LANDSLIDES IN ALAMEDA COUNTY, 
CALIFORNIA, A DIGITAL DATABASE EXTRACTED 
FROM PRELIMINARY PHOTOINTERPRETATION 
MAPS OF SURFICIAL DEPOSITS BY T.H. NILSEN IN 
USGS OPEN-FILE REPORT 75-277

by

Sebastian Roberts1, Michelle A. Roberts1, and Eileen M. Brennan1

with a preface by

Richard J. Pike1 and Sebastian Roberts1





Open-File Report  99-504
Version 1.1

1999







U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards 
or with the North American Stratigraphic Code.  Any use of trade, product, or firm names is for descriptive purposes 
only and does not imply endorsement by the U.S. Government.  

This database, identified as 'Landslides in Alameda County, California, a digital database extracted from preliminary 
photointerpretation maps of surficial deposits by T.H. Nilsen in USGS Open-file Report 75-277', has been approved 
for release and publication by the Director of the USGS. Although this database has been reviewed and is 
substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. This 
database is released on condition that neither the USGS nor the U.S. Government may be held liable for any damages 
resulting from its use.


1 Menlo Park, CA




































Cover illustration:  Reactivated 40-hectare (100-acre) earth flow on 
Mission Peak, in southern Alameda County, which threatened 
several homes in the town of Fremont in the spring of 1998, when 
it moved downslope at the rate of a meter per day.  The Mission 
Peak slide is part of a much larger landslide complex initially 
mapped by Nilsen (1973b) and included in the digital database 
released in this report.   Photo 1998 by J.D. Rogers




ABSTRACT

All or part of 25 7.5-minute quadrangles identifying 8465 
landslidesÑlargely slow-moving slides and earth flowsÑin 
Alameda County, California, have been converted to a 
digital-map database, compiled at 1:24,000 scale and plotted 
at 1:62,500 scale, that can be acquired from the U.S. 
Geological Survey over the Internet or on magnetic tape.




PREFACE


Public agencies in the 1960Õs became increasingly 
aware of their responsibility to take geologic 
hazards into consideration when planning future 
regional development.  One reason for this 
growing awareness was a series of court decisions 
that increased the liability of public agencies in 
cases where property was damaged by natural 
hazardsÑand the agency had been involved in 
approving the affected site for development.  
Another was the realization that many geologic 
hazards can be anticipated and thus can be used 
to make informed development decisions 
(Camerotto and Johnson, 1972).  As a result of this 
shift in attitude the San Francisco Bay Region 
Environmental and Resources Planning Study 
(SFBRS) was formed in 1970 with support from 
the U.S. Geological Survey (USGS) and the 
Department of Housing and Urban Development's 
Office of Policy Development and Research.  The 
study sought to provide, in forms understandable 
and usable by non-specialists, the earth-science 
information needed to solve problems related to 
growth and development in the nine-county, 
7,400-square-mile San Francisco Bay Region 
(Nilsen and others, 1979).  

Among the scores of publications resulting from 
the SFBRS effort was a 1:24,000-scale 
photointerpretive inventory of (predominantly) 
old and ancient landslides measuring over 200 
feet across and other surficial geologic deposits in 
the southeastern part of the Bay Region (Nilsen 
and others, 1975).  The present report is a 
1:24,000-scale digital database that reproduces 
the landslide content (n = 8465) of that inventory 
for Alameda County.  The area included by the 
database was mapped by Nilsen only.
Several series of vertical aerial photographs 
taken for USGS were used by Nilsen to prepare 
the original maps;  series BUU taken in 1958, 
1959, and 1966 at the scale of 1:20,000, series GS-
Yf taken in 1953 at the scale of 1:23,600, and 
series GS-JL taken in 1949 at the scale of 1:23,600.  
In addition larger scale maps at the scale of 
1:80,000 from series GS-VCMI were used over the 
entire county.

The 1:24,000-scale landslide-inventory maps 
that cover Alameda County had been drafted 
prior to 1975 and released in the SFBRS 
publications series as six paper maps at 1:62,500-
scale (Nilsen, 1971, 1972a-c, 1973a, b; see also, 
Nilsen and others, 1976).  These coarser-scale 
maps do not contain the fine detail of the 1:24,000 
source mapsÑall of which is, however, preserved 
in the digital database of this report.  A 
generalized broad-scale compilation of landslide 
deposits in Alameda and nine other Bay Region 
counties was also published as a paper map by 
Nilsen and others (1979) and in digital-map 
format by Wentworth and others (1997).

Although the information in this database has 
not been updated or amended beyond that in the 
original publication, we are releasing it in 
digital form because it is the only systematic 
landslide-inventory that covers Alameda County 
in its entirety.  Users are cautioned that these 35- 
to 40-year-old data have shortcomings in 
addition to caveats cited in the original map text 
(see Part I, below).  Information on the hazard is 
incomplete; no slope failures since 1966 are 
shown.  The many landslides from the severe 
winters of 1982, 1983, and 1998 (Coe and others, 
1999) are absent.  Detailed mapping by private 
consultants also is not included here, nor are 
landslides mapped more recently by the 
California Division of Mines and Geology 
(Majmundar, 1991a, b; 1995a, b).  

