The most common cause of contorted bedding in 

chert probably is submarine slumping of beds prior 

to hardening. In places, disturbance of incompletely 

hardened beds may have been caused by volcanic 

activity, such as flow movement or intrusive action. 

In other places, crumpling of chert appears to have 

preceded volcanic activity. Contorted bedding also 

occurs in the vicinity of large faults. Small faults that 

show displacements of a few inches to 1 foot are confined to the axis of small folds and evidently ruptured 

during folding. Possible drag folding related to the 

formation of major folds in the Franciscan Formation 

can be seen in some localities, particularly on Marin 

Peninsula (fig. 29).

Massive chert generally produces large bold bare 

exposures. In favorable exposures it is seen to be interbedded with thin-bedded radiolarian chert and shale. 

In less favorable exposures it commonly is isolated 

and is surrounded by slope debris and other surficial 

deposits that obscure its relation to other rock units. 

Massive chert bodies are generally wedge shaped with 

blunt ends and may have very irregular borders within 

thin-bedded radiolarian chert and shale sections. At 

many exposures thin-bedded chert and shale terminate 

abruptly against massive, thick-bedded, or obscurely 

bedded chert lenses.

The largest known exposure of a single chert bed 

in a section of thin-bedded chert and shale is on Sunset 

Heights in San Francisco along 14th Avenue, 600 feet 

east of the intersection of Noriega Street and 16th 

Avenue (fig. 30). It is approximately 500 feet long 

and 25 feet in maximum thickness. It has an irregular 

bottom and lies partly on thin-bedded chert and 

partly on pale-green shale. The shale is as much as
40 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 29. Radiolarian chert containing possible drag folding. 

Above, Radiolarian chert at Lime Point, as seen looking 

northwest along the strike of the Franciscan Formation. 

Greenstone lies under the left side of the Golden Gate Bridge 

tower and the high cliff on the left of the tower. Chert lies 

below the right side of the bridge tower and is exposed in the

1 foot thick where it fills the irregular space between 

massive and thin-bedded chert, and its bedding is 

parallel to that of the underlying thin-bedded chert. 

Some chert immediately adjacent to greenstone or 

chert completely enclosed by greenstone differs from 

typical bedded chert. The differences may be from 

heat and chemical effects of the erupting molten 

greenstone acting on the typical thin-bedded chert or 

on siliceous gel prior to consolidation. The effect on 

some chert is brecciation and quartz veining, the 

absence of the shale partings, and no change in color 

(fig. 31). The chert layer next to greenstone is generally 1 foot or more thick; adjoining chert beds are 

less than 4 inches. Stronger effects, usually seen on 

chert enclosed by greenstone, are extensive brecciation, quartz veining, a color change to pale or bright 

red, orange, yellow, green, and brown, and a drusy 

coating of tiny quartz crystals on joints. Microscopic 

examination of thin sections of this rock show that 

new minerals have formed.

cliff to the right of the tower. A light-colored zone in the cliff 

across the road from the buildings is chert containing jarosite 

derived from the alteration of pyrite. Opposite, Possible drag 

folding in the chert exposed in the cliff to the right of the bridge 

tower. Chert beds are 1 4 inches thick. Dark part at upper 

right-hand corner is bottom of bridge.

Fractures are numerous in thin-bedded chert and 

shale, especially in the chert. They are commonly 

nearly perpendicular to the bedding. Fractures intersect each other at various angles up to 90°, but most 

of them are more or less alined in a limited number of 

directions. Parallel fractures are generally Ys-Yz inch 

apart.

Some fractures are slightly open fissures and commonly are lined with manganese and (or) iron oxides. 

The rock splits readily along such fractures. Other 

fractures are tightly cemented, generally with quartz, 

or less commonly with calcite, gypsum, zeolites, and 

other minerals. About half the fractures in a typical 

chert bed are filled with white quartz veins that are 

generally less than one-eighth in thick. Thicker quartz 

veins penetrate the interbedded shale about an eighth 

inch; thus bedding surfaces of many chert beds from 

which the shale has been removed have a prominent 

reticulate pattern of raised quartz veins. Chert rarely 

breaks along quartz-cemented fractures; the brittle-
RADIOLARIAN CHERT AND SHALE 41

FIGURE 29. Continued.

ness of chert stems from its tendency to break along 

incipient uncemented fractures. At faults, chert may 

be so minutely fractured that it can easily be shattered by the fingers into small splinters generally 

elongated perpendicular to the bedding.

Fractures parallel to the bedding in chert are uncommon. They do appear in some chert beds as one 

or two conspicuous uncemented joints. Many cherts 

contain numerous tiny discontinuous quartz veins or 

possible fossil spicules, either in a plane parallel to 

the bedding or very irregular in shape but generally 

parallel to the bedding. They are generally less than 

0.5 mm wide and less than 10 mm long, but they give 

the chert a laminated appearance.

Most massive chert bodies are not as brittle as thinbedded chert. On close examination massive chert 

bodies are generally found to be highly brecciated and 

thoroughly recemented with one or more generations 

of quartz and chalcedony. Massive chert at most contacts with greenstone contains numerous quartz veins 

as much as 1 2 inches thick. These veins are more 

irregular than quartz veins in thin-bedded chert.

CHEMICAL COMPOSITION

The chemical composition of chert and related shale 

is given in table 7. Silica is the predominant constituent. In chert, silica is almost entirely chalcedony and 

quartz. In shale, the content of silica, chalcedony, and 

quartz varies with the Radiolaria content. In shale 

containing no Radiolaria, only about half the silica is 

represented by these minerals and the rest is combined in silicates. In the cherts, Si02 content is well 

above 90 percent; a massive chert at Grand View Park 

is nearly pure Si02 (table 7, analysis 6).

Fe-jOs and A12 03 are generally next in order of abundance, except in green chert which evidently was 

subjected to iron leaching and reducing conditions 

(analysis 4). The three constituents, SiO2 , A12O3 , and

534-039 O - 74 -
42 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

-. - *\

' » ''H -I,-* sK. '.j t HL-^£ H *\-**^^'-S*''>flE.^f;^ 'ms !%** - Ai* '. * ' -: >,

 .^ . ^%^^^^^ti^wc?:-mm^-tr v '

Tr":i"^«*flra w=*r' '"-'« * Tn *.«t .tV^iV*i3K»**^.v »-« ^k!^^^ v». '1

FIGURE 30. Massive chert in the Franciscan Formation. The containing quartz, mica, and kaolinite. The chert bed below the

bottom of a massive chert bed, 25 feet thick, interbedded with shale in the middle of the photograph is about 2 inches thick,

thin-bedded chert and shale. Material between massive chert West side of Sunset Heights, San Francisco; 14th Avenue near

and thin-bedded chert is green hydrothermally altered shale Ortega Street.

FlGL'RE 31. Quartz veins in radiolarian chert lying on altered 

greenstone. Twin Peaks, San Francisco.

Fe2 03 , make up more than 97 percent of the cherts. 

Besides Si02 , the shales contain substantial quantities 

of A12 03 , Fe2O3 , and K^O. As in the red cherts, iron 

is mostly in the ferric state of oxidation, though FeO 

is nearly 1 percent in a green shale which appears to 

have been a red shale reduced by hydrothermal fluids. 

Shale differs from chert in having less Si02 and more 

Fe2 03 , FeO, MgO, K,0, H2 0, P2O5 , and MnO. Minorelement content as measured by spectrochemical 

analysis also differs. Shale contains more Cr, Sr, V, 

Y, and Zr and contains the following elements which 

are either absent from chert or are below the limits 

of detection: Be, Ce, Ga, La, Mo, Nb, Nd, Pb, Sc, and 

Yb. Minor-element content of chert generally substantiates Krauskopf's (1955, p. 427) statement that 

the few analyses available to him indicate negligible
RADIOLARIAN CHERT AND SHALE 43

TABLE 7. Analyses of radiolarian chert and shale samples of the Franciscan Formation

[Chemical analyses (rapid rock methods) by Paul Elmore, Samuel Botts, I. H. Barlow, and Gillison Chloe. Semiquantitative spectrochemical analyses by 

H. W. Worthing. Looked for but not found: As. Au, Bi, Cd, Cs, Dy, Er. Eu, Gd, Ge, Hr, Hg, Ho, In, Ir, Li, Lu, Os, Pd, Pr, Ft, Rb, He, Eh, Ru, Sb, 

Sm, Ta. Tb, Te, Th, Tl, Tm, U, W, Zn]

1 2 3 4 6

Chemical analyses (weight

SiOz ................ ...-......... ........

AkCh

FeaOs ..........................................

FeO

MgO

CaO ..............................................

Na,O _._......__... ..........

K2O ..............................................

H2O+

H20- ............................................

TiOz

P2Os ....-......-..... -......-..--.....

MnO .....................................

COs ..........................................

Total ..................._....._....

Specific gravity (powder) ~.

93.5 

.96 

2.8

<.05

.11 

.42 

.01 

.08 

.72

.22 

.04 

.03 

1.3 

<.05

100

0.01 

2.68

95.9 

1.1

1.7 

.34

.10 

.05 

.02 

.26 

.70

.11

.06 

.02 

.05 

<.05

100

0.00 

2.66

84.7 

1.1 

2.7 

.22

.14 

.06 

.01 

.37 

.63

.16 

.06 

.03 

.05 

<.05

100

0.01 

2.65

Semiquantitative

Ag A~ ...............................................

Ba ................. ..............................

Be ................................... _ .......

Ce

Co

Cr . ......... .. ....

Cu ........... __ ...............................

Ga

La .-......---..-...............-.-.........--

Mo .. ............. . ....

Nb

Nd ................................................

Ni .................

Pb ...................................... .........

Sc .................. .... ........ .

Sn ......................... __ ...... ........

Sr

V

Y

Yb ... .........

Zr ................... ... ....... ........

0.00007 

0 

.07 

0 

0

.0003 

.0003 

.015 

0 

0

0 

0 

0 

.007 

0

0 

0 

.0007 

.0015 

.0007

0

.0007

0 

.003 

.03

0 

0

0 

.0003 

.0015 

0 

0

0 

0 

0 

.003 

0

0 

0 

.0003 

.0015 

0

0 

.0015

0 

0 

.015 

0 

0

.00015 

.0003 

.0015 

0 

0

0 

0 

0 

.003 

0

0 

0 

.0003 

.0015 

0

0 

.0015

96.5 

1.5 

.34 

.38

.16 

.17 

.11 

.26 

.65

.15 

.08 

.04 

.03 

<.05

100

0.01 

2.63

93 

2 

2.4 

<.05

.13 

.11 

.11 

.41

1

.26 

.12

.05 

.40 

<.OB

100

0.01 

2.64

6

percent)

97.4 

.47 

1.3 

.26

<.05 

.05 

.01 

.04 

.55

.07 

.03 

.04 

.02 

<.05

100

0.02 

2.64

7

40.1 

10.9 

27.6 

<.05

3.5 

.53 

.12

4.5 

4.8

2.2

.76 

.14 

4.8 

.08

100

0.00 

2.97

8

60.9 

13.1 

9.2 

1

2.3 

<.05 

.08 

4.9 

4.6

2 

.92 

.14 

.11 

<.05

99

0.02 

2.73

9

58.4 

14.3 

7.4 

<.05

3.3 

2

1.7 

3.9 

3.1

2 

.66

1.3 

1.4 

<.05

99

0.03 

2.66

10

66.7 

14.1 

6.5 

.58

1.6 

.05 

.10 

3.8 

4

1.2

.81 

.09 

.10 

<.05

100

0.01 

2.69

11

66.3 

15.9 

3.3 

.96

1.8 

<.05 

.18 

4.9 

4.3

1.3 

.76 

.27 

.08 

<.05

100

0.02 

2.66

spectrochemical analyses (weight percent)

0 

.003 

.15 

0 

0

.00015 

.0003 

.0016 

0 

0

0 

0 

0 

.003 

0

0 

0 

.0015 

.0007 

0

0 

.0015

0.000015 

.003 

.15 

0 

0

.003 

.0003 

.003 

0 

0

0 

0 

0 

.007 

.00015

.0007 

.0007 

0 

.0015 

.0003

0

.0015

0 

.003 

.007 

.00015 

0

.0007 

.0003 

.003 

0 

0

0 

0 

0

.003 

0

0

0 

0

.0007 

0

"0 

.0007

0.0007 

.007 

.3 

.00015 

.015

.003 

.003 

.03 

.0007 

.007

.0015 

.0003 

.015 

.07 

.015

.0015 

.003 

.007 

.007 

.003

.0003 

.015

0 

.003 

.03 

.00015 

.015

.0015 

.007 

.015 

.0007 

.007

.0007 

.0003 

.015 

.007 

.003

.0015 

.0007 

.0007 

.15 

.003

.0003 

.015

0.00015 

.003 

.15 

.00015 

.03

.03 

.0015 

.015 

.0007 

.015

.0003 

.0003 

.015 

.03 

.0015

.003 

.0003 

.15 

.003 

.015

.0015 

.016

0.000015 

.007 

.07 

.00015 

015

.0015 

.003 

.007 

.0007 

0

0 

.0007 

0 

.007 

.0003

.0015 

0 

.15 

.007 

.003

.0003 

.015

0 

.015 

.015 

0 

.015

.003 

.003

.0015 

.0007 

.003

0

.0003 

0 

.015 

0

.0015 

0 

.007 

.03 

.0015

.00015 

.015

1. Chert, thin-bedded; much iron and manganese oxides on joints; Twin 

Peaks, San Francisco (sample No. 60-800).

2. Chert, thin-bedded; on Sausalito Lateral, 3,500 ft west of Yellow Bluff, 

Marin Peninsula (sample No. 60-802).

3. Chert, thin-bedded; on Sausalito Lateral, 3,000 ft northwest of Yellow 

Bluff. Marin Peninsula (sample No. SF-1970).

4. Chert, thin-bedded, green; west cut U.S. Highway 101, 8,500 ft northwest 

of Yellow Bluff (sample No. SF-2043).

5. Chert, thin-bedded; 600 ft east of 16th Ave., 1,100 ft north of Ortega St., 

Grand View Park, San Francisco (sample No. SF-2145).

amounts of rare metals (minor elements) in chert. 

However, the shales interbedded with chert of the 

San Francisco North quadrangle show the following 

maximum enrichment factors (the ratio of the minor 

element content to the crustal abundance of the element, see Krauskopf, 1955, p. 417-428): Ba, 12; Co, 13; 

Cu, 4; Mo, 15; Ni, 11; Pb, 9; rare earths including Y, 5; 

Sr, 5; V, 10. The shales interbedded with cherts differ 

from gray shales interbedded with sandstones of the 

Franciscan Formation (table 7, analysis 8) mostly in 

having more Fe2O3 and K2O and less FeO and Na2O.

MICROSCOPIC FEATURES 

CHERT

Thin-bedded chert consists of a matrix of chalcedony and cryptocrystalline to microcrystalline quartz

6. Chert, massive; 600 ft east of 16th Ave., 500 ft north of Ortega St., San 

Francisco (sample No. SF-2143).

7. Shale bedded with chert of analysis 1 (sample No. 60-801).

8. Shale bedded with chert of analysis 2 (sample No. 60-803).

9. Shale bed; 1 ft thick; bedded with chert; east cut U.S. Highway 101. 3,750 

ft west of Yellow Bluff (sample No. SF-2111).

10. Shale bedded with chert of analysis 5 (sample No. SF-2145A).

11. Shale, green; 1 ft thick; below massive chert of analysis 6 and above thinbedded chert (sample No. SF-1941).

enclosing Radiolaria and a small amount of spicules. 

The fossils also consist of chalcedony and quartz. 

Veinlets that cut the matrix and the fossils consist of 

quartz, chalcedony, and subordinate amounts of calcite, gypsum, stilpnomelane, chlorite, kaolinite, rareearth phosphates, and zeolites. In some cherts the 

matrix appears to be isotropic or shows only scattered 

specks of light under crossed nicols. This material is 

thought to be largely quartz of exceedingly small 

grain size for the following reasons: (1) The index 

of refraction is approximately 1.535, slightly lower 

than those of quartz, but far above that of opal and 

(2) X-ray diffraction powder analysis shows only 

quartz spacings and does not contain spacings of betacristobalite, which are obtained by X-ray diffraction 

analysis of most opals. Previously, Lawson (1895, p. 

422), Davis (1918b, p. 255), and Taliaferro and Hudson (1943, p. 231) reported that some cherts of the
44 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

Franciscan Formation are composed of amorphous 

silica.

Most grains of chalcedony and quartz in the fossils 

and veinlets are at least 3 and generally more than 

10 times larger than grains in the matrix. The veinlets 

contain the largest grains of quartz, some being more 

than 0.5 mm in diameter. Veinlets in some chert are 

calcite or gypsum with a thin border of quartz. Small 

patches of stilpnomelane and chlorite are common. 

Iron and manganese oxides and rare-earth phosphates 

rich in cerium coat joints in some cherts.

The matrix of the typical red thin-bedded chert is 

heavily charged with tiny brownish-red particles. The 

fossils and the veinlets are relatively free of these 

particles.

The proportion of fossils to matrix varies within a 

single chert bed and from bed to bed. In some chert 

beds fossils are so crowded that they touch each other. 

Few if any are seen in light-colored pigment-free chert 

beds or in the pigment-free parts of otherwise red 

chert beds rich in fossils. Radiolaria generally appear 

in thin sections as sharply defined clear circular or 

conical areas, 0.04 0.5 mm in diameter, of relatively 

coarse chalcedony and more rarely of microcrystalline 

quartz. The rim of many fossils is equigranular microcrystalline quartz enclosing a large core of one or 

several grains of chalcedony, each grain consisting of 

a sheaf of fibers of negative optical elongation (length 

fast) radiating from the edge or center of the grain. 

The chalcedony and quartz that preserve the former 

chambers and pores within radiolarian tests usually 

contain some red pigmentation, in contrast with the 

clear parts of the rest of the test. Spines and other 

delicate surface ornamentation are generally absent.

As would be expected, the color of the chert varies 

with the amount of pigment in the matrix: the dusky 

cherts contain more pigment than the light-colored 

ones. Parts of the matrix of dusky-red chert appear 

opaque under the microscope because they are so 

heavily charged with pigment. The pigment particles 

are mostly embedded in chalcedony and quartz grains 

and to some extent between these grains. At high 

magnification the pigment in red chert is seen to consist mostly of equidimensional and elongated rounded 

transparent reddish and brownish crystals that average 1 micron in diameter. They have a moderate to 

high index of refraction. Most of them appear isotropic under crossed nicols, but some, particularly the 

thin laths, show a moderate to high birefringence. 

Definitive studies of the pigment have not been reported, but published descriptions of these red chert 

beds identify the pigment as hematite or iron oxide 

(Davis, 1918b, p. 254; Taliaferro and Hudson, 1943, 

p. 150, 227, 231; Williams and others, 1954, fig. 124A,

p. 363). Taliaferro and Hudson (1943, p. 232) found 

that manganese oxide and carbonate are also coloring 

agents. Davis (1918b, p. 258, 260) reported finding 

pyrite in green and gray chert and a glauconitelike 

mineral in chert from Point Richmond, 3.5 miles north 

of Angel Island.

X-ray diffraction powder patterns of typical red 

thin-bedded shale-free chert show much quartz, small 

amounts of hematite (alpha-Fe2O3 ), goethite, possibly 

lepidocrocite, and small amounts of minerals with 

spacings suggestive of poorly crystalline micalike and 

chloritelike structures. These minerals no doubt 

account for much of the A12O3 , MgO, K2O, Na2O, and 

H20, and for some of the iron shown in table 7. Taliaferro and Hudson (1943, p. 232) suggested that "the 

alumina, magnesia, lime, and alkalies represent the 

small amount of fine clayey detritus mechanically 

entangled when the colloidal silica was flocculated." 

Silicates in the veinlets also account for part of these 

components.

Microscopic examination of thin sections of the 

brown cherts of Angel Island and vicinity reveal 

abundant Radiolaria remains in the form of clear 

circular quartz-crystal aggregates that are slightly 

coarser in texture than the inclusion-filled quartz 

crystals that surround them. Also, some radiolarian 

structures are outlined by lepidocrocite, which may 

represent altered pyrite. The quartz surrounding the 

Radiolaria is crowded with tiny pale-yellow-green to 

colorless slightly pleochroic hornblende (rarely aegerine) needles, 2-20 microns long, and stumpy, euhedral 

hexagonal prisms of apatite, 3-10 microns in diameter. 

The apatite may be recrystallized from the chitinous 

organic remains found in shale interbedded with unmetamorphosed chert. Veins of magnetite, mostly 

altered to lepidocrocite, and of stilpnomelane are 

common.

Pale-blue chert is interbedded, or irregularly interspersed, with the brown variety. It appears to be 

slightly more metamorphosed than the brown variety, 

for no radiolarian remains were found in it and much 

of the quartz appears to be recrystallized. It also contains abundant hornblende needles, some of them 

partly altered to stilpnomelane, and small amounts of 

epidote, crossite, and lepidocrocite pseudomorphing 

magnetite. Shale, interbedded with either pale-blue 

or brown chert, is dark blue in color and is completely 

recrystallized. It consists predominantly of various 

types of blue amphiboles of the glaucophane-riebeckite 

series and stilpnomelane and minor amounts of aegerine, hornblende, sphene, and lepidocrocite.

The metacherts described here probably formed 

under conditions similar to those for Hutton's (1940, 

p. 27-28) subzone Chlorite 1 of the chlorite zone.
RADIOLARIAN CHERT AND SHALE 45

Microscopic examination of baked chert shows 

differences from unbaked chert. Most of the baked 

rock consists of microcrystalline quartz or large 

spherules of chalcedony. Radiolaria are generally 

obliterated but are recognized in some baked cherts. 

Colorless isotropic material is absent. Baking tended 

to recrystallize the pigment into larger particles than 

exist in the unbaked chert. The particles may take 

the form of long irregular veinlets of magnetite, 0.4 

mm in maximum thickness, or of euhedral crystals of 

magnetite, and unidentified opaque or deep yellowishbrown isotropic crystals of high refractive index. The 

pigment in the baked cherts is generally segregated 

into sharply defined clots and concentric shells leaving nearly clear irregularly shaped masses of quartz 

crystals between the pigmented clots. The pigment 

is generally confined to chalcedony and cryptocrystalline quartz of the finest grain size in the chert, 

whereas relatively unpigmented quartz is the coarsest 

in grain size, some crystals being nearly 1 mm in 

diameter.

Silicate minerals are common products of baking. 

Unidentified microlites and unidentified long hairlike 

crystals, as well as larger crystals of hornblende, 

pumpellyite, epidote, aegerine, and stilpnomelane, are 

often found in these rocks. Hornblende with Z = dark 

to moderate yellow brown, X = Y = pale yellow 

brown to pale brownish yellow, Z A c = 15°, birefringence = 0.010 is common in chert found lying 

between pillows of basalt in a roadcut 2,000 feet northwest of Horseshoe Bay.

Massive chert lenses are petrographically different 

from the thin-bedded chert that encloses them. Microscopic examination shows that massive chert consists 

largely of microcrystalline quartz and relatively coarse 

chalcedony spherules; the pigment appears to be in 

crystals larger than those in the typical red thinbedded chert. Taliaferro and Hudson (1943, p. 260), 

however, found some massive chert to be partly opaline. Irregular veins and clots of unpigmented relatively coarse anhedral to euhedral quartz crystals, as 

large as 1 mm in diameter, are found throughout the 

massive chert; so, the rock appears to be a breccia of 

older pigmented rock recemented with the clear coarse 

quartz. Radiolaria are not recognized with the hand 

lens and are obscure and sparse in thin sections 

examined under the microscope. Their apparent 

scarcity may be explained by the difficulty of seeing 

them against generally pale background of massive 

chert. Their scarcity, however, may be real and may 

be related to the origin of massive chert. Massive 

chert may contain a greater proportion of chemically 

precipitated silica than is found in typical thin-bedded 

chert.

SHALE

A few thin sections of shale interbedded with chert 

from the Twin Peaks area show that shale there consists mostly of (1) flakes and wavy veins of a micaceous 

mineral showing dark-reddish-brown (Y, Z) to moderate-reddish-brown (X) colors and (2) lens-shaped 

aggregates of tiny micaceous crystals that show a weak 

aggregate pleochroism in moderate brown hues. Present in smaller but variable amounts are veins and disseminated particles of dark-red-brown nearly opaque 

hematite and silt-size grains of quartz and colorless 

mica. The mica flakes and micaceous crystals, the lensshaped aggregates, and the veins parallel the bedding, 

except in shale containing Radiolaria and spicules 

where their orientation near the fossils is parallel to 

the surface of the fossils.

X-ray diffraction analysis of the shale indicates the 

presence of large amounts of mica, most of which is 

poorly crystallized or randomly interlayered with other 

micaceous minerals, moderate amounts of a chloritelike mineral, and hematite, goethite, and quartz. No 

expanding-lattice clay mineral was detected. Feldspar 

is present in very small amounts or is absent.

Radiolaria and spicules are sparse in most shale, but 

thin beds of shale containing as much as 70 percent 

Radiolaria and spicules are interbedded with Radiolaria-free shale. Radiolaria range in diameter from 0.2 

to 0.05 mm. As in the chert, the Radiolaria are preserved in microcrystalline quartz and chalcedony which 

contain red pigment inclusions in former chambers of 

the test. Radiolaria replaced by calcium carbonate 

were also found in the chert of a phosphatic nodule near 

Laguna Honda. Taliaferro and Hudson (1943, p. 260) 

found that Radiolaria in their "Franciscan-Knoxville" 

radiolarian shales collected at a number of localities in 

the Coast Ranges are invariably replaced by calcium 

carbonate.

Most shales also contain a small amount of carbon 

having relict plant-cell structures. Some red shales 

contain a small amount of light-bluish-white curved 

thin shells, as much as 1 mm in size, that resemble 

chitinous parts of arthropods and tiny shark's teeth. 

These remains are preserved as carbonate fluorapatite.

Green shale found between massive and thin-bedded 

chert on Sunset Heights in San Francisco consists of 

about 40 percent hydrous mica, 40 percent quartz, and 

20 percent kaolinite.

HYDRO-THERMAL ALTERATION

Radiolarian chert and shale are hydrothermally 

altered along faults and along greenstone bodies. Chert 

becomes either grayish orange or light gray, and shale 

generally is grayish orange. In some localities the rock
46 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

is green from ferrous iron alteration products. In some 

chert sections the color changes and other effects of 

hydrothermal alteration are confined to bands along 

joints; these give the rocks a mottled appearance. 

Hydrothermal effects may follow the bedding of sections 1 25 feet thick in the midst of a section of unaltered beds or may cross the bedding (fig. 32).

Altered chert beds generally preserve much of the 

hardness of fresh chert beds. Altered shale beds soften 

and are generally plastic when wet. X-ray diffraction 

analyses of the <2-micron fraction of hydrothermally 

altered shale show mica, kaolinite, and montmorillonite. The mica is poorly crystalline or randomly interlayered with expandible clay minerals.

In some places hydrothermal alteration has converted 

both chert and shale into a soft clayey material containing quartz, montmorillonite, kaolinite, mica, and 

the hydrous iron silicate hisingerite. The clay is plastic 

and weak when wet.

At some wide fault zones the chert is thoroughly 

shattered and has the properties of a friable sandy 

material or a clayey, moderately plastic material. Clay 

minerals in fault gouge consist mostly of mica, with 

subordinate amounts of random mixed-layered micamontmorillonite (mostly mica) and kaolinite.

Fresh radiolarian chert near zones of hydrothermal 

alteration commonly contains veinlets of quartz and 

well-crystallized kaolinite.

WEATHERING

Most exposures of radiolarian chert and shale show 

virtually no weathering effects except for raveling of the 

shale towards the ground surface (fig. 33). The physical 

properties of the chert beds appear to be little affected

FIGURE 32. Irregular band of hydrothermally altered radiolarian chert of the Franciscan Formation within a section of 

unaltered chert. Light chert is altered; dark chert is fresh. 

Northwest Mount Sutro, University of California Medical 

Center, San Francisco.

._> , ,*^f

FIGURE 33. Radiolarian chert and shale. Near the east side of 

U.S. Highway 101, three-fourths mile north of Golden Gate 

Bridge, Marin Peninsula. Note the thin soil cover and raveling of thin-bedded chert and shale in foreground, thick chert 

beds in background.

by weathering, although the slight disintegration of 

the interbedded shale decreases the shearing strength 

of the whole rock mass. At some exposures, weathering 

has leached out iron, removed some of the silica, and 

colored the surface of the chert light red brown to white 

to a depth of 1 mm. Radiolaria stand out in relief on 

such surfaces (Davis, 1918b, p. 261). In discussing 

oxidation of manganese ores in Franciscan chert, Taliaferro and Hudson (1943, p. 270) evidently believed 

that manganiferous chert may be oxidized by weathering processes to a depth of several feet, although they 

gave no depth figures and stated that the maximum 

migration of manganese, leaving behind porous cores 

of silica, is only a few inches.
RADIOLARIAN CHERT AND SHALE 47

ORIGIN

Chert is a marine deposit, but its environment of 

deposition on the sea floor, the source of silica, and the 

manner in which the silica becomes chert are largely 

unsettled questions.

The probability of a deep-sea origin for the graywacke sandstone and shale through turbidity current 

transportation of detritus also dictates a probable 

deep-sea environment for the radiolarian chert with 

which they are interbedded (Sanders and Swinchatt, 

1957). The ultimate source of the silica is believed to 

be the greenstones of the Franciscan Formation. The 

close association in time and space of greenstone and 

radiolarian chert of the Franciscan Formation of the 

Coast Ranges is given by Taliaferro and Hudson (1943, 

p. 273-274) as the strongest evidence for this belief. 

They stated that chert did not begin to form until volcanism started, that the maximum development of the 

chert coincides with maximum volcanism, and that 

where volcanic rocks decrease laterally and disappear, 

interbedded chert does likewise. In the San Francisco 

North quadrangle, chert is indeed generally found near 

greenstone and is absent or sparse where greenstone is 

absent.

ORIGIN OF SILICA

Silica in chert can be supplied by silica-secreting 

organisms or by inorganic processes. Bramlette (1946, 

p. 37 55) presented strong evidence for the derivation 

of porcelaneous and cherty rocks of the Miocene Monterey Formation in the California Coast Ranges from 

diatoms through a diagenetic alteration process that 

dissolves finely divided opal of diatoms in diatomaceous 

deposits and reprecipitates it nearby and that is accompanied by moderate load and dynamic metamorphism.

Taliaferro and Hudson (1943, p. 233-234) believed 

the silica is derived from submarine springs, which 

they expect are common during volcanism, and from 

the reaction of hot sea water and hot lava or pyroclastics. Rhythmically bedded chert and shale lenses may 

have been originally bottom oozes consisting of colloidal 

silica and small amounts of colloidal iron, aluminum, 

manganese compounds, and other substances, mixed 

with clayey material and the siliceous organisms that 

nourished in the silica-rich waters. When colloidal 

silica changes from a sol to a gel, it tends to free itself 

of impurities. Thus the shale interbedded with the 

chert represents layers of impurities segregated by 

flocculation of silica. In aging and syneresis, water is 

lost and the gel hardens rapidly (Taliaferro, 1933, p. 

50; 1934, p.221-227).

Results of experimental investigations of the solubility and precipitation of silica by Krauskopf (1956; 

1967, p. 166-170) and White, Brannock, and Murata

(1956, p. 27-59) show that silica forms true solutions 

in water, mostly in the form of dispersed molecules of 

monosilicic acid, H4SiO4 . Amorphous and colloidal 

silica is more soluble than crystalline silica (tridymite, 

cristobalite, chalcedony); quartz has the lowest solubility. In supersaturated solutions, colloidal silica, in 

amounts equal to the excess over true solution equilibrium concentration, forms slowly at low temperatures 

and rapidly near and above 100°C. The amount of soluble silica found in sea water (1-10 ppm) is far below 

the experimentally determined solubility of amorphous 

silica in sea water (100-110 ppm at 22°-27°C; 280-310 

ppm at 85°-95°C, according to Krauskopf, 1956, p. 13). 

Removal of silica by Radiolaria, diatoms, and siliceous 

sponges probably accounts for much of this difference. 

The remarkable undersaturation of soluble silica in sea 

water would seem, at first glance, to be strong evidence 

against the inorganic origin of chert by flocculation of 

colloidal silica. But the undersaturation is true only of 

sea water that has been analyzed. Sampling has not 

been extended to sea water in regions of submarine 

volcanic activity where hot sea water is in contact with 

hot molten or solidified volcanic volcanic rocks and 

where springs are expected to be numerous and the 

silica content is expected to be high, especially in quiet 

basins. Determination of the release of silica, aluminum, iron, magnesium, and other elements from molten 

lava to heated sea water should yield data especially 

pertinent to the origin of chert. The high temperature 

of such waters would greatly increase the solubility of 

silica, and cooling of the water would cause supersaturation and rapid formation of colloidal silica. Thus 

Krauskopf (1956, p. 23) suggested that the cherts 

commonly associated with pillow lavas may have an 

inorganic origin by flocculation of colloidal silica, 

although he added that Bramlette's hypothesis of diagenetic alteration for the opaline cherts of the 

Monterey Formation is in good agreement with experimentally determined behavior of silica.

ORIGIN OF CHERT

Both organic and inorganic processes may have 

formed the chert of the Franciscan Formation. The 

Franciscan cherts are closely associated in the field 

with contemporaneous volcanic rocks and also contain 

abundant Radiolaria. Part of the silica in Franciscan 

chert is obviously radiolarian skeletons and spicules. 

Thus the problem of the origin of the additional silica 

that fills and encloses the Radiolaria and spicules and 

that has no recognizable organic structure remains 

unsolved. The amount of silica is large in some cherts 

in which no fossils are apparent, but even those 

crowded with Radiolaria and spicules probably contain
48 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

30-50 percent of silica showing no organic structures. 

Hinde remarked (1894, p. 238) "the fossilization, 

which has been sufficient to obliterate most of the 

radiolarian structure, would completely destroy all 

traces of the smaller and more delicate diatoms," and 

he suggested this process also occurred in the cherts. 

In addition, Bramlette (1946, p. 55) suggested "that 

the Radiolaria preserved in the cherts of the Franciscan 

Formation represent only the heavier-shelled forms of 

the Radiolaria or other siliceous organisms originally 

present." This difference in behavior of silica in different organisms has been confirmed experimentally by 

D. E. White (oral commun., 1956), who stated that the 

form and stability of silica vary among silica-secreting 

organisms and that the silica of diatomite is unstable, 

fairly soluble, and very readily crystallized to chalcedony or quartz. He also suggested that in Radiolaria 

skeletons the silica of the spines is more soluble than 

the remaining silica. Thus some of the silica of the 

Franciscan chert not involved in organic structures 

may represent diatoms and other unstable organic 

silica remains whose organic origin has been obliterated.

ORIGIN OF SHALE

The shale that is rhythmically bedded with Franciscan chert differs considerably from the shale that is 

interbedded with graywacke sandstone of the Franciscan Formation. Individual shale beds in chert seldom 

reach 25 feet in length, whereas a shale bed in sandstone extends 100 feet or more. Shale in chert generally 

has more total iron, commonly ferric, as hydrated or 

anhydrous iron oxide. Shale in sandstone has mostly 

ferrous iron in silicate minerals. Na2O is lower and K2O 

is higher in the shale in chert. There is a notable lack 

of feldspars, as shown by X-ray diffraction analysis. 

These peculiarities indicate to the writer that the shale 

in chert probably did not form by sedimentation of 

detritus. Its formation by the little-known colloidal 

process by which impurities in silica gel tend to be 

pushed aside and segregated into bands better explains 

its characteristics. Other than the fossils, most of the 

constituents of the shale, as well as of the chert, are 

thought to have been derived by rapid chemical precipitation from cooling sea water. The sea water was able 

to acquire these constituents in solution by reaction 

with hot molten or freshly solidified volcanic rocks 

(Bailey and others, 1964, p. 66-67). The precipitates 

would be mostly colloidal in size and would be mixed 

with skeletons of siliceous organisms, and possibly a 

small amount of detritus sinks to the sea floor. Most of 

the micaceous minerals may have formed incipient 

crystals during precipitation from the cooling sea water 

and may have grown later by diagenesis. The bulk of 

the deposit consisted of silica gel, which lost water,

shrank, and coagulated under the weight of overlying 

sediments; eventually it hardened into chert.

ORIGIN OF COLOR

The red color of the chert and the shale is probably 

the original color of these rocks. It is caused by fine 

particles of ferric oxide, probably precipitated from sea 

water as colloidal ferric hydroxide. Two factors may 

have induced precipitation: (1) a lowering of temperature and consequent lowering of solubility of Fe with 

respect to the minerals and (2) a change from acidic 

conditions of sea water in contact with erupting volcanic rocks to the alkaline conditions of average sea 

water (Hem and Skougstad, 1960, p. 96). The high 

oxygen content of sea water at great depths (Sverdrup 

and others, 1946, p. 748, 753) would promote the oxidation of any ferrous ions in sea water, especially at 

the high pH of average sea water (Huber, 1958, p. 133). 

Nevertheless, shales associated with graywacke sandstone deposited at the same depth of sea water contain 

more ferrous iron, in the form of silicates, magnetite, 

and pyrite, than ferric iron probably because of a local 

negative Eh (redox potential) created by the presence 

of detrital carbonaceous material.

AGE

Radiolaria and spicules are common (as much as 70 

percent) in the chert and in some shale but are sparse 

in most shale. Radiolaria can be seen in chert by handlens examination of wet fresh fractures. They appear as 

dark-gray clear round or conical masses tightly 

cemented in the rock. On weathered surfaces they have 

relief. In shale they appear with the hand lens as tiny 

pellets or leave pits where they were detached. Most 

of them are between 0.5 and 0.05 mm in diameter. Delicate structures, such as slender external spines, are 

rare and are seen only in thin sections of chert or on 

chert surfaces that have been etched for a few minutes 

in hydrofluoric acid. Obscure chitinous material is 

found in some shale.

Radiolaria in the cherts of the Franciscan Formation 

were first described by Hinde (1894), who identified 

several genera in cherts from Angel Island and from 

Buri-Buri Ridge, 6-10 miles south of the quadrangle. 

Preservation was too poor to permit identification of 

species. Hinde (1894, p. 237-238) noted a similarity 

in character of the rock, mode of preservation of the 

Radiolaria, and abundance of the genus Dictyomitra 

in the cherts of the Franciscan Formation and in the 

red radiolarian jaspers and chert of Jurassic and Cretaceous age from the Tyrol, from Switzerland, from 

Hungary, and other places.
RADIOLARIAN CHERT AND SHALE 49

Riedel and Schlocker (1956) described several 

genera of Radiolaria found in shale interbedded with 

red radiolarian chert of the Franciscan Formation at 

Belmont, 18 miles southeast of the San Francisco 

North quadrangle. The Radiolaria there are somewhat 

better preserved than those found in cherts in the San 

Francisco North quadrangle. Some are similar to 

Jurassic species of other parts of the world, others are 

similar to Cretaceous species. Radiolaria are not yet 

useful in dividing the Franciscan Formation. Pessagno 

(1970, p. 130) recognized tentative radiolarian zones 

in the Great Valley sequence and suggested that the 

zone may be used for the Franciscan Formation.

Two typical red thin-bedded radiolarian chert and 

shale sections, 10-20 feet thick, are interbedded with 

sandstone and shale west of James D. Phelan Beach 

State Park. They are separated stratigraphically 400- 

600 feet from the Douvilleiceras-bearing sandstone of 

Albian age. Part or all of the sedimentary rock sequence 

in this area is overturned, and it is not known whether 

the radiolarian chert and shale sections are older or 

younger than the Albian sandstone.

OCCURRENCE 

MARIN PENINSULA

On Marin Peninsula, radiolarian chert and shale 

occur in thick zones interbedded with sandstone and 

greenstone at several stratigraphic positions (fig. 34). 

Between Lime Point and Horseshoe Bay, the two radiolarian chert sections, separated by a thick sandstone 

section, vary in thickness from about 50 to more than 

500 feet. They can be followed along the strike for 

about 2 miles, which, according to Taliaferro (1943, 

p. 149), is an exceptional persistence for chert beds of 

the Franciscan Formation. Several other thick sections 

of radiolarian chert lies stratigraphically above and 

below these sections (fig. 34).

ANGEL ISLAND AND VICINITY

Relatively small bodies and lenses of chert are widely 

distributed on Angel and Belvedere Islands and on 

Tiburon Peninsula. Most of the chert shown in 

these areas is slightly to moderately metamorphosed, 

although it retains most of the field appearance of 

bedded chert and associated shale. The chert north of 

Hospital Cove on Angel Island is the least metamorphosed. These chert bodies are unsheared. They are 

pale reddish or greenish brown or pale blue. The largest 

mass is 2,000 feet east of Stuart Point. Sheared metachert is shown as metamorphic rocks. These rocks are 

brown and rich in aegerine and (or) stilpnomelane or 

are dark blue and rich in amphiboles of the glaucophane-riebeckite series.

FIGURE 34. Franciscan Formation exposed on north shore of 

Golden Gate, Marin Peninsula. A, View between Golden 

Gate Bridge and Point Diablo. Cliff left (west) of bridge is 

greenstone. Cliff near left edge exposes radiolarian chert on 

left and greenstone on right. B, Close view of wooded ravine 

(see A for location) 0.6 mile west of Golden Gate Bridge. 

Radiolarian chert exposed as barren rock east (right) of the 

ravine and forms hogback west (left) of the ravine. Pyroclastic greenstone exposed immediately west of ravine. Note 

dip to west in chert and pyroclastic rocks in 400-foot cliff. 

Colluvium generally underlies dark brush.

SAN FRANCISCO

The largest exposures of radiolarian chert in San 

Francisco are the central highlands area near the south 

border of the quadrangle. Sunset Heights, Mount 

Sutro, Twin Peaks, Mount Olympus, and Corona 

Heights owe their height and boldness in large part to 

the presence of radiolarian chert (figs. 35, 36, 37). The 

maximum thickness is exposed on Mount Sutro, where 

this unit may be as much as 1,600 feet thick. The same 

section is exposed on Mount Olympus and Twin Peaks. 

It appears to lens out rapidly westward and is separated by sandstone and greenstone from a younger 

radiolarian chert section, approximately 800 feet thick, 

exposed at Corona Heights and on 18th Street at Mission Park. The sandstone underlying this younger section contains numerous lenses of radiolarian chert, 

many of them less than 50 feet thick and 400 feet long. 

Small lenses of radiolarian chert are also found interbedded with sandstone along the shore between Fort 

Point and Lands End. Somewhat larger radiolarian 

chert bodies are in the east half of Golden Gate Park.
50 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 35. Central highlands area of San Francisco, viewed 

west from Potrero Hill. Peaks underlain by radiolarian chert 

in the Franciscan Formation are, from left to right, Twin 

Peaks, Mount Sutro, and Corona Heights (barren hill below 

and to right of small cloud).

Although contortions are common in the radiolarian 

chert of Sunset Heights, persistent bedding attitudes 

indicate that this rock dips eastward, which would 

place it below sandstone of the Franciscan Formation 

exposed at the base of Mount Sutro at Laguna Honda 

Reservoir and also below the radiolarian chert of 

Mount Sutro. On the other hand, it may be part of the 

Mount Sutro chert section repeated by a normal fault 

now concealed by dune sand, west of Seventh Avenue. 

If the radiolarian chert of Sunset Heights is a separate 

section, it is at least 400 feet thick. Lawson (1914, San 

Francisco quadrangle,structure-sections plate) showed 

it to be 950 feet thick in Sunset Heights.

METAMORPHIC ROCKS

Metamorphic rocks of the Franciscan Formation 

include slate, phyllonite, phyllite, fine- to coarsegrained schist, and fine- to medium-grained granular 

hornfelslike or granulitelike rocks that lack foliation, 

for which the name granofels (Goldsmith, 1959, p. 109) 

is useful. On the basis of field appearance, rocks mapped 

as metamorphic rocks were judged to be of somewhat 

higher metamorphic grade than the semischists. metacherts, and metagreenstones described. Rock types 

mapped as metamorphic rocks in one area, however, 

may be mapped as other bedrock units elsewhere. 

Relict textures and structures that suggest parentage 

are present in many of the metamorphic rocks, but 

because details of mineralogy, chemical composition, 

and fabric of the rocks are lacking, ideas as to parentage 

and metamorphic processes are speculative.

Metamorphic rocks are most prevalent on Angel and 

Belvedere Islands and on Tiburon Peninsula. Almost 

everywhere in these places Quarry Point excepted 

the clastic sedimentary rocks are semischists, and other 

Franciscan rock types are metamorphosed to varying

FIGURE 36. Radiolarian chert and sandstone in the Franciscan 

Formation, 17th Street, Mount Olympus area, San Francisco, 

viewed northwest.

FIGURE 37. Radiolarian chert and shale, Sunset Heights, San 

Francisco, near Ortega Street and 14th Avenue. Dune sand 

lies on the chert at top of hill at elevation of 600 feet above 

sea level.

degrees. Completely recrystallized schists are also 

common. Metamorphic rocks from Tiburon Peninsula 

north of this quadrangle are described by Lawson 

(1914, p. 7) and Taliaferro (1943, p. 162-166). Several 

bodies 2-10 feet in diameter in San Francisco are 

included on the geologic map (pi. 1) in the sheared 

rocks unit or in serpentine. Some offshore rocks north 

of Bakers Beach and west of Lands End are metamorphic rocks within the Fort Point-Hunters Point or 

City College shear zones, respectively.

On the basis of their field occurrence, the metamorphic rocks are placed into three categories: (1)
METAMORPHIC ROCKS 51

Rocks on serpentine borders; (2) rocks included within 

serpentine; and (3) rocks distant from known serpentine bodies.

METAMORPHIC ROCKS ON SERPENTINE BORDERS

The major metamorphic rock types near serpentine 

borders are fine-grained tough greenish-brown, bluishgray, and dark-yellowish- and dark-reddish-brown 

phyllites, schists, and less-foliated rocks. Many of the 

rocks have little tendency to split along the foliation. 

Foliation generally strikes northwestward, parallel to 

the bedding of nearby strata. Near the south shore of 

Angel Island dips are northeastward. Minor crenulations have maximum amplitudes and wave-lengths of 

about 8 inches. They generally trend and plunge northwest, but they vary considerably over distances of less 

than 5 feet.

Metamorphic rocks on serpentine borders include 

the following assemblages (minerals listed in decreasing order of abundance; minerals making up less than 

5 percent of rock listed as minor): 

Group I:

Tremolite-albite-chlorite with minor pumpellyite,

sphene, and oxidized pyrite cubes. 

Tremolite-albite-sphene with minor muscovite,

chlorite, and oxidized pyrite cubes. 

Albite-stilpnomelane-tremolite with minor sphene

and calcite.

Tremolite (sodic)-albite-stilpnomelane-sphene. 

Tremolite (sodic)-albite-sphene with minor epidote, muscovite, chlorite. 

Albite-stilpnomelane-actinolite with glaucophane

rims, chlorite with minor pyrite. 

Quartz-tremolite-stilpnomelane with minor muscovite, sphene, magnetite, calcite, and garnet. 

Group II:

Tremolite-chlorite. 

Tremolite. 

Group III:

Quartz - muscovite - stilpnomelane - crossite schist 

(fine-grained, silvery to brownish-gray with 

dark-blue rosettes of crossite). 

Quartz-garnet-hematite-stilpnomelane with minor 

clinozoisite and glaucophane (fine grained, dark 

brownish gray) with thin micaceous layers of 

muscovite-chlorite-iron oxide. 

Quartz-albite-crossite schist with minor muscovite, sphene, stilpnomelane, magnetite, oxidized 

pyrite cubes, and apatite (fine grained, silvery 

to dark bluish gray). 

Quartz-chlorite with minor muscovite (fine grained,

green). 

Quartz-garnet with minor stilpnomelane, muscovite, glaucophane, and sphene (fine grained, 

dark brownish gray).

Stilpnomelane-quartz-albite with minor glaucophane (fine grained, reddish brown, with thin 

blue glaucophane-rich streaks). 

Group IV:

Pumpellyite-albite-sphene rock with minor clinopyroxene (relict microphenocrysts) and chlorite (very fine grained, tough, slightly schistose, 

greenish gray).

Clinopyroxene (diopside - jadeite) - albite - sphene 

rock with minor chlorite (very fine grained, 

tough, slightly schistose, greenish gray). 

Tremolite - albite - clinozoisite - epidote rock with 

minor sphene, stilpnomelane, and pyrite (fine 

grained, tough, slightly schistose, greenish 

gray).

Group II rocks are less abundant but more conspicuous than group I rocks. They are very fine grained 

white to silvery light-gray schists with small-scale 

crenulations (%-%-in. amplitude) and well-developed 

cleavage parallel to the foliation. Some are in elongated 

pods and veins in shear zones between serpentine and 

the darker rocks of group I. Tremolite and tremolitechlorite rock are generally soft and talcose. Groups III 

and IV are not as common but form distinct metamorphic assemblages. The largest exposed bodies of 

metamorphic rocks are on the borders of serpentine 

along the south shore of Angel Island (fig. 38). Inland 

to the northwest along the west side of a large serpentine body, however, the bodies are isolated and do not 

form a continuous belt adjacent to the serpentine.

Along the south shore of Angel Island, metamorphic 

rocks border both sides of the serpentine dike (pi. 1). 

The smaller body, east of the serpentine, is mostly fine 

grained light-green rock consisting of the first two 

assemblages listed in group I. Schistosity in this body 

is shown by layers 1 or 2 inches thick, but individual 

layers are tough and only slightly schistose. The larger, 

western body consists of a large variety of metamorphic 

rocks and includes most of the assemblages in groups 

I-IV. Schistosity is developed to various degrees. 

Some rocks show closely spaced highly schistose micaceous layers; others, rich in quartz, albite, and other 

nonmicaceous minerals, are tough and show only moderate or little tendency to break along the Schistosity. 

Rocks of group IV are present in minor volume.

PARENT ROCK TYPES

Metamorphic rocks along serpentine borders are 

believed to be metamorphic equivalents of sedimentary 

and igneous rocks of the Franciscan Formation. The 

highly tremolitic rocks of groups I and II may have
52 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 38. Serpentine body and metamorphic rocks on Angel 

Island. Upper photograph is view northwest across Raccoon 

Strait and along Tiburon Peninsula. Serpentine is light band 

in center partly covered with trees. Metamorphic rocks form 

headlands on shore at right. Lower photograph is of the same 

part of the serpentine body showing its ridge topographic 

expression, as viewed from a boat southwest of Knox Point 

(white lighthouse building on shoreline at left). Serpentine 

ridge is light-colored mostly barren ridge between the shore 

cliffs and the skyline.

been subjected to magnesium metasomatism, or they 

may be metamorphosed serpentine or peridotite. Group 

III rocks are probably metamorphosed cherts or arkosic wackes. Group IV rocks are probably metamorphosed greenstones.

A rock abundant near the west border of the serpentine along the south shoreline of Angel Island consists 

 of alternating reddish-brown stilpnomelane-rich beds 

1-3 inches thick and bluish- and brownish-gray quartzrich beds cut by numerous quartz veinlets and pigmented with crossite, glaucophane, garnet, and stilpnomelane. Mineral composition and bedding strongly 

indicate that the rock was originally interbedded chert 

and shale. Thus, quartz veinlets in the quartz-rich 

layers appear to be relicts of quartz veinlets in the 

original chert. This metamorphic rock differs from 

typical Franciscan chert in that the quartz-rich layers 

and stilpnomelane-rich layers are nearly the same 

thickness, whereas unmetamorphosed chert layers are 

typically 3 inches thick and the shaly layers are only 

one-fourth inch thick. The difference in thickness may 

be caused by metamorphism.

METAMORPHIC ROCKS WITHIN SERPENTINE BODIES

A great variety of metamorphic rocks occur as inclusions in all the serpentine bodies. They are especially 

abundant on Angel Island but are sparse in San Francisco. The inclusions vary in size from about 2 inches 

to 20 feet in diameter and include a variety of textures 

and compositions. They appear to have a random distribution with respect to serpentine borders; foliation 

and lineation are random in some inclusions. All these 

features are compatible with the belief that the inclusions formed under varying physical and chemical conditions before reaching their present position. Similar 

tectonic inclusions in serpentine are described by 

Brothers (1954, p. 616) from the Berkeley Hills, 8 

miles northeast of Angel Island. The inclusions in the 

San Francisco North quadrangle are believed to be 

metamorphosed rocks of the Franciscan Formation and 

metamorphosed serpentine and gabbro-diabase.

Inclusions are classified here according to their mineralogy and probable parentage into five groups: 

Group I:

Quartz-crossite. 

Quartz-crossite-stilpnomelane. 

Quartz-hornblende (with crossite aureoles)- 

sphene-garnet with minor stilpnomelane and 

cut by veins of albite-stilpnomelane-muscovite. 

Quartz-stilpnomelane-hornblende (with glaucophane and crossite aureoles) with minor sphene 

and albite.

Quartz-albite-hornblende (with crossite aureoles)- 

aegerine with minor sphene, stilpnomelane, and 

hematite. 

Group II:

Garnet-chlorite-vesuvianite with minor sphene.

Garnet-chlorite-diopsidicaugite-sphene.

Garnet - chlorite - vesuvianite - calcite with minor

magnetite and pyrite. 

Diopside-chlorite with minor sphene. 

Diopside-chlorite with minor sphene and garnet. 

Chlorite-diopside-zoisite-sphene. 

Clinozoisite-diopside-garnet-vesuvianite with

minor sphene and chlorite. 

Diopside-garnet-prehnite-chlorite-sphene. 

Group III:

Pumpellyite-pigeonite-chlorite-sphene with minor

stilpnomelane. 

Pumpellyite-albite-sphene and minor calcite and

chlorite.

Chlorite-sphene-andesine (An44 ). 

Group IV:

Hornblende - albite - clinozoisite - diopside - sphene

with minor stilpnomelane and muscovite. 

Clinozoisite-diopside-sphene with minor chlorite 

and stilpnomelane.
METAMORPHIC ROCKS 53

Group V:

Tremolite-chlorite.

Actinolite-glaucophane-chlorite-pumpellyite with 

minor calcite, muscovite, and sphene.

Albite-actinolite with minor chlorite and sphene.

Chlorite-actinolite with minor sphene.

Chlorite-magnetite-apatite.

Chlorite-magnetite-sphene-apatite.

Chlorite-sphene with minor stilpnomelane.

Talc - chlorite - hornblende (sodic) with minor

sphene, pyrite, and stilpnomelane. 

Rocks in group I are medium- to dark-bluish-gray or 

brownish-gray fine-grained somewhat brittle schists. 

Most of them contain more than 50 percent quartz and 

in addition are cut by numerous quartz veins. They are 

probably metamorphosed cherts or arkosic wackes.

Rocks in group II are dark greenish gray to light 

yellowish gray, aphanitic to medium grained, and 

exceedingly tough. Typically, they are spheroidal 

inclusions l%-4 feet in diameter. They show no foliation. Most of them have a light-colored core and a 1 3- 

inch-thick medium- to dark-gray rim. The dark rim 

consists mostly of chlorite and magnetite. Garnet composition, determined by use of Sriramadas' (1957, p. 

295-296) and Winchell and Winchell's (1951, p. 493) 

graphs, shows grossularite-andradite (40-70 percent 

grossularite) and garnet-hydrogarnet of the same grossularite-andradite range, but with 1-1.2 moles of water. 

Many rodingites show a relict porphyritic texture with 

phenocrysts replaced by metamorphic minerals, althrough some of the clinopyroxene appears to be primary phenocrysts. One inclusion contained euhedral 

plagioclase laths replaced by chlorite, garnet, and vesuvianite in a groundmass that shows structures identical 

to those found in tachylitic, pyroclastic, and pillow 

greenstone preserved mostly by cloudy garnet-hydrogarnet (of grossularite-andradite composition). Greenstone structures are also preserved, mostly as chlorite 

and magnetite, in the dark rim.

Rocks in group II are believed to be metamorphosed 

igneous rocks and possibly sediments of the graywacke 

type. Their mineralogy indicates that calcium was 

added to them. Their composition and their occurrence 

as inclusions in serpentine suggests that they are 

rodingites. Rodingites have been described throughout 

the world as calcium-enriched gabbros or diorites intrusive into serpentine. Rodingites on Angel Island do not 

intrude serpentine; rather, they are tectonic inclusions 

in serpentine and are intruded by serpentine (antigorite) veins. Thus the rock from which they were altered 

to rodingite is older than serpentine (Schlocker, 1960). 

The source of additional calcium needed to form 

rodingite and the conditions under which calcium was 

concentrated in the cores of the inclusions and magnesium was concentrated in the dark rim are not 

known. Calcium is thought to come either from solutions left after crystallization of ultrabasic "magma" 

or from pyroxenes in peridotite and may have been 

released during serpentinization (Turner and Verhoogen, 1951, p. 488). The presence of calcium-bearing 

pyroxenes such as diallage (diopside) in the serpentine 

of the San Francisco area indicates that the latter 

source is possible here. The diopside-garnet-prehnitechlorite-sphene rock and the diopside-chlorite-zoisitesphene rock were inclusions in serpentine and landslides north of Bakers Beach.

Rocks of group III are grayish-green aphanitic to 

fine-grained exceedingly tough unfoliated metagreenstones. They are generally cut by blue veinlets of 

crocidolite.

Rocks in group IV are green to greenish-gray fineto medium-grained granoblastic metadiabase-gabbros. 

They are commonly cut by pumpellyite, quartz, and 

albite veins.

Rocks of group V include types that are rich in magnesium minerals. They have various colors that range 

from dark greenish gray to pale pearly green and various textures that range from aphanitic hornfelsic at 

one extreme to coarse-grained porphyroblastic schistose at the other. They may have been subjected to 

magnesium metasomatism. Parentage is unknown, but 

some of them may be metaserpentines or metaperidotites. The actinolite-glaucophane-chlorite-pumpellyite 

rock was found in serpentine in San Francisco near the 

intersection of Euclid and Masonic Avenues.

METAMORPHIC ROCKS DISTANT FROM 

KNOWN SERPENTINE BODIES

Some metamorphic rocks have erratic distributions 

within semischists and relatively unmetamorphosed 

rocks of the Franciscan Formation of Angel and Belvedere Islands and Tiburon Peninsula. They are particularly abundant on Angel Island, within and near the 

centrally located greenstone body, although a number 

of them are found near Blunt Point, near a small exposure of pillow greenstone (pi. 1). They are generally 

elongated bodies less than 1 to more than 5 feet thick 

and mostly too small to show on the geologic map (pi. 

1). Schistosity, where shown, is more or less parallel to 

the bedding of nearby stratified rocks. Some of the 

metamorphic rocks occupy shear zones, and high shearing stresses may have been important in their formation. A hydrothermal origin is also suggested by the 

following characteristics: (1) Crocidolite on joints in 

unmetamorphosed unsheared greenstone; (2) veins of 

albite, quartz, calcite, pumpellyite, amphiboles, chlorite, stilpnomelane, and opaque ore minerals in meta-
54 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

morphic rocks; (3) rapid changes in texture and mineralogy over short distances; and (4) concentration of 

metamorphic rock bodies in and near the largest greenstone body exposed on Angel Island. (For more information on these processes, see Turner and Verhoogen, 

1951, p. 244,472.)

Similar observations were made by Borg (1956, p. 

1581). She used these as evidence that hydrothermal 

metamorphism formed glaucophane schists and eclogites in the Franciscan Formation near Healdsburg, 

Calif., 60 miles northwest of San Francisco. Nevertheless, Borg (1956, p. 1563, 1581) concluded that the 

metamorphism was not accompanied by metasomatism, 

after she had compared chemical analyses of unmetamorphosed parent rock types and the metamorphic 

equivalents and found them to be similar. The analyses 

given in table 5 show that this is also true of unmetamorphosed sandstone and jadeitized sandstone of 

Angel Island.

The rocks not directly related to serpentine bodies 

are further segregated according to their mineralogy 

and (or) parentage into the following groups: 

Group I:

Quartz-crossite with minor stilpnomelane. 

Quartz-crossite-aegerine. 

Quartz-aegerine-stilpnomelane with minor riebeckite and sphene.

Quartz-aegerine-magnetite with minor stilpnomelane. 

Quartz-garnet-stilpnomelane with minor muscovite, hornblende (sodic), and hematite. 

Group II:

Blue amphibole (glaucophane-riebeckite series)- 

lawsonite - pumpellyite - stilpnomelane - albite - 

chlorite with minor sphene 

Blue amphibole (glaucophane-riebeckite series)-

lawsonite-sphene with minor stilpnomelane 

Glaucophane - pyroxene (diopside - jadeite) -chlorite-sphene with minor clinozoisite, muscovite, 

and stilpnomelane

Clinozoisite - chlorite - lawsonite - stilpnomelane -

pyroxene (diopside-jadeite)-sphene with minor

muscovite, cut by veins of lawsonite-stilpnomelane-chlorite with minor muscovite

Glaucophane - lawsonite - albite - sphene cut by

quartz and calcite veins

Hornblende (hastingsite)-lawsonite with minor 

muscovite and sphene, cut by veins of lawsonite 

with minor chlorite and by veins of albite with 

minor hornblende.

Tremolite (sodic)-lawsonite-pyroxene (diopsidejadeite) with minor chlorite, stilpnomelane, 

glaucophane, and lepidocrocite.

Group III:

Glaucophane-albite-chlorite-muscovite with minor

sphene. 

Glaucophane - chlorite - pumpellyite - sphene with

minor muscovite, stilpnomelane, lawsonite, and

apatite (garnet porphyroblasts completely

altered to chlorite, muscovite, pumpellyite,

sphene, and stilpnomelane). 

Glaucophane-clinozoisitic epidote-stilpnomelane

with minor muscovite and sphene, cut by veins

of calcite. 

Albite-hornblende (with glaucophane aureoles)-

muscovite with minor chlorite, clinozoisite, garnet, pyrite, and lepidocrocite. 

Albite-hornblende-chlorite with minor sphene,

leucoxene, ilmenite, and magnetite, cut by veins

of albite, hornblende, and chlorite. 

Albite-amphibole (sodic hornblende to tremolite)

with minor chlorite, muscovite, and sphene, cut

by veins of coarse-grained albite. 

Group I rocks are predominantly fine grained metacherts. Quartz makes up three-fourths of most of them. 

Those rich in crossite are blue; those rich in aegerine 

are yellowish or reddish brown or grayish red. Some 

rocks contain both minerals as segregated masses or 

as yellowish and reddish brown rosettes streaked with 

blue. The last assemblage listed in group I is mostly 

light gray with subordinate brown flakes. Some rocks in 

group I are interbedded with relatively unmetamorphosed chert, especially on Belvedere Island, where 

blue semischistose metacherts are interbedded with 

colorless to red nonschistose chert.

Group II rocks are predominantly fine- to coarsegrained metagreenstones that are within, and at the 

borders of, the large greenstone body on Angel Island. 

The first two assemblages listed are blue fine-grained 

breccia fragments in a sheared partly metamorphosed 

greenstone that it enclosed within relatively unmetamorphosed greenstone breccia. The glaucophane-lawsonite-albite-sphene rock is a blue fine-grained tectonic 

inclusion more than 20 feet in diameter in the shear 

zone west of Lands End.

Group III rocks are fine- to coarse-grained blue or 

grayish-green generally micaceous completely recrystallized schists. On Angel Island they are spatially 

related to the large greenstone body and crop out at 

the unnamed point 3,000 feet north of Blunt Point. 

Parentage and conditions of metamorphism are uncertain. Because of the lack of chemical analyses, guesses 

about their parentage cannot be very selective. Their 

association with greenstone and sandstone and their 

mineralogy suggest that they are probably metamorphosed greenstone or arkosic wacke. The presence of 

late stilpnomelane and late pumpellyite suggests that
SHEARED ROCKS 55

chlorite zone conditions prevailed for some of them at 

a late stage of metamorphism.

TECTONIC SETTING OF METAMORPHIC ROCKS

Metamorphic rocks of Angel Island are part of an 

irregular northwestward-trending zone of metamorphic 

rocks, occurring mostly as tectonic inclusions, that 

follows the Tiburon Peninsula syncline for at least 6 

miles and includes Tiburon Peninsula and Belvedere 

Island. Perhaps this belt was an exceptionally strong 

axis of downwarping in which the Franciscan rocks 

were metamorphosed to varying degrees, depending on 

local conditions of shear, water-vapor pressure, temperature, and chemical environment. The tectonic inclusions within and on the border of serpentine evidently 

were emplaced during shearing. Lack of schistosity in 

the numerous rodingite inclusions in serpentine suggests that they were enriched in calcium after reaching 

their present position. Metamorphic rocks within semischists and relatively unmetamorphosed rocks of the 

Franciscan Formation in most places appear to be 

tectonic inclusions, but exposures are not complete 

enough to confirm this for all occurrences.

Metamorphic rocks in San Francisco are tectonic 

inclusions within the Hunters Point-Fort Point and 

the City College shear zones.

Ransome (1894, p. 211, 222) believed that the metamorphic rocks of Angel Island were formed by "local 

contact metamorphism" along intrusive borders of 

greenstone and serpentine. He explained the greater 

abundance of metamorphic rocks on the west side of 

the serpentine dike as due to greater intensity of metamorphic processes on the upper side of the dike. His 

report on the geology of Angel Island first clearly 

described the transition of the unmetamorphosed rocks 

into metamorphosed rocks and the close spatial relations of the metamorphic rocks and the mafic and 

ultramafic intrusive rocks. Early thinking on the origin 

of the Franciscan metamorphic rocks was also indicated by Ransome (1894, p. 211):

As the existing literature on Coast Range geology makes no 

mention of "glaucophane schists" arising from local contact 

metamorphism, and generally assigns them, together with the 

radiolarian cherts and much of the serpentine, to widespread 

regional metamorphism, the results arrived at in this paper 

have been to a certain extent forced upon the writer against 

certain preconceived notions drawn from reading.

Taliaferro (1943, p. 168, 182) believed that the 

metamorphic rocks are the result of local pneumatolytic metamorphism or metasomatism accomplished by 

emanations from mafic and ultramafic intrusives and 

that the schistosity is relict bedding. The writer 

believes that metamorphism was accomplished by one 

or more of the following: Hydrothermal solutions, 

metasomatism, local shear, or hydrostatic pressure.

HYDROTHERMAL ALTERATION AND WEATHERING

The metamorphic rock body exposed about 750 feet 

northeast of Knox Point is altered to a grayish-orange 

crumbly schist composed mostly of muscovite, vermiculte, quartz, and albite, and of a small amount of 

glaucophane. Small patches of partly altered bluish 

glaucophane-rich schist within the altered schist suggest that some or all of the vermiculite was derived 

from glaucophane. The schist and underlying greenstone are believed to be hydrothermally altered.

Weathering of metamorphic rocks has been slight.

SHEARED ROCKS

Zones of intense shearing that include several types 

of bedrock are distinguished on the geologic map (pi. 

1) from zones of intense shearing in but one rock type. 

These shear zones generally consist of a mixture of 

hard blocks of bedrock, from less than 1 inch to 25 feet 

or more in diameter, in a soft intensely sheared matrix. 

The largest of these blocks is about 0.4 mile by 0.6 

mile, in the Potrero Hill area. The blocks are generally 

rounded and surrounded by slickensided shear surfaces. The matrix is cut by abundant closely spaced 

shear surfaces. It consists generally of incompetent 

serpentine or shale that has broken down and generally 

moved plastically. Other incompetent material, such as 

hydrothermally altered rock, is found locally, but 

sheared shale is the only matrix material in many 

zones (fig. 11). Sheared serpentine and sheared shale 

resemble each other superficially, and they are difficult 

to distinguish in the field. Because of this resemblance, 

serpentine has been erroneously reported in previous 

literature in some areas.

Shearing of Franciscan rocks is greatest in shale, 

especially at and near serpentine borders, and the 

width of shear zones varies considerably. In some 

places, shear zones between the Franciscan Formation 

and serpentine are gradational, are more than 100 feet 

wide, and consist of an intimate mixture of serpentine 

and other rock types. In other places, shear zones are 

sharply defined and are only 3-5 feet wide, and the 

Franciscan rocks beyond are relatively undisturbed. 

Serpentine in many shear zones is converted to black 

waxy bentonite veins consisting predominantly of 

montmorillonite mixed with fragments of unaltered 

serpentine.

The color of shear zones is generally black or gray 

tinged with green. The color is determined by the 

sheared matrix. In sheared shale, carbon in thin disseminated streaks and films is believed to be partly 

responsible for the black and gray color. Chlorite is 

formed, in part, during shearing and gives the green 

tinge.
56 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

OCCURRENCE

The largest bodies of sheared rocks form the ridge 

between the U.S. Mint and the intersection of Presidio 

Avenue and Geary Boulevard. These rocks are predominantly clastic rocks and serpentine, but they 

include small bodies of radiolarian chert, greenstone, 

gabbro-diabase, and metamorphic rocks. Some of the 

clastic rock bodies are more than 1,000 feet long. Rocktype relationships are similar to those on the northeast 

slopes of Potrero Hill, where tectonic inclusions of 

sandstone occur in serpentine. Franciscan rocks, however, appear to be more abundant than serpentine. 

Depicting rock types within shear zones on the geologic 

map (pi. 1) generally is not feasible because the zones 

are very complex and mostly concealed by homes and 

streets in this area. The sheared rock relationship 

south of California Street and west of Presidio Avenue 

is considerably generalized from a detailed field map 

at a scale of 1:2,400 prepared by M. G. Bonilla and the 

writer when this area was being subdivided.

In a shear zone in the Point Lobos Lands End area, 

a large variety of Franciscan bedrock is represented, 

including a few glaucophane-bearing metamorphic 

rocks with subordinate amounts of silica-carbonate 

rock and serpentine. The shear zone is tentatively 

thought to be the northern extension of the City College fault. (See p. 92.) Bodies of sheared rocks also 

occur on the north slope of Twin Peaks, Lone Mountain, 

on the west slope of Mount Sutro, and at Point Diablo.

SERPENTINE

Serpentine derived from peridotite occurs in San 

Francisco, on Angel and Belvedere Islands, and on 

Tiburon Peninsula. The peridotite was largely harzburgite; a small part of it was dunite. The present position of much of the serpentine is believed to be due to 

tectonic movement at relatively low temperatures. 

Most of the serpentine is sheared, but in a few places 

it is largely massive and unsheared. A few small bodies 

of clinopyroxenite and gabbro-diabase are found in 

the serpentine.

Serpentine in San Francisco is exposed in a shear 

zone about 1 ^ miles wide that extends from the southeast corner of the quadrangle across the city to the 

shoreline southwest and east of Fort Point. Other 

occurrences are below the south tower of Golden Gate 

Bridge, at James D. Phelan Beach State Park, in shear 

zones in the Lands End area, and in Lincoln Park. A 

serpentine body, about 600 feet thick, crosses the 

southwest part of Angel Island (fig. 38). Small serpentine exposures occur southwest of this body, along 

the southwest shore of Belvedere Island, and on Tiburon Peninsula.

In the San Francisco North quadrangle, as in the 

Coast Ranges generally, large bodies of serpentine 

form ridges (fig. 39). Serpentine and mixtures of serpentine and sheared rocks of the Franciscan Formation 

form the ridges of Potrero Hill, Alamo Square, and 

much of the high area of Fort Scott in the Presidio and 

continue for half a mile northwest of Fort Point as a 

submarine ridge to a point about midway in the Golden 

Gate channel (Carlson and others, 1970; Carlson and 

McCulloch, 1970). On Angel Island, serpentine is generally exposed in ridges (fig. 44) and spurs except at 

the northwest end of the island where it underlies a 

valley. Small bodies of serpentine do not generally 

form ridges.

Serpentine is apparently resistant to erosion, despite 

its generally sheared character. This is evidently due 

to its stability in the weathering environment and its 

consequent lack of a thick soil mantle. It is also due 

partly to its property of retaining cohesiveness after 

shearing by yielding to tectonic forces in generally 

plastic manner, instead of by shattering. It yields 

easily, however, to strong and continuous erosion processes, such as wave attack at the base of the cliffs. Most 

of the serpentine cliffs south of Bakers Beach are 

involved in landslides.

Another possible explanation for the tendency of 

serpentine to form ridges is that some Coast Ranges 

serpentine bodies appear to be under stress and are 

moving upward and laterally over adjoining more competent rock.

MEGASCOPIC FEATURES

Most serpentine is strongly sheared and fractured 

(fig. 40). Typical exposures include spheroidal knobs 

of hard serpentine with a polished, slickensided rind in 

a waxy, crumbly, thoroughly sheared matrix (fig. 41).

FIGURE 39. Shoreline along Bakers Beach, the Presidio, Fort 

Point, and vicinity, view northeast. The cliffs south of Fort 

Point (far left) are huge landslides mostly in serpentine. The 

hill on the skyline is mostly underlain by serpentine. Bakers 

Beach is in the middle distance. Raised beach sands were 

found in the excavation for sewers for the houses on the far 

right on the distant slope of the Presidio. Franciscan sandstone exposed in near cliffs.
SERPENTINE 57

FIGURE 40. Serpentine in a landslide. Fort Point Rock area, 

San Francisco, viewed southwestward from Golden Gate 

Bridge. The serpentine was strongly sheared before landsliding.

The knobs vary in diameter from a fraction of an inch 

to 6 feet or more. The sheared matrix generally constitutes 30 percent or more of the entire rock. In some 

places, bands of serpentine several hundred feet wide 

consist largely of sheared serpentine and a few unsheared nodules. In a few exposures, however, the serpentine consists of massive blocks, 2 feet or more in 

size, and less than 10 percent sheared serpentine. 

Such serpentine is shown separately on the geologic 

map (pi. 1) for the Potrero Hill serpentine mass and 

for the small serpentine body on Divisadero Street near 

Duboce Avenue and is also present within other serpentine bodies.

The color of serpentine varies widely and is evidently 

dependent on the composition of the parent peridotite, 

on the serpentinization process, and on the degree and 

type of alteration and shearing. Most commonly, coherent knobs and massive serpentine derived from harzFlGURE 41. Serpentine nodule in sheared serpentine. West end 

of Crissy Field, Presidio, San Francisco.

burgite are bluish gray mottled with dark green and 

brown. Some especially hard ones are black, or greenish or bluish black. Somewhat crumbly ones are grayish 

green, and the abundant joints are black or dark blue. 

Hard, massive serpentine thought to be derived from 

dunite is mostly grayish green, grayish yellow green, 

or mottled grayish green and reddish brown. The predominating surface color of a knob, generally lighter 

than the interior color, is light greenish or bluish gray, 

but some knob surfaces are black or dark blue. The 

hard knobs with a black interior may have a white or 

yellow rind. The color of the sheared matrix surrounding the knobs, also generally lighter than the interior of 

the knobs, is mostly light greenish or bluish gray, or 

grayish yellow. Strongly sheared and altered serpentine may be dark gray or white.

On Angel Island many slickensided ellipsoidal knobs 

are embedded in sheared serpentine. These knobs are 

tough dark-gray to grayish-green serpentine that 

shows a very fine to fine-grained drusy to sugary
58 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

appearance on a fresh fracture. Hard inclusions of 

metamorphic rocks are exceedingly common in sheared 

serpentine of Angel Island, also, but they are sparse 

in sheared serpentine in San Francisco.

MINERALOGY AND PETROGRAPHY

The spheroidal knobs and blocks of serpentine 

derived from harzburgite are generally studded with 

green, submetallic bronze-brown, or dark-brown crystals as much as 1 cm in diameter. These crystals have 

a good cleavage or parting, and they constitute 5 15 

percent of the rock. They are either unaltered or partly 

altered pyroxene, or they are bastitic serpentine completely replacing pyroxene. Where the pyroxene is 

largely fresh, relict olivine appears as tiny corroded 

crystals in the cores of mesh-texture serpentine. The 

pyroxene is orthorhombic, generally enstatite. Palache 

(1894,p. 166-167),Ransome (1894,p.221), andTaliaferro (1943, p. 154) also reported the presence of a 

small amount of the clinopyroxene diallage. The pyroxene is completely serpentinized in some bastite pseudomorphs, and so the original pyroxene could not be 

identified; however, the clinopyroxene content is 

believed to have been small, and thus the peridotite 

prior to serpentinization was harzburgite.

Serpentine believed to have been derived from 

dunite shows no relict pyroxene. It is not as common 

as serpentine derived from harzburgite. It is present on 

the west side of Potrero Hill.

Microscopic examination of thin sections shows that 

most serpentine from San Francisco consists predominantly of fibrous serpentine minerals. They commonly 

show mesh texture with length-fast alpha serpentine 

borders and length-slow gamma serpentine cores. The 

description and illustrations of this texture and its 

variations given by Francis (1956, p. 201-226) for a 

serpentine body in Scotland are equally applicable to 

the serpentine of this quadrangle. In some San Francisco serpentine, the cores of the meshes are isotropic. 

This material has been referred to previously as an 

amorphous mineraloid "serpophite," but Francis (1956, 

p. 219) explained that the isotropism is caused either 

by a matted growth of gamma serpentine fibers or by 

the 2V of the serpentine mineral passing through 90°. 

Alpha serpentine handpicked by Francis (1956, p. 

208-210) yielded an X-ray diffraction pattern of 

chrysotile.

Several color and textural varieties of hard nodules 

from the serpentine of the San Francisco North quadrangle were investigated by X-ray diffraction. The criteria given by Whittaker and Zussman (1956, p. 107- 

126) were used to distinguish serpentine minerals. The 

results, given in table 8, show that clinochrysotile, 

lizardite, and orthochrysotile are the predominating

serpentine minerals in most serpentine in San Francisco. Antigorite predominates in the serpentine 

sampled on Angel and Belvedere Islands. Brucite is 

absent except in some serpentines, where it may be 

present in trace amounts. Table 8 also gives bulk density of serpentines. The bulk density of serpentines 

from the Potrero Hill and U.S. Mint areas ranges from 

2.1 to 2.4. In the Fort Point area serpentine partly 

altered to montmorillonite has a value of 1.14, whereas 

serpentines containing unserpentinized enstatite reach 

values of 2.65. The antigorite-bearing serpentines of 

Angel and Belvedere Islands have a bulk-density range 

of 2.44-2.66.

Mesh-texture serpentine is evidently derived from 

olivine, for some of it contains tiny corroded olivine 

relicts in the cores of the meshes and magnetite veinlets, 0.01-0.03 mm thick, which divide the rock into 

polygonal areas and which are believed to have been 

derived by exsolution along fractures in olivine at the 

time of serpentinization. The areas outlined by the 

magnetite veinlets generally include five to 10 meshes, 

but the magnetite veinlets usually follow the border 

of the outer meshes of the group. Nontronite and possibly bowlingite or stilpnomelane are present in some 

veinlets instead of magnetite. It is not known whether 

they are alteration products or magnetite or whether 

they formed in lieu of magnetite during serpentinization. Opaque iron ore dust is found in the cores of some 

of the meshes. Chromite or picotite appears sparsely in 

irregular masses, some of which, especially the ironrich opaque chromites, have rims of nontronite or, more 

commonly, kammererite.

BASTITE

Altered pyroxene or "bastite" serpentine is readily 

recognized in thin section by (1) the presence of 

closely spaced straight parallel veinlets of magnetite, 

bowlingite, or nontronite that follow cleavage or parting fractures in the original pyroxene, (2) one or two 

common extinction directions of the serpentine within 

the altered pyroxene, and (3) lack of mesh texture that 

is common in the matrix surrounding replaced pyroxene. An X-ray diffraction powder pattern of a serpentinized pyroxene, handpicked from a brown serpentine 

from the U.S. Mint area, showed mostly spacings of 

lizardite. Zussman (in Whittacker and Zussman, 1956, 

p. 123) reported that two of three bastites he studied 

gave X-ray diffraction powder patterns of lizardite; the 

third bastite gave a pattern of chrysotile.

RELICT OLIVINE AND ORTHOPYROXENE

Optical measurements (table 9) of the relict olivine 

and orthopyroxene confirm the presence of iron in the 

original peridotite, although the FeO/MgO ratio, indi-
SERPENTINE

TABLE 8. Partial mineral composition and bulk density of serpentine

[Mineral composition based on X-ray diffraction and thin section examination; magnetite and chrome-spinel present in all serpentine. 

M major constituent; m=minor constituent; Tr. (?) =i den tin cation of minor constituent is uncertain; n.d.=not determined]

59

Sample No. Description

Potrero Hill area: 

SF- 237; -1525..- ...Light yellow green, compact.

2050 ___ .. . .... Mottled dark green and yellowish brown,

2213 ....Dark greenish gray, strongly sheared,

U.S. Mint area:

1953 __ . _ . __ . Greenish gray with pale greenish

Fort Point-Presidio area:

Angel Island and Belvedere Island:

51 .................... Mottled dark greenish gray and light

hard ..............................................................

hard ..........................................._.._...............

Bulk

density

2.41

n.d.

2.31

2.30

2.40

2.30 

2.28 

2.36 

2.37

2.37 

2.01

2.07 

2.2

2.28 

2.42 

2.18 

2.21

1.14

2.58 T

2.34 

2.65

2.62

2.54

2.66 

2.65

2.62 

2.52

2.44 

2.63

1On coating.

TABLE 9. Optical properties of olivine and orthopyroxene 

and suggested composition

Refractive index 2V and 

Mineral Locality and composition composition

Duboce Ave. 75

MgSiOs=flfl percent;2 81 

gamma=1.666 

MgSiOs=97 percent. 

Orthopyroxene ..South edge of Alpha=1.669 Note 

Potrero Hill. MgSiOa=88 percent; 

gamma=1.676 

MgSiOs=90 percent.

MgSi04= 

percent.1 

MgSiOs= 

percent.2

etermined.

Trom charts of Poldervaart (1950, p. 1073) . 

2From charts of Kuno (1964, p. 40) .

cate by optics of enstatite, is lower than that indicated 

by olivine.

£ £ £

i 1 1 ! G{ * * s . i f

! 1 ! t 1! ! ! ! 1 ! I j u o < to d m o w o f^ S

M M

M M ....-..- ............ ............ ............ ............ ............ ............ ............ ............

M

M

M M m ........................ Tr.(?) ............................................................

M M

M M

M m M

M M . ...... ....... . m ............ .... _ .... .......

M M ............ ............ ............ ............ ....... .. . ____ ....... _ _ ....... ............

MM TV (')

MM M

MM Tr (')

M M

m M __ . . __ .. _ .-

MM m M

r.(?) M .................................... Tr. (?) .... __ .. M ....................................

M M ........................ _ ....... Tr. (?)............ m ................... _ . ___ ..

m M

M m

M m

M M M

m m ............ M ............ ___ .... ............ ..... ... ... .... . . ..

BLUE COLOR OF SERPENTINE

The characteristic blue coloring of much serpentine 

is in thin films and crusts along shear surfaces. Its 

composition was not investigated in detail, but it 

appears to be a finely disseminated mixture of magnetite crystals, smaller than 2 microns in diameter, and 

serpentine minerals.

ANTIGORITIC SERPENTINE

Serpentine composed largely of antigorite is found 

on Angel and Belvedere Islands as hard rounded knobs 

within soft sheared serpentine or as sheared, but coherent, serpentine containing tight slickensided joints
60 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

throughout. The hard knobs are mottled dark-gray and 

pale-yellow-green fine-grained rock resembling basalt 

in hand specimen appearance. Under the microscope 

they are seen to consist mostly of plates of antigorite 

that average 0.1 mm in diameter and 0.006 mm in thickness. The largest plates are 0.4 mm in diameter and 

0.05 mm in thickness. Veins and vague clots and segregations of antigorite crystals of the same general size 

make up the rock. Some rocks appear to show a vague 

foliation of the antigorite plates; others show a rectangular arrangement in which some plates are perpendicular to others. Magnetite crystals are generally 

larger than in mesh-texture serpentine, though in some 

serpentine they appear to outline meshes or are parallel 

to relict olivine or pyroxene crystallographic directions. 

In other antigoritic serpentine magnetite is in large 

isolated crystals or clots as much as 2 mm in diameter 

surrounded by 2 5 mm of magnetite-free antigorite. 

Chlorite appears to pseudomorph pyroxene and contains parallel bands of magnetite euhedra and antigorite plates.

Evidence for the partial conversion of mesh-texture 

serpentine to antigoritic serpentine is seen in thin sections of some serpentines from Angel Island consisting 

of a matrix of antigorite plates within which walls of 

the meshes appear collapsed and broken into subparallel groups containing very little mesh core material. 

The supposed collapsed mesh-wall material is similar 

to the sheared mesh-texture described by Francis 

(1956, p. 218-220). X-ray diffraction analysis of serpentine consisting partly of collapsed mesh walls shows 

the presence of antigorite, clinochrysotile, and lizardite 

(entries S-ll and S-44, table 8).

VEINS IN SERPENTINE

Late veinlets of cross-fiber chrysotile are common in 

the mesh-texture serpentine but are sparse in the antigoritic serpentine of Angel and Belvedere Islands. These 

veinlets vary from dark-green through light-yellowgreen silky asbestiform fibers to white and pale-blue 

compact porcelaneous material. X-ray diffraction analysis shows clinochrysotile is present in all and orthochrysotile and lizardite in some. The porcelaneous rinds 

on hard serpentine knobs are also largely clinochrysotile.

Many knobs have a strongly fibrous rind, as much as 

one-half inch thick, of fibers alined parallel to the knob 

surface. In a few places this material has the appearance of cross-fiber chrysotile asbestos, but the fibers 

lack the flexibility of asbestos and break when bent. 

The fibrous rinds generally give the X-ray diffraction 

pattern of splintery antigorite (Whittaker and Zussman, 1956, p. 121), though a few are mixtures of 

chrysotile and lizardite. The fibrous rinds may have

formed by shearing of cross-fiber chrysotile veinlets, 

which evidently are planes of weakness that become 

shear surfaces during tectonic movement. Some rinds 

show gradation from cross-fiber chrysotile veinlets to 

parallel-fiber splintery antigorite. The clinochrysotile 

rinds may have formed during shearing of massive serpentine and may not be related to the presence of previously existing veinlets.

On many serpentine knobs, late cross-fiber chrysotile veinlets, about 2 mm in maximum thickness, are 

oriented radially, at right angles to the knob surface. 

Such veinlets characteristically wedge out 1 or 2 inches 

from the surface of the knob. A pleochroic green and 

brownish-yellow mineral, chlorite or ferrostilpnomelane, accompanies the chrysotile in some veins. These 

veins appear to have formed by filling of shrinkage 

fractures, but no cause for such shrinking is known. 

Many knobs are cut by three-dimensional rectangular 

networks of late subparallel chrysotile veinlets (fig. 42).

Veins of carbonates and magnetite are common in 

serpentine. The common carbonate veins are hydromagnesite (Mg4CO,(OH)2'3H2O) andmagnesite (MgCO:t)- They are conspicuously white veins and zones of 

nodules that reach 3 inches in thickness. Xonotlite, 

Cae (Si6O1T ) (OH2 ), (ffl =j8=1.582±0.002; r=1.593± 

0.002), occurs as radial fibrous sheaves resembling pectolite in veins with carbonates.

Calcite (CaCO3 ) in brown and white veins is common in some places. Aragonite (CaCO3 ) is found as 

bladed crystals on joints in serpentine nodules at several places on the east slope of Potrero Hill. Aragonite 

also occurs as anastomosing cross-fiber veinlets, as 

much as one-half inch thick, and as euhedral crystals 

in the serpentine near Fort Point Rock. Aragonite is 

also reported in serpentine at Hoboken, N.J. (Palache 

and others, 1951, p. 190). The crystallization of aragonite rather than calcite from aqueous solution is 

evidently favored by the presence of magnesium salts 

and a temperature of 30°-70°C (Palache and others, 

1951, p. 191). Such conditions were evidently met during hydrothermal alteration of the serpentine.

Along Webster Street, at the U.S. Mint, sockets 

remaining in sheared serpentine matrix after the removal of hard knobs have a % -inch-thick lining of magnesite, possibly derived from chrysotile or splintery 

antigorite. Thin sections of serpentine from Potereo 

Hill show veinlets of cross-fiber chrysotile grading to 

fibrous magnesite.

SHEARED SERPENTINE

In San Francisco the sheared matrix enclosing the 

hard ellipsoidal knobs of serpentine contains the same 

serpentine minerals that are found in the knobs. Soft 

sheared matrix of antigoritic serpentine knobs on Angel
SERPENTINE 61

2cm 

J

FIGURE 42. Clinochrysotile veinlets in bastitic serpentine. Fort Point, San Francisco.

Island and Belvedere were not analyzed. Montmorillonite is common and in some sheared serpentine may 

be a major constituent. Talc is present locally. A lightblue to white strongly sheared friable serpentine is 

found in several places in the quadrangle. It swells and 

becomes plastic when wet and consists mostly of Clinochrysotile and pyroaurite. No montmorillonite could be 

found in this material. The swelling and plasticity 

seems to be related to the Clinochrysotile which forms 

a fluffy sediment and colloidal suspension when agitated in water.

CHEMICAL COMPOSITION

The approximate chemical composition of serpentines was obtained by semiquantitative emission spectrochemical analysis (table 10). The nickel, chromium, 

cobalt, and scandium contents are compatible with the 

results of Faust, Murata, and Fahey (1956, p. 318-320) 

on ultrabasic rocks of eastern United States and of 

Europe. Page (1968) and Page and Coleman (1967) 

gave partial chemical composition of serpentine from 

Tiburon Peninsula and complete chemical composition 

of serpentine minerals from other localities.
62 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

TABLE 10. Approximate semiquantitative spectrochemical analyses of serpentine and nontronite veins in serpentine

[Analyst: H- W. Worthing. Looked for but not found: As. Au, Be. Bi, Cd, Ce, Cs, Dy, Er, Eu, Gd, Ge, Hf, Hg, Ho, In, Ir, La, Lu, Nb, Nd, Os, P, Pd, Pr, 

Ft, Rb, Re, Rh, Ru, Sb, Sm, Ta, Tb, Te, Th, Tl, Tm, U, W, Yb, Zn. M=major constituent > 10 percent]

SI

Al

Fe

MB

Ca - -

Na _.._.__..._... ....-.. _._..-

Ti .__.._._.._...._..........._..._....

Mn ................... _ ...................

Ag

B

Ba

Co

Cr

Ga ........................................ _

Mo ......

Ni

Pb . _ .

Sc

Sn .............................................

Sr

V

Y

Zr

1

M

........................... 0.3

,. ... 7

.. _ .................. M

.015

0

........................... 0

.015

.07

0

.003

........................... .0003

.007

........................... .07

__ . __ .......... .0003

0

3

0

0

003

0

0007

2

M

0.3

7

M

.07

0

0

.0015

.07

.00015

.007

.07

.0007

0

.3

.0003

0

.003

0

.0015

3

M

0.3

M

.7

0

.003

.16

0

.003

.0003

.0015

0

.3

0

.0007

.0015

0

.0007

4

M

0.3

M

.IB

0

.003

.15

.00015

.07

.0007

0

0

.3

.0007

.0003

.003

.0015

5

M

M

.03

.016

0

.0007

0

.0003

.007

0

.3

.0003

0

0

.003

0

.0015

6

M

3

7

3

.07

.03

.07

0

.003

.0015

.003

.0007

.00016

.3

0

.0016

.0003

.003

0

.003

7

M

1.6

7

1.6

.15

.15

.07

.07

.007

.003

.003

.015

.00015

.3

.0015

.0003

.003

.0003

.007

1. Serpentine; mostly antig-orite; Angel Island {sample No. SF-1604).

2. Serpentinized harzburgite; no primary minerals; shattered bluish nodule; 

U.S. Mint, San Francisco (sample No. SF-1956).

3. Serpentinized harzburgite; tough brownish nodule. Most of enstatite is 

unserpentinized; small amount of primary olivine; U.S. Mint, San 

Francisco (sample No. SF-1957).

4. Serpentinized harzburgite; tough rock. Most of enstatite is unserpentinWEATHERING AND HYDROTHERMAL ALTERATION 

Soil development on serpentine is very slight, and 

only in a few places does the surficial mantle, generally 

less than 1 foot thick, appear to be soil formed by 

weathering processes. The white or yellow rind on 

black knobs of hard harzburgite serpentine may have 

formed during weathering.

The serpentine at and near Fort Point is altered to 

hydromagnesite, pyroaurite (Mg6Fe2CO:i (OH) 16 '4H2 

O), coalingite (Mgu,Fe2CO:i (OH) 24 -2H,O), and nesquehonite, MgCO3 -3H,O (Mumpton, 1965). The hydromagnesite forms white nodules as much as half an inch 

in diameter in a talcose matrix of serpentine and pyroaurite. Coalingite occurs in golden brown veinlets 1-2 

mm thick in serpentine largely altered to pyroaurite. 

Nesquehonite forms a white soft surface efflorescence 

at moist places in serpentine. Eakle (1901, p. 316) 

reported barite and gypsum in fissures in serpentine 

near Fort Point. Sheared serpentine at the Hunters 

Point area is altered to a white soft talcose mixture of 

pyroaurite and clinochrysotile. The altered serpentine 

is in bodies as much as 3 feet thick. When wet the mixture is plastic.

Mixtures of epsomite and hexahydrite commonly 

occur as a white soft powdery efflorescence on protected surfaces of serpentine. Chromite, shiny black on 

fresh fractures, is sparsely scattered in the serpentine 

in crystals as much as 4 mm in diameter. Locally, in 

serpentinized dunite, it is concentrated in thin bands 

of blebs 1-10 mm in diameter. Millerite and pyrite are

ized; small amount of primary olivine; late veins of chrysotile and 

stilpnomelane; Fort Point^San Francisco (sample No. SF-2056).

5. Serpentinized dunite showing mesh texture; no primary minerals; Potrero 

Hill near San Francisco Hospital (sample No. SF-2133).

6. Nontronite vein in serpentine; Potrero Hill, 18th and James Lick Freeway 

(sample No. SF-1861).

7. Nontronite vein in serpentine; Ellis and Gough Streets, San Francisco 

(sample No. SF-2134).

commonly disseminated on sheared surfaces in serpentine, especially near contacts with the Franciscan Formation; they have not been found together. Octahedra 

of magnetite as much as 2 mm in diameter commonly 

stud shear surfaces. Gray and brown montmorillonitic 

clay minerals are common in serpentine.

At many places in San Francisco, serpentine contains 

a variety of alteration products in the form of opaline 

spheroidal nodules, mammillary and botryoidal masses, 

and crusts. In hand specimen some of these are colorless, glassy, and transparent. Others are white, vitreous, 

and opaque; pale yellowish brown porcelaneous to 

earthy; or mottled blue, brown, and green and opaline 

with a greasy cast. Many are coated with a thin blue 

opaline film. The clear glassy ones are opal; the others 

contain large amounts of opal. The white ones also contain montmorillonite as well as manganese oxide dendrites in addition to opal. The pale-yellowish-brown porcelaneous to earthy ones contain larger mounts of 

montmorillonite admixed with opal. Earthy nodules in 

some places have been largely destroyed, leaving 

thoroughly pitted serpentine. The mottled opaline material is a mixture of opal and serpentine minerals. 

Serpentine containing the opaline material generally 

shows other signs of alteration, such as gossanlike cellular iron oxide-rich segregations and abundant soft waxy 

brown segregations of montmorillonite. Alteration to 

opal appears to be related to weathering.

At many places serpentine is hydro thermally altered 

as well as strongly sheared along its contact with the
SERPENTINE 63

Franciscan Formation. A common product of these 

processes is a dark-gray bentonite which is moderately 

hard when dry but exceedingly plastic when wet. This 

material consists largely of montmorillonite. Montmorillonite masses vary in thickness from 1 or 2 inches 

to more than 20 feet. The large bodies of this material 

contain knobs of serpentine and Franciscan rocks. In 

some places, such as on Parker Avenue near DeAnza 

Street, white talc segregations are associated with the 

montmorillonite.

Hydrothermal alteration within serpentine bodies 

has also changed serpentine into soft crumbly porous 

brown-to-white material of bulk density less than 2. 

The alteration zones vary from distinct veins, i£-l foot 

in thickness, to bodies several hundred feet in size having indistinct borders and containing segregations of 

fresh hard serpentine. Alteration of the sheared matrix 

of serpentine is apparently more pronounced and more 

widespread than alteration of the knobs.

Nickelian nontronite is found as brown crumbly 

veins 1 12 inches thick; semiquantitative X-ray spectrochemical analyses by the writer indicate that the 

nickel content of the nontronite is several times greater 

than that in unaltered serpentine. Semiquantitative 

analyses of two nontronite veins are given in table 10 

(samples 6, 7). The analyses indicate that the altering 

solutions added Al, Na, K, Ti, Ba, Ga, Mo, Sc, and Zr 

to the serpentine and diluted or removed Mg, Co, and 

Cr.

Silica-carbonate rock, a product of extreme hydrothermal alteration of serpentine, is rare. It is an exceedingly tough rock consisting of a complex network of 

veins and masses of quartz, chalcedony, opal, carbonates (generally magnesite), and limonite. Silica-carbonate bodies, 5-15 feet in diameter, are seen at the 

U.S. Mint and at the Lands End landslide area.

SHAPE OF SERPENTINE BODIES

At one stage of the field mapping, shear orientation 

in the serpentine seemed to give a clue to the manner 

of intrusion and the shape of the serpentine. Shear 

orientation is best shown by the elongation of sheared 

hard knobs. In the soft matrix between the knobs, 

shear orientation is influenced by the knobs and is generally parallel to the surface of the knobs. In general, 

however, shear orientation varies greatly over short 

distances, and no meaningful pattern evolved except, 

perhaps, in Potrero Hill, where most of the shearing 

dips eastward or is horizontal or vertical and could be 

interpreted to suggest that the serpentine was intruded 

by low-angle shearing or thrusting (fig. 43).

Unfortunately, because of poor exposures, the structure of the Franciscan Formation adjacent to the serpentine belt of San Francisco is unknown. Lawson

(1895, p. 450-453; 1914, p. 6) believed the serpentine 

between Potrero Hill and Fort Point consists of two 

nearly flat lying sheets separated by a continuous sheet 

of sandstone about 160 feet thick and "in places so 

warped that they are more or less discordant with the 

stratification planes of the Franciscan rocks" (1914, 

p. 6). No field evidence was found for Lawson's continuous sheet of sandstone. Instead, the sandstone 

bodies in the serpentine are believed to be tectonic 

inclusions completely within serpentine (fig. 44).

FIGURE 43. Sheared serpentine containing large augens of 

altered and sheared gabbro-diabase. West side of Potrero 

Hill, James Lick Freeway near 19th Street, San Francisco.

FIGURE 44. Serpentine, light gray, on the left and sandstone, 

dark gray, on the right, separated by a shear zone containing 

both rock types. Boys are at the edge of the serpentine and 

are left of sheared serpentine, sandstone, and shale. The 

sandstone is a huge tectonic inclusion in serpentine. East 

side of Potrero Hill, San Francisco. Iowa Street, south of 

20th Street.
64 GEOLOGY OP THE SAN FRANCISCO NORTH QUADRANGLE

Irwin's (1964, p. C-l, C-9) suggestion that serpentine belts mark westward-overriding low-angle thrust 

faults may very well apply to the Hunters Point-Fort 

Point and City College shear zones. The hypothesis 

would be strengthened if the Franciscan Formation 

lying northeast of the Hunters Point Fort Point shear 

zone should prove to be considerably different in age 

from those west of the zone. No evidence to support an 

age difference was found.

The border of the serpentine body on Angel Island 

appears to dip southwestward, whereas the adjoining 

sandstone dips northeastward. Ransome (1894, p. 219) 

described this body as a dike dipping about 55° southwestward.

Conclusions could not be reached regarding the subsurface shape of the serpentine bodies in the quadrangle.

ORIGIN

Microscopic examination shows that most of the serpentine was derived from harzburgite and some from 

dunite. Evidence is sparse on the early intrusive relations of the peridotite and serpentine and on the time 

of serpentinization. The peridotite and serpentine in 

the quadrangle are classified as of the alpine type 

because they generally occur with spilitic volcanic 

rocks, radiolarian chert, and graywackes in an orogenic 

zone (Hess, 1955, p. 392; Turner and Verhoogen, 1951, 

p. 239-240). The peridotite and serpentine are believed 

to come from the upper mantle, from which they were 

raised into the crust and eventually to the surface along 

deep faults or folds that extended into the mantle 

(Irwin, 1964; Hess, 1955, p. 403; DeSitter, 1956, p. 

360-361). Bailey, Blake, and Jones (1970) believed 

the peridotite and serpentine were derived from the 

oceanic mantle and became the basal part of the ophiolite sequence of the oceanic crust upon which the 

Great Valley sequence and the Franciscan Formation 

were deposited. They stated that since middle-Cretaceous time the Franciscan Formation has been thrust 

(subducted) under the serpentine of the oceanic crust 

upon which the Great Valley sequence was deposited. 

The serpentine bodies of the quadrangle, however, do 

not appear to be part of an ophiolite sequence, for they 

lack an overlying layer of mafic volcanic rocks. The 

serpentine bodies apparently lie wholly on or within 

the Franciscan Formation, with the possible exception 

of the serpentine in the shear zone of Lands End which, 

as part of the City College shear zone, separates the 

Great Valley sequence of San Bruno Mountain and 

Point Lobos from the Franciscan Formation to the 

northeast. (See fig. 62.) Serpentine in the Fort PointPotrero Hill-Hunters Point shear zone may have been 

derived from the upper mantle, for the shear zone may 

be a northeasterly dipping thrust fault created by

underthrusting (due to sea-floor spreading) the predominantly radiolarian chert and greenstone block 

southwest of the zone below the predominantly graywacke and shale block on the northeast. (See fig. 62.)

Although the borders of serpentine bodies in this 

quadrangle are poorly exposed and hydrothermally 

altered, some evidence was found to suggest that the 

serpentine reached its present position by tectonic 

stresses acting on more or less solid serpentine. Contacts between serpentine and the Franciscan Formation, wherever they were seen, are strongly sheared. 

Blocks of Franciscan Formation lying within serpentine 

have sheared slickensided exterior surfaces. The random attitude of bedding planes in such blocks suggest 

that they are tectonic inclusions. Furthermore, the 

common occurrence of serpentine as a sheared matrix 

enclosing ellipsoidal slickensided knobs is best explained as a response of solid material of low strength 

to shearing stresses. Finally, the adjoining rocks of the 

Franciscan Formation do not show the metamorphism 

that would be expected at the high temperatures 

needed to maintain a molten peridotite or serpentine 

magma (Turner and Verhoogen, 1951, p. 244-252).

Serpentine forms by hydration of peridotite by chemical reactions similar to the following for olivine and 

water (Turner and Verhoogen, 1951, p. 250): 

5Mg2SiO4 + 4H,O^2Mg3Si2O5 (OH) 4 + 4MgO + SiO2

(olivine) (water) (serpentine)

If MgO and SiO2 are removed in solution, the volume of 

olivine is almost the same as the volume of the serpentine formed from it. If the MgO and SiO2 contents 

remain the same before and after serpentinization, a 

volume gain of more than 50 percent is possible. The 

preservation of cleavage and parting of serpentinized 

enstatite in the harzburgite knobs in San Francisco 

indicates little or no volume change on a microscopic 

scale during serpentinization. Coleman (1971, p. 910) 

suggested that both constant-volume and constantcomposition processes may operate in serpentinization. 

He finds that the most common products of serpentinization of alpine-type ultramafics are clinochrysotile, 

lizardite, antigorite, brucite, and magnetite. Coleman 

(1971, p. 908) gave reactions for serpentinization of 

olivine that show that SiO2 must be added or MgO 

removed in order to convert olivine to serpentine by 

hydration. The excess MgO can be removed from the 

reaction by forming brucite. Judging from the absence 

of brucite in serpentine derived from dunite in the 

quadrangle, SiO2 was added or MgO was removed during serpentinization. After using data from laboratory 

investigations of the system MgO-SiO2 -FeO-Fe2O3- 

H-O, Coleman suggested that most serpentine derived 

from alpine-type ultramafic rocks in orogenic zones 

probably formed between 100° and 300°C and that
GABBRO 65

antigorite-bearing serpentine may have formed between 300° and 550°C in deeper levels of the crust 

than those at which lizardite and clinochrysotile form. 

Barnes and O'Nefl (1969) suggested that serpentinization is taking place at present at and near the surface.

AGE

Serpentine in the quadrangle is probably slightly 

younger than the rocks in which it was injected, but 

almost no time data are available. The serpentine at 

James D. Phelan Beach State Park probably reached 

its present position by tectonic injection into sandstone 

containing Douvilleiceras sp. of Early Cretaceous 

Albian age. Thus, the serpentine injection here may be 

post-Albian.

The presence of rare detrital grains of picotite and 

orthopyroxene in some Franciscan sandstone means 

that ultramafic rocks were exposed, though scarce, during Franciscan time.

In his study of the Coast Ranges, Taliaferro (1943, 

p. 153-154) found sills and plugs of serpentine in the 

Franciscan Formation and also in and nearly to the top 

of the Late Jurassic Knoxville Formation (Great Valley sequence of Bailey and others, 1964). He believed 

they "were emplaced as peridotite before the sediments 

had been uplifted from the basin of deposition," 

but also stated that "some of the strati graphically 

higher sills were largely serpentinized before complete 

emplacement***."

In a study of the Klamath Mountains and northern 

Coast Ranges, Irwin (1964, p. C-l) suggested that 

ultramafic rocks, including serpentine, were emplaced 

during the Nevadan (Late Jurassic) and Coast Range 

(probably Late Cretaceous) orogenies.

PYROXENITE

Several masses of pyroxenite as much as 10 feet 

across were found in the serpentine on the south slope 

of Potrero Hill and along the shore north of Bakers 

Beach. The pyroxenite is a tough grayish- to yellowishgreen coarse-grained rock that contains crystals as 

much as 7 mm long. It consists largely of diallage with 

small amounts of tiny euhedral to subhedral magnetite 

crystals. The diallage is slightly altered to chlorite and 

leucoxene. Pyroxenites have a hypidiomorphic texture, 

modified by postsolidification forces to a cataclastic 

texture. This texture is shown by breccia zones between 

some of the diallage crystals and by veinlike breccia 

zones.

A tough greenish rock on the shore about half a mile 

south of Fort Point is regarded as a metapyroxenite. 

The rock contains relict pyroxene that is ragged and 

has been largely replaced by chlorite and by smaller

amounts of clinozoisite and sphene under chlorite-zone 

metamorphic conditions that may have existed during 

serpentinization.

A tough pale-green rock encased in altered serpentine, one-eighth mile east of Fort Point, consists of 

diopside, chlorite, and prehnite. It may be an altered 

gabbro. Ransome (1894, p. 221-222) found pyroxenite 

in the serpentine on Angel Island. He stated that the 

pyroxene crystals are "10 mm or so in length***" and 

identified them as diallage. Crandall (1907, p. 19) 

found both diallage and enstatite in pyroxenite from 

the Presidio area.

GABBRO

Small bodies of fine- to medium-grained gabbro are 

widely scattered in the serpentine. They vary in size 

from 5 feet in diameter for small equidimensional 

bodies to elongate bodies 100 feet by 20 feet. Only the 

larger ones are shown on the geologic map (pi. 1). 

Gabbro is dark gray where freshest and brown where 

weathered. Most bodies have sharp slickensided contacts with serpentine; the small bodies are rounded by 

shearing, and the elongated ones are randomly oriented. They are probably tectonic inclusions. Because 

some of the bodies have a linear arrangement and a 

decrease in grain size from their centers outward, 

Palache (1894, p. 172) suggested that they are intrusive bodies that were disrupted by movement of the 

serpentine. At most places no contact effects are seen 

on the adjoining serpentine.

The gabbro is tougher and less fractured than the 

serpentine and tends to crop out in relief 2-5 feet above 

the serpentine. On Angel Island metagabbro is so 

tough, so fracture free, and so rounded that it is difficult 

to obtain a hand specimen. At one locality on Potrero 

Hill, gabbro altered mostly to prehnite is in sharp contact with fresh gabbro.

The texture of the gabbro varies from allotriomorphic 

granular to subophitic; rocks with the latter texture 

have the field appearance of a diabase. The predominating constituents are plagioclase laths (white to 

colorless), which generally make up one-half to twothirds of the rock, and ferromagnesian minerals (dark), 

which rarely make up more than half of the rock. The 

gabbro of Potrero Hill in San Francisco has a slight 

foliation. The plagioclase is labradorite, An60 , mostly 

altered and replaced by chlorite, muscovite, zoisite, and 

unidentified clay minerals. The ferromagnesian mineral 

is predominantly hornblende, in generally equidimentional crystals about 1 mm in diameter (ZAC=20°; 

X=pale brown; Y=light brown; Z=moderate to 

slightly greenish brown to light brown). All the hornblende may be secondary after pyroxene, for a few 

crystals of colorless pyroxene (ZAC=48°), partly
66 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

altered to hornblende, are identified in thin sections. 

Part of the ferromagnesian minerals is replaced by 

chlorite and nontronite. Present in small amounts are 

stumpy apatite prisms, rounded blebs of monazite, and 

magnetite and ilmenite filling the space between plagioclase and hornblende crystals.

A medium-grained gabbro at the Presidio, San Francisco, is about 90 percent hornblende (ZAC=21°; 

X=pale brown; Y=light brown; Z=light bluish 

green). Most of the remainder is clinozoisite, actinolite, 

epidote, and albite. Monazite blebs are abundant in 

the hornblende. Actinolite needles have grown into the 

albite from the borders of hornblende crystals.

Xonotlite and pumpellyite veinlets are common in 

gabbro. In a sheared and altered unidentified "basic 

rock" inclusion in serpentine near Fort Point, Eakle 

(1901, p. 316) found veins of pectolite and datolite. 

The rock was not found by the writer, but it has been 

described (Murdock and Webb, 1956, p. 137) as an 

"altered diabase dike." On Angel Island a fresh metagabbro consists of about two-thirds andesine laths 

(An42 ) and one-third pigeonite. The pigeonite generally has rugged edges where it is partly replaced by 

actinolite and chlorite; in one body the pigeonite was 

completely replaced by actinolite. The andesine is 

crowded with needles and stumpy prisms of a colorless 

to pale-brown amphibole. About 2 percent leucoxene 

replaces skeletal ilmenite. Skeletal pyrite is also present. The gabbros of Angel Island and Tiburon Peninsula are also cut by pumpellyite and chlorite veinlets.

Palache (1894, p. 173-178) found orthorhombic and 

monoclinic pyroxene in some of the gabbro of Potrero 

Hill and named the rock hypersthene diabase. He also 

found the hornblendic variety, which he classed as 

epidiorite.

Gabbro generally crops out as fresh tough rock. 

In some places it has a weathering mantle, 1-3 feet 

thick, consisting of material high in nontronite and 

vermiculite.

The gabbro may represent acidic segregations of a 

periodotite magma richer than usual in SiO2 , CaO, and 

Na2O. If such segregations formed they probably had 

a lower temperature of crystallization that the peridotite and thus may be slightly younger than the peridotite.

If serpentinization quickly followed crystallization of 

peridotite, gabbro may also be younger than serpentine. This possible age difference may explain why it is 

not serpentinized. As seen today, most of the bodies 

are tectonic inclusions in serpentine.

Because the gabbro of Angel Island shows no contact effects on the adjoining serpentine, Ransome 

(1894, p. 227-231) suggested that it is older than the 

peridotite. Palache (1894, p. 172) suggested that the

gabbro bodies of Potrero Hill are parts of one or more 

continuous dikes or sills that intruded the serpentine 

or peridotite and were broken into the present detached 

masses by movement of the serpentine. Some rodingites 

on Angel Island and the prehnitized gabbro of Potrero 

Hill are metamorphosed gabbro.

SURFICIAL DEPOSITS

Surficial deposits of late Pleistocene and Holocene 

age cover approximately 80 percent of the land area of 

the quadrangle. They have a maximum thickness of 

approximately 300 feet. The Tertiary Period is not 

represented in the quadrangle, other than the possible 

occurrence of Pliocene deposits in the oldest deposits 

below San Francisco Bay. The oldest of the surficial 

deposits, the Colma Formation, is a complex of Pleistocene coastal sediments, which include marine estuary 

deposits, beach deposits, and at higher elevations 

eolian, stream, and colluvial (slope debris) deposits. 

Dune sands cover more than half the city of San Francisco. They are the most widespread surficial deposit 

and reach thicknesses of 150 feet. Most dunes were 

actively moving in historic time.

Most hill slopes are mantled with debris (colluvium) 

derived from underlying rock and represent in part old 

landslide deposits. Ravines are partly filled with colluvium from adjoining slopes. Modern alluvium is 

uncommon in the quadrangle because of the absence of 

large streams. Alluvium is shown on plate 1 only in 

small deposits on Twin Peaks where it is related to a 

slightly older than modern drainage system that 

existed there when rainfall was greater than it is now. 

Modern landslide deposits are widely distributed on 

bedrock hills and along sea cliffs, though many of them 

are too small to show on plate 1.

Mud and clay of San Francisco Bay are unconsolidated sediments containing clay- and silt-size detritus 

and large amounts of water. They have accumulated to 

thicknesses greater than 100 feet in the bay along the 

east shore of San Francisco and along the north shore 

of Sausalito and Belvedere Island.

Artificial fill covers more than 3 square miles and is 

as much as 60 feet thick in some places. It consists of 

dune sand, colluvium, spoil from excavations and 

quarries, bay mud, and the general garbage of an urban 

area.

COLMA FORMATION

The Colma Formation is a group of unconsolidated 

sandy estuarine and coastal deposits of Pleistocene age. 

It was first named by Schlocker, Bonilla, and Radbruch 

(1958). Its type locality is south of the town of Colma, 

approximately 6 miles south of the quadrangle. Typical
COLMA FORMATION 67

exposures are on the west side of El Camino Real 

extending 0.7 miles southeast of the intersection of El 

Camino Real with Hickey Boulevard. The lower part of 

the Colma Formation is exposed in depositional contact 

on the Merced Formation, approximately 3.6 miles 

south of the quadrangle. This exposure, designated a 

reference locality, is on the east side of the road leading 

to Thornton Beach State Park, about 0.15 miles southwest of the intersection of Alemany and Skyline Boulevards and between 200 and 225 feet above sea level. 

The Colma probably records several different periods 

of sedimentation related to periods of relatively high 

sea level. The isolated exposures within the San Francisco North quadrangle can be correlated with each 

other and with the type Colma on the basis of gross 

similarity in physical properties and stratigraphic 

position.

The Colma Formation was first recognized by the 

author and his colleague, M. G. Bonilla, in the Lake 

Merced area near the Pacific Ocean, about 1 mile south 

of the quadrangle. There the Colma is a nearly flat 

lying friable sand that lies unconformably on the 

steeply tilted fossiliferous marine Merced Formation 

of Pliocene and early Pleistocene age and unconformably below Pleistocene (?) and Holocene dune sand. 

In the San Francisco North quadrangle, widespread 

poorly consolidated bedded sand deposits are correlated with the Colma Formation rather than with the 

Merced Formation of the Lake Merced area because 

they are approximately horizontally bedded and lie 

immediately below latest Pleistocene and Holocene 

deposits. They also lack the shale beds and marine 

invertebrate megafossils that are common in the Merced Formation. For practical mapping purposes, silt, 

clay, and poorly sorted rubble intercalated with bedded 

sand were placed in the Colma Formation. Silt, clay, 

and rubble not associated with bedded sand were generally mapped as slope debris and ravine fill, though 

they may have been deposited contemporaneously with 

part of the Colma Formation.

MEGASCOPIC FEATURES

Bedding in the Colma Formation is generally easy to 

discern, for individual beds are 1-3 inches thick. Some 

of the Colma deposits, however, such as the one at map 

locality 6 in the Presidio (pi. 2A), are obscurely 

bedded. Bedding is revealed mostly by variations in 

the proportion of the silt-clay matrix relative to sand 

grains and the resulting variations in cohesiveness and 

cementation. Cohesiveness increases as the proportions 

of silt and clay increase; however, the silt-clay matrix 

does not generally exceed 20 percent by weight. The 

variations in cohesiveness are soon revealed in fresh

cuts by rainwash sculpturing. The sandy beds are more 

readily eroded than the clayey beds, which stand out 

in relief a fraction of an inch to several inches from the 

sandy beds. Bedding is accentuated in some places by 

color differences which probably are caused by variations in the permeability of individual beds and by 

resulting differences in the amount of iron oxide stain 

deposited by interstratal solutions. The source of the 

iron is largely grains of iron-bearing heavy minerals 

concentrated in thin nonpersistent layers. Bedding 

may appear as an abrupt change in sand grain size. 

Rarely, a few pebbles, as much as 1 inch in diameter, 

are sparsely distributed in a one-pebble-thick layer in 

the midst of clayey sand. Bedding is rarely shown by 

clay beds.

Thinly bedded clayey sands in the Colma Formation 

commonly display a slight waviness of bedding with an 

amplitude of 1-6 inches and a wavelength of %-3 feet 

(fig. 45). The waviness probably records uneven compaction rather than the environment of deposition. 

Crossbedding or inclined torrential bedding intercalated with near-horizontal bedding is especially common on the south slopes of Twin Peaks (fig. 46). 

Locally crumpled bedding between parallel nearly horizontal undisturbed beds 1-2 feet apart probably 

records subaqueous slumping.

The Colma Formation is generally yellowish or reddish brown. On Angel Island it is predominantly light 

gray, as are beds on the west slope of Russian Hill 

uncovered in the excavation for the Broadway Tunnel. 

On Twin Peaks the Colma Formation is mostly yellowish or reddish brown, but cuts disclose gray uncemented sand deposits resembling channel fillings, 15- 

25 feet wide. In a few places in San Francisco, the top 

3-5 feet of the Colma Formation immediately underlying dune sand is gray to black, owing to organic carbonaceous material. Richard Janda (written commun., 

1967) believed that the fresh Colma Formation is light 

gray or light brownish gray. The common yellowishand reddish-brown colors, he believed, are due to 

weathering. He also believed soil stratigraphy may be 

useful in dividing the formation.

The Colma Formation is chiefly moderately sorted 

fine to medium sand with small to moderate amounts 

of detrital silt and clay (pi. 2B-E). Beds of silty clay 

%-5 feet thick and coarse rubble containing gravel as 

large as cobbles are interbedded with the sand in a few 

places. Locally, rubble beds make up a minor part of 

the formation.

By using the method described by Inman (1952), 

Inman and Chamberlain (1955, p. 109), and Folk and 

Ward (1957), 13 samples of the Colma Formation were 

analyzed; the results are given on plates 2D and 2E. 

They show the following ranges in characteristics:
68 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FIGURE 45. Colma Formation (Qc) overlying greenstone of 

the Franciscan Formation (KJg), Sutro Reservoir, San 

Francisco. The light-colored streak dipping to left is interpreted as a zone of considerable ground-water chemical activity that altered the Colma Formation to clay and ironstone, 

possibly an old soil developed on a surface cut on older rocks 

of the Colma Formation to the right and later buried by 

younger Colma to left. Photograph below is a closeup view 

showing bedding and the inclined clay-ironstone zone.

FIGURE 46. Rubble deposits and crossbedding in the Colma 

Formation. In upper photograph contact with greenstone of 

the Franciscan Formation is 3 feet to left of man. Lower 

photograph is closeup view of well-bedded sand overlying 

poorly bedded rubble deposit containing large greenstone 

pieces. The rubble may be an old landslide deposit; the sand 

is a stream or lake deposit. South slope Twin Peaks, San 

Francisco.

Median diameter:

Md 0.183-0.270 mm

Md<j> 2.45-1.89 phi units 

Sorting:

Trask So 1.18-1.85

Phi deviation a<j> 0.44-1.91 

Skewness:

Trask Sk 0.625-1.019

Phi m£ + 0.02-0.76

Conclusions on the significance of these characteristics are given elsewhere.
COLMA FORMATION 69

COMPOSITION AND PHYSICAL PROPERTIES

Most of the sand grains of the Colma Formation are 

(in decreasing order of abundance) rock fragments, 

quartz, and feldspar (pi. 2F). Rock fragments are 

generally locally derived. For example, those in the 

Colma Formation at map locality 2 on Angel Island 

consist mostly of metamorphic rocks similar to those 

found in the underlying Franciscan Formation. Sand 

grains from locality 9 in San Francisco consist mostly 

of chert and from locality 11A of altered greenstone, 

nontronitic clay, and some chert. Heavy minerals make 

up 10-25 percent of the sand grains (pi. 2F). The predominant heavy minerals (in decreasing order of abundance) are green hornblende, augite, clinozoisite-epidote, brown hornblende, hypersthene, and ilmenite. 

Locally, dark layers of heavy minerals MG-^ inch thick 

are abundant. Particles smaller than 2 microns consist 

of varying proportions of montmorillonite, chlorite, and 

mica and random and regular mixed layered clay minerals of all three. In most sands, black, carbonaceous 

silt-size material, probably representing plant remains, 

is present, generally in amounts less than 1 percent. 

In a few exposures in San Francisco, dark-gray to black 

peat, clayey sand, or sandy clay occurs at the top of 

the Colma Formation immediately below dune sand. 

Peaty layers less than 1 foot thick were also penetrated 

by a few borings in the Colma Formation. Rubble beds 

were derived locally and consist of whatever rocks were 

available from nearby Franciscan Formation exposures 

in Colma time.

Most sand grains in the formation are medium size 

and coarser, are generally subangular to subrounded, 

and have polished surfaces. About 5-15 percent of the 

grains are angular; an equal portion are well rounded 

and have frosted surfaces. Feldspar, heavy minerals, 

serpentine, and rock fragments including chert and 

microporphyritic igneous rocks are generally better 

rounded than quartz; however, a small number of 

quartz grains also are well rounded and have frosted 

surfaces.

Cementation and porosity of the sands of the Colma 

Formation vary with the clay content. Most sands of 

the Colma Formation are friable or only weakly 

cemented. A 25-foot-thick deposit of gray fairly clean 

sand interbedded with brown clayey sand at Sutro 

Reservoir is uncemented. Porosity of the Colma Formation sand is generally moderate to high because of 

the moderately good sorting, the large proportion of 

subrounded and rounded grains in the sand sizes, and 

the lack of enough fine sizes to fill the spaces between 

sand grains. Some porous sands are cemented only at 

points of contact of the % 1-mm-thick clay shells 

around each grain. In a few places the clay matrix completely fills the space between sand grains, and the 

sediment has low porosity. Sand grains and clay cement 

are generally iron stained. Oxidation of black layers of 

iron-bearing heavy minerals produces a tough, lowporosity ironstone 1 3 inches thick.

WEATHERING

Widespread orange and brown iron stains, with green 

and gray patches, and sporadic ironstone layers are 

evidence for oxidation and reduction which, in surficial 

deposits such as the Colma Formation, are probably 

related to weathering processes. Most of the clayey 

material in sand of the Colma Formation appears to 

be detrital, though a small amount of clay may represent alteration of sand grains. Evidence that the clay 

is detrital is indicated by the freshness of the feldspar 

and rock grains and by fairly abrupt variations in clay 

content from bed to bed. Soil development varies, but 

it usually appears to be slight to moderate, for distinct 

soil horizons generally are present only as a 1-footthick light- to medium-gray top layer. In a few places 

the soil-forming process has been intense, such as on 

the west side of Russian Hill and near Lombard and 

Divisadero Streets, where clayey sand grades upward 

into a sandy silty clay layer about 10 feet thick. The 

bottom half of the sandy silty clay layer is compact, 

but the top half is vesicular, has columnar structure, 

resembles loess in appearance, and is evidently the B 

soil horizon. The A horizon is poorly represented by 

small 1-foot-thick patches of gray clayey silty sand.

Weathering is also shown by etching of pyroxenes 

and hornblende. Almost all pyroxene grains in the 

Colma Formation have well-developed, long, delicate 

sawtooth terminations. In contrast, the pyroxene grains 

in the modern beaches and dunes have only tiny, short 

sawtooth terminations or no such terminations. Hornblende grains in the Colma Formation generally show 

good cleavage faces, in contrast with grains in modern 

beach and dune sand which are generally well rounded.

Strong etching of pyroxene comparable to that found 

in the Colma Formation was illustrated by Bradley 

(1957, p. 434, fig. 10). Though he examined modern 

beach sands, he found etched pyroxene only in old 

marine deposits lying on an elevated 100-foot marine 

terrace at Santa Cruz, Calif.

The dark-gray to black, carbonaceous clayey material found at the top of the Colma, underlying dune 

sand in a few places in San Francisco, may represent an 

old soil that developed in or near local marshes or lakes.

OCCURRENCE

The Colma Formation is widely distributed on Angel 

Island and in San Francisco. It was not recognized on
70 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

Tiburon and Marin Peninsulas or on Belvedere Island, 

although Quaternary deposits at these localities may 

be of the same age as the Colma Formation. The thickness of the Colma Formation varies greatly over short 

distances and is difficult to estimate. On many slopes 

its nearly horizontal bedding may be exposed in a continuous section through elevation differences of 100 or 

200 feet, yet thicknesses measured at right angles to 

the bedding or the slope at individual points are considerably less. Thicknesses are less variable and generally are greater in broad valleys at low elevations 

than on the slopes.

The Colma Formation is exposed discontinuously on 

the east and southeast shores of Angel Island (fig. 47), 

where exposures range in altitude from sea level to 

about 325 feet. It may also be represented in the 

unconsolidated deposits that fill the valley of Hospital 

Cove. Maximum observed thickness in the quadrangle 

is 75 feet.

FIGURE 47. Colma Formation on the southeast slope of Angel 

Island, about 250 feet above sea level.

In San Francisco the Colma Formation is exposed 

discontinuously from about sea level to an elevation 

of about 550 feet, and it is recognized in boreholes more 

than 100 feet below sea level. The most extensive exposures are on the southwest and northeast slopes of the 

ridge occupied by the Presidio Golf Course and reservoir, where the deposits are confined to elevations 

below 280 feet (pi. 1). Sands of the Colma Formation 

near Mountain Lake are at least 30 feet thick. In the 

Presidio north of Kahn Playground, small youthful 

valleys appear to be cut entirely in the Colma Formation, which is at least 60 feet thick.

On the slopes between the Presidio and Russian and 

Nob Hills, the Colma Formation is found in small 

sporadic exposures below about 200 feet elevation and 

in boreholes. The top of a 42-foot-thick clayey sand, 

believed to be the Colma Formation, was penetrated in 

a boring at 42.2 feet below mean sea level at Fort 

Mason near the southeast corner of the pier at the end 

of Laguna Street. The formation was found to be more 

than 30 feet thick in the excavation for the twin vehicular tunnels on Broadway near Hyde Street.

The Colma Formation is believed to be present on 

the lower slopes and valleys in eastern San Francisco 

below an elevation of 100-200 feet, in the area between 

North Point and the south edge of the quadrangle. At 

the Ferry Building in a boring below 102 feet of mud 

and clay, a sand layer, 38 feet thick, is correlated with 

the Colma Formation.

Small patches of the Colma Formation rest on the 

bluff behind Bakers Beach and on the sea cliffs westward and southward towards the Cliff House. These 

are slightly consolidated nearly horizontally bedded 

sand. Most of these patches are too small to show on 

the geologic map (pi. 1). In the valley north of the Cliff 

House, the formation is 30 feet thick. Southeast of the 

Cliff House the Colma Formation overlies sandstone 

of the Franciscan Formation and is overlain by dune 

sand. Here, the basal part consists of coarse sandstone 

rubble derived from the underlying Franciscan Formation. The Colma Formation is at least 40 feet thick; it 

appears to thicken southward, and it dips gently southward toward Golden Gate Park.

On the north slope of Mount Sutro, the Colma Formation is exposed north of Parnassus Avenue. It was 

penetrated south of Parnassus Avenue in borings made 

to investigate foundation conditions below the University of California Medical Center Building (Moffet 

Hospital). South of Mount Sutro and west of Twin 

Peaks, the formation is found at elevations as high as 

550 feet. On the south slope of Twin Peaks, the formation fills a channel cut in greenstone of the Franciscan 

Formation (fig. 48). In the excavation for Sutro reser-
COLMA FORMATION 71

FIGURE 48. Colma Formation filling a channel cut in greenstone. South slope of Twin Peaks, San Francisco.

voir, a 75-foot-thick section of the formation was 

exposed (fig. 45).

ORIGIN

The Colma Formation appears to have been deposited 

mostly by water and gravity and, to a lesser extent, by 

wind in a variety of coastal environments. At elevations above 250 feet, the Colma Formation commonly 

is massively bedded or steeply crossbedded and is 

believed to consist mostly of stream, colluvial, and 

eolian deposits (figs. 46, 48). In contrast, at lower 

elevations the Colma Formation consists mostly of 

moderately well sorted sands, usually with persistent 

horizontal stratification but in some places with intercalated steeply inclined crossbeds. The lower sediments 

probably accumulated in an extensive complex of 

shallow bays, inlets, and channels such as would exist 

if sea level were 35-100 feet higher than now. Deposition in water is also suggested by local contorted layers, 

1-2 feet thick, interbedded with undisturbed beds; the 

contortions appear to have formed by slumping in water.

Intercalations of bedded moderately sorted marine 

sands with clay and with massive poorly sorted continental sediments indicate fluctuations in the transportation and sedimentation processes, in the strength 

of water currents at the site of deposition, in the location of the shoreline, and in the character and amount 

of material supplied to the basins of deposition.

The environment of deposition of the Colma Formation can be inferred from the size-distribution diagram 

(pi. 2D). Most of the data shown are for marine sands 

believed to have been deposited at the lower elevations. 

Data for dune sand and modern beach sand are also 

shown on the diagrams. Dune sands are distinguished

from beach sand by their smaller median grain diameter and by better sorting. (<rc£ for dune sands is less 

than 0.5; v<f> for beach sands is greater than 0.5.) Sediments of the Colma Formation whether the whole 

sediment or only sand fraction is considered have 

about the same range in median grain diameter as 

dune sands; however, they are generally not as well 

sorted, and their frequency distribution curve is generally skewed towards finer grain sizes. Stewart (1958, 

p. 2589, 2595) found that marsh sediments of lagoon 

borders have sorting and skewness values comparable 

to those of the Colma Formation, though his samples 

were of finer grain (median phi diameter about 3 6) and 

the Colma does not contain enough organic matter to 

be a marsh sediment. Inman and Chamberlain (1955, 

p. 114,119) referred to sediments of median phi diameter about 3-4, sorting a<j> values greater than 1.5, and 

positive skewness, a<j>, values as transition sand and 

suggested that such sediments probably result from 

basic differences between modes of transport of sand 

and of silt and clay. They found transition sand about 

}4-l mile offshore from a beach north of Point La Jolla, 

San Diego County, Calif., and within shallow bays 

behind barrier beach sand islands near Rockport, Tex.

The grain-size distribution of the Colma Formation 

may be due to deposition and mixing of silt and clay 

in estuaries to which sand was transported before or 

during the existence of the estuary. The sites of Colma 

estuaries, before the rise in sea level that created them, 

may have been covered by dune sand, by the Merced 

Formation, or by sand brought in by the ancient Sacramento-San Joaquin River. The similarity in ranges of 

median diameters of the sand fraction of the Colma 

Formation and of dune sand suggests that some sand 

may have reached Colma estuaries by wind. Sand of 

median diameters like those of the Colma Formation 

may also have been brought by water currents less 

vigorous than breakers on a beach.

The poorly consolidated Merced Formation is rich 

in sand grains of sizes found in the Colma Formation 

(M. G. Bonilla, oral commun., 1961) and is the most 

likely source of much of the Colma Formation. The 

Merced Formation was deposited over a wide area west 

of the present shoreline and as far north as Point Reyes 

Peninsula and is approximately 5,000 feet thick. Its 

initial volume appears to have been large enough to 

have supplied detritus for a large part of the Colma 

Formation. The folding of the Merced probably brought 

it above sea level and made it available as a source of 

the Colma. The predominance of hornblende in the 

Colma Formation in San Francisco (pi. 2F) is evidence 

that its main source was not the local Franciscan Formation, in which hornblende is scarce (table 3). How-
72 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

ever, the content of hornblende and other minerals in 

the Merced Formation is similar to that in the Colma 

Formation and supports the belief that some or most 

of the Colma Formation was derived from the Merced 

Formation or from the same source as the Merced Formation. Where the Colma Formation lies on the Merced Formation, west of Lake Merced, it is difficult to 

distinguish one formation from the other.

Another possible source of the Colma Formation is 

the detritus carried by the Sacramento-San Joaquin 

River. Drainage from the Great Valley into the San 

Francisco Bay area is believed to have started by early 

Pleistocene time and to have been the source of the 

upper member (Pleistocene) of the Merced Formation 

(Hall, 1965, p. 152, 153). Because sea level may have 

been high when the Colma was deposited, the ancient 

Sacramento-San Joaquin River may have reached the 

estuary that existed then far upstream from the site of 

deposition of the Colma Formation on the San Francisco Peninsula, much as the present river reaches tidewater east of Suisun Bay. Thus the silt and clay sizes 

in the Colma Formation may have been partly contributed by the ancient Sacramento-San Joaquin 

River drainage from the Great Valley of that time.

The sandiness of the Colma Formation may be 

explained also by its deposition in basins and channels 

swept by tidal currents. Sands are deposited where 

tidal currents are vigorous, such as those in the central 

parts of San Francisco Bay and in parts of Golden Gate 

channel. Muds are deposited along the fringes of the 

bay where currents are weak. Muddy sands are 

deposited under intermediate conditions or where 

rocky headlands furnish sand that mingles locally with 

muds (Louderback, 1939, p. 784-793; 1951, p. 91; 

Inman and Chamberlain, 1955, p. 119). The few frosted 

and well-rounded sand grains in the Colma Formation 

suggest wind action on a beach, or they may have been 

derived from the Merced Formation.

The Colma Formation was thought previously to 

have been deposited in an offshore marine environment 

(Lawson, 1895, p. 463; Ashley, 1896, p. 354) or in 

swamps, lagoons, lakes, flood plains, and dune fields 

(Martin, 1916, p. 226). The author believes the Colma 

Formation now found below an elevation of 200 feet 

was deposited in a complex estuary swept by vigorous 

tidal currents.

A eustatic rise of sea level and a local rise of the crust 

after deposition explain the presence of marine Colma 

deposits of late Pleistocene age several hundred feet 

above sea level. Sea level should have been higher than 

now whenever less glacial ice existed than exists today. 

During at least one interglacial period, ice was less 

extensive than it is today (Louderback, 1951, p. 86; 

Kuenen, 1955, p. 201; Kuenen, 1950, p. 539). Hopkins,

MacNeil, and Leopold (1960) and McCulloch, Taylor, 

and Rubin (1965, p. 446) showed that in the Nome 

and Kotzebue Sound areas, Alaska, the rise in sea level 

during Sangamon time was only about 35-40 feet. 

Hopkins, MacNeil, and Leopold (1960, p. 49) showed 

that in the Nome area the greatest eustatic rise of sea 

level during a Pleistocene (preKansan(?)) interglacial 

period was approximately 100 feet. Thus the marine 

deposits in the quadrangle presently above an elevation 

of 100 feet probably reached their present position 

partly by tectonic uplift.

If sea level were 35-100 feet higher than it is today, 

sandy sediments of the Colma Formation could have 

accumulated on beaches or in shallow marine water 

near the shore, as well as in basins several miles offshore covered by sea water several hundred feet deep. 

Thus the Colma Formation in the Presidio area, which 

is now more than 200 feet above sea level, may have 

been deposited at the same time as the Colma Formation now lying more than 100 feet below sea level at 

the Ferry Building.

Smith (1960, p. 160) suggested that the Colma Formation records depositional episodes related to several 

different sea levels. He separated a unit from the Colma 

Formation which he believed represents beach and 

dune deposits related to his Colma marine terrace. He 

traced this marine terrace for more than 15 miles at 

elevations between 200 and 300 feet above sea level on 

the bay side of San Francisco Peninsula south of Lake 

Merced. Above this lower terrace he found terraces 400 

feet and 500 feet above sea level. Deposits on the 400- 

foot terrace resemble the Colma Formation. No 

deposits were found on the higher terrace. Smith 

believed the terrace between 200 and 300 feet above 

sea level was cut during the high sea level of the Sangamon Interglaciation.

Lawson (1895, p. 463) recognized the beds here 

called the Colma Formation as his "Terrace Formations." He described these beds as occurring from an 

altitude of about 750 feet down to sea level and thought 

they represented marine deposits. In the San Francisco 

Folio, Lawson (1914, p. 15) abandoned the term "Terrace Formations" but mentioned a deposit in the Lake 

Merced area of "light-yellow sands, about 200 feet 

thick, which probably lies unconformably upon the 

Merced***" and suggested that it may be the correlative of the Alameda Formation, a Pleistocene marine 

and continental formation found on the east shore of 

and in San Francisco Bay. He explained that he did 

not show the deposit in the Lake Merced area on a 

geologic map because it could not be easily distinguished from the underlying Merced Formation.

Marine megafossils have not been found in the Colma 

Formation at lower elevations where the formation is
COLMA FORMATION 73

believed to have been deposited in a marine environment. Perhaps strong currents and a sandy bottom 

were unfavorable for marine life, as they are today. 

Evidence for the presence of leaching acidic solutions 

is found in decomposed sand grains bearing iron such 

as magnetite, which occur almost everywhere in the 

Colma Formation. Such acidic solutions could decompose marine shells, which are largely calcium carbonate, more readily and more completely than they could 

decompose magnetite and other iron oxides. Smith 

(1960, p. 153) also suggested leaching by ground water 

as an explanation for the lack of fossil shells in marine 

sediments of Colma age on former wave-cut platforms 

2-25 miles south of the quadrangle.

CORRELATION WITH NEARBY DEPOSITS

Sand lying below bay mud along the bay shore east 

of San Francisco is included in the Colma Formation, 

although it could not be traced continuously from the 

Colma Formation on the Pacific Ocean and Golden 

Gate channel sides of San Francisco Peninsula. On 

their geologic cross section that extends from the bay 

onto San Francisco near the south border of the quadrangle, Trask and Rolston (1951, p. 1085) designated 

the sand below bay mud as the Merritt Sand. It was 

designated the "sand layer" in the Islais Creek area at 

the southeast corner of the San Francisco North quadrangle by Radbruch and Schlocker (1958); they correlated the upper part of the layer with the Merritt 

Sand and Temescal Formation. On the east side of San 

Francisco Bay, Radbruch (1957) showed that the 

marine type Merritt Sand grades into and interfingers 

eastward with alluvial-fan deposits of the Temescal 

Formation, which she found to be the same unit as the 

upper member of the San Antonio Formation. Similar 

facies are found on the west shore of the bay in San 

Francisco where the marine Colma Formation of lower 

elevations here correlated with the Merritt Sand 

interfingers with and grades into the continental Colma 

Formation of mostly higher elevations.

Pleistocene marine deposits are also above sea level 

on the east side of San Francisco Bay (Louderback, 

1951, p. 86). Similar deposits were not seen on the 

Marin Peninsula, though small patches of them were 

seen in the vicinity of Point Bonita west of the quadrangle. They are lacking from Marin Peninsula because 

such unconsolidated deposits could easily have been 

removed by erosion of the steep slopes when sea level 

dropped. The same cause may explain the absence of 

the Colma Formation on the steep slopes on the north 

side of Angel Island. Some of the deposits at elevations 

of 200 feet and lower on Yerba Buena Island, designated "reworked colluvium" by Radbruch (1957),

resemble the Colma Formation and may have a marine 

origin.

Abraded marine and nonmarine diatoms and sponge 

spicules are present, but no specific identifications have 

been made. Part or all of the diatoms could have been 

derived from older sediments. Vertebrate remains are 

reported from what may be the Colma Formation. Hay 

(1927, p. 5, 210) reported that a bone now lost, apparently a fragment of a tibia of a large ground sloth, was 

found before 1853 at a depth of 23 feet on Pacific Street 

between Kearney and Montgomery Streets. Stock 

(1925, p. 201-202) reported that a humerus of Mylodon 

(probably Paramylodon, a ground sloth) was found in 

excavations for the Twin Peaks Tunnel at a depth of 

60 feet, about 300 feet west of 18th Street. Hay (1927, 

p. 210) concluded that the material from the two localities comes from Lawson's San Antonio Formation and 

represents the Aftonian Interglaciation.

Peabody (1945, p. 60-63) described a vertebrate 

fauna of late Pleistocene age from a deposit that he 

believed to be slightly older than Lawson's "Terrace 

Formations" (the Colma Formation of Schlocker and 

others, 1958, and of this report). The bones were found 

above Mussel Rock on the ocean shore 6 miles south 

of the quadrangle. Peabody (1945, p. 60) reported that 

the bones are in a stream deposit of gray sand and 

gravel resting on the Merced Formation and overlain 

by Lawson's "Terrace Formations" of reddish-brown 

horizontally bedded sand. Field mapping in the Mussel 

Rock area by M. G. Bonilla (oral commun., 1958) indicates an obscure relationship of the fossiliferous sand 

and gravel and the overlying reddish-brown sand to 

the Colma Formation. Savage (1951) listed other 

vertebrates from the San Francisco Bay region to which 

he assigned an "undifferentiated Pleistocene" age. 

They were found in deposits that may be the same age 

as the Colma Formation.

A tree identified by Rowland W. Brown as a juniper 

or redcedar, probably Juniperus californica, was found 

in the excavation for the Broadway Tunnel on the west 

slope of Russian Hill near Hyde Street in a sand 

thought to be the Colma Formation. Duplicate carbon14 age determinations by J. L. Kulp in 1954 gave an 

age greater than 30,000 years (sample designation: 

Lament No. 227).

The Colma Formation is tentatively assigned a late 

Pleistocene age.

AGE RELATION TO THE ANCIENT SACRAMENTO RIVER

The shape of the exposed and buried parts of the 

bedrock surface shown on the bedrock-surface map 

(pi. 3) is strikingly similar to the pattern of valleys 

and ridges made by stream erosion. Most of the valleys
74 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

are tributaries of the large valley, now Golden Gate 

channel, that was carved between San Francisco and 

Marin Peninsulas by the ancient Sacramento River 

which emptied into the Pacific Ocean at various distances west of Point Lobos. The ancient channel of 

the Sacramento River and some of its tributaries is 

shown on maps by Trask (1956, p. 17). The bedrocksurface map of central San Francisco Bay by Carlson 

and McCulloch (1970) shows channels in bedrock that 

may represent old stream channels. Louderback (1951, 

p. 82) suggested that Golden Gate channel was carved 

perhaps in early Pleistocene time or perhaps during a 

middle Pleistocene disturbance inasmuch as it maintained its flow across the slowly rising land west of the 

Great Valley. Thus, some of the sedimentary deposits 

mapped as Colma Formation may have been transported by the ancient Sacramento River before the 

crustal rise or at an early stage of the rise.

Sea level is estimated to have been about 228 406 

feet below present sea level during the Wisconsin Glaciation and 325-390 feet below present sea level during a pre-Wisconsin glaciation (Flint, 1947, p. 437; 

Moore and Shumway, 1959, p. 373; Curray, 1965, p. 

725; Milliman and Emery, 1968). The present shoreline would shift 34 39 miles westward if sea level were 

lowered to the present 300-foot-depth contour (U.S. 

Coast and Geodetic Survey, 1941), and this lower base 

level would result in vigorous erosion of the present 

bottom. The present-day buried valleys on San Francisco and Marin Peninsulas must have been carved 

when sea level was lower, during a maximum glacial 

stage, or earlier during a middle Pleistocene disturbance, as postulated by Louderback (1951, p. 82).

The Colma Formation and its age equivalent, the 

Merritt Sand, are the upper part of the natural fill in 

these buried valleys. For example, the bedrock floor of 

the buried valley below the Ferry Building is more 

than 270 feet below sea level; however, the bottom of 

the Colma Formation at this place is 127 feet higher. 

Thus, at the Ferry Building the Colma Formation was 

deposited after the valley was carved, and it subsequently was filled with 127 feet of sediments older than 

the Colma. It is also likely that several periods of erosion and filling during successive glacial and interglacial stages intervened between the time of the carving of the valley and the deposition of the Colma 

Formation. After its deposition, the Colma Formation 

was also cut by valleys graded to a sea level lower than 

the present level.

RELATED DEPOSITS

Some Quaternary deposits around San Pablo, Suisun, Tomales, and Monterey Bays (fig. 1) are tentatively correlated with the Colma Formation.

The Millerton Formation of late Pleistocene age 

at Tomales Bay consists of interbedded marine and 

continental beds (Weaver, 1949a, p. 99-103; Weaver, 

1949b, p. 51-52). The marine beds contain a large 

molluscan fauna, and the continental beds contain a 

large flora. Weaver (1949b, p. 52) believed the fossils 

"indicate a climate slightly warmer than that which 

prevails today, possibly representing an interglacial 

epoch." Richards and Thurber (1966, p. 1092) determined an apparent age of 55,000 years by the Th230/ 

U2M method on mollusks from the Millerton Formation 

and suggested that the mollusks "could easily have 

been deposited during the last interglacial stage or 

earlier." The Millerton Formation contains seven vertebrate genera, including Bison (Weaver, 1949a, p. 

103), and is therefore of Rancholabrean mammalian 

age (Savage, 1951, p. 289), which spans the Illinoian 

Glaciation, the Sangamon Interglaciation, and the Wisconsin Glaciation (Hibbard and others, 1965, p. 514).

Fossiliferous marine sand and associated sandy clay 

and gravel, assigned to the Millerton Formation by 

Weaver (1949a, p. 101), are found in nearly horizontal 

beds in San Pablo and Suisun Bays. Weaver (1949a, 

p. 103-106) also named several nearby continental 

deposits, the Huichica, Glen Ellen, and Montezuma 

Formations, which are partly contemporaneous with 

the Millerton Formation.

The Aromas Red Sands of Alien (1946, p. 18,43-45), 

exposed over a wide area near Monterey Bay, are similar in field appearance and lithology to the sands of 

the Colma Formation. Alien believed "They were laid 

down by the action of both wind and waves, on a lowlying plain, as lagoonal deposits, sand dunes, and bars. 

After uplift, the oxidation and solution of the magnetite in the sand resulted in the development of red 

colors, cementation, and reprecipitation of hematite." 

Although no fossils have been found, Alien (1946, p. 

45) suggested that the Aromas is "at least as late as 

middle Pleistocene in age."

OLDER BEACH DEPOSITS

In May 1952 a fresh-appearing beach sand about 100 

feet above sea level was exposed below a 5-10-foot 

cover of dune sand in a deep trench on the Presidio 

Military Reservation. The sand is pale yellowish brown 

(WYR 6/2), loose, and fine to medium. It is believed 

to be a beach deposit because of the nearly horizontal 

bedding and the persistence of thickness and inclination of the beds for several hundred feet (fig. 49). 

These features are in marked contrast to corresponding 

features of dune bedding (Thompson, 1937, p. 747- 

751; McKee, 1953, p. 20, 25). In many places the bedding resembles Thompson's (1937, p. 732) type A
OLDER BEACH DEPOSITS 75

FIGURE 49. Raised older beach deposit covered by dune sand. 

Southwest corner of the Presidio, San Francisco. Bakers 

Beach is one-fourth of a mile to the left. A, The raised beach 

deposit below dune deposits as exposed in north wall of sewer 

trench. Elevation approximately 150 feet above sea level. 

B, Closeup view of the raised beach deposit showing regularity of bedding. C, Closeup view of dune sand covering 

raised beach deposit. Note crossbedding in dune sand.

cross-lamination found on the upper foreshore of 

modern California beaches, where laminae of low landward dip are truncated by an erosional surface of low 

seaward dip. In the Presidio the dip of the bedding of 

the raised beach deposit increases to as much as 20° 

eastward, suggesting a facies change that may represent backshore beach deposits (McKee, 1953, p. 14, 

19). These sands, in turn, grade into sands showing 

classic dune crossbedding.

Grain-size distribution of sand from the raised beach 

is similar to modern beach sands and modern dune 

sands (pi. 2C-E). Most of the deposit has a median 

diameter near the lower limit of the medium sand size, 

but sands of maximum eastward dip (pi. 2A, map locality 19, sample 1434) have a median diameter near the 

lower limit of the coarse size. Sorting is good, except 

for the coarse sand, which is only moderately sorted.

The shape, roundness, and frosting of grains of the 

raised beach sand are more similar to those of dune 

sand than to those of modern beach sand; mineral 

composition is similar to that of both modern beach 

sand and dune sand. Intensity of etching of pyroxenes 

is between that shown by dune sand and that shown by 

modern beach sand. About 1-2 percent of the pyroxene 

grains show long sawtooth terminations with blunted 

points. Most of the pyroxene grains show only tiny 

sawtooth terminations or show none.

AGE

The older beach deposit, like the Colma Formation, 

was probably deposited during an interglaciation when 

sea level was higher. Properties of the grains and their 

size distribution suggest that the raised beach sand 

consists mostly of dune sand blown from a beach west 

of it and below it and that the dune sand entered the 

beach environment during a high stand of the sea. The 

raised beach sand may be younger than the Colma 

Formation, for it appears to be completely unaltered 

and unconsolidated and contains tiny shell fragments, 

whereas sands of the Colma Formation are generally 

weathered to a darker brown color, have a fair cohesion, 

and have no shell fragments. The contrast in etching of 

pyroxene grains is also striking, for the strong etching 

seen in almost all pyroxene in the Colma Formation is 

rare in pyroxene of the raised beach sand. On the 

other hand, the raised beach sand may be a relatively 

unweathered part of the Colma Formation, the weathered part having been removed by wind action.

The stratigraphic relations of the raised older beach 

sand and the Colma Formation were not directly observed and remain obscure. The Colma is exposed at 

the same elevation 2,500 feet east of the raised beach 

sand and also near sea level 1,000 feet west of it. If the
76 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

raised beach deposit is younger than the Colma Formation, why is it not more extensive and why is it not 

found elsewhere above the Colma? Perhaps some 

deposits thought to be dune sands, penetrated in boreholes, are part of the raised beach deposit.

Because it is unaltered, the raised beach sand 

appears to be younger than marine sands on the Half 

Moon Bay marine terrace, described by Smith (1960, 

p. 46). If the raised beach sand is younger and if 

Smith's (1960, p. 186) Sangamon age designation for 

the Half Moon Bay terrace is correct, the raised beach 

sand is Wisconsin in age. Sea level during Wisconsin 

time, however, was lower than present sea level (Hopkins, 1959, p. 7). Consequently, if the raised beach 

sand is indeed Wisconsin in age, it reached its present 

elevation by uplift rather than by deposition during a 

sea level higher than present sea level. On the other 

hand, the raised beach sand may have been deposited 

during Sangamon time, which was the latest time when 

sea level was substantially above present sea level. If 

the sand is Sangamon in age, then the Colma Formation and possibly the Half Moon Bay terrace are related 

to earlier interglaciation.

MODERN BEACH DEPOSITS

Sands and coarser beach deposits are numerous 

along all the shores except the east edges of San Francisco and Sausalito. Most beaches are small and are 

confined to coves and inlets. The largest, Ocean Beach, 

extends for nearly 8 miles south of the Cliff House 

(figs. 50, 51). The next largest, Bakers Beach, is more 

than half a mile long, west of the Presidio. Beach 

deposits that at one time bordered nearly all the shore 

between Fort Point and Telegraph Hill are now largely 

buried under artificial fill.

Beach deposits vary in thickness and areal extent, 

depending on the nature of the waves and the supply 

of sediments, both of which vary from hour to hour and 

season to season. An excess of sediments permits beach 

building, and an inadequate supply of sediments for 

some wave conditions causes beach erosion. Energetic 

storm waves may cause considerable erosion in a few 

hours (Trask, 1959, p. 21). Deposits on the smaller 

beaches probably are not more than 20 feet thick, but 

those along Ocean Beach and Bakers Beach may be 

considerably thicker.

Beaches of predominantly coarse gravel and cobbles 

with some sand and fine gravel are found in a few coves 

on both sides of the Golden Gate and on Tiburon 

Peninsula and Belvedere and Angel Islands. The principal materials of gravel beaches on the north side of 

the Golden Gate channel are radiolarian chert and 

greenstone. Some beach deposits consist of boulders

FIGURE 50. Ocean Beach, viewed southeastward from the Cliff 

House area. The seawall serves as a barrier to the eastward 

movement of windblown sand.

FIGURE 51. Ocean Beach looking northward from a point about 

1 mile south of the San Francisco North quadrangle. Note 

slight development of berm. Photograph taken in February 

1962.

of intact greenstone pillows that have become detached 

and fallen from exposures above the shore. Rocks 5-10 

feet in diameter are common on the beach between 

Fort Point Rock and Bakers Beach and west of Lands 

End where debris of sheared rocks or serpentine has 

fallen to the shore. Locally abundant are concrete 

pieces derived from construction waste dumped into 

the sea.

Trask (1959) examined Ocean Beach at five stations 

(pi. 2A) at 2-6-week intervals from July 1956 to June 

1957 and found many changes during that time. 

Beaches generally were built upward and seaward during summer and fall and were eroded during winter 

and spring.

Sand, the predominant beach deposit, is yellowish 

brown (WYR 6/4) to light gray (N7) speckled with a 

few white shell fragments and abundant dark-gray, 

green, and brown grains. The median diameter of sand
MODERN BEACH DEPOSITS 77

grains is in the medium to coarse size ranges. Sorting is 

moderate as measured by the phi deviation measure, 

<7<f>, which ranges from 0.60 to 0.95 (pi. 2C-E). The 

Trask sorting coefficient ranges from 1.34 to 1.55. The 

phi skewness measure, <*<£, shows that most of the 

analyzed sands have a size-frequency curve skewed 

slightly to moderately on the larger grain size side of 

the mode.

Trask (1959) gave median diameters and Trask 

coefficients of sorting for about 500 sand samples collected along Ocean Beach on various parts of the 

beaches at his stations (pi. 2A) during 1 year. The 

average median diameters ranged from 0.191 mm (fine) 

to 0.497 mm (medium). These extremes were obtained 

from sands collected on one beach (station J) in September (fine) and April (medium). The median diameters were generally larger in winter and spring than 

in summer and fall. Average median dfameter for the 

year at all the stations were in the medium sand size 

range. At any given time of sampling, grain size did not 

vary significantly at different parts of the beach.

The sorting values suggest a progressive increase in 

sorting to the south along Ocean Beach. Sorting was 

best in summer and fall.

Sand grains are mostly quartz, feldspar, and rock 

fragments. The rock fragments from the sample taken 

at map locality 15 (pi. 2A) consist mostly of chert, 

quartzite, porphyries, felsite, and some serpentine. 

Heavy minerals, chiefly ilmenite, chromite, green and 

brown hornblende, augite, and hypersthene, make up 

about one-fifth to one-third of the sand. Black sands 

natural concentrates of heavy minerals are found 

from time to time on Ocean Beach about 1 mile south 

of the quadrangle. More than half of the dark grains 

have a secific gravity below 2.82; these include chert, 

quartzite, porphyry, felsite, and serpentine. A small 

amount of gold has been recovered from these sands 

(DeGroot, 1890, p. 545-547; Day and Richards, 1905, 

p. 1188; Davis, 1949, p. 107). Modern mollusk and 

echinoid shells are abundant locally on the beach surface, but they are generally rare in the sand below 

the surface. Flat rounded cobbles of fossiliferous sandstone of the Merced Formation are occasionally found 

on Ocean Beach. Sand along the north shore of the 

Golden Gate are mostly dark brown and consist mostly 

of radiolarian chert and greenstone grains.

Most of the quartz and feldspar grains are slightly 

elongated, are subangular to subround, and have polished surfaces. A small percentage are angular or well 

rounded and have dull pitted surfaces. Some grains 

show these features on only part of the grain. The dark 

grains, whether heavy or light, are generally better 

rounded than quartz and feldspar grains; a large percentage of the dark grains are well rounded and have

polished surfaces. Many of the heavy mineral grains, 

chiefly hornblende, pyroxene, and zircon, are rod 

shaped. Pyroxene grains generally have tiny or blunted 

sawtooth terminations, a few have long, delicate sawtooth terminations, and a few have no such terminations.

SOURCE OF SAND ON OCEAN BEACH

The chief sources of sand on Ocean Beach and in the 

related dunes that extend 6 miles eastward are the 

poorly consolidated Merced Formation along the shore 

south of this quadrangle and the Colma Formation and 

younger Quaternary rocks. These sources are exposed 

for several miles along the shore to the south in steep 

cliffs that reach heights of 500 feet. Active landsliding 

of these cliffs furnishes large volumes of sediment to 

the shore at their feet (Bonilla, 1959, p. 29; 1960, landslide location map).

Direct evidence for the derivation of beach sand from 

these older formations is furnished by the sand grains. 

The similarity in mineral composition of sand of Ocean 

Beach and sand of the Colma Formation (pi. 2F) and 

Merced Formation suggests these formations are 

sources of the beach sand. Hornblende is abundant in 

the beach sands and indicates that the local hornblendepoor sandstone of the Franciscan Formation is not an 

important source. Hornblende, however, as well as other 

minerals, may have reached Ocean Beach from other 

sources, such as the quartz diorite of Montara Mountain 9 miles south of San Francisco. Bradley (1957, p. 

434) indicated that pyroxene grains are not etched in 

the active beach environment. The abundance of etched 

pyroxene grains, some strongly etched, on Ocean Beach 

indicates a derivation from older sand deposits such 

as the Colma and Merced Formation, in which almost 

all pyroxenes are strongly etched.

In the past, beach sand may also have been derived 

from the vast Sacramento-San Joaquin drainage area. 

During Quaternary glaciations sea level is thought to 

have been 228-406 feet lower than now (Moore and 

Shumway, 1959, p. 373; Hopkins, 1959, p. 7; Kuenen, 

1955, p. 197; Kuenen, 1950, p. 537; Flint, 1947,p. 437). 

The shoreline during these times would have been 

about 30 miles west of Ocean Beach. Thus, the Sacramento River would have carried sediments into the sea 

to form a delta many miles west of the Golden Gate. 

Sand from the river would probably have furnished 

much material for beaches near its mouth and for dunes 

east of the shore. With rising sea level, the locus of 

deltaic, beach, and dune deposition would have shifted 

eastward. The delta then would have retreated to its 

present location east of Carquinez Strait as the ocean 

covered the lower course of the river. The wide sandy 

sea-bottom plain between the present shoreline and the
78 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

Farallon Islands is probably a relict of these former 

deltas, beaches, and dunes. Some or much of the sand 

on Ocean Beach, Bakers Beach, and the beaches 

between Fort Point and Black Point and the sand in 

the offshore bar across the mouth of the Golden Gate 

may, thus, be derived from land far to the east.

Although direct measurements of sand drift have not 

been made, some northward movement parallel to the 

shore probably has occurred for several reasons. Waves 

frequently strike Ocean Beach at an angle from -the 

south (Trask, 1959, p. 24). Richard Janda (written 

commun., 1967) also suggested that a north-moving 

current may be related to strong prevailing winds from 

the southwest and west and the relatively straight 

north-south trend of the coast. The U.S. Coast and 

Geodetic Survey (1943, p. 32-33; Gilbert, 1917, p. 69) 

reported a weak offshore current that flows southsoutheastward, following the trend of the coast, but 

near the shore the Survey reported a weak current that 

flows northward, except when winds blow southward. 

The north-flowing inshore current in the San Francisco 

area is explained in part as an eddy, formed in the lee 

of the Point Reyes salient of the coastline, counter to 

the general southward circulation of the California part 

of the eastern Pacific Ocean (Davis, 1933, p. 19-20; 

Howard, 1951, p. 105).

Moore (1965, p. 55-56) concluded from a study of 

heavy minerals in recent coastal sediments west of 

Marin and San Francisco Counties that (1) longshore 

transport is only of local importance at the present time; 

(2) sand in more than 90-120 feet of water originates 

from an earlier period of sedimentation than that of 

the modern coast; (3) the bay bar sand is derived from 

San Francisco Bay; (4) the beaches south of the Golden 

Gate owe their composition at least partly to offshore 

sands mixed with local sediments by the surf; and (5) 

longshore movement south of the bay bar is to the 

south.

Yancey and Lee (1972) placed the modern beach 

sediments of the west and north coast of San Francisco in their hornblende-augite-hypersthene heavymineral assemblage, which they believe results from 

mixing of sediments derived from volcanic, metamorphic, and sedimentary rocks of the Central Valley 

of California.

DUNE SAND

Dune sand underlies more than half of the city of San 

Francisco, but the only other occurrence in the quadrangle is a small deposit behind a small sandy beach on 

the east shore of Angel Island south of Quarry Point. 

Prevailing westerly winds swept the sand from Ocean 

Beach, Bakers Beach, and possibly from a former beach 

west of Black Point as far eastward as the former Yerba

Buena Cove, which was between Rincon and Telegraph 

Hills and has been artificially filled. Winds from the east 

created the small dune on Angel Island. Dune sand is 

more than 600 feet above sea level at Sunset Heights 

and Grand View Parks and covers the 575-foot summit 

of Buena Vista Park. On Nob Hill, near its east border, 

dune sand is nearly 300 feet above sea level.

Dune sand in the Civic Center area (fig. 52) and 

surrounding parts of Hayes Valley reaches thicknesses 

greater than 100 feet. Dune sand, 5-40 feet thick, 

covers the west slope of Mount Sutro north of Kirkham 

Street. Maximum thickness of dune sand is approximately 150 feet. Elevation differences of 75 feet were 

measured from dune crests to interdune troughs in the 

area now occupied by the Mark Twain School and A. P. 

Giannini Junior High School on Ortega Street prior to 

the development of that area in 1952, and the total 

thickness of sand was greater than 100 feet. Dune sand 

mantling the prominent hills and ridges is much thicker 

on the east (lee) side than on the west and is thin or 

absent on the crests.

Dune sand is absent from the west slopes of Mount 

Sutro on the lee side of the Grand View-Sunset Heights 

Parks ridge. The extensive area of hills and lowlands 

east of the Mount Sutro-Mount Olympus-Twin Peaks 

highland was also protected from windblown sand.

At the present time, manmade barriers confine most 

wind transportation of sand to a narrow zone near 

Ocean Beach. Active dunes were observed by the 

author in 1937 in the Sunset District west of 33d Avenue and as late as 1951 in the area west of Sunset 

Boulevard and south of Ortega Street (figs. 53, 54). 

They were of the transverse-ridge type (Cooper, 1958, 

p. 27-49; 1967, p. 42-52). The U.S. Coast Survey map

FIGURE 52. Dune sand exposed in excavation for Brooks Hall. 

Civic Center, San Francisco.
79

FIGURE 53. Sand dunes in Sunset District of San Francisco. 

Notch in skyline in center of photograph is site of San Andreas fault in Marin County. View northwest toward Bolinas 

Bay.

FIGURE 54. Unstable embankment of dune sand on the lee 

side of Sunset Heights, near Moraga Street and Funston 

Avenue, San Francisco, left by excavation for pad for house. 

Sand was blown over top of hill, which is 666 feet above sea 

level. View west.

of San Francisco published in 1853 (fig. 62) shows 

ridges that probably are longitudinal dunes in the present Civic Center-Market Street area. The dunes near 

Aquatic Park are shown in a photograph taken about 

1878 (fig. 55).

Dune sand is fine to medium and well sorted (pi. 

2C-F). Median diameter decreases sharply from beach 

sands to adjoining dune sands. A decrease in median 

grain diameter and an increase in sorting were expected 

from west to east across the peninsula; however, no 

consistent variation was seen in the dune sand samples. 

Dune sand in the central part of the peninsula generally 

shows positive phi skewness, whereas samples closer to 

the Pacific Ocean beaches and samples closer to the

- -J.--T r i* . i***M 

. ^'\f -\. 'Fir ' ^g r^.^atx'^^"".. 1' -

FIGURE 55. Sand dune in Aquatic Park area near intersection 

of Columbus Avenue and Beach Street, San Francisco. 

Photograph taken in 1878 or earlier looking northwest to 

Golden Gate from Telegraph Hill. (Photograph from Professor William S. Cooper, Boulder, Colo.)

San Francisco Bay show negative phi skewness or zero 

phi skewness. Because variations in parameters are 

expected in different parts of a single dune, the inconsistency may be due to a failure to select samples from 

the same parts of dunes, though a negative phi skewness of the western dune sand may be an inheritance 

from beach sands. It is also possible that samples were 

taken from different generations of dunes; these may 

have formed under slightly different conditions of wind 

and source of sand.

Dune sands show considerable local variations in 

shape, roundness, surface texture, and mineralogy. As 

in the beach sands, most grains are generally equant in 

shape, though such heavy minerals as hornblende, 

pyroxene, and zircon are rod shaped. Most grains are 

subround or subangular. Polished surfaces are more 

common than dull pitted surfaces. In all dune deposits, 

however, some well-rounded grains with dull pitted 

surfaces are present; in a few deposits they are predominant. In general, the larger grains are more rounded 

than the smaller grains, and dark grains are more 

rounded than light-colored grains of the same size. The 

dark grains have greater roundness because they are 

generally of greater specific gravity or are more easily 

scratched than the light-colored grains (Pettijohn, 

1949, p. 428). Textures of rocks that are now grains 

in the dune sand may have some influence on rounding, 

for dark-red-brown fine chert grains in the dune sand 

are generally more rounded than grains of single quartz 

crystals of the same size and specific gravity. Magnetite, ilmenite, chromite, sphene, monazite, and apatite
80 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

are especially well rounded, even in sizes smaller than 

100 mesh (0.149 mm in diameter), whereas in the same 

deposit quartz and feldspar grains of the same size are 

generally angular or subangular.

Roundness of quartz grains between 50 and 70 mesh 

(0.295-0.206 mm in diameter) was determined by O. 

M. Schmidt, using Power's (1953) visual method for 

beach sands (pi. 2A, E, samples 1688S, 1690) and dune 

sands (samples 1293, 811, 1483, 1253, 2001, 1249). 

Roundness increases sharply between beach sands and 

the nearest dune sands and increases slightly and gradually from west to east in the dune sands. The sharp 

increase in roundness between beach sand and dune 

sand was also observed in other areas by Beal and 

Shepard (1956), who suggested that the wind tends to 

select the more rounded grains for transportation. 

Pettijohn (1949, p. 422) also suggested that shape 

sorting may be more important than abrasion in causing a greater roundness of dune sands compared with 

sands from which they were derived.

Pyroxene grains with sawtooth terminations are 

common in the dune sands near Ocean Beach. According to R. C. Gervacio, graduate student at Stanford 

University (oral commun., 1957), such pyroxene grains 

progressively decrease in abundance eastward.

Bradley (1957, p. 434) found a progressive increase 

of etching of pyroxene grains with increasing time of 

subaerial weathering in a marine terrace deposit at 

Santa Cruz, Calif. He indicated that etching of pyroxene does not take place on active modern beaches.

Mineral composition of sand grains in two dunes 

from the quadrangle and from a dune at Edgemar, 8.7 

miles south of Point Lobos, is given on plate 2F. Feldspar and rock fragments, chiefly chert and quartz in 

varying proportions, make up 70-85 percent of the 

dune sands. The remaining grains are heavy minerals, 

in approximate order of abundance, hornblende, clinozoisite-epidote, ilmenite, magnetite, augite, hypersthene, and chromite. Hornblende, chromite, and ilmenite are the most common heavy minerals in the Edgemar dune sand. A very slight iron staining of the surface of quartz and feldspar grains and the abundance 

of dark grains give dune sand its prevailing color of 

moderate yellowish brown, though active dunes commonly are light gray.

ORIGIN AND AGE

The extensive dunes in San Francisco are due to a 

favorable combination of wind, supply of sand, coastal 

topography, and limited plant cover. Strong eastward 

onshore winds are frequent along the west shore of San 

Francisco Peninsula. The greatest source of dune sand, 

Ocean Beach, is continuously replenished by longshore 

currents and by the surf carrying sand shoreward.

Bakers Beach, too, is probably supplied by sand from 

the surf and tidal currents. Some of the sand for Bakers 

Beach and the beach east of Fort Point may come from 

the large area of sand waves on the bottom of the bay 

between Angel Island and San Francisco, formed by 

the action of tidal currents moving from the bay to the 

ocean (pi. 1).

The topography east of Ocean Beach and Bakers 

Beach is either nearly level, such as at Golden Gate 

Park, or rises gently. The level topography and the 

present and possibly past absence of a forest of 

large trees offer no marked obstruction to wind and 

sand movement within at least 2 miles of the shore. 

Dune sand above the high cliffs between Point Lobos 

and Bakers Beach was probably blown by occasional 

east winds from dune sand lying on the gentle slopes 

in back of the beaches.

Two sections of dune sand separated by bay mud 

and clay are penetrated in borings in the Market Street 

area, east of the Civic Center. This indicates a long 

history of dune activity. Intensive development of 

dunes in San Francisco took place more than once during the Holocene and perhaps in the Pleistocene 

(Cooper, 1967, p. 49-50). However, belts of progressively older dunes away from the beach such as those 

in the Santa Maria coast area, 230 miles south of San 

Francisco, were not seen in San Francisco (Woodring 

and Bramlette, 1950, p. 116; Cooper, 1958, p. 136). 

Numerous accounts of San Francisco during and following the gold rush of 1849 illustrate and discuss 

dunes between Nob and Rincon Hills and in the western part of San Francisco near the present Golden Gate 

Park (Brewer, 1930, p. 499; Weinstein, 1967, p. 38-39).

Cooper (1958, p. 130; 1967, p. 113-115) suggested 

that during periods of rising sea level, dune development is especially active. He believed that the coastal 

dunes of California, Oregon, and Washington are related 

to major sea level changes a rise in sea level, some 

5,000-6,000 years ago, and a succeeding period of virtually stable sea level. He also suggested that sea level 

changes from earlier Pleistocene glaciations would have 

influenced dune activity. Thus, it is probable that dunes 

of San Francisco are not all of the same age. For 

example, some of the dunes on the east side and central 

part of the peninsula may be older than dunes closer to 

the ocean on the west side of the peninsula.

SLOPE DEBRIS AND RAVINE FILL

Most slopes in the quadrangle are mantled by unconsolidated debris derived from weathered underlying 

material and transported by mass-movement processes. 

These deposits have been mapped wherever they are 

known to be more than about 5 feet thick. Locally, this 

unit includes soils developed on bedrock as well as
SLOPE DEBRIS AND RAVINE FILL 81

minor amounts of alluvial, eolian, and landslide materials. Slope debris thickens progressively downhill to 

an observed maximum of 18 feet; locally, it probably 

attains thicknesses three or four times as great.

Most of the ravines are partly filled with material 

derived from the adjoining slopes. These deposits vary 

in thickness from a few feet to more than 30 feet. 

Locally, especially on Marin Peninsula, they are interbedded with and grade into stream alluvium. In many 

places throughout the quadrangle, roadcuts in gently 

rounded, mature slopes have revealed deep, steepsided ravines that have been completely buried by 

slope debris (fig. 10).

Slope debris and ravine fill consist of a mixture of 

angular rock fragments in a matrix of sand, silt, and 

clay (pi. 25). The physical properties of the slope 

debris are partly dependent upon the underlying bedrock. On many slopes in northern San Francisco, where 

the bedrock is predominantly sandstone and shale of 

the Franciscan Formation, most of the material is in 

the sand-to-clay size range with minor amounts of 

gravel-size material. Elsewhere in San Francisco and 

on Marin Peninsula, where radiolarian chert is a common bedrock and where slopes are generally steeper 

and transportation of surface materials more vigorous, 

slope debris and ravine fill contain a large percentage 

of gravel- and cobble-size radiolarian chert pieces and 

appreciable quantities of greenstone and sandstone 

(fig. 56). Slope debris on or below greenstone or sandstone and shale bedrock is generally much thicker than 

that derived mostly from radiolarian chert and shale 

bedrock.

Slope debris may include deposits of different ages. 

Some roadcuts in slope debris on Marin Peninsula

FIGURE 56. Slope debris on weathered greenstone of the Franciscan Formation. Gravel pieces are chert derived from upslope. Note slight soil development at top.

show unconformable layers of distinct lithologies. In 

many areas slope debris is apparently the youngest 

material of the slope. On some of the lower slopes and 

along the wide shelving shores, however, the ravine fill 

is overlain by marine deposits and is probably late 

Pleistocene in age. Material identical in lithology with 

the modern slope debris has also been observed below 

and interbedded with the Colma Formation. Material 

in the buried ravines is somewhat older than modern 

slope debris. The buried ravines were evidently formed 

during a period of vigorous erosion, such as would occur 

at the time of a lower Pleistocene sea level when, also, 

the intensity and amount of precipitation was probably 

greater than now. The ravines were probably then filled 

during the early part of the succeeding interglacial 

period. Transportation of material downslope and into 

ravines was mostly by such colluvial processes as creep, 

mud flowage, and debris flowage.

SAN FRANCISCO BAY SEDIMENTS

San Francisco Bay and the Golden Gate channel, 

which cover more than half the quadrangle, are underlain by modern sediments, though in places these are 

only a few inches thick. Bay sediments also underlie 

areas of artificial fill along the fringes of the bay. Sediments and sedimentation in San Francisco Bay are 

currently (1969) under investigation by the U.S. Geological Survey. This report summarizes results of other 

investigations and results of the author's examination 

of San Francisco Bay sediments.

Sediment studies in San Francisco Bay have been 

reported by Sumner, Louderback, Schmitt, and Johnston (1914), Gilbert (1917), Packard (1918), Bailey 

(1921), Miller, Ramage, and Lazier (1928), Louderback (1939; 1951,p.88-93), TraskandRolston (1951), 

Trask (1956), Arden (1961), George Porterfield, N. L. 

Hawley, and C. A. Dunham (written commun., 1961), 

Einstein and Krone (1961), Krone (1962; 1963), 

Arnal and Conomos (1963), Kvenvolden (1962), U.S. 

Army Corps of Engineers (1963), Smith (1963; 1966), 

Conomos (1963), Treasher (1963), Mitchell (1963), 

Storrs, Selleck, and Pearson (1965), Slater (1965), 

Reese (1965), Means (1965), Gram (1966), Selleck, 

Pearson, Glenne, and Storrs (1966), Meade (1967), 

and Quinterno (1968).

Data in this paragraph on sediment volume and 

movement is taken from Smith (1966, p. 10) and Selleck, Pearson, Glenne, and Storrs (1966, p. 34). The 

annual sediment inflow to the bay system, including 

the delta area east of Suisun Bay, is estimated to be 

between 6.2 and 8 million cubic yards. The combined 

San Joaquin Sacramento Rivers supply about 85 percent by weight of sediments that enter the bay; much 

of the remainder is carried into the bay by Napa River

534-039 O - 74 - (
82 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

and Alameda Creek. About 1.8 million cubic yards of 

sediment is deposited each year in the delta area. More 

than half the total sediment volume enters the bay 

during February, March, and April. Ebb tides tend to 

move sediments towards the Golden Gate; approximately 30 percent of the sediments entering the bay is 

swept out to sea.

Smith (1966, p. 15) gave the following data on grain 

size of sediments. The approximate grain-size distribution of sediments entering the bay from all streams is 

57 percent clay, 29 percent silt, and 14 percent sand. 

The composition of sediments in suspension in bay 

water is 57 percent clay and 43 percent silt. Much of 

the sand-size material is deposited in the delta and in 

Suisun Bay, but some is carried into San Pablo Bay 

and southward.

Colloidal clay particles suspended in fresh water 

flocculate where the salinity is approximately 1,000 

parts per million. The interface between fresh water 

and water of this salinity value varies in position 

depending on the volume of fresh water entering the 

bay. During periods of severe storms and high runoff, 

the fresh-water saline-water interface may reach the 

Golden Gate, but it is generally in San Pablo Bay or 

in the adjoining Carquinez Strait. A large proportion 

of suspended sediments settle out in San Pablo Bay 

because of flocculation of colloidal particles and 

because of the large areas of relatively quiet water 

there compared to Carquinez Strait and Suisun Bay. 

Deposition in the bay system is generally greatest at 

water depths of 10-30 feet and outside of tidal channels. Deposition also takes place at times in water less 

than 10 feet deep, but these shallow-water deposits are 

generally eroded by water made turbulent by wind. 

Part of the material kept in suspension by tidal currents and other turbulent water, and deposits stirred 

up, eroded, and resuspended by these waters, are carried out to sea through the Golden Gate. Or they are 

carried to the south arm of the bay, where they form 

thick deposits in places where the water is quiet.

Sand and locally gravel, therefore, are deposited in 

the tidal channels. Bedrock, as well as gravel and sand, 

may be exposed where strong ebb currents are concentrated into narrow powerful currents by shoreline projections. In the San Francisco North quadrangle, such 

areas are found near Campbell and Stuart Points on 

Angel Island, Peninsula Point on Belvedere Island, east 

and south of Alcatraz Island, and in the Golden Gate 

channel. Smith (1963, figs. 6D, E) presented data indicating considerable scour in the period 1855-95 in much 

of the area between Belvedere Island and San Francisco west of Alcatraz Island and in the period 1850 

1956 along the east shore of San Francisco between 

China Basin and Hunters Point.

Vigorous tidal currents prevent deposition of silt and 

clay in much of the part of San Francisco Bay lying 

within the quadrangle. Tidal currents are especially 

strong and widespread in this part of the bay because 

this area is the distribution point of the flood tidal currents and the joining point of the ebb tidal currents 

passing through the Golden Gate channel. Bottom 

sampling by the U.S. Coast and Geodetic Survey (1959) 

yielded rock, gravel, and sandy sediments over much 

of this area. Mud and clay deposits, however, are thick 

in other places where tidal currents and water turbulence have been weak during the Holocene. Trask 

(1956, p. 18) showed the general distribution of 

present-day sediments in San Francisco Bay, based on 

U.S. Coast and Geodetic Survey charts. The author 

found about 1 foot of gravel overlying Franciscan 

sandstone and shale near the northwest end of Raccoon Strait. The gravel pieces, 1-6 inches in diameter, 

consist largely of Franciscan sandstone and some gastropod shells. Most gravel pieces were coated with 

Bryozoa.

Holocene mud and clay deposits in the San Francisco North quadrangle reach thicknesses of more than 

80 feet along the east shore of the city of San Francisco, including the part now covered by artificial fill 

(Radbruch and Schlocker, 1958; Goldman, 1969a, pi. 

4). These fine-grained deposits are more than 120 feet 

thick about 2,500 feet northeast of the Ferry Building. 

Mud is more than 100 feet thick in the center of Richardson Bay at the northwest corner of the quadrangle. 

Mud is about 40 feet thick near the shore of Sausalito 

and may be 40-60 feet thick in the northeastern part 

of the quadrangle, east of Angel Island.

TRANSVERSE SAND BARS

Soundings by the U.S. Geological Survey and the 

U.S. Coast and Geodetic Survey suggest that between 

Angel Island and San Francisco, sand is distributed in 

transverse bars or giant ripple marks resulting from 

turbulent interaction of ebb and flood tidal currents 

(Gibson, 1951; Shepard, 1952, p. 1909; Carlson and 

others, 1970). The approximate boundary of the area 

of transverse sand bars is shown on plate 1. Detailed 

soundings of a small area about 1 mile southwest of 

Alcatraz Island show sand bars oriented approximately 

due north to N. 30° E. Bar crests are at depths of 50-60 

feet, and the average distance between crests is 240 

feet. Their height is approximately 6 feet. In Raccoon 

Strait similar bars are 17 feet high. The swifter 

westerly ebb current is believed to make the east slopes 

of the bars gentler than the west slopes. Shallow borings by the U.S. Bureau of Mines and the State of 

California Division of Bay Toll Crossings, observed by 

the author in 1965, show that the youngest deposit
SAN FRANCISCO BAY SEDIMENTS 83

located about 1 mile northeast of Alcatraz Island is 

medium sand at least 8 feet thick.

BAY MUD AND CLAY

Bay mud and clay are being deposited in San Francisco Bay at the present time. In their natural state 

they are generally olive gray (5Y 3/2) to dark bluish 

gray (5B 3/1), and they range from soft, plastic, and 

nearly fluid to moderately firm about like modeling 

clay. When dried, the color lightens typically to light 

greenish gray (5GY 7/1) or light olive gray (5Y 6/1), 

and the material becomes fairly tough and somewhat 

brittle. Bedding is not readily apparent in some mud 

and clay, but in others it is shown by sand partings, 

1-3 mm thick, separating clay layers 0.5-20 cm thick 

or by flattened plant remains. The general particle-size 

content of the bay mud and clay is 30-60 percent claysize particles (<2 microns), 30-65 percent silt-size 

particles (<62->2 microns), and 1-10 percent sandside particles (<2,000->62microns).

Lenses and irregular segregations, 1 cm to several 

meters thick, of fine to coarse sand or mollusk shells 

are common in the mud and clay in some places. The 

sand generally has a low silt and clay content. The 

shell lenses range from a 1-inch-thick layer of a few 

isolated clam shells in mud or sand, generally both 

valves being present, to concentrations of shells 3 feet 

thick in a clay or sand matrix. Shell lenses are mainly 

in the top 30 feet of the bay mud and clay, according 

to Hart (1966, p. 43). The shells are mostly pelecypods 

and minor amounts of gastropods. Although radiocarbon analysis indicates that shells in thickest lenses are 

approximately 2,400 years old, the shells are the same 

species as those living in the bay today (Packard, 1918, 

p. 236-244). Shell layers as much as 3 feet thick, composed entirely of whole or fragmentary separated pelecypod valves, are found on beaches along the bay south 

of San Francisco. They were probably concentrated by 

strong waves and tidal currents that washed away the 

mud and silt. Buried and preserved, such lenses in time 

will become coquinas. Goldman (1969b, fig. 1) showed 

the distribution of shell deposits in San Francisco Bay.

Organic remains such as diatoms, peaty plant fragments, Radiolaria, Foraminifera, sponge spicules, Bryozoa coccoliths, mollusk shells, and so forth are common 

and even abundant at some localities. Diatoms are 

especially common and make up 20 percent by volume 

of some mud and clay. Carbonaceous plant remains 

commonly make up 3-10 percent of the volume of bay 

muds and clays. Conspicuously peaty sediments, in 

scattered lenses about 1 inch to about 1 foot thick, contain brownish-black layers of plant remains Vie-% inch 

thick and as much as 5 inches long. A persistent peat 

zone is found at the base of the bay mud and clay in

the vicinity of the San Mateo Bridge, commonly as a 

concentrated layer less than 1 foot thick but locally as 

a disseminated zone 4 feet thick (Story and others, 

1966, p. 48-49).

Water content in bay mud and clay ranges from 

about 30 to 92 percent; percentages between 50 and 60 

percent are common in the uppermost 60-100 feet. The 

high percentage of water and the plasticity of the sediments stem from their montmorillonite content. When 

wetted, montmorillonite absorbs water, expands, and 

becomes plastic. X-ray diffraction analysis of particles 

smaller than 2 microns from these sediments, from near 

the east shore of San Francisco and from Richardson 

Bay, shows about 50-60 percent montmorillonite, about 

20-30 percent mica, and about 10-20 percent chlorite. 

Smaller percentages consist of mica and chlorite randomly interlayered with montmorillonite or regularly 

interlayered montmorillonite and mica (1:1), quartz, 

and plagioclase. The presence or absence of a small 

amount of kaolinite could not be ascertained because 

of the difficulty of detecting small amounts of kaolinite 

in the presence of much larger amounts of chlorite. A 

moderate amount of kaolinite, however, was found by 

the author in mud collected north of Suisun Bay. The 

silt-size fraction of the mud and clay also has a high 

montmorillonite content, but it differs from the claysize fraction in having a much larger proportion of nonclay minerals mostly quartz and plagioclase feldspar 

and minor potassium feldspars and organic remains. 

Sand grains are mostly quartz and plagioclase feldspar 

and minor potassium feldspar, rock fragments, and 

shell fragments.

Preliminary examination of bay muds from a limited 

number of localities from depths of 1-30 feet shows that 

the sand and silt grains generally include ^-5 percent 

heavy minerals (specific gravity >2.86). These minerals include, in approximate order of abundance: green 

hornblende, augite, tremolite-actinolite, pyrite, brown 

hornblende, micas, clinozoisite, epidote, chlorite, glaucophane, jadeite, lawsonite, hypersthene, sphene, oxyhornblende, garnet, zoisite, zircon, apatite, tourmaline, 

calcite, and anatase. Pyrite, crystallized around 

diatoms and other organic remains, occurs in drusy 

spheres, 10-30 microns in diameter, intergrown 

spheres, and elongate barbs. Irregular round grains, 

resembling glauconite in form, green or brown in color, 

and consisting of aggregates of tiny translucent crystals, make up about 1-3 percent of the heavy minerals. 

Analyses by X-ray diffraction and optical microscopy 

showed that some of them are ankerite. Gram (1966, 

p. 131) reported 3.1-23.5 percent glauconitic material 

in sand-size grains of bottom samples obtained from an 

area between Hunters Point and the San Mateo Bridge. 

The jadeite, which was abundant in one sample, is the
84 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

variety showing strong dispersion of the bisectrices and 

a texture of crystal aggregates similar to the jadeite in 

metasandstones of Angel Island and the Diablo Range. 

A program of heavy-mineral analyses of bay sediments 

from widespread localities and depths would yield 

information on sediment source and geologic history 

of the bay.

ENGINEERING PROPERTIES OF BAY MUD AND CLAY

Use of the bay mud and clay for foundation purposes 

demands careful engineering design to avoid shear 

failures and differential settlement problems. The 

problems are related to the high content of both water 

and the swelling clay mineral montmorillonite, which 

together cause low shearing strengths, high voids ratios, 

low specific gravity, high consolidation under load, and 

high drying shrinkage. Mitchell (1963, p. 26) summarized these and other properties in a table that shows 

a range of shearing strength of 100-1,200 pounds per 

square foot. Engineering properties of bay mud and 

clay are also discussed in Goldman (1969a, p. 20-23), 

Lee and Praszker (1969), Seed (1969), Steinbrugge 

(1969), Schlocker (1969, p. 25), Wigginton (1969), 

San Francisco Bay Conservation and Development 

Commission (1967), Treasher (1963), U.S. Army 

Corps of Engineers (1963), Roberts and Darragh 

(1963), Langston, Trask, and Pask (1958), Radbruch 

and Schlocker (1958), California Department of Public 

Works (1955), Lee (1953), and Trask and Rolston 

(1951) (fig. 57).

Treasher (1963, p. 23) found a marked increase in 

strength of bay mud 40-70 feet below the top of the 

mud. The lower, slightly stiffer mud is slightly overconsolidated. According to him, the increased strength 

is the result of desiccation during a glacial period of 

low sea level. The top 2 4 feet of bay mud loses water 

and gains strength in areas that have been diked off 

and drained.

STRATIGRAPHIC CORRELATION, FOSSILS, AND AGE

Bay mud and clay lie on other Quaternary deposits 

or on bedrock (fig. 56; pi. 1, section C-C')- Trask and 

Rolston (1951, p. 1085, 1086) showed bay mud lying 

on Merritt Sand in an east-west cross section at the 

southeast corner of the San Francisco North quadrangle and bay mud overlying undifferentiated Merritt 

Sand, Posey, and San Antonio Formations in a cross 

section parallel to and 300 feet north of the San Francisco-Oakland Bay Bridge. The foundation excavation 

for the Headworks Building of the Southeast Sewage 

Treatment Plant, near the shore of the bay half a mile 

south of the quadrangle, and logs of nearby boreholes 

indicate that the bay mud and clay lie unconformably 

on the eroded surface of sand deposits that may belong

FIGUEE 57. Bay mud, showing desiccation cracks, lying under 

artificial fill (dune sand). Foundation area Crown-Zellerbaeh 

Building, Bush and Battery Streets. San Francisco.

to the Colma Formation. These sand deposits are massive, brown, and clayey (described by the author in 

Radbruch and Schlocker, 1958). They may be part of 

the same fossil soil zone that is developed also on sand 

below bay mud and clay found by Story, Wessels, and 

Wolfe (1966, p. 49) in numerous boreholes in the 

vicinity of the San Mateo Bridge. Clays older than the 

soft uppermost bay mud and clay (informally called 

younger bay mud and clay by Radbruch and Schlocker, 

1958) lie below or are interbedded with the sand 

deposits (Radbruch and Schlocker, 1958). Older bay 

clays also lie below a sand layer in the area between 

Rincon Hill and Telegraph Hill in San Francisco, but 

according to Treasher (1963, p. 21) this sand forms 

scattered lenses in San Francisco Bay rather than a 

continuous blanket on older bay clay. In some areas 

the younger bay mud and clay (Treasher's semiconsolidated mud) directly overlie older, stiffer bay clay. 

The older bay clay is distinct from the overlying 

younger clay and mud because it is generally highly 

overconsolidated, from 1.5 to 3 tons per square foot 

(Trask and Rolston, 1951, p. 1096-1097).
ALLUVIUM 85

Fossils found in the younger bay mud and clay are 

of species living today. They include mollusks and a 

large assemblage of microscopic flora and fauna. Preliminary work on diatoms obtained from boreholes in 

eastern San Francisco shows both marine and freshwater species in some mud samples and only marine 

species in others. Kenneth E. Lohman (oral commun., 

1955) briefly examined the diatoms and suggested that 

the muds containing both marine and fresh-water diatoms were deposited under brackish-water conditions.

For two core samples of bay mud obtained by the 

Corps of Engineers, U.S. Army, about 1 mile east of 

the quadrangle and 2 miles south of its north boundary 

and collected 11 and 20.5 feet below the sediment surface, Kvenvolden (1962, p. 1645) reported the following radiocarbon dates from unspecified organic material: 6,210±175and7,925±810B.P. (before present), 

respectively.

Shell and peat in bay mud obtained 6-9 miles southeast of the quadrangle were dated by Story, Wessels, 

and Wolfe (1966, p. 47). Radiocarbon ages ranged from 

2,420±180 to 7,360±320 B.P. for samples 2-50.5 feet 

below the top of the bay mud. These radiocarbon ages 

confirm the belief that the modern deposition of mud 

began after the Wisconsin Glaciation about 14,000 

years ago, when sea level began to rise with the melting 

of ice (Milliman and Emery, 1968, p. 1121-1123). 

Treasher (1963, p. 23) suggested that semiconsolidated bay mud was deposited during a pre-Wisconsin 

interstadial high stand of sea level, was slightly overconsolidated by desiccation during a brief exposure to 

air during the subsequent Wisconsin Glaciation, and 

was inundated again and covered by the uppermost 

bay mud after the Wisconsin Glaciation. Older bay 

clays may have been deposited during periods of high 

sea level associated with the Sangamon and earlier 

interglaciations.

Great volumes of the uppermost soft mud were no 

doubt deposited during the days of hydraulic mining 

of gold when huge amounts of debris were fed into the 

Sacramento River. Gilbert (1917, p. 48^9) stated that 

the chief deposit in the bay from mining debris is mud. 

He also estimated that in the period 1849-1914, 50 

million cubic yards of sediments, mostly of grains 

smaller than fine sand and mostly derived from mining 

debris, reached the ocean. As early as 1862, many years 

before hydraulic mining reached its peak, Brewer 

(1930, p. 295) wrote as follows:

Previous to 1848 the river (the Sacramento) was noted for the 

purity of its waters, flowing from the mountains as clear as 

crystal; but, since the discovery of gold, the "washings" render 

it as muddy and turbid as is the Ohio at spring flood in fact it 

is perfectly "riley", discoloring even the waters of the great bay 

into which it empties.

ALLUVIUM

Alluvium, which is material transported and deposited 

by running water, is only sparsely present in the quadrangle. Alluvium is distinguished with difficulty from 

slope debris and ravine fill, which were transported by 

creep and landsliding. Evidently streams are too small 

to transport enough slope debris and ravine fill to 

create substantial deposits of alluvium. In San Francisco the high porosity of the dune sand may have 

inhibited substantial surface flow. Alluvium that 

formed in the last few hundred years may be represented by the deposits in the valley at Horseshoe Bay 

and in the southwest-trending valley of an intermittent 

stream in the west-central part of Marin Peninsula. 

The bed of the largest intermittent stream in San 

Francisco, Lobos Creek, is dune sand, artificial fill, and 

Colma Formation. Valleys on the east flank of Twin 

Peaks contained streams that flowed into the bay when 

the city was founded in 1776, but the stream beds now 

are covered with slope debris or artificial fill.

Several deposits of alluvium were mapped on the 

west slopes of Twin Peaks (fig. 58). They have a maximum observed thickness of 15 feet and are interbedded 

with and grade laterally and vertically into slope debris 

and ravine fill. Most of these deposits are related to 

slightly older drainage systems than the present ones 

and are now moderately dissected. Similar alluvial 

deposits occur in many other parts of the quadrangle, 

but they are not shown on the map (pi. 1) because 

they are largely or entirely concealed by other surficial 

deposits or by artificial fill and manmade structures. 

Most of the alluvium is composed of medium silty 

clayey sand; clean medium sand occurs locally. AlluFlGURE 58. Alluvium (Qal) lying on Colma Formation (Qc). 

Greenstone (Kjg) at right. West slope of Twin Peaks, San 

Francisco.
86 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

vium is generally better sorted than slope debris and 

ravine fill. Chert and greenstone pebbles are locally 

abundant in the alluvium, especially near the heads of 

the valleys.

LANDSLIDE DEPOSITS

Landslide deposits are widespread throughout the 

quadrangle, and they indicate that landsliding is one of 

the principal agents of erosion. The deposits mapped 

as landslides, however, are rather local features that 

were formed by relatively recent landsliding. Landslide 

topographic features, such as shear-failure scarps or 

hummocky topography, are visible, even though some 

of them are considerably modified by erosion. Spoonshaped cliffs like the one just west of the Golden Gate 

Bridge tower (fig. 19) may have been shaped by 

landsliding.

In the downward and outward movement of slopeforming material that constitutes landsliding, part of 

the potential energy of the slope is converted to kinetic 

energy and friction; the subsequent landslide deposit 

has correspondingly less potential energy, and so many 

slopes are more stable after landsliding than before. 

Thus, if a slope is to remain unstable over a long period 

of time, the landslide deposits must be removed. In the 

San Francisco North quadrangle, instability is usually 

perpetuated by rainwash and stream erosion, especially during large storms, that rework and move landslide deposits downslope or completely remove them 

from the landslide-sculptured slope. For this reason the 

activity of landsliding is directly proportional to the 

activity of other agents of erosion. Consequently, some 

other Quaternary deposits, especially some of the 

Colma Formation and slope debris and ravine fill, are 

reworked landslide deposits, but they are older than 

the landslide deposits designated as such on the map 

(pl.l).

Though landslides are widely distributed in the 

quadrangle, the small volume of landslide deposits 

shown on the geologic map (pi. 1) may seem to belie 

the important role of landsliding as an agent of erosion. 

Local construction activities have obscured many of 

the distinctive topographic features of landslides. On 

the other hand, abundant manmade structures afford 

numerous known survey points for measuring even 

slight landslide movements.

Landslide processes are succinctly discussed by 

Varnes (1958, p. 42-45), who pointed out the following: 

The process of landsliding is essentially a continuous series of 

events from cause to effect- * * *

Very seldom, if ever, can a slide be attributed to a single 

definite cause. The process leading to the development of the 

slide has its beginning with the formation of the rock itself, 

when its basic physical properties are determined, and includes 

all the subsequent events of crustal movement, erosion, and 

weathering, until some action, perhaps trivial, sets a mass of it

in motion downhill. The last action cannot be regarded as the 

one and only cause, even though it was necessary in the chain 

of events.

Because most slides involve failure of slope materials 

under shear stress, a logical account of causes of landsliding would deal with (1) factors that contribute to 

high shear stress and (2) factors that contribute to 

low shear strengths. These factors are outlined in detail 

by Varnes (1958, p. 42-45). Removal of lateral and 

underlying support is the most common factor that 

contributes to high shearing stress in a slope. It includes 

actions of erosion, of previous landsliding, and of man. 

A variety of factors leads to low shear strengths, including the initial state of the slope material, subsequent 

changes brought on by weathering, and other physicochemical reactions. In addition, water is an important 

factor in slope stability, for it contributes both to high 

shear stress and low shear strength.

The numerous landslides in the quadrangle owe their 

existence to several factors: an irregular hilly terrain 

with gentle to steep slopes underlain in many areas by 

unconsolidated surficial deposits or by badly sheared 

and shattered bedrock, a relative abundance of highly 

plastic and swelling clay in all the foregoing materials, 

occasional periods of prolonged rainfall, occasional 

earthquakes, and the continuous disturbance and alteration of the original terrain by man.

The distribution of landslides in the quadrangle is 

shown on plates 2 and 3. Slopes underlain by strongly 

sheared Franciscan rocks, by serpentine, and by the 

sheared-rock map unit are especially prone to landsliding. The largest landslides are in sheared serpentine

FIGURE 59. Steeply cut slopes in serpentine, Potrero Hill, San 

Francisco. Cut face to right of stairway has remained intact 

for many years except for minor sloughing and raveling. 

Slope beneath and to left of stairway was probably the site 

of a small landslide.
LANDSLIDE DEPOSITS 87

and in Franciscan rocks in the Lands End area and in 

the sheared serpentine south of the Golden Gate 

Bridge. Wave action periodically reactivates some of 

these slides by removing the supporting material at 

their bases. Serpentine, contrary to popular belief, is 

not especially prone to landsliding everywhere, because 

the large serpentine body of Potrero Hill is almost completely free of landslides (fig. 59). Harding (1969, p. 

66) pointed out that "areas of obvious potential instability can be mapped on a regional scale, however, the 

corollary, mapping whole hillside areas as stable on 

the basis of rock type, cannot safely be done."

Many landslides too small to be shown on the geologic map (pi. 1) are numerous in the slope debris and

ravine fill, especially along roadcuts. Soil moisture in 

ravine fill is believed to be a primary cause for sliding, 

although the actual movement may be triggered by 

other factors.

CHARACTERISTICS OF LANDSLIDES

Slides range from rockfalls at one extreme to mudflows at the other; earthflows and debris flows are the 

commonest types. Many landslides, however, are complex; their upper parts are rockslides, debris slides, or 

rotational slumps, and their lower parts are earthflows 

or debris flows (figs. 60, 61). A few rockfall deposits are 

found below steep quarry faces and along steep rocky 

shores. Characteristics of landslides are given on plate 3.

FIGURE 60._Landslide in center of photograph is a slump and debris slide in its upper part and a debris flow in its lower part (No. 

93, pi. 3). Note fracturing of sandstone of the Franciscan Formation. South Bay part of Golden Gate, San Francisco, looking 

southeast. (Barney Peterson, photographer, used with permission of the San Francisco Chronicle.)
88 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

FlGUKE 61. Landslide in which soft sheared sandstone and 

shale, below radiolarian chert, is moving downhill as a rotational slump and debris slide (No. 51, pi. 3). Toe of slide 

above roadcut has bulged the surface and is calving onto road. 

One mile northwest of Lime Point, Marin Peninsula.

The frequency of landsliding differs greatly for the 

various lithologic units. About 60 percent of the landslides involve surficial deposits and weathered and 

sheared bedrock. More than half the landslides in bedrock, including sheared and weathered rock, involve 

sandstone of the Franciscan Formation, 21 percent 

involve greenstone, 8 percent involve radiolarian chert, 

and 9 percent involve serpentine. Bonilla (1960) 

obtained similar results in an analysis of landslides in 

the San Francisco South quadrangle. These percentages, however, do not take into account the size of 

slides, nor the relative area of exposure of each type. 

Serpentine slides are among the largest in the quadrangle. Landslides involving radiolarian chert are generally of small size. A more satisfactory statistical comparison of the tendency of various materials to slide 

would evaluate dimensions or volume of slides and the 

relative abundance of the various lithologic units. 

Proper evaluation of slope stability can be made only 

by considering such local factors as degree of shearing 

and chemical alteration, steepness of slope and groundwater conditions, as well as the initial state of the rock.

The frequency of types of landslide movement follows. Each type of movement in a complex landslide is 

counted as a separate landslide in the tabulation.

Type of movement Percent of total landslides

Earthflow ._..........__......._.._..._............. .... .... 28

Debris flow . ....................... ... ..... ......... 24

Debris slide .................................................. 22

Rotational slump ........................................ 13

Rockfall ........................................................ 5

Sand flow ...................................................... 3

Block glide .................................................... 2

Mudflow ........................................................ 2

Debris avalanche ........................_.... ........ 1

1 100

The slope angle before sliding ranges widely for 

sandstone of the Franciscan Formation, but it averages 

35°; for greenstone it averages 31°. These averages, of 

course, cannot be used to design slopes at individual 

construction sites. The lower slope angle of greenstone 

probably reflects the tendency of greenstone to alter by 

weathering and related processes more readily than 

does sandstone. Greenstone alteration products, moreover, consist mostly of easily sheared clay minerals 

including large proportions of swelling clay.

Landslide deposits vary widely in composition. 

Because earthflows and debris flows are the commonest 

types of slides, most deposits consist of heterogeneous 

unstratified mixtures of rock, sand, silt, and clay in 

proportions that vary within each landslide and from 

one landslide to another.

In the large landslide half a mile north of Lime Point 

on Marin Peninsula (pi. 3, Nos. 12, 13, 48, 52), most 

of the landslide debris is stabilized, though some of it 

near the bases of the slides has moved since about 1940.

One of the earliest published accounts of slope instability was that of Captain Beechey who visited San 

Francisco in 1827. In his "Narrative of a Voyage to the 

Pacific and Beering's Strait" (Beechey, 1831, p. 345), 

he reported the following at Fort Point: "The fort, 

which we passed upon our right, mounts nine guns, and 

is built upon a promontory on the south side of the 

entrance, apparently so near to the precipice, that one 

side will, before long, be precipitated over it by the 

gradual breaking away of the rock."

ARTIFICIAL FILL '

The practice of creating land by dumping artificial 

fill on the gently shelving tidal flats along the east and 

north margins of the San Francisco Peninsula was 

begun before 1850. Flatland has been at a premium 

since the Gold Rush first made San Francisco a center 

of growth and development. More than 3 square miles 

of the most valuable land in San Francisco originated 

in this way (pi. 1; fig. 62). The average thickness of 

the fill north of China Basin is about 10 feet; south of 

China Basin it reaches a maximum thickness of about 

60 feet. Similarly, large areas of land have been made 

in the Sausalito-Tiburon area. The thicknesses of arti-
ARTIFICIAL FILL 89

p

FIGURE 62. Map showing shoreline of San Francisco in 1853, present shoreline, and areas formerly covered by water 

that are now artificially filled (shaded). Base modified from Chart 627, U.S. Coast and Geodetic Survey (formerly 

U.S. Coast Survey).
90 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

ficial fill at several places are shown on the geologic map 

and cross sections (pi. 1).

The earliest artificial fills consisted of nearby dune 

sand and, less frequently, colluvium and weathered 

rock, which were simply dumped on the soft wet bay 

mud. A seawall, built to prevent the fill from being 

washed away now extends more than 3 miles from Fort 

Mason to China Basin and supports the Embarcadero. 

The first sections of the wall were made by building a 

long ridge of rock and earth. Subsequent better 

designed sections consist of a concrete wall supported 

on piles set in a rock embankment. For a long time, no 

restrictions were placed on the type or quantity of fill; 

spoil from excavations, debris from quarries, dune sand, 

and mud dredged from the bay were dumped indiscriminately. Some parts of the fill area were also used 

for public dumps; so, fairly large deposits of manmade 

debris were added to the mixture, including miscellaneous discarded objects of a large urban population, 

such as shoes, bottles, broken pottery, and bed frames.

The Marina Park fill area in San Francisco was 

created in 1913 for the Panama-Pacific Exposition. 

Treasure Island was created from sandy bay sediments 

bbtained southwest of Blunt Point and piped to the 

site by hydraulic transport. The site was enclosed by 

sheet piling.

Maps showing historical development of reclaimed 

land are found in "Future Developments of San Francisco Bay Area 1960-2020" (U.S. Office of Area Development, 1959, pi. 19), which also includes a discussion 

of reclamation of marshland, tideland, and submerged 

land (p. 75-94), and in "The San Francisco Bay Area 

 A Metropolis in Perspective" (Scott, 1959, p. 37).

The hilly parts of the quadrangle are also dotted 

with thousands of small fills made in the course of road 

and building development. Most of these fills are too 

small to be shown on a map at a scale of 1:24,000, but 

the largest ones are shown. They are generally along 

the downhill sides of major roads or are in large deep 

ravines that are used as dumps for rock and soil waste 

from nearby excavations. A large area of artificial fill 

between Mount Olympus and Golden Gate Park is now 

largely concealed by urban development. Evidence for 

the filling was found by studying logs of boreholes and 

by comparing old and modern topographic maps. The 

comparison showed that the valley was formerly deeper 

and its walls were steeper.

^s

STRUCTURE

The largest conspicuous structural features in and 

bordering the San Francisco North quadrangle are 

northwest-trending faults and shear zones (figs. 1, 63), 

including the San Andreas fault one of the most persistent structural features in California (fig. 6) (Dickinson and Grantz, 1968). The San Andreas fault is 

known to extend more than 600 miles northwestward 

from the Gulf of California to Point Arena. It is the 

boundary between the North American and Pacific 

Ocean crustal plates. The North American crustal plate 

extends to the east of the fault and contains the Franciscan Formation as the oldest known rock in this area; 

the Pacific Ocean crustal plate extends westward and 

contains Cretaceous granitic rocks that intrude older 

gneisses, schists, marbles, limestone, and dolomite. On 

San Francisco Peninsula the Pilarcitos fault, which lies 

west of the San Andreas fault, is the local west boundary of the Franciscan Formation. The San Andreas 

fault intersects the coast north of Mussel Rock, approximately 5]/2 miles south of the quadrangle. The 

Pilarcitos fault intersects the coast at the Pedro Valley 

district of Pacifica, 10% miles south of the quadrangle.

The San Francisco North quadrangle lies near the 

west edge of the crustal plate made up of the Franciscan Formation; the Golden Gate Bridge is about 6^ 

miles east of the San Andreas fault. The Hayward 

fault, another prominent active northwest-trending 

shear zone, lies 10 miles east of San Francisco and, in 

the bay area, is nearly parallel to the San Andreas fault. 

Strong historic earthquakes were centered on the San 

Andreas fault in 1838,1865, and 1906 and on the Hayward fault in 1836 and 1868. Movement along these 

active faults is mostly horizontal and right lateral, such 

that points along the west side of the faults move 

northwestward relative to points along the east side. 

Part of the fault movement is vertical, for small vertical 

displacement of the surface accompanied the 1868 

faulting on the Hayward fault (Radbruch, 1967) and 

the 1906 faulting on the San Andreas fault. Additional 

evidence for vertical movement on these faults is the 

associated fault scarps. The San Andreas fault is at the 

base of a high ridge, Bolinas Ridge, north of Bolinas 

Lagoon, about 12 miles northwest of the quadrangle. 

The Hayward fault lies mostly at or near the base of 

the Berkeley Hills, and small scarps are within the fault 

zone in the Centerville area. At least some of the vertical movement on these faults is post-Pliocene, for the 

faults disrupt Pliocene sediments.

Lawson (1914, p. 14) believed that the San Francisco North quadrangle is part of a structural block in 

which bedrock was tilted down towards the northeast 

from San Bruno Mountain, Bolinas Ridge, and Mount 

Tamalpais to the foot of the Berkeley Hills. The crust 

between San Bruno Mountain and the Berkeley Hills, 

however, probably is cut by numerous faults and probably has not acted as a rigid block. The presence of 

bedrock far above sea level on Angel Island, at El Cerrito, at Potrero San Pablo in Richmond, at Red Rock
STRUCTURE 91

122° 30' 122" 22' 30"

5 SAN FRANCISCO CITY

" AND\J COUNTY

QUATERNARY CRETACEOUS AND JURASSIC!?)

Geology in San Francisco 

South quadrangle modified 

after Bonilla (19651

Surficial rocks

r 

Great Valley

Sandstone and shale

Rocks in the Point

Lobosarea. west

of the City College

shear zone, are

ed to the Great

Valley sequence

Franciscan Formation

r \

m ^^

Radiolarian chert, Sandstone Sheared rocks

greenstone, and and shale Includes fragments

some sandstone of Franciscan

and shale t-'ormaiioti, Great

* \'allcv sequence.

Contact

Anticline Syncline

FIGURE 63. Generalized geology of parts of the San Francisco North and South quadrangles showing major shear zones 

and fold axes. Geology south of latitude 37°45' by M. G. Bonilla (1965).
92 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

Island, and at Coyote Hills near Newark is not compatible with Lawson's tilted-block hypothesis, which 

was based mostly on the presence of bedrock more than 

1,000 feet above sea level at San Bruno Mountain and 

more than 1,000 feet below sea level west of the Berkeley Hills in the vicinity of Berkeley. Gravity observations by Taylor (1957) in San Francisco Bay south of 

the San Mateo Bridge are also incompatible with 

simple northeast tilting of the crust. Cross sections by 

Taylor (1957, pi. 5-7) showed that the top of bedrock 

is generally deepest in the middle of San Francisco Bay 

and that steeply-dipping faults with a strong vertical 

component of displacement break the bedrock into six 

or more blocks. Furthermore, epicenters of many smallmagnitude earthquakes are located in the area between 

the San Andreas and Hayward faults. More than 100 

were recorded in 1968 by the U.S. Geological Survey 

(Roller and others, 1968). Tocher (1959, p. 48) wrote 

that a small shock (magnitude 2.1) in December 1956 

had an epicenter near Yerba Buena Island and noted 

that Townley and Alien (1939) mentioned that many 

shocks were felt on Russian Hill in San Francisco in 

April 1905. An epicenter of a magnitude 4 earthquake 

was determined to be in San Francisco Bay near Red 

Rock Island, 8 miles north of San Francisco (California 

Department of Water Resources, 1964). Thus, instead 

of simply being tilted northeast, the block between 

San Bruno Mountain and the Hayward fault is structurally complex and is cut by many faults, some of 

which are probably active. The position of the bedrock 

is related more to local tectonism within the area rather 

than to simple tilting.

The structure of the Franciscan Formation in the 

San Francisco North quadrangle is poorly known, 

owing in large part to a lack of exposures and also to 

its structural complexities. The character of the formation further contributes to the difficulties, for distinctive and persistent marker beds are lacking, fossils 

are almost nonexistent, lateral variations in texture, 

thickness, and lithology are the rule, and bedding is 

poorly developed in many places. Repeated crustal 

movements have left their mark in the intensity to 

which the Franciscan is sheared and shattered. Almost 

every exposure has one or more sets of slickensided slip 

planes and gouge zones. Moreover, a large part of the 

Franciscan is covered by water, surficial deposits, and 

manmade structures.

The rocks between Fort Point and Potrero Hill (fig. 

62) are believed to be part of a major shear zone that 

extends northwest to the middle of Golden Gate channel and southeast toward Hunters Point about 1 mile 

southeast of the quadrangle. Airborne magnetometer 

studies (U.S. Geological Survey, 1966) suggest that 

this shear zone extends southeast across San Francisco

Bay to a point north of Coyote Hills about 16 miles 

from Hunters Point. Orientation of shear planes in 

Potrero Hill and the Presidio suggest that the zone may 

dip eastward at a low angle. A second shear zone 

exposed at Lands End is believed to be part of another 

major northwest-trending shear zone that is largely 

concealed by dune sand in the San Francisco North 

quadrangle but that is also found in the Sunset Reservoir area at the south edge of the quadrangle. It is correlated with the City College fault mapped by Bonilla 

(1961) in the San Francisco South quadrangle. In the 

central highlands of San Francisco, rocks in the block 

bounded by these shear zones are mostly radiolarian 

chert and greenstone folded primarily on east-west 

axes and secondarily on north-south axes, but rocks 

northeast of the Fort Point-Hunters Point shear zone 

and southwest of the City College shear zone are 

mostly graywacke sandstone and shale folded mostly 

along northwest-trending axes.

The potassium feldspar-bearing sandstone at Point 

Lobos and the Cliff House and the concealed bedrock 

southwest of the City College shear zone at Lands End 

are probably part of the San Bruno Mountain block of 

the Great Valley sequence, which is the same age as 

the Franciscan Formation. The Great Valley sequence 

has either been thrust westward over the Franciscan 

(Irwin, 1964, p. 66; Bailey and others, 1964, p. 123- 

141, fig. 72) or the Franciscan Formation has been 

thrust eastward under the Great Valley sequence 

(Bailey and others, 1970). Thus, the City College shear 

zone may also be the locus of Late Cretaceous thrusting that carried miogeosynclinal rocks of the Great 

Valley sequence over eugeosynclinal rocks of the Franciscan Formation. Later movement along the same 

zone may have been vertical or horizontal.

The rocks between Bakers Beach and the shear zone 

at Lands End consist mostly of graywacke and minor 

radiolarian chert, greenstone, and serpentine. The sedimentary rocks dip northeastward; however, graded 

bedding and channeling indicate that part or all of the 

section is overturned. Similar features in the lithologically similar sedimentary rocks at the north end of 

Bakers Beach indicate that the northeast-dipping sedimentary rocks are not overturned. These attitudes 

suggest the presence of a northwest-trending anticline 

whose axis lies within Bakers Beach. No additional 

evidence for such a fold was seen to the southeast because bedrock is concealed by beach and dune deposits.

A syncline that plunges northwestward along Columbus Avenue between Russian Hill and Telegraph Hill in 

San Francisco is suggested by bedding attitudes and 

by the disposition of an apparently persistent unit of 

shale and thin-bedded sandstone of the Franciscan 

Formation. Orientation of graded bedding and small-
SEISMICITY 93

scale channeling suggest that locally the west limb of 

the syncline is overturned. Opposing flanks of a large 

anticline are suggested by northeasterly dips on Alcatraz Island and southwesterly dips on Telegraph Hill. 

Meager structural data indicate one or more northwesttrending large folds between the Columbus Avenue 

syncline and the belt of sheared rocks between Potrero 

Hill and Fort Point. On Marin Peninsula the immense 

section of southwestward-dipping sediments and greenstone apparently forms the limb of a large anticline 

whose axis lies in Richardson Bay (pi. 1, section A-A'). 

On Angel Island the principal structure is a broad syncline whose axis plunges northwestward to Hospital 

Cove and beyond along the middle of Tiburon Peninsula.

No satisfactory structural correlation exists between 

Marin and San Francisco Peninsulas. The marked contrast in stratigraphy, lithology, and local structures 

between the two suggest that a major fault exists in 

the Golden Gate channel, but no other supporting 

evidence was found, with the possible exception of the 

westward-trending shear zone at Point Diablo. The 

alinement of the Richardson Bay anticlinal axis with 

the axis between Alcatraz Island and Telegraph Hill is 

one possible link. The radiolarian chert and greenstone 

southwest of the Fort Point-Hunters Point shear zone 

may be represented by similar rocks in the Point 

Bonita quadrangle. Rocks in both areas are folded 

mostly on east-west fold axes (Lawson, 1914, geologic 

map of Tamalpais 15' quad.).

Another interpretation of the structural link across 

the Golden Gate is that the Columbus Avenue syncline 

is a part of the Angel Island-Tiburon Peninsula syncline offset 2 miles westward by a northeast-trending 

fault south of Angel Island. Under such an interpretation the Fort Point-Hunters Point shear zone could 

continue in offset as the serpentine of Angel Island and 

Tiburon. However, fieldwork by the author in the Point 

Bonita quadrangle indicates that the Fort PointHunters Point shear zone may continue northwestward 

across the Golden Gate to the shear zone of the Bonita 

Cove area.

SEISMICITY

The San Francisco North quadrangle lies between 

the active San Andreas and Hayward faults and is 

within a region of high seismic activity, in which earth 

tremors are frequent and unpredictable. The five largest earthquakes since 1800, as well as many other 

strong shocks, originated from movement on or near 

these two faults. Parts of the Calaveras fault zone, 

located about 20 miles east of the quadrangle, are also 

active. Epicenters located by seismographs suggest

that other faults in the bay area are also active, b'lt 

data are too meager to identify them.

Surface ground breakage by historic fault movement 

during earthquakes has occurred on the San Andreas 

fault (Lawson and others, 1908) and on the Haywrrd 

fault (Radbruch, 1967). No field evidence of recent 

fault movement has been found in the quadrangle.

Epicenters for small earthquakes in and near the 

quadrangle are shown on maps by Byerly (1951, p. 

159) for the periods 1930-41 and 1947-48 and by 

Tocher (1959, p. 46) for the period 1942-57. None of 

these epicenters are located in the quadrangle, though 

several are shown in nearby areas. The epicenter of 

the strongest shock recorded between 1930 and 1957, 

that of March 22, 1957, Richter magnitude 5.3, is near 

Mussel Rock where the San Andreas fault intersects 

the Pacific Ocean shore 5 l/2 miles south of the quadrangle. Several other epicenters of weaker shocks are 

also near Mussel Rock and west of Golden Gate, generally along the alinement of the San Andreas fault; 

one epicenter is about 1 mile south of Twin Peaks; one 

is on the shore of the bay near Islais Creek; two are 

near Hunters Point, 2 miles southeast and 3 miles 

southwest of the point; and three are within 1-4 miles 

west and northwest of the northwest corner of the 

quadrangle. Except for the March 22, 1957, earthquake, all shocks were only strong enough to rattle 

windows and doors and occurred during the period 

1930-41. During 1968 the U.S. Geological Surrey 

recorded six epicenters of microearthquakes in the 

quadrangle (Roller and others, 1968), all located n^ar 

the northwest shore of San Francisco between Fort 

Mason and Hunters Point.

Earthquake history of the San Francisco Bay area 

is discussed by Tocher (1959, p. 39-48) and Byerly 

(1951, p. 141-160). The five largest known earthquakes are those of June 10, 1836, and October 21, 

1868, which originated from movements along the Hayward fault, and those of June 1838, October 8, 1£35, 

and April 18, 1906, which originated from movements 

along the San Andreas fault. Serious damage to strictures in San Francisco is recorded for the 1865, If-8, 

and 1906 earthquakes. Almost no information is available on damage to structures from the 1836 and 1838 

earthquakes because the population of San Franc:'sco 

was small and structures were few, though the walls of 

the Presidio and Mission Dolores were reported to 

have been seriously damaged by the 1838 earthquake 

(Louderback, 1947).

Effects of the 1906 earthquake have been extensively 

documented in many publications. Some of the more 

comprehensive reports are by Gilbert, Humphrey, 

Sewell, and Soule (1907), by the American Society of 

Civil Engineers (1907), and by Lawson and others
94 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

(1908). Though structures were damaged in all parts 

of the city, much of the damage was moderate. Nevertheless, in the heavily built-up northeastern part of the 

city, from North Point to Townsend Street and from 

the Embarcadero to an irregular line approximately 

along Van Ness Avenue to the intersection of Dolores 

and 20th Streets, destruction by the earthquake and 

the resulting fire was a great catastrophe. About 400 

lives were lost in the city, and estimates of total 

damage range from $350 million to $1 billion. Estimates of the loss due to the earthquake exclusive of 

the fire range from 5 to 20 percent of the total damage.

In San Francisco the relationship between damage 

to manmade structures and geology was investigated 

by Wood (1933, p. 67-82; 1908, p. 220-245; see also 

Lawson and others, 1908, atlas map 19). Though Wood 

postulated a general increase in intensity of shaking 

towards the fault, he found that damage to structures 

was influenced mostly by the geology of their foundations. The least damage was in hilly areas of the city 

where buildings were founded on or near Franciscan 

Formation bedrock. Damage was 5-10 times greater 

in the areas where structures were founded on artificial 

fill lying on bay sediments (Duke, 1958, p. 9). Wood 

found that some of the damage in the filled-in areas 

was caused by settling of poorly consolidated material. 

Another pertinent observation on damage to structures 

in the filled-in areas was made by the American Society 

of Civil Engineers (1932, p. 40): "No class 'A' building 

founded on well-driven piles or well-placed concrete 

piers suffered any material injury due to the earthquake; serious damage or collapse occurred only in 

poorly constructed buildings erected on soft foundations in the 'filled-in' areas." Steinbrugge (1968) discussed relations of building damage to foundation 

material, earthquake risk zoning, and building code ordinances requirements for earthquake resistant design.

For the Alaska earthquake of March 27, 1964, damage was also greatest in areas underlain by thick saturated unconsolidated deposits, whereas there was no 

significant damage to structures founded on indurated 

bedrock or on bedrock with a thin veneer of unconsolidated deposits (Eckel, 1970, p. 29). Damage was 

related more to local geology than to distance from the 

epicenter (Eckel, 1970, p. 29).

A rough appraisal of earthquake stability of the 

geologic units in the quadrangle is given in table 11.

A cooperative effort of the Earthquake Engineering 

Research Institute, the National Science Foundation, 

the National Center for Earthquake Research (NCER) 

of the U.S. Geological Survey, and several State of 

California agencies is being made to place more than 

40 instruments in and near the quadrangle to record 

motion during strong local earthquakes. These instruments will obtain data on shaking characteristics of 

various types of geologic rock units, surficial and bedrock, in various geologic environments.

Measurements made by the National Center for 

Earthquake Research in and south of this quadrangle 

of ground motion generated by underground nuclear 

explosions in Nevada show good correlation of motion 

amplitude with geologic setting of the recording site. 

Maximum horizontal ground velocities at sites underlain by bay mud generally increased with thickness of 

bay mud and were as much as ten times greater than 

those recorded on nearby bedrock (Borcherdt, 1970).

Additional information on the behavior of foundations in the San Francisco Bay area during earthquakes 

is given by Bonilla (1970), Seed (1970), and Cluff and 

Bolt (1969).

ENGINEERING GEOLOGY

The San Francisco North quadrangle, particularly 

the city of San Francisco, is blanketed by various kinds 

of unconsolidated surficial deposits through which protrude scattered patches of more durable bedrock. The 

quadrangle is between two major active faults and is 

therefore subject to frequent earth tremors. Other 

geologic phenomena peculiar to the quadrangle are two 

broad zones of sheared rock, many landslides, and large 

areas of land that have been reclaimed by covering soft 

mud with artificial fill. All these features are of concern 

to the engineering geologist and the engineer because 

of the special problems they pose individually and in 

combination with each other.

The differences in permeability, shearing strength, 

bearing capacity, and other engineering properties 

between bedrock and surficial deposits are generally 

large, and in addition, these properties may vary considerably over short distances within a single bedrock 

or surficial deposit map unit. At some site? on the 

heavily utilized land areas, heavy structures h ave been 

built partly on soft bay mud and partly on hard sandstone of the Franciscan Formation. Proper us^ of such 

sites for foundation purposes demands careful exploration and engineering design. Even construction sites 

entirely on the Franciscan Formation encompass shear 

zones consisting of soft clayey material studded with 

hard rocks. These zones are a few inches to many feet 

wide. Thus, many building sites are founded on rocks 

with differing bearing capacities and resistances to 

seismic shocks.

Wood (1933; 1908) concluded that the damage to 

structures in the 1906 earthquake was controlled 

mostly by the foundation of the building. Puildings 

founded on bedrock were the least damaged, and buildings on well-designed and well-placed piles or concrete
ENGINEERING DEPOSITS 95

footings or mats in soft saturated poorly consolidated 

bay or marsh sediments did not suffer material damage. 

Serious damage occurred only in poorly constructed 

buildings erected on soft foundations in the filled-in 

areas. On the other hand, Steinbrugge (1968, p. 30) 

observed that damage from the July 28, 1957, earthquake in Mexico City, which is founded on soft wet 

expansible lake clays, was mostly to tall reinforced 

concrete buildings, whereas 1-2 story "collapse hazard" 

buildings performed well. These and other observations made in a worldwide study make it apparent that 

the natural period of vibration of the foundation and 

the engineering structure, as well as the nature of the 

earthquake vibrations and the foundation material, 

must be considered in design and construction.

In addition to the increased earthquake effects to 

buildings on artificial fill, differential subsidence is a 

second serious problem. In less than 60 years, poorly 

made fill has settled more than 7 feet (fig. 64) (Bonilla 

and Schlocker, 1966, p. 452). To avert possible subsidence, construction of modern fills should be preceded 

by careful study of the properties and ground-water 

conditions of the fill foundation, and only selected 

materials should be laid under controlled conditions. 

Engineering problems of reclaimed land in the northeastern part of the city are discussed in "Subsidence

FIGURE 64. Differential settlement of buildings on artificial fill 

in former Mission Swamp. Photograph taken shortly after 

street, sidewalk, and sewerline were raised to former level 

before subsidence. Near Sixth and Folsom Streets, San 

Francisco.

and the Foundation Problem in San Francisco" (American Society of Civil Engineers, 1932, p. 24-36). Less 

serious engineering problems in areas of artificial fill 

along the fringes of the bay are raised by the old pilings 

and sunken ships which are found in pile driving and 

caisson sinking. Also, old garbage dumps present 

foundation excavation problems because of the variation in strength and bearing capacity of miscellaneous 

objects in the dump.

Because of the great contrast in engineering behavior 

between bedrock and surficial deposits, the location of 

the buried bedrock surface (pi. 3) and the generalized 

engineering characteristics of all the exposed rock units 

(table 11) are important in planning structures. The 

contours shown on the bedrock map are based largely 

on records of borings made for foundation purposes, 

water wells, and on records of tunnel excavations. Most 

of them are unpublished. Some of the logs are given 

as abbreviated notations on the geologic map (pi. 1), 

and some are published in "Selected Logs of Borings" 

(Institute of Transportation and Traffic Engineering, 

1951) and in "Subsidence and the Foundation Problems in San Francisco" (American Society of Civil 

Engineers, 1932). As a further aid in evaluating the 

exposed bedrock surface for possible construction sites, 

the locations of faults, shear zones, and known landslides are also shown on plate 3.

Engineering properties of San Francisco Bay sediments are discussed briefly in the section "San Francisco Bay Sediments," where the reader will find references to other more detailed discussions. Engineering 

properties of other geologic map units in the quadrangle are discussed by Trask and Rolston (1951), Lee 

(1953), California Department of Public Works 

(1955), and Schlocker (1969; 1970). The reader should 

also consult the sections "Landslide Deposits" and 

"Seismicity" for additional material on engineering 

geology.

In conclusion, each construction site within the 

quadrangle presents a unique combination of such 

conditions as topography, hydrology, and foundation 

materials. Foundation materials, in turn, vary in degree 

of consolidation, fracturing, and chemical alteration. 

The bedrock surface map (pi. 3) and the table summarizing the characteristics of rock units (table 11) 

provide only a general framework on which to base 

plans for particular site investigations. The information 

is generalized, covers a large area, and does not supplant detailed site investigations both field and laboratory that are necessary for evaluating specific sites 

for specific structures. It is hoped that the presentation 

of these data will serve as a reminder to stimulate the 

acquisition of precise geologic data for use in the proper 

location and design of structures.
96 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

TABLE 11. Generalized description of

[Franciscan Formation extends

Name and 

map symbol 

(Pl.l)

Artificial fill 

(Qaf).

Weathering, soil development 

Lithology alteration

Mostly dune sand but None, 

includes silt, clay, rock 

waste, manmade debris, 

and organic waste.

Permeability

High, except where 

clayey materials 

predominate.

Workability

Generally easy to remove 

except locally in tangles 

of ship timbers and other 

manmade debris.

Slope stability

Generally 

fills are 

near or 

table.

low because most 

uncemented and lie 

below the water

Landslide 

deposits 

<Q1, Qlo, 

Qly).

Alluvium 

(Qal).

Modern and 

older beach 

deposits 

(Qb,Qob).

Slope debris 

and ravine 

fill 

(Qsr).

Bay mud and 

clay (Qm).

Variable. Rock pieces of None or very little, 

all sizes in sand, silt, 

and clay matrix.

Gray silty clayey medium No well-developed soils; top

Variable, but 

generally high.

Variable, but generally 

easy to excavate and 

compact.

Dune sand 

(Qd).

Colma

Formation 

(Qc).

Serpentine

1-2 ft of older alluvium has 

abundant plant fragments and 

higher silt content than lower 

portions.

sand; fine to medium 

sand; clayey silt; some 

pebbles. Grades to slope 

debris and ravine fill. 

Older alluvium is rich 

in plant fragments. 

Bedding obscure.

Gray well-sorted medium None. 

to coarse sand. A few 

small gravel beaches.

Light-yellow to reddish- Generally none, but a slight

brown unsorted rock 

fragments, gravel, 

sand, silt, and clay in 

various proportions. 

Clayey parts exhibit 

moderate swelling and 

plasticity when wet.

Gray silty organic (lig- None, 

nitic and diatomaceous) 

clay with minor 

amounts of sand; locally, lenses of sand, 

peat, or shell fragments. Soft and plastic 

near top, moderately 

stiff at depth. Shrinks 

and becomes hard when 

dried during excavation; plastic and swells 

when rewetted.

weathering of top 2-4 ft seen 

in older material on Marin 

Peninsula.

Light-brown to orange 

fine to medium sand 

with minor amounts of 

clay. Evenly spaced 

horizontal or nearly 

horizontal bedding and 

crossbedding. Beds 1-3 

in. thick. Cobble-size 

rubble beds rare. Low 

swelling and plasticity 

when wettedMostly soft friable

sheared rock enclosing 

hard spheroidal knobs 

of unsheared serpentine ( sp). Rarely 

massive and tough 

(sph). Various colors 

but generally greenishgray, blue, or brown.

Generally moderate; Easily excavated with hand

fairly high in sand.

High.

or power equipment, such 

as bulldozer, front-end 

loader, backhoe, or scraper. 

Generally easy to compact, 

except where silt and 

organic content is high.

Easily excavated with hand 

or power equipment. Compacts rapidly; compaction 

density increased by water 

flooding and vibration. 

Owing to high water table, 

deep excavations require 

pumping.

Variable, but Generally easily excavated

generally moderate and compacted with power

to low. equipment.

Slopes cut in landslide

deposits generally unstable. 

Although some undisturbed 

natural slopes of landslide 

deposits are stable for many 

years, sliding may be reactivated by changes in stress 

or strength conditions.

Clayey material stands in steep 

or vertical cuts for several 

months when dry. Subject to 

severe gullying. Sandy 

alluvium unstable in steep 

cuts.

Generally unstable and free 

running, especially on slopes 

greater than about 30°. 

Susceptible to wind and rain 

erosion- Excavation walls 

more than 2 or 3 ft in height 

require support.

Stands in steep to vertical cuts 

for several months when dry. 

Generally unstable and 

prone to sliding when wet. 

Gullying severe.

moderate stability at 1:1 cut 

slope for foundation excavations for several months 

during dry season.

Yellowish-brown to light- Slight to none. Most grains coated 

gray well-sorted fine to with film of iron oxide. Minor 

medium sand. Quartz, amounts of carbonaceous plant 

feldspar, and horn- matter disseminated locally in 

blende are chief top 2-3 ft. 

minerals.

Very low, except Excavated with power equip- Generally unstable; where 

sand lenses. ment, such as dragline and above water table, has 

clamshell bucket, bulldozer, 

front-end loader, or backhoe. Sheet piling generally 

required for excavation. 

Highly compressible, 

difficult to compact. One 

method of placing artificial 

fill is to push aside surface 

layers of soft mud to avoid 

trapping it below the fill. 

Another method is to place 

a thin layer of fill on the 

undisturbed top layer of 

mud that has been somewhat consolidated by drying for a few years.

Moderate to slight soil development; in places soil identified 

only by presence of organic 

matter, increase in silt and clay 

content, and iron staining.

High. In most places 

zone of saturation 

is deep.

Variable from low to 

high in adjacent 

beds. Found above 

and below water 

table.

Easily excavated. Compacts 

rapidly; compaction 

density increased by water 

flooding and vibration.

Easily excavated by hand or 

power equipment. Scraper 

with light ripping or no 

ripping used on large 

grading jobs. Easily 

compacted.

Generally unstable and free 

running. Slopes steeper than 

30° generally unstable. 

Susceptible to wind and rain 

erosion. Lagging required 

to support excavation walls 

more than 2 or 3 ft high.

Fair to good, except for siltand clay-free layers, which 

are unstable in cut slopes 

greater than 30°. Excavated 

vertical faces stand for 

several weeks to several 

months when dry.

Soil generally absent or less than Low to moderate 

1 ft thick. Locally, 5-10-ft-thick 

mantle of dark-gray clayey 

material containing much highswelling montmorillonite; 

probably derived from sheared 

and hydrothermally altered 

serpentine. This material is 

"adobelike" and develops deep 

shrinkage cracks on drying.

Sheared serpentine excavated 

readily with power equipment. Massive serpentine 

may require heavy ripping, 

blasting, or equivalent. 

Mixtures of massive and 

sheared serpentine 

excavated with light to 

moderate ripping.

Massive serpentine stable in 

steep or vertical cuts. Cut 

slopes in sheared serpentine 

should not exceed 1:1, and 

embankment height should 

be limited. Serpentine at and 

near other bedrock types 

generally sheared and 

altered, and slope stability 

low. Nodules of hard 

serpentine tend to fall out 

of sheared matrix.
ENGINEERING DEPOSITS 97

engineering properties of map units

from KJss through KJm]

Earthquake stability

Shearing strength; 

foundation conditions

Possible or 

reported use

Unit weight 

(pounds per 

cubic foot) 1

Unified soil 

classification 

group symbol2

Poor to fair. Most movement Generally moderate shearing strength, but

where thick, poorly compacted, 

and overlying soft bay mud 

and clay. Fair where thin, well 

compacted, and overlying firm 

materials.

Low.

exceedingly variable depending on 

composition, method of placement, age, 

thickness, underlying material, and 

history following placement, such as 

ground-water conditions, loading and so 

forth. In 1906 earthquake the greatest 

damage to structures was inflicted in 

areas of artificial fill overlying bay mud 

and clay along east shore of city.

Generally unsuitable for foundations.

Used extensively for construction Variable within Variable within, 

material and foundation purposes. wide limits. wide limits. 

SeePutnam (1947, p. 271-278) for 

estimating storm wave heights in 

San Francisco Bay and for planning 

building locations on artificial fill on 

edge of bay.

Rockfalls possible local source of Highly variable. Variable. GC to 

pervious fill, riprap, and so forth. CH.

Moderate. Moderate to high in sandy alluvium; low 

where deposits are predominantly clay 

and silt or high in plant fragments. 

To safeguard foundations, clay-filled 

surface and subsurface channels should 

be adequately drained.

Surface material possible source of 

topsoil for lawns and gardens.

100-114 SM, SC, SW, OL.

Probably moderate. Moderate to high shearing strength where 

confined. Susceptible to wave erosion on 

beach. Uniform sands have lower 

shearing strength than well-graded sands.

Blending sand for concrete aggregate; 

fill.

105-110 SP.

Moderate. Variable. Deposits with relatively high clay Fill, 

content are weak and plastic when wet; 

sandy and gravelly deposits have 

moderate to high strength.

104-124. CL, SM, SC, rarely 

GC and GM.

Very low; structures erected on 

artificial fill overlying bay 

mud and clay severely 

damaged in earthquakes of 

1865, 1868, and 1906.

Low shearing strength. An older bay clay, 

lying below Colma Formation, is firm 

and preconsolidated in most places and 

has moderate shearing strength.

Although of very poor quality, bay mud 43-98 (older bay 

and clay have been used for fill behind mud). 

part of Embarcadero seawall. Recent 

shell deposits and underlying clay''and 

mud dredged from bay for manufacture of cement. Blended with better 

quality clay to make structural clay 

ceramic products. May be suitable for 

making foundry sand.

CL, CH.

Probably moderate. Moderate to high shearing strength when 

confined.

Fair quality fill. Admixed with clay to 65-102 at surface; 

make foundry sand. Small tonnages 110 compacted, 

used as blending sand in concrete 

aggregate.

SP.

Probably moderate to high. Moderate to high shearing strength. Used 

for pile and caisson support.

Good quality fill. 105-130. Mostly SP; som3 is 

SC, SM.

High. Shearing strength high in massive rock but Massive and moderately sheared

decreases with increasing proportion of 

shearing and alteration. Thoroughly 

sheared and altered serpentine has low 

shearing strength. Veins of soft altered 

material in hard serpentine may present 

special problems.

serpentine widely used for fill; highly 

sheared and altered rock is unsuitable.

78 (sheared and 

altered) 

-158 (massive) 

(See table 8).

Some is CL, CH.

See footnotes at end of table, p.

534-039 O - 74 -
98 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

TABLE 11. Generalized description of

Name and

map symbol

(Pi. 1) Lithology

Weathering, soil development, 

alteration Permeability

Observed weathering depth is less Low, except where 

than 3 ft. Moderately altered in fractured, 

places, mostly to prehnite. 

Weathered rock is speckled 

brown and orange and ranges 

from crumbly to moderately 

hard; generally nonswelling.

Maximum depths of weathering 

observed, 60 ft; average 30 ft. 

Soils well developed locally, with 

1-18-ft-thick B horizon of sandy 

clay. A and B horizons high in 

swelling clay minerals 

montmorillonite and vermiculite, 

but sand content keeps swelling 

low to moderate. Partly altered 

rock, C horizon, is brown or 

orange, friable, and nonswelling.

Workability Slope stability

Generally low; 

moderate to high 

fractured rock.

Gabbro and Gray coarse- to finediabase grained equigranular 

(gb). and diabasic igneous 

rock that occurs as 

segregations in 

serpentine.

Sandstone Gray tough nonporous 

(KJss). fine- to coarse-grained 

thick-bedded sandstone 

and minor thin-bedded 

shale. Conglomerate 

beds rare. Pervasive 

nearly random fracturing to J/£-2 in. blocks, 

but locally, blocks 

between major fractures are 1 ft or more 

in size.

Shale and Dark-gray shale interthin-bedded bedded with dark-gray 

sandstone fine-grained sandstone. 

(KJsh). Beds generally 2-5 in.

thick; paper-thin

laminations locally

common. Sheared rock

is slaty or reduced to

soft, friable, and

mashed material

enclosing hard nodules.

Radiolarian Reddish-brown alternate Slight; cherts whiten from removal Low, except where 

chert and beds of ha_rd chert, of iron and become more broken fractured, 

shale 1-5 in. thick, and along joints; shale more broken 

(KJc). brittle friable shale as than chert. Hydrothermal

much as % in. thick. alteration to orange and white. 

Includes thick irregular but durable, rock; pronounced in 

masses of unstratified some places, especially in fault 

chert with brecciated zones, to white plastic clay of 

structure. Some altered little or low swell when wetted, 

chert is badly fractured 

and splintery.

Maximum depth of weathering Low, except where 

observed, 30 ft. Well-developed fractured, 

soils with thick clayey B horizon. 

A and B horizons of moderate 

to high swelling and (or) 

plasticity when wetted. Altered 

and sheared shale generally of 

high swelling and (or) plasticity 

when wetted.

Greenstone Greenish-gray aphanitic 

(KJg). to medium-grained 

volcanic rocks. Predominantly basalt 

flows, agglomerates, 

and tuffs. Pillow lavas, 

locally interbedded 

with radiolarian chert, 

are common. Most 

natural exposures are 

reddish-brown soft 

crumbly altered rock; 

hard tough unaltered 

rock limited to deep 

excavation. Some rock 

altered to soft clay that 

swells when wet.

Metamorphic Hard fine- to coarserocks grained slate, schist, 

(KJm). and granofels.

Maximum depth of weathering 

observed, 40 ft. Soil well 

developed and reddish brown or 

grayish orange in color, containing iron-rich swelling clay, 

though swelling and plasticity 

low to moderate when wetted. 

Hydrothermal alteration 

common to clayey material 

containing halloysite and of low 

to moderate swelling and 

plasticity when wetted.

Low, except where 

fractured.

Only moderate to slight soil 

development observed in 

quadrangle. Partly altered rock 

on Angel Island contains much 

vermiculite.

Low, except where 

fractured.

Sheared rocks, Hard rocks of the Fran- Moderate to well-developed soil. Low.

enciated much as hundreds of 

(Ks). feet in diameter, in a 

soft and crumbly 

matrix of sheared shale 

and serpentine.

common. Swelling and plasticity 

on wetting is moderate to high 

in shale and serpentine matrix 

as well as in soils and hydrothermally altered materials.

Generally requires heavy 

ripping or blasting except 

where altered.

Altered rock excavated 

readily by ripping. Backhoe trenching is generally 

slow; highly altered rock 

trenched rapidly by backhoe. Fresh to moderately 

fresh rock generally 

excavated by heavy ripping; massive rock usually 

requires blasting.

Variable; high where fresh; 

moderate to low where 

altered.

Fresh or moderately fresh rock 

stable in vertical cuts. Blocks 

may fall from vertical faces 

in jointed sandstone. 

Moderately sheared and 

fractured sandstone stable in 

cut slopes of 55°. Badly 

altered, sheared, fractured 

sandstone tends to slump and 

slide, especially when wet.

Fresh rock excavated by 

moderate to heavy ripping; 

blasting required in s_ome 

places. Support required 

for excavation walls and 

tunnels. Altered rock 

excavated by ripper, frontend loader, bulldozer, or 

equivalent.

Steep cut slopes tend to rave], 

but are fairly stable for long 

periods except where extensively sheared, fractured, 

and altered. Slid'ng likely 

on bedding dipping in same 

direction as cut slope.

Bedded chert generally 

excavated by moderate 

ripping. Massive chert may 

require blasting.

Altered greenstone can be 

excavated by light to 

moderate ripping. Fresh 

massive greenstone 

requires heavy ripping or 

blasting.

Generally excavated by 

heavy ripping or blasting.

Soft material easily 

excavated by light to 

moderate ripping; large, 

hard inclusions generally 

require heavy ripping or 

blasting.

Generally stable in steep cuts, 

except for mino~ raveling. 

Sheared and hydrothermally 

altered zones may slide in 

steep cuts. Dip slopes should 

be cut at lower angle than 

d_ip of beds. Commonly chert 

lies on badly altered greenstone and may slide on slip 

surface developed in greenstone.

Fresh and moderately fresh 

rock stable in steep cuts, but 

lava pillows may fall out of 

weak matrix. Alfred rock 

stable at 1:1 or g-entler 

slopes depending on degree 

of alteration and1 fracturing 

and such local conditions as 

ground water and height of 

slope.

Steep cut slopes ar-? stable. Dip 

slopes should be cut lower 

than angle of scl fstosity.

Generally low, especially when 

wet (matrix); stable in steep 

cuts where hard rock 

inclusions are large enough.
ENGINEERING DEPOSITS 99

engineering properties of map units Continued

Earthquake stability

Shearing strength; 

foundation conditions

Possible or 

reported use

Unit weight 

(pounds per 

cubic foot) 1

Unified soil 

classification 

group symbol2

Generally high. Generally moderate to high shearing 

strength.

Good quality fill. Possible limited source 180-192 (fresh); 

of concrete aggregate and large size 160 (altered). 

riprap.

High. Shearing strength is high except in badly 

shattered and altered rock.

Fresh rock suitable for good quality 

fill, road metal, riprap, concrete 

aggregate. Moderately altered rock 

suitable for fill. Highly altered rock 

has been used for impervious lining 

of reservoir.

128-144.

High in fresh rock. Probably 

moderate in thoroughly 

sheared or altered rock.

Shearing strength high in fresh rock. 

Foundations on badly sheared, altered 

rock may require pile support.

Fresh rock and much moderately 127-142. 

altered rock suitable for fill. Calcined, 

expanded shale is source of lightweight aggregate. Shale used to make 

common bricks. Possible use in 

manufacturing of cement.

High. Generally moderate to high shearing 

strength, except where badly altered.

Widely and successfully used as fill 

and road metal. Suitability of chert 

for concrete aggregate questionable 

(Goldman and Klein, 1959).

166 (fresh);

151 (moderately 

altered); 110 

(badly altered).

High. Shearing strength high in relatively fresh 

rock, low in altered clayrich rock.

Moderately altered greenstone with 

associated chert and shale, known 

locally as "redrock," is used as fill and 

road metal. Fills of badly altered 

greenstone are prone to sliding on 

moderate or steep slopes. Relatively 

fresh rock is possible source of 

concrete aggregate and riprap.

113 (thoroughly 

altered) to 

185 (fresh).

High. High shearing strength except where 

parallel to cleavage or schistosity.

Good quality fill, road metal, largesize riprap, concrete aggregate.

169-195.

Moderate. Matrix has low to moderate shearing Used extensively for low quality fill. 78-110 (matrix); 

strength. Large rock fragments found in Hard rock inclusions used for good 125-170 (rock 

exploratory borings may give false quality fill and large riprap. fragments). 

impression of sound foundation 

conditions.

Some is CL, CT-T.

'Pounds of dry material per cubic foot of original material.

Classification used by U.S. Bureau of Reclamation (1953) and U.S. Army Corps of Engineers (1953).
100 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

REFERENCES

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INDEX

[Italic page numbers indicate major references]

A Page

Age, beach deposits............ ............................... 75

chert, radiolarian ...................................... 48

clastic sedimentary rocks ........................ 27

clay .............................................................. 84

Colma Formation ...................................... 7S

dune sand .................................................... 80

Franciscan Formation ............................ 9

gabbro .......................................................... 66

mud .............................................................. 84

ravine fill .................................................... 81

serpentine .................................................. 65

shale, radiolarian ...................................... 48

slope debris ................................................ 81

surficial deposits ..... .................................. 66

Agglomerate .................................................... 32, 36

Alameda Creek, sediments .............................. 82

Alameda Formation, obsolete name ............ 72

Alamo Square .................................................... 3

serpentine .................................................. 56

Alcatraz Island, clastic

sedimentary rocks ....... . ....... 26

fossil locality .............................................. 27

metamorphosed sedimentary rocks ...... 21

sand bars .................................................... 82

sandstone detrital grains ........................ 15

sandstone matrix ...................................... 14

sediment deposition area .................... 82

structure ...................................................... 93

Alien, J. E., quoted .......................................... 74

Alluvium .......................................................... 66, 85

Alteration, hydrothermal, greenstone ........ 29,

33, 36 

hydrothermal, radiolarian chert and

shale .............. .... .... ......... ... .... 45

sandstone ............................................ 18

serpentine .......................................... 62

shale .................................................... 20

Angel Island .................................................... 3, 5

beach deposits .......................................... 76

chert, radiolarian ..... . .................... ..... 48, 49

clastic sedimentary rocks ........................ 26

Colma Formation .... . ........... ........ 67, 69, 70

conglomerate .............................................. 20

dune sand ................................................ 78, 80

Franciscan Formation ............................ 9

gabbro ........................................................ 65, 66

greenstone ............................ 29, 30, 32, 33, 35

metamorphic rocks .............. 50, 52, 53, 54, 55

metamorphosed sedimentary rocks.... 20, 21,

	22 

sandstone detrital grains ........................ 15

sandstone matrix ... .................................. 14

sediment deposition area ......... .............. 82

serpentine .. ............. 51, 56, 57, 58, 60, 64, 65

antigoritic .......................................... 59

shale, radiolarian .................................... 49

structure ...................................................... 93

Arkose .................................................................. 14

Aromas Red Sands .... ....................................... 74

Artificial fill ................................ 66, 76, 81, 8S, 94

Bakers Beach .................................................... 3

beach deposits ............................................ 76

clastic sedimentary rocks ...................... 24

Page 

Bakers Beach Continued

dune sand .................................................... 80

Franciscan Formation ............................ 9

metamorphic rocks .. ........................... 50, 53

sandstone detrital grains ........................ 15

serpentine ................................................ 56, 65

structure ...................................................... 92

Barlow, I. H., analyst ...................... 19, 22, 35, 43

Basalt ................................................................ 29, 32

Bastite .................................................................. 58

Beach deposits ................................................ 74, 76

Bedding ...................................................... 39, 67, 92

sandstone .................................................... 11

Bedrock .............................................................. 9, 94

Belvedere Island ................................................ 3

beach deposits ............................................ 76

chert, radiolarian ...................................... 49

clastic sedimentary rocks ........................ 26

Colma Formation ...................................... 70

conglomerate .............................................. 20

greenstone .................................... 29, 30, 35, 36

metamorphic rocks .................... 50, 53, 54, 55

metamorphosed sedimentary rocks ...... 20

sediment deposition area ............. ........ 82

serpentine ................................................ 56, 61

antigoritic .......... ........ ... ................ 59

shale, radiolarian ............ ......................... 49

Berkeley Hills, metamorphic rocks .............. 52

structure ...................................................... 90

Black Point ........................................................ 3

Bloxam, T. W., quoted .................................... 22

Blunt Point, artificial fill .............................. 90

clastic sedimentary rocks ...... .... ............ 26

greenstone ........................ ......... ............... 29

metamorphic rocks ............................... 53, 54

metamorphosed sedimentary rocks ........ 21

Bolinas Ridge, structure ................................ 90

Bonita Sandstone, obsolete name .................. 10

Botts, S. D., analyst ....................... 19, 22, 35, 43

Bramlette, M. N., quoted . .............................. 48

Breccia ....... ...................................................... 30, 36

Brewer, W. H., quoted .......... . ................... 85

Broadway Tunnel area, Colma Formation .. 67 

shale ............................. ......... .................... 20

Buena Vista Park, clastic sedimentary

rocks ........ ................................... 24

dune sand .................................................... 78

greenstone .............. .............. .................... 34

Buri-Buri Ridge, radiolarian chert .............. 48

Cahil Sandstone, obsolete name ............ 10, 25, 28

Calaveras fault zone ....... . . ........................... 93

California Current .......................................... 5

Campbell Point, clastic sedimentary

rocks . ......................................... 26

greenstone ................................................ 32, 36

metamorphosed sedimentary rocks ...... 22

sediment deposition area ....................... 82

Carbonaceous matter .................. 15, 19, 45, 69, 83

Carquinez Strait, sediments ............. ............ 82

Chert .... ............................................................. 27, 86

radiolarian ............ 9, 24, 26, 33, 36, 43, 76, 81,

87, 92, 93 

origin .................................................. 47

Chert grains, sandstone .................................. 15

China Basin, artificial fill ............................ 88, 90

Page

Chloe, Gillison, analyst .................... 19, 22, 35, 43

City College fault .................................... 24, 56, 92

City College shear zone ........................ 50, 55, 64

Clay ............................................................ C6, 82, 83

Cliff House area, clastic sedimentary

rocks ............................................ 24

Colma Formation ...................................... 70

Franciscan Formation ............................ 9

structure ...................................................... 92

Climate ................................................................ 5

Coast Miwok Indians ...................................... 6

Coast Range thrust fault .............................. 9

Coast Ranges ........................................ 3, 26, 28, 56

Colluvium ........ .............................................. 66, 90

Colma Formation .............................. 66, 77, 81, 86

Conglomerate .... ... ........................................ 20, 26

Corinthian Island .............................................. 3

clastic sedimentary rocks ........................ 26

metamorphosed sedimentary rocks ...... 21

Corona Heights, radiolarian chert

and shale .................................... 49

sandstone detrital grains ........................ 14

Correlation, clay ................................................ 84

Colma Formation .................................. 73, 74

mud .............................................................. 84

Costanoan Indians ............................................ 6

Crocidolite .......................................................... 30

D, E

Debris flows .......................................................... 87

Deformation, shale ................. .......................... 20

Depositional environment, clastic

sedimentary rocks .................... 26

Colma Formation .................................. 71, 72

Detrital grains, sandstone ............................ 14

Detrital source, clastic sedimentary

rocks ............................. .............. 27

Colma Formation ...................................... 72

Dikes ............... .................................... 27, 36, 51, 66

Dune sand ................................ 3, 66, 71, 75, 78, 90

Earthflows .......................................................... 87

Earthquakes .............................................. 90, 93, 94

Elmore, Paul, analyst ...................... 19, 22, 35, 43

Engineering geology ......................... ............ 94

Engineering properties, mud and clay ........ 84

Exploration ........................................................ 6

Faults ......... . ...................................................... 90

Ferry Building area, Colma

Formation .......................... 70, 72, 74

Fieldwork ............................................................ 7, 8

Flows ..... ........... .............................................. 29, 36

Folds ........... .... ................................................... 39

Foliation .................................................... 21, 51, 52

Fort Baker, clastic sedimentary rocks ........ 26

greenstone pillows .................................... 30

radiolarian chert........................................ 39

Fort Mason, artificial fill ................................ 90

Colma Formation ...................................... 70

seismic activity .......................................... 93

Fort Point .......................................................... 3

beach deposits ........................................... 76

clastic sedimentary rocks ...................... 25

dune sand .................................................... 80

107
108 INDEX

Page

Fort Point Continued

Franciscan Formation ............................ 9

radiolarian chert and shale .................... 49

serpentine .................................... 56, 62, 63, 65

structure .................................................. 92, 93

Fort Point-Hunters Point shear zone .... 50, 55,

64,92

Fort Point Rock, beach deposits .................. 76

Fort Scott, serpentine ... .................................. 56

Fossils:

Bison ............................................................ 74

Bryozoa ................ ....................................... 83

chert ................................ -.... .. .. .. 44

clastic sedimentary rocks ........... ............ 27

clay .............................................................. 84

Colma Formation ...................................... 73

Diatoms ...................................... 47, 73. 83, 85

Douvilleiceras .............................. 12, 27. 49, 65

sandstone detrital grains ................ 15

echinoids .................................................... 77

Foraminifera .............................................. 83

Franciscan Formation ............................ 9

Inoccramus ellioti .................................... 28

Juniperus californica .............................. 73

Lucina alcatrazis ..................................... 28

Mantelliceras .......... ................................... 27

Marthasterites tribrachiatus ................ 28

mollusks .................................. 26, 74, 77, 83, 85

mud .............. ............................................... 84

Mylodon ...................................................... 73

Paladmete perforata ................................ 27

plant fragments ........................................ 83

Radiolaria ................ 15, 20, 26,28, 43, 44, 45,

48, 83 

shale ............................................................ 45

sloth fragment ...............>........................... 73

sponge spicules .................... 43, 45, 48, 73, 83

Fractures .......................................................... 30, 40

Franciscan Formation ................ 9, 55, 88, 90, 94

structure .................................................... 92

Franciscan Group, obsolete name .............. 10, 28

Gabbro ..........................

Glen Ellen Formation 

Gold ................................

............................ 65

........................... 74

........................... 77

Golden Gate ........................................................ 3

clastic sedimentary rocks ........................ 25

landslide deposits ...................................... 86

sandstone detrital grains ........................ 15

seismic activity ......................................... 93

serpentine .................................................. 56

Golden Gate channel .................................... 3, 5. 56

beach deposits ............................................ 76

formation .................................................... 74

greenstone ................................. ............ 29, 31

sandstone detrital grains ......... .............. 15

sediment deposition area ........................ 82

sediments .................................................... 81

structure ..... ............................................ 92, 93

Golden Gate Park .............................................. 3

artificial fill ................................................ 90

dune sand .................................................... 80

radiolarian chert and shale .................... 49

Grand View Park, dune sand ........................ 78

Granitic rock grains, sandstone .................. 15

Granitic rocks .... ............................................... 27

Granodiorite, as Franciscan Formation

source ............... ... ..................... 27

Granofels ............................................................ 50

Graywacke ............................................ 9, 14, 15, 92

Great Valley ...................................................... 9, 72

Great Valley sequence ...................................... 92

Greenstone ................ . 9, 26, 29, 81, 86, 88, 92, 93

H

Hayes Valley .................................................... 3

dune sand .................................................... 78

Hayward fault ................................................ 90, 93

Page 

Heavy-mineral analyses, sandstone .............. 15

Hinde, G. J., quoted ........................................ 48

Horseshoe Bay, alluvium ................................ 85

greenstone ............................ 29, 30, 32, 33, 34

radiolarian chert and shale .................... 49

Hospital Cove, clastic sedimentary

rocks ............................................ 26

Colma Formation . .... ............................... 70

conglomerate .............................................. 20

metamorphosed sedimentary rocks .... 21, 22

radiolarian chert and shale .................... 49

structure .................................................... 93

Huichica Formation ........................................ 74

Hunters Point ................................................. 93

Franciscan Formation ............................ 9

seismic activity .......................................... 93

serpentine .................................................. 62

structure .................................................... 92

Hunters Point-Fort Point

shear zone .................... 50, 55, 64, 92

I, J, K

Ingleside Chert, obsolete name ...................... 10

Introduction ......................................... ............ 2

Iron oxidation, greenstone ............................ 30

Islais Creek, seismic activity ........................ 93

Jadeitization .................................................... 22, 36

James D. Phelan Beach, clastic

sedimentary rocks ................... 25

fossil locality .............................................. 27

radiolarian chert and shale .................... 49

sandstone .................................................... 12

serpentine ................................................. 56

Joints .......................................................... 12, 18, 30

Kahn Playground, Colma Formation .......... 70

Klamath Mountains, Franciscan

Formation border ...................... 9

Knox Point, greenstone .......................... 29, 32, 35

metamorphic rocks .................................. 55

metamorphosed sedimentary rocks .... 21, 22

Lafayette Square .............................................. 3

Lake Merced area, Colma Formation .......... 67

Laguna Honda, clastic sedimentary rocks .. 24 

fossil locality ............................................. 28

radiolarian chert and shale .................... 50

shale ............................................................ 20

Lands End, beach deposits ............................ 76

clastic sedimentary rocks ........ ............... 24

greenstone ................................................ 29, 34

landslide deposits ......... ............................ 87

metamorphic rocks ................................ 50, 54

radiolarian chert and shale.................... 49

serpentine .................................................. 56

structure ...................................................... 92

Lands End Point Lobos ................... ............ 3

sheared rocks ............................................ 56

Landslides .............................. 11, 56, 66, 77, 86, 94

Lime Point, clastic sedimentary rocks ........ 25

greenstone ........................................ 29, 31, 34

landslides ................................... ................ 87

radiolarian chert and shale .................... 49

sandstone matrix ...................................... 14

Lincoln Park, serpentine ................................ 56

Location ... .......................................................... S

Lone Mountain, greenstone ........... ............ 30, 33

sheared rocks .............................................. 56

M

Mack, M. S., analyst ........................................ 22

Manganese ...................................................... 37, 40

Manganese oxidation, greenstone ................ 30

Mapping, methods ............................................ 8

Marin Peninsula .............................................. 3

alluvium ...................................................... 85

chert, radiolarian ............................ 37, 39, 49

clastic sedimentary rocks ...................... 25

Page 

Marin Peninsula Continued

Colma Formation ......................... ............ 70

fossil locality ........................................... 27

greenstone .................................... 29, 31, S3, 35

landslides .... ............................................... 87

metamorphosed sedimentary rocks ...... 21

ravine fill .................................................... 81

sandstone detrital grains ....................... 14

sandstone matrix ...................................... 14

shale, radiolarian .................. ............... 37, 49

slope debris ................................................ 81

structure .................................................... 93

Marin Sandstone, obsolete name ................ 10, 28

Marina Park, artificial fill .............................. 90

Masonic Temple area, sandstone .................. 12

Matrix, Colma Formation .............................. 69

sandstone .................................................... 1Z

serpentine .................................................. 57

Merced Formation .................................. 67, 71, 77

Merritt Sand .............................................. 73, 74, 84

Metamorphic rock grains, sandstone .......... 15

Metamorphic rocks ............................ 25, 27, 50, 55

Metamorphism .... ..................................... 21, 29, 36

Millerton Formation ........................................ 74

Mission Dolores, clastic sedimentary rocks 24 

seismic activity ........................... ............ 93

Montezuma Formation .................................... 74

Mount Caroline Livermore ............................ 5

clastic sedimentary rocks ........................ 26

greenstone ................................................. 35

Mount Olympus, artificial fill ........................ 90

clastic sedimentary rocks ...................... 24

radiolarian chert and shale .................... 49

Mount Sutro ...................................................... 3

clastic sedimentary rocks ...................... 24

Colma Formation ...................................... 70

dune sand .................................................... 78

Franciscan Formation ............................ 9

radiolarian chert and shale .................. 49, 50

sheared rocks .............................................. 56

Mount Tamalpais ........................................... 5

structure ...................................................... 90

Mountain Lake, Colma Formation ................ 70

Mud, bay .............................................. 66, 82, 8S, 94

Mussel Rock, seismic activity ..................... 93

N

Napa River, sediments .................................. 81

Nob Hill, clastic sedimentary rocks ............ 23

Colma Formation ...................................... 70

dune sand ................................................ 78, 80

sandstone detrital grains ........................ 15

North Point, Colma Formation .................... 70

O

Ocean Beach ..........

beach deposits 

dune sand ........

sand source ....

Olivine ....................

................................. 3

................................. 76

.................................. 80

.................... ............ 77

.................................. 58

Origin, chert, radiolarian ............... ............ 47

clastic sedimentary rocks ...................... 26

clay ... .......................................................... 85

Colma Formation ...................................... 71

dune sand ................................... .............. 80

gabbro ......................................................... 66

greenstone .................................................. 36

landslides .................................................... 86

metamorphic rock .................................... 51

mud .............................................................. 85

San Francisco Bay sediments ................ 81

serpentine ................................................ 58, 64

shale, radiolarian .................................... 48

Orthopyroxene .................................................. 58

Peninsula Point, sediment deposition area 82 

Phosphate nodules, shale ................................ 20
INDEX 109

Page 

Phyllite ................................................................ 50

Pilarcitos fault .................................................. 90

Pillows .................................... 26, 29, 30, 32, 36, 76

Plugs ... ............................................................ 36, 65

Point Bonita, Franciscan Formation .......... 9

Point Diablo, greenstone ............................... 29

sheared rocks ............................. ............. 56

structure .............................. ..................... 93

Point lone, sandstone detrital grains ........ 16

Point Lobos ......................................... .............. 3

dune sand .................................................... 80

structure ......................................... ............ 92

Point Lobos-Lands End ................................. 3

sheared rocks ........................................... 56

Posey Formation .............................................. 84

Potassium feldspar, relation to age,

clastic sedimentary rocks ........ 28

Potrero Hill ....... ........................ ..................... 3

clastic sedimentary rocks ...................... 26

gabbro ....................................................... 65, 66

serpentine ........................ ........... 56, 60, 63, 65

sheared rocks ................................. ........ 55, 56

structure .................................................. 92, 93

Precipitation .................................................... 5

Presidio .............................................................. 6

beach sand .................................................. 74

clastic sedimentary rocks ........................ 25

Colma Formation ............................ 67, 70, 72

gabbro ......................................................... 66

greenstone .................................................. 34

sandstone detrital grains ....................... 15

seismic activity ......................................... 93

serpentine ................................................ 56, 65

structure ..................................................... 90

Previous work .................................................. 7

Pyroxenite, in serpentine ............................. 65

Q

Quarry Point, dune sand ..........................

metamorphic rocks ..................... ......

metamorphosed sedimentary rocks

sandstone detrital grains ..................

sandstone matrix ................................

Quartzito ......................................................

.. 78

.. 50

21,22

15

.. 14 

.. 27

R

Raccoon Strait .................................................. 5

sand bars .................................................... 82

Rainfall ............................................................. 5

Ransome, F. L., quoted ................................ 20, 54

Ravine fill ........................................................ 80. 86

Richards, H. G., quoted ....................... .......... 74

Richardson Bay .................................. ............. 3

Franciscan Formation ............................ 9

structure .................................................... 93

Richmond District ............................................ 3

Rincon Hill ................. .................................... 3

clay .............................................................. 84

dune sand .................................................. 80

greenstone ................................. ........... 33, 34

Rockslides ......................................................... 87

Rodeo Lagoon .................................................... 5

Russian Hill ....................................... .............. 3

clastic sedimentary rocks ........................ 23

Colma Formation ............................ 67, 69, 70

greenstone .................................................. 33

sandstone matrix ...................................... 14

structure .................................................... 92

Sacramento River ............................................ 3

ancient .............................................. 71, 74, 77

sediments .................................................. 81, 85

San Andreas fault .......................................... 90, 93

San Antonio Formation .............................. 73, 84

San Bruno Mountain, clastic

sedimentary rocks .................... 24

structure...................................................... 90

Page 

San Francisco .................................................... 60

clay ............................................. ................ 84

clastic sedimentary rocks .................... 23, 24

chert, radiolarian ................ ................. 39, 49

Colma Formation ............................ 67, 69, 70

dune sand ................ .................. .............. 80

greenstone .................................................. 34

metamorphic rocks ................................. 53

ravine fill .................................................... 81

serpentine .......................................... 56, 58, 62

shale, radiolarian .................................... 39, 49

slope debris ................................................ 81

structure .................................................... 92

San Francisco Bay .............................. .......... 3

sediments ..................................... ............ 81, 94

San Francisco Peninsula ................... ............ 3

metamorphosed sedimentary rocks ...... 21

structure .................................................... 93

San Joaquin River .......................................... 3

ancient ........................................................ 71

sediments .................................................... 81

San Pablo Bay, sediments .............................. 82

Sand bars ........................................................... 82

Sand drift ............................................. ............ 78

Sandstone ........ 9, 11, 23, 24, 25, 26, 27, 35, 81, 92

Sausalito Chert, obsolete name .................. 10, 28

Sausalito-Tiburon area, artificial fill ............ 88

Schistosity ...................................................... 51, 53

Sedimentary rock grains, sandstone ............ 15

Sedimentary rocks, clastic ........................... 11

metamorphosed ........................................ 20

structure ...................................................... 92

Sedimentation .................................................... 8, 67

Sediments, bay .................................................. 81

Seismicity .......................................................... 93

Serpentine ................................ 9, 27, 56, 76, 88, 92

antigoritic .................................................. 59

related to metamorphic rockb ...... 51, 52, 53

sheared ................................................ 55, 56, 86

Serpentinization ................................................ 64

Settlement .......................................................... 6

Shale .......................................... 9, 18, 23, 26, 27, 81

radiolarian ........................................ 36, 45, 48

sheared .............................................. 12, 20, 55

Shear zones ....................................................... 90

Sheared rocks .................................... 55, 76, 86, 94

Sheared serpentine ............................ ............ 60

Silica, origin, radiolarian chert and shale .. 47 

Sills ............................................................. 36, 65, 66

Simpton Point, conglomerate ........................ 20

greenstone .......................................... 32, 35, 36

metamorphosed sedimentary rocks ...... 22

Slate ................................................................... 50

Slope debris .................................................... 80, 86

Soils, sedimentary rocks ............................... 11

Source, dune sand ............................................ 80

sand ............................................................. 77

South Bay, fossil locality ................................ 27

sandstone detrital grains ........................ 15

South Twin Peak ............................................ 3

Stratigraphy ..................................................... 9

sedimentary rocks . .. ............................... 11

Structure ............................................................ 90

sedimentary rocks .................................... 11

Stuart Point, conglomerate ............................ 20

greenstone ........................................ 29, 32, 35

radiolarian chert and shale .................... 49

sediment deposition area ........................ 82

Suisun Bay, sediments .................................. 81, 82

Sunset Heights .................................................. 3

dune sand .................................................. 78

Franciscan Formation ............................ 9

greenstone .................................................. 32

radiolarian chert and shale .... 39, 45, 49, 50

Sunset Reservoir, structure .......................... 92

Surficial deposits ............................................ 66, 94

Sutro Reservoir, Colma Formation .............. 69

Telegraph Hill

Telegraph Hill-Continued

beach deposits ..................

clastic sedimentary rocks 

clay ......................................

sandstone matrix ..............

structure ............................

Temescal Formation ................

Page

............. 76

............ 23

............. 84

............. 14

............. 92

............. 73

Terrace Formation, obsolete name .............. 72

Thickness, artificial fill ..................... ............ 90

beach deposits ............................................ 76

chert, radiolarian ...................................... 37

clastic sedimentary rocks ........ 23, 24, 25, 26

clay .............................................................. 82

Colma Formation ...................................... 70

conglomerate .............................................. 20

dune sand .................................................... 78

Franciscan Formation .... ....................... 9

gabbro .......................................................... 66

greenstone .......................................... 33, 34, 35

mud .............................................................. 82

sandstone ................................................... 11

Thurber, D. L., quoted .................................... 74

Tiburon Peninsula .......................................... 3, 5

artificial fill ................................................ 88

beach deposits ............................................ 76

chert, radiolarian ............................... ..... 49

clastic sedimentary rocks ...................... 26

Colma Formation ...................................... 70

conglomerate ................................. ............ 20

Franciscan Formation ............. .............. 9

greenstone .......................................... 29, 30, 36

metamorphic rocks ........................ 50, 53, 55

metamorphosed sedimentary rocks ...... 20

serpentine .................................................. 56

shale, radiolarian ...................................... 49

structure .................................................... 93

Tiburon Peninsula'syncline .......................... 55

Tidal currents- ................................................ 72, 82

Topography ...................................................... *

Treasure Island, artificial fill ........................ 90

Tuff ......... .................................................... 31, 32, 36

Turbidity currents .......................................... 26, 47

Twin Peaks ........................................................ 3

alluvium .................................................... 66, 85

chert, radiolarian .................................. 39, 49

Colma Formation .................................. 67, 70

Franciscan Formation ............................ 9

greenstone ................................................ 32, 34

sandstone detrital grains ........................ 14

seismic activity ........................................ 93

shale, radiolarian ...................................... 49

sheared rocks ............................... ............. 56

U, V, W, Y

U.S. Mint area, clastic sedimentary rocks.. 25 

serpentine .................................................. 60

sheared rocks .............................................. 56

Varnes, D. J., quoted ...................................... 86

Veins in serpentine ....................................... 60

Volcanic rock grains, sandstone ................. 15

Volcanic rocks ................................................ 25, 27

Volcanisra .......................................... 29, 34, 36, 47

Waldo Tunnel, fractured greenstone .......... 30

Water depths .................................................... 3

Weathering, chert, radiolarian ...... ............. 46

Colma Formation ................................... 69

greenstone .................................................. 33

metamorphosed sedimentary rocks ...... 23

sandstone .................................................... 18

serpentine .................................................. 62

shale ............................................... ............ 20

radiolarian .......................................... 46

Weaver, C. E.. quoted ...................................... 74

Whitney, J. D., quoted ................................... 26

Worthing, H. W., analyst .............. 19, 35, 43, 62

Yellow Bluff, greenstone ................................ 33

Yerba Buena Island, metamorphosed

sedimentary rocks ................... 21

sandstone matrix ...................................... 14

U. S. GOVERNMENT PRINTING OFFICE : 1974 O - 534-039
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Kvenvolden, K. A., 1962, Normal paraffin hydrocarbons in 

sediments from San Francisco Bay, California: Am. Assoc. 

Petroleum Geologists Bull., v. 46, no. 9, p. 1643-1652.

Langston, R. B., Trask, P. D., and Pask, J. A., 1958, Effect of 

mineral composition on strength of central California sediments: California Jour. Mines and Geology, v. 54, no. 2, 

p. 215-235.
REFERENCES 103

Lawson, A. C., 1895, Sketch of the geology of the San Francisco 

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 1967, Approximate location of fault traces and historic 

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U.S. Geol. Survey Misc. Geol. Inv. Map 1-264.

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104 GEOLOGY OF THE SAN FRANCISCO NORTH QUADRANGLE

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 1969, The geology of the San Francisco Bay area and its 

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 1966, Sedimentation aspects of San Francisco Bay: San 

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 1969, Seismic risk to buildings and structures on filled 

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Health, California Univ., Berkeley, SERL Rept. 65-2, 

319 p.

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no. 3, p. 47-50.

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San Francisco Bay: California Univ. Pubs. Zoology, v. 14, 

no. 1, p. 1-198.
122°22'30"

37°52'30"

UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

1559 IV NW 

M9 I (SAN OUENTIN> 552 11450000 FEET

MOUNT TAMALPAJS

,\@AME X REFUGE

\ \ cDowell \

Sausalfta V ^_6 -J \ ^- * x

/ / V*- ' / HTMaritir MILITARY RES , gSte HUl

68' Qu^ 15' clay, gravel. y"KJsh (shale) 

(.23' clay "~~^ ',

n [26' sandy day 

78' QiK 43' sand, cla" 

19' clay

jt«bt BUILDING i38' sand

-26'clay, sand, Uu

^| 68' water

) \ \ 96' Qm 

 ight?

PoinfLeabos

KJs

ssion Rock

Terminal

Qaf

Light

^^^rr^^M/'

iSp&^'feW

-?,-, <^- ,,.*s;ic,^safldy tlay;, _ «.JJfri

fS4/V FRANCISCO SOUTH) 

1559 III

37°45'

122 1 22'30"

FLEISHHACKER ZOO I.I Ml.

Base by U.S. Geological Survey, 1956 

Polyconic projection. 1927 North American datum 

10,000-foot grid based on California coordinate system, zone 3 

1000-meter Universal Transverse Mercator grid ticks, zone 10 

Include revision from aerial photograph taken 1968.

Geology mapped in 1948-61 by Julius 

Schlocker, M. G. Bonilla, D. H. Radbruch, 

C. A. Kaye, and W. I. Konkoff

APPROXIMATE MEAN 

DECLINATION, 1973

CONTOUR INTERVAL 25 FEET 

DASHED LINES REPRESENT 5-FOOT CONTOURS

DATUM IS MEAN SEA LEVEL

DEPTH CURVES IN FEET DATUM IS MEAN LOWER LOW WATER 

SHOUELINE SHOWN REPRESENTS THE APPROXIMATE LINE OF MEAN HIGH WATER 

THE MEAN RANGE OF TIDE IS APPROXIMATELY 4 FEET

CALIF N 

QUADRANGLE LOCATION

 O

§

:t)

il 

' e

Artificial fill

Predominantly dune sand but includes silt, clay, rock waste from 

excavations, manmade debris, and 

organic waste. Probable fill material indicated in parentheses. 

Maximum thickness approximately 60 feet

Modern beach deposits 

Predominantly well sorted medium 

to coarse gray sand; coarse gravel 

and large cobbles in some places. 

Maximum thickness approximately 

JtOfeet

PROFESSIONAL PAPER 782 

PLATE 1

EXPLANATION 

SURFICIAL DEPOSITS

Landslide deposits 

Generally unstratified mixtures of

bedrock, sand, silt, and clay in

varying proportions. 

Qly, younger landslide deposits 

Qlo, older landslide deposits

Alluvium

Predominantly clayey silty sand and 

clayey silt, some pebbles; locally 

clean medium sand, generally gray 

to brown. Maximum thickness 

approximately 25 feet; observed 

thickness 15 feet

Slope debris and ravine fill 

Angular rock fragments in sand, 

silt, and clay matrix; generally 

light yellow to reddish brown. 

Maximum thickness approximately 80 feet

Dune sand

Clean well-sorted fine to medium 

sand; yellowish- brown to light 

gray. Maximum thickness approximately isofeet

Qob

Older beach deposits 

Predominantly well sorted medium 

to coarse gray sand. Maximum 

thickness approximately 30 feet

Colma Formation

Unconsolidated fine to medium sand 

with small to moderate amounts 

of silt and clay; in places includes 

clay beds, 6 inches to 5ft thick; 

cobble-size rubble rare; commonly 

light brown to gray. Observed 

thickness 75 feet; probably maximum thickness approximately 

300 feet

Ul

is PUBLIC PROPERTY and 1? ndt 

removed frc-.t the officiol f««es:

POSSESS 

(R. S. Sup. Vol. 2 pp. 360, Sec.

-w

U.S. Geological Survey

504 Custom House

555 Battery Street

San Francisco, Califorttl*

Bay mud and clay 

Plastic gray silty clay; some lenses 

of sand, peat, and shell fragments; 

fluid to soft upper layers; moderately stiff clay at depth Concealed except for exposures north 

of Corinthian Island and north of 

Sausalito Point. Maximum 

thickness approximately 140 feet

Surficial deposits, undivided

BEDROCK

Sheared rocks, undifferentiated

Coherent blocks and pieces of hard rock as much as several 

hundred feet in diameter, in a matrix of intensely sheared 

shale and serpentine. Clasts predominantly sandstone, shale, 

and serpentine, but also include all other rock types known 

in Franciscan Formation. Matrix gray to greenish gray, 

moderately firm to soft and clayey; generally expansive 

and plastic when wet

Serpentine

Mostly soft sheared rock containing hard knobs of unsheared serpentine, rodingite, and rocks of the 

Franciscan Formation. Derived from peridotites 

(mostly harzburgite, some dunite).

sph, hard serpentine, slightly sheared. Shown separately on Potrero Hill only. Variously colored, but 

generally greenish gray, blue, or brown. Includes 

sheared rocks (Ks) where that unit is mostly serpen-- 

tine

Gabbro

Fine- to coarse-grained gabbro; 

includes diabase where texture is 

subophitic. Occurs as inclusions 

or segregations in serpentine

I

c 7

KJss KJsh KJs L

KJg KJm

Clastic sedimentary rocks

KJss, sandstone; thick-bedded and massive graywacke sandstone interbedded with thin layers of shale and fine-grained sandstone;

some thick conglomerate lenses. 

KJsh, shale and thin-bedded sandstone; predominantly interbedded and

laminated shale and fine-grained graywacke sandstone; beds generally

2 to 5 inches thick. 

KJs, sandstone and shale,undifferentiated; consists of units KJss or

KJsh so poorly exposed that predominant lithology could not be

determined. 

Sandstone and shale at Point Lobos and vicinity west of City College

fault zone is probably Great Valley sequence of Bailey, Irwin, and

Jones (1964, p. 123)

Radiolarian chert and shale 

Reddish-brown; alternate beds of 

hard brittle chert, 1 to 5 inches 

thick, and firm brittle shale, Vs to 

% inch thick. Locally includes 

bodies of massive chert

Greenstone

Greenish-gray aphanitic to mediumgrained altered volcanic rocks; 

predominantly basalt. Includes 

flows, agglomerate, and tuff; 

pillow lavas common; less commonly massive; interbedded with 

radiolarian chert and sandstone

Metamorphic rocks 

Fine- to coarse-grained slate, schist, 

and granofels of the blueschist 

fades

u

I

fa

Contact

Long dashed where approximately located; short dashed 

where indefinite; dotted where concealed

Fault, showing dip and relative movement 

Long dashed where approximately located; short dashed 

where inferred; dotted where concealed. Query 

indicates greater uncertainty as to existence of fault 

than does dashed line. U, upthrown side; D, downthrown side

Showing average dip Approximately horizontal 

Shear zone

Anticline Syncline

Folds

Showing axis and direction of plunge. 

Dashed where approximately locat - 

ed; dotted where concealed

35 ,

Strike and dip of beds

60 _

Strike and dip of foliation

' 'X 

Fossil locality

Approximate b - ?; .... ... _., , _., ,

San Francisco i3ay (Modified from Gibson, 1951, p. 6)

Quarry

47' Qd

Top of landslide scarp

100' Qu (clay, sand,

sand-clay)

Abbreviated columnar section

Stratigraphic sequence and approximate thickness in 

feet of geologic map unit based on logs of boreholes 

and excavation. Sand*clay may include one or both 

of the following: sand, silt, and clay in various 

proportions; alternating thin beds of sand and silty 

clay

-r- SEA LEVEL 

200' 

400' 

600' 

800' 

1000'

SAN FRANCISCO BAY

Franciscan Formation

\ \

400' -i 

200' - 

SEA LEVEL - 

200' - 

400'

SEA LEVEL

100'

SEA LEVEL

150' -

200' 

250' 

300' 

350'

INTERIOR GEOLOGICAL SURVEY. WASHINGTON, D.C. 1974 G72Z97

350'

GEOLOGIC MAP OF THE SAN FRANCISCO NORTH QUADRANGLE, SAN FRANCISCO AND MARIN COUNTIES, CALIFORNIA

cr 

<

z 

tr

UJ

<

D 

O

cn

D 

0 

UJ 

U

£

LU

IT 

U

3 

O 

LU 

U

U

Q

Z.

C"

U

DC

D
UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

PROFESSIONAL PAPER 782 

PLATE 2

122° 30' 

37°52'30"

122° 22' 30"

37° 52'30"

1.0

0.5

0.5

1.0

+

5, 12 ,7

~+~ + -30 ?Q ' 

I0i« » 29

"+.. \ 3*

1685 25 

5

 22

16B

19A

PHI MEDIAN DIAMETER, 1

MILLIMETERS

2.0

.5

3

.125

GRAIN SIZE, IN MILLIMETERS Mechanical analyses by O.M. Schmidt, 1958

B. REPRESENTATIVE CUMULATIVE CURVES OF GRAIN SIZES

1.5

CLAY

37° 45' 37° 45' 

122°22'30"

A. LOCATION OF SAMPLES

EXPLANATION

Symbols used on A, C, and D 

Numbers are map localities

EXPLANATION

52 

 (10)

Number in parentheses is percent 

of gravel included in sample

Gravel, > 2 

Sand, 2-0.0625 

Silt, 0.0625-0.0039 

Clay, <0.0039 

Sizes, in millimeters

u

Colma Formation

14

Beach sand

A 19

Raised beach sand

26

Dune sand

Slope debris and 

ravine fill

o 62 

Alluvium

Beach sands of 

Trask(1959) 44 

(.40) SANDY

48 SILT 

(13)

SAND

13+ 9

/3,9^6 4 

SAI\m/gJ2+ ?61 + ^

11 2 10 8

C. SAND, SILT, AND CLAY COMPOSITION, BY WEIGHT

E. GRAIN SIZE DISTRIBUTION MEASURES

Map

No.

Sample 

No.

Median diameter

mm

Md

phi

Md 0

84th percentile

mm

P84

phi

$84

16th percentile

mm

P16

phi

#16

Sorting, 

deviation 

measure phi

<V-l/2(#84 -#16 )

Median diameter, 

phi

M 0=l/2(*84+*1 6 )

Skewness 

measure,phi

M0-Md<£

ad> 

V 0-0

75th 

percentile

mm

Q 3

25th 

percentile

mm

Qi

Sorting, 

Trask 

coefficient * r^7

VQ3

Skewness, 

Trask 

coefficient

QiQ 3

kJJV Md 2

One

percentile

mm

C

Colma Formation

1

2

3

4

5

6

1

8

9

10

11

12

13

SF1304

1306

1169

335

1844

2068

2069

431

1460

414

775

1770

1357

0.206

.183

.258

.236

.270

.220

.204

.208

.220

.24

.247

.243

.200

2.28

2.45

1.95

2.08

1.89

2.18

2.29

2.27

2.18

2.06

2.02

2.04

2.32

0.043

.104

.140

.03

.156

.086

.120

.135

.097

.130

.163

.17

.123

4.54

3.27

2.84

5.06

2.68

3.54

3.06

2.89

3.37

2.94

2.62

2.56

3.02

0.315

.279

.308

.42

.389

.328

.288

.317

.34

.383

.349

.31

.254

1.67

1.84

1.70

1.25

1.37

1.61

1.80

.66

.56

.38

.52

.69

.98

1.44

.72

.56

1.91

.66

.97

.63

.62

.91

.78

.55

.44

.52

3.12

2.56

2.27

3.16

2.03

2.58

2.43

2.28

2.31

2.16

2.07

2.13

2.50

0.58

.15

.57

.52

.21

.41

.22

.02

.76

.13

.09

.20

.35

0.094

.133

.193

.107

.186

.147

.160

.152

.155

.171

.187

.203

.155

0.282

.235

.290

.367

.358

.287

.265

.280

.297

.328

.320

.282

.230

1.73

1.33

1.23

1.85

1.39

1.39

1.28

1.36

1.38

1.39

1.31

1.18

1.22

0.625

.933

.841

.71

.914

.872

1.019

.983

.951

.973

.982

.968

.892

0.6

.54

.48

.8

.58

.62

.50

.57

.54

.8

.56

.54

.38

Beach sand

14

ISA

15B

16A

16B

16C

17

18

*

**

SF1691

1690S

1690B

1688S

1688B

1689

2129

2130

1685

1687

0.44

.340

.355

.369

.384

.508

.350

.56

.318

.420

1.18

1.56

1.49

1.44

1.38

.98

1.52

.84

1.65

1.25

0.270

.225

.240

.235

.252

.30

.229

.262

.203

.265

1.88

2.15

2.06

2.09

1.99

1.74

2.13

1.93

2.30

1.92

0.675

.54

.55

.78

.82

1.09

.558

.98

.52

.68

0.57

.89

.86

.36

.29

.12

.84

.03

.94

.56

0.66

.63

.60

.87

.85

.93

.65

.95

.68

.68

1.23

1.52

1.46

1.23

1.14

.81

1.49

.98

1.62

1.24

0.08

-.06

-.05

-.24

-.28

-.18

-.05

.15

-.04

-.01

0.31

.255

.269

.264

.282

.349

.255

.345

.226

.306

0.60

.46

.48

.60

.623

.840

.495

.84

.52

.70

1.39

1.34

1.34

1.51

1.49

1.55

1.39

1.56

1.52

1.51

0.96

1.04

1.02

1.16

1.19

1.14

1.03

.92

1.16

1.21

1.5

1.1

1.1

2.3

2.6

2.6

1.5

1.5

.85

2.4

Raised beach sand

***

19A

19B

19C

19D

SF1432

1434

1435

1436

0.248

.490

.260

.265

2.01

1.03

1.94

1.92

0.197

.247

.185

.213

2.34

2.02

2.43

2.23

0.33

.80

.421

.35

1.60

.32

1.25

1.52

0.37

.85

.59

.36

1.87

1.17

1.84

1.87

-0.38

.16

-.17

-.12

0.212

.299

.202

.228

0.295

.71

.351

.317

1.18

1.54

1.34

1.22

1.02

.884

1.05

1.03

0.54

1.15

.64

.53

Dune sand

20

21

22

23

24A

24B

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42A

42B

42C

43

SF1653

1126

2001

2127

1249A

1249B

2004

1902

2002

880

1253

1094

1581

1353

1354

1580

1257

1483

1582

1259

1355

1575

1295

1293B

1293S

1294

811

0.256

.240

.187

.205

.215

.208

.187

.240

.258

.259

.223

.223

.217

.20

.20

.215

.209

.233

.220

.216

.226

.228

.245

.252

.243

.244

.244

1.97

2.06

2.42

2.29

2.22

2.27

2.42

2.06

1.95

1.95

2.16

2.10

2.20

2.32

2.32

2.22

2.26

2.10

2.18

2.21

2.15

2.13

2.03

1.99

2.04

2.04

2.04

0.210

.168

.153

.16

.163

.165

.147

.190

.187

.190

.161

.169

.159

.150

.152

.160

.159

.163

.162

.158

.164

.165

.207

.188

.183

.193

.188

2.25

2.57

2.71

2.64

2.62

2.60

2.77

2.40

2.42

2.40

2.63

2.57

2.65

2.74

2.72

2.64

2.65

2.62

2.63

2.66

2.61

2.60

2.27

2.41

2.45

2.37

2.41

0.338

.332

.256

.271

.286

.275

.251

.318

.355

.35

.286

.295

.280

.272

.278

.277

.275

.316

.295

.283

.3

.315

.299

.34

.35

.308

.325

1.57

1.59

1.97

1.89

1.81

1.86

1.99

1.65

1.49

1.51

1.81

1.76

1.84

1.88

1.85

1.85

1.86

1.66

1.76

1.82

1.74

1.67

1.74

1.56

1.51

1.70

1.62

0.34

.49

.37

.38

.41

.37

.39

.33

.47

.45

.41

.41

.41

.43

.44

.40

.40

.48

.44

.42

.44

.47

.27

.43

.47

.34

.40

1.91

2.08

2.34

2.27

2.22

2.23

2.38

2.03

1.96

1.96

2.22

2.17

2.25

2.31

2.29

2.25

2.26

2.14

2.20

2.24

2.18

2.14

2.01

1.98

1.98

2.04

2.02

-0.18

.04

-.16

-.05

0

-.11

-.10

-.09

.02

.02

.15

.17

.12

-.02

-.07

.08

0

.08

.05

.07

.07

.02

-.07

-.01

-.13

0

-.05

0.219

.189

.162

.172

.175

.170

.159

.20

.213

.208

.175

.183

.173

.160

.164

.173

.172

.180

.175

.172

.182

.180

.218

.203

.20

.207

.20

0.305

.295

.232

.25

.265

.256

.232

.288

.321

.32

.267

.275

.262

.249

.252

.263

.255

.288

.275

.283

.275

.282

.282

.311

.295

.287

.3

1.18

1.25

1.20

1.21

1.23

1.23

1.21

1.20

1.23

1.24

1.24

1.23

1.23

1.25

1.24

1.23

1.22

1.27

1.25

1.28

1.23

1.25

1.14

1.24

1.22

1.18

1.22

1.02

.968

1.08

1.02

1

1.01

1.05

1

1.03

.992

.939

.927

.963

.996

1.03

.985

1

.955

.995

1.04

.980

.977

1.024

.994

.99

.998

1.01

0.40

.41

.35

.38

.39

.39

.28

.40

.52

.55

.39

.46

.37

.37

.40

.41

.38

.43

.44

.40

.41

.52

.52

.58

.6

.48

.56

* Sample from beach 1 mile south of San Francisco North quadrangle

** Sample from beach 8.7 miles south of San Francisco North quadrangle

*** Elevations of samples from map locality 19 are 123, 130, 175, 

and 100 feet, respectively

-eb

1.0

0.5

16C,

19B 16BA

,10

15B

A1685 _i_5

1 A

15A 19C, 

19DA

25

22

26

41

Map locality numbers are not shown 

for all dune sand samples

PHI MEDIAN DIAMETER, 1 

MILLIMETERS .5

2 

.25

3

.125

D. SIZE DISTRIBUTION DIAGRAM

F. COMPOSITION OF SAND GRAINS

Map unit

Map locality number

Colma Formation

2 9 3 13 6 11A

Beach sand

15

(*)

Dune sand

29 22

/**)

 73 .S

O, »-i

c/3 c

57A

Approximate frequency** 51

Heavy minerals

(specific gravity >2.82):

Actinolite-tremolite

Allanite

Andalusite

Apatite

Aragonite

Augite

Biotite

Brookite

Cassiterite

Chlorite

Chromite

Chrysotile

Clinozoisite-epidote

Composite, fine-grained, unidentified

Enstatite

Garnet, brown, green, or pink

Garnet, colorless

Glaucophane

Hornblende, brown

Hornblende, green

Hornblende, oxy

Hypersthene

Idocrase

Ilmenite

Jadeite

Kyanite

Leucoxene

Lawsonite

Magnetite (+200 mesh)

Monazite

Nephrite

Opaque, undifferentiated

Pumpellyite

Pyrite

Rutile

Sphene

Spinel

Staurolite

Stilpnomelane

Thorite (uranoan)

Tourmaline

Xenotime

Zircon

Zoisite

Percent of heavy minerals (+200 mesh)

-

-

-

-

-

4

-

-

-

1*

3

-

6-

4

-

2

-

2

5

7+

1

5

1

3

-

-

-

-

4

3

-

-

lx

1

-

4

-

-

-

-

-

-

1

_

10.3

2

-

-

1

-

6-

1

-

-

-

3

-

3

2

-

-

1

-

5

6

4

6-

-

5

-

-

-

-

3

4

-

-

lx

1

1

4

-

-

-

-

-

-

1

.

23

-

-

-

1

-

5

-

-

-

lx

3

-

5

3

-

-

-

1

4

7-

1

4

-

5

lx

-

-

-

2

2

-

-

-

-

1

4

-

-

-

-

-

-

1

3

14.5

-

-

-

1

-

6+

1

-

-

1

3

-

5

1

-

3

lx

3

5

7-

2

5

1

4

-

-

-

-

4

3

-

-

1

1

-

3

1

-

-

-

1

-

2

.

12

1

-

-

1

-

6-

-

-

-

-

2

1 *

6

4

2

lx

lx

1

4

6+

4

5

-

4

-

-

-

1 x

5

2

-

-

1 x

-

-

3

-

-

1

-

-

-

2

.

***** 

10.4

-

-

-

lx

-

6-

-

-

-

-

lx

-

5

2

-

lx

lx

2

4

6

4

6

-

5

-

-

-

-

6-

2

-

-

-

-

1

5

-

-

-

-

-

lx

lx

lx

25.7

-

-

-

1

-

5

lx

-

-

lx

5

lx

4

2

-

1

2

1

4

5

2

5

-

6+

-

-

-

-

4

2

2

-

-

-

lx

3

-

-

-

-

-

-

1

.

8.3

-

-

-

-

lx

4

-

lx

-

-

7-

-

4

4

-

lx

-

lx

2

6+

1

2

-

-

-

-

-

-

-

-

-

-

lx

-

lx

4

-

-

-

lx

lx

-

6

lx

70

2

-

lx

1

-

5

-

lx

-

-

4

-

5

4

-

1

1

lx

4

6+

4

5

-

5

lx

-

2

-

5

1

-

-

1

-

1

3

-

1

-

-

-

lx

3

-

17.7

1

-

-

-

-

5

-

1

1

-

5

-

6-

2

-

2

-

2

4

6

4

5

-

5

-

-

2

-

5

3

-

-

-

-

1

4

-

-

-

-

1

-

3

-

28

-

lx

-

-

-

5

-

lx

-

-

6

-

5

2

-

lx

lx

lx

4

5

2

5

-

6

-

-

2

-

5

3

-

-

-

-

-

4

lx

-

-

-

-

lx

2

.

15.3

1

-

-

-

-

2

1

-

-

-

3

-

6****

4

-

2

3

2

5

6+

5

2

-

5

-

-

-

-

2

3

-

-

2

-

-

3

-

-

-

-

-

-

2

.

-

Approximate percentage

Light components

(specific gravity <2.82):

Rock fragments

Quartz

Feldspar (plagioclase: potassium

feldspar ratio is 10:1 or larger)

40

40

20

65

15

20

60

35

5

25

70

5

50

30

20

60

10

30

30

50

20

-

-

-

40

25

35

30

30

40

10

30

60

-

-

-

* Black sand, Princeton Beach, 21 miles south of Point Lobos. -115+ 250 mesh only, modified from Hutton (1959, p. 10)

** Sample from 8.7 miles south of Point Lobos, 700 feet east of Pacific Ocean

*** The Evans, Hayman, and Majeed scale given by Hutton (1950, p. 650), as shown below, is used:

Frequency Approximate percentage

8+ 90-100

8 75-89

8- 60-74

7+ 45-59

7 35-44

7- 28-34

6+ 23-27

Frequency Approximate percentage

6 

6-

5

4

3

2

1

18-22 

14-17

7-13

4-6

2-3 

1-2

'/2-1

1 grain only

  Mostly common epidote

** *** -50+70 mesh

COMPOSITION AND GRAIN SIZE OF SURFICIAL DEPOSITS, SAN FRANCISCO NORTH QUADRANGLE,

SAN FRANCISCO AND MARIN COUNTIES, CALIFORNIA

534-039 O - (Inside back cover)
122°30' 

37°52'30"

UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

 545ooom E , 545 ' R. 6 Wi R. 5 W.

MILL VALLEY 6.5 Ml.

U.l Ml. TO U-S.

122°22'30"

37°52'30"

7559 IV NW 

549 I (SAN QUENTIN)

MOUNT TAMALPAIS

.VJAME REFUGE

Exposed bedrock 

Includes Franciscan Formation and serpentine

Contours on buried bedrock

Contour interval above mean sea level is 100 feet; below mean sea level, 50 feet. 50-foot intermediate 

contours are dashed. Contours not shown on Marin 

and Tiburon Peninsulas and Angel Island where 

bedrock surface is generally less than 20 feet below 

the topographic surface. Reliability of contours is 

greatest near surface exposures and near boreholes 

that reached bedrock. Offshore contours where borehole data are lacking are based on U.S. Coast and 

Geodetic Survey soundings, bottom sampling, tidal 

current data, and subjective geologic criteria and are 

only approximate. Bedrock-surface contours for 

most of the Golden Gate channel and San Francisco 

Bay in this quadrangle, obtained by subbottom 

acoustical profiling, are given in U.S. Geological 

Survey open file report, 1970, entitled "Bedrocksurface map of central San Francisco Bay, California" by Paul R. Carlson and David S. McCulloch

-70

Borehole 

Number indicates elevation of top of bedrock

Borehole that did not reach bedrock 

Number indicates elevation of bottom of hole

Landslide 

Number refers to accompanying table

VREAS-LH

>*;

°fef;

,< *. m

27 30 I c>ea FLEISHHACKER ZOO 4. I Ml

PROFESSIONAL PAPER 782 

PLATE 3

FLEISHHACKER ZOO I.I Ml.

Base by U.S. Geological Survey, 1956 

Polyconic projection. 1927 North American datum 

10,000-foot grid based on California coordinate system, zone 3 

1000-meter Universal Transverse Mercator grid ticks, zone 10 

Include revisions from aerial photographs taken 1968.

(SAN FRANCISCO SOUTH) 

(559 ;// NW

SCALE 1:24000 

0

DALY CITY 4 Ml 

SAN BRUNO IO Ml.

1 MILE

.5 1 KILOMETER

APPROXIMATE MEAN 

DECLINATION 1973

CONTOUR INTERVAL 25 FEET 

DASHED LINES REPRESENT 5-FOOT CONTOURS

DATUM IS MEAN SEA LEVEL 

DEPTH CURVES IN FEET DATUM IS MEAN LOWER LOW WATER

SHORELINE SHOWN REPRESENTS THE APPROXIMATE LINE OF MEAN HIGH WATER 

THE MEAN RANGE OF TIDE IS APPROXIMATELY 4 FEET

122°22'30"

INTERIOR GEOLOGICAL SURVEY, WASHINGTON, D.C. 1974 G72297

Prepared by J. Schlocker, 1959-61; 

assisted by R. F. Harty, 1959

QUADRANGLE LOCATION

LANDSLIDES IN THE SAN FRANCISCO NORTH QUADRANGLE

Map 

No.

Map unit 

before sliding

Type of movement Probable main cause

Approximate

slope 

before 

sliding

(degrees)

Angel Island

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

Qaf /KJss

KJss

KJss

Qsr/KJss

Qsr

Qsr

Qsr

KJss

KJss

KJss

KJss

KJss

KJss

KJg

KJss

KJss

KJss

KJss

KJg

Qsr/KJss

KJss

Qsr/KJg

KJm

KJss

Rockfall, debris flow

Debris slide

do.

Debris flow

Earthflow, debris flow

do.

Debris slide

Debris flow

do.

do.

do.

do.

Earthflow

Debris slide

do.

do.

do.

do.

do.

Debris flow

Debris slide

Debris flow, earthflow

Debris slide

Earthflow

Removal of support by wave cutting

Water, removal of support by wave cutting

Removal of support by wave cutting

do.

Water, removal of support by wave cutting

do.

Water

Water, removal of support by wave cutting

do.

do.

do.

do.

do.

Removal of support by wave cutting

do.

Water

Removal of support by wave cutting

do.

Removal of support

Removal of support by wave cutting

do.

Removal of support by road cut

Prolonged heavy rains

Water, removal of support by wave cutting

65

35

40

30

20

25

20

30

30

30

30

15

25

10

15

31

45

30

30

35

40

30

25

35

Tiburon Peninsula and Belvedere Island

Marin Peninsula

Alcatraz Island

San Francisco

25

26

27

28

29

30

31

Ks

Qsr/KJss

Qsr/KJss

Qsr/KJss

Qsr/KJss

Qsr/KJss

Qsr/KJss

Rotational slump, debris flow

Debris slide

do.

do.

do.

do.

do.

Removal of support by manmade cut other than roadcut

Removal of support by wave cutting

do.

do.

do.

do.

do.

30

40

20

25

30

45

40

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

Qsr/KJss

Qsr/KJss

Qsr/KJss

Qsr/KJss

Qsr/KJss

Qsr/KJg

KJg

Qsr/KJg

Qsr/KJss

Qsr/KJg

Qsr/KJg

Qsr(KJg)/KJg

Qsr(KJg)/KJg

Qsr/KJg

Qsr/KJg

Qsr/KJg

Qlo (Qsr)/KJss

Qly/Qlo

Qsr(KJg)/KJg

KJss

KJsh/KJc

Qlo (Qsr)/KJss

Qly/Qlo

Qsr(KJc)/KJss

Qsr (KJc)/ KJss

Qsr(KJc)/KJss

Qsr/KJg

Qsr(KJc)/KJg

Qsr/KJg

KJg

Earthflow, debris flow

do.

do.

do.

do.

Earthflow

Block glide, debris slide

Earthflow, debris flow

Debris avalanche

Rotational slump, earthflow

Earthflow

Earthflow, debris flow

do.

do.

Earthflow

do.

Debris flow, rotational slump

Earthflow

Debris slide, debris flow

Rotational slump, debris flow

do.

Earthflow, debris flow

do.

do.

Debris slide, earthflow

Debris flow

Earthflow

Block glide, debris slide

Water

do.

do.

do.

do.

do.

Removal of support by manmade cut other than roadcut

Water, removal of support by manmade cut other than roadcut

Prolonged heavy rain

Water removal of support by manmade cut other than roadcut

Removal of support by loading of slope with Qaf and wave cutting

Prolonged heavy rain

do.

Removal of support by wave cutting

Removal of support by roadcut and wave cutting

Removal of support by wave cutting

Water, removal of support by roadcut

Prolonged heavy rain

Water

Fault surface, removal of support by roadcut

Water, removal of support by roadcut

do.

do.

do.

Removal of support by roadcut

Water, removal of support by roadcut

Water, removal of support by loading of slope with Qaf

Removal of support by manmade cut other than roadcut

15

15

20

20

15

20

35

40

35

35

25

25

25

45

30

30

*33

**40-50

35

30

*21

**45

*32

**40-50

25

25

20

45

35

35

35

60

61

62

KJss

Qaf

Qsr/KJss

Rockfall

Debris slide

Soil fall

Removal of support by wave cutting

Water, removal of support by wave cutting

Water .

85

55

70

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

KJss, KJsh

KJss

KJss

Qsr/KJss

KJss

KJss

Qsr

Qsr/KJsh

Qsr/sp

Qd/Qc/KJg

Qsr

Qsr

Qsr/KJss

Qsr

KJss

KJc/KJg

Qsr

Qsr/ KJss or KJc

Qd

Qd/KJc

Qd

Qd

Qsr

sp

sp

sp

KJss

sp/KJss

sp/KJss

KJss

Qaf, Qd, KJss, sp

KJg/ KJss

KJg/KJss

KJss, KJg

Ks

KJss

KJsh

KJss

KJss, sp

Debris slide

Rockfall

do.

Rockfall, mudflow

Debris slide

do.

do.

do.

Debris flow, earthflow

Rotational slump

Earthflow

do.

Rotational slump, earthflow

Debris flow

Debris slide

do.

Rotational slump

Earthflow

Sandflow

do.

Rotational slump, sandflow, debris slide

Sandflow

Debris slide

Rotational slump, debris slide

Rotational slump, debris slide, earthflow

do.

Rockfall

Rotational slump

do.

Rotational slump, debris slide

Debris flow, debris slide, mudflow

Rotational slump

Debris slide

Rotational slump

Rotational slump, earthflow, debris flow

Earthflow

do.

Rotational slump

Not known, probably rotational slump

Removal of support by manmade cut other than roadcut

Water, removal of support by manmade cut other than roadcut

do.

do.

Prolonged heavy rains

do.

Removal of support by manmade cut other than roadcut

Water, removal of support by manmade cut other than roadcut

do.

Removal of support by roadcut, vibration

Removal of support by roadcut

do.

Water, removal of support by manmade cut other than roadcut

Water

do.

Weakening of underlying KJg, now altered to clay

Water, removal of support by manmade cut other than roadcut

Water, removal of support by manmade cut other than roadcut,

loading of slope with Qaf

Water

Desiccation and loss of cohesion

Removal of support by manmade cut other than roadcut

Water, earthquake

Removal of support by roadcut

Water, removal of support by wave cutting

do.

do.

Removal of support by wave cutting and by manmade cut other than roadcut

Removal of support by wave cutting and by water

do.

Removal of support by wave cutting, loading slope with Qaf

do.

Removal of support by wave cutting

Water, removal of support

Removal of support by wave cutting and by water

Water, removal of support by wave cutting

Removal of support by wave cutting

Water, removal of support by wave cutting

Removal of support by roadcut

Removal of support for roadcut

45

90-95

70

70

3Q

30

30

50

30

30

20

20

10

15

33

45

20

30

35

50

45

25

30

35

35

20

50

30

30

10 (Upper)

45 (Lower)

17 (Upper)

60 (Lower)

25

25

30

30

40

45

40

45

Letter symbols indicate map units on plate 1, as follows: 

Surficial deposits:

Qaf, artificial fill

Qly, younger landslide deposits

Qlo, older landslide deposits

Qd.dune sand

Qsr, slope debris and ravine fill

Franciscan Formation: 

KJss, sandstone

KJsh, shale and thin-bedded sandstone 

KJg, greenstone

KJc, radiolorian chert and shale 

KJm, metamorphic rocks 

Ks, sheared rocks

sp, serpentine

Qaf (KJc)/KJg indicates slope debris and ravine fill composed 

predominantly of debris of radiolarian chert and shale of 

the Franciscan Formation overlying greenstone of the Franciscan Formation

* Natural slope

**Roadcut

MAP SHOWING AREAS OF EXPOSED BEDROCK, CONTOURS ON BEDROCK SURFACE, AND LANDSLIDES IN THE

SAN FRANCISCO NORTH QUADRANGLE, SAN FRANCISCO AND MARIN COUNTIES, CALIFORNIA