Because landslides in the database were mapped 
remotely, from aerial photographs, Nilsen 
ascribed two levels of certainty to his 
interpretations.  Deposits are queried "?" where 
identification was uncertain.  The 8465 deposits 
are not attributed as to type of movement, but 
consist primarily of slow-moving slides and earth 
flows (Wentworth and others, 1997).  Their time 
of formation ranges from perhaps a few hundred 
thousand years ago to 1966.  Deposit thickness 
ranges from about three meters to a few hundred 
meters; larger deposits are thicker; many small 
deposits may be thin and involve surficial 
materials only.  Debris flows are absent from the 
database (The January 3-5, 1982, storm alone 
triggered many such failures; Ellen and others, 
1997).  Locations in Alameda County that might 
be susceptible to this type of small landslide are 
indicated in a recent terrain-based mathematical 
model that predicts source areas for debris flows 
(Ellen and others, 1997).  Nilsen and others (1976) 
offer further background on slope-failure 
problems in Alameda County.

This report comprises three separate and distinct 
parts.  The first reproduces excerpts from T.H. 
Nilsen's explanatory text printed on the source 
maps.  Part II describes compilation of the source 
maps as a digital database and the conventions 
adopted in encoding the landslide information in 
a geographic information system (GIS).  The final 
section describes details of the digital-map 
database, its constituent data files, and their 
computer formats.  Part III further lists the steps 
required to obtain the contents of the landslide-
map database, and the 1:62,500-scale paper plot 
derived from it, from USGS.  Roberts and others 
(1998) is an earlier report in this series of digital 
re-releases of mapped USGS and other landslide 
inventories for San Francisco Bay Region counties.





REFERENCES


Camerotto, O.G., and Johnson, S.S., 1972, Public 
works engineering: San Francisco Bay Region 
StudyÑmulti-disciplinary study, first meeting, 
Menlo Park, CA, October 11, 1972, unpublished 
typescript of presentation, 16 p.
Coe, J.A., Godt, J.W., Brien, Dianne, and Houdre, 
Nicolas, 1999, Map showing locations of 
damaging landslides in Alameda County, 
California, resulting from 1997-98 El Ni–o 
rainstorms: U.S. Geological Survey, 
Miscellaneous Field Studies Map MF-2325-B, 
scale 1:125,000.
Ellen, S.D., Mark, R.K., Wieczorek, G.F., 
Wentworth, C.M., Ramsey, D.W., and May, 
T.E., 1997, Map showing principal debris-flow 
source areas in the San Francisco Bay region, 
CA: U.S. Geological Survey, Open-File Report 
97-745 E.  <http:wrgis.wr.usgs.gov/open-
file/of97-745/index.html>.
Majmundar, H.H., 1991a, Landslides and related 
features, in Landslide hazards in Livermore 
Valley and vicinity, Alameda and Contra 
Costa Counties, California: Calif. Div. Mines 
and Geology, Open-file Report 91-2, plates B1 
and B2, map scale 1:24,000.
Majmundar, H.H., 1991b, Landslides and related 
features, in Landslide hazards in the Tassajara 
and Byron Hot Springs 7.5' quadrangles, 
Alameda and Contra Costa Counties, 
California: Calif. Div. Mines and Geology, 
Open-file Report 92-5, plate B, map scale 
1:24,000.
Majmundar, H.H., 1996a, Landslides and related 
features, in Landslide hazards in the Hayward 
quadrangle and parts of the Dublin quadrangle, 
Alameda and Contra Costa Counties, 
California: Calif. Div. Mines and Geology, 
Open-file Report 95-14, plate B, map scale 
1:24,000.
Majmundar, H.H., 1996b, Landslides and related 
features, in Landslide hazards in the Las 
Trampas Ridge quadrangle and parts of the 
Diablo quadrangle, Alameda and Contra Costa 
Counties, California: Calif. Div. Mines and 
Geology, Open-file Report 95-15, plate B, map 
scale 1:24,000
Nilsen, T.H., 1971, Preliminary 
photointerpretation map of landslide and 
other surficial deposits of the Mount Diablo 
area, Contra Costa and Alameda Counties, 
California: U.S. Geol. Survey Misc. Field 
Studies map MF-310, scale 1:62,500.
Nilsen, T.H., 1972a, Preliminary 
photointerpretation map of landslide and 
other surficial deposits of the Altamont and 
Carbona 15-minute quadrangles, Alameda 
County, California: U.S. Geol. Survey Misc. 
Field Studies map MF-321, scale 1:62,500.
Nilsen, T.H., 1972b, Preliminary 
photointerpretation map of landslide and 
other surficial deposits of the Byron area, 
Contra Costa and Alameda Counties, 
California: U.S. Geol. Survey Misc. Field 
Studies map MF-338, scale 1:62,500.
Nilsen, T.H., 1972c, Preliminary 
photointerpretation map of landslide and 
other surficial deposits of the Mount Hamilton 
quadrangle and parts of the Mount Boardman 
and San Jose quadrangles, Alameda and Santa 
Clara Counties, California: U.S. Geol. Survey 
Misc. Field Studies map MF-339, scale 1:62,500.
Nilsen, T.H., 1973a, Preliminary 
photointerpretation map of landslide and 
other surficial deposits of the Concord 15-
minute quadrangle and the Oakland West, 
Richmond, and part of the San Quentin 7.5-
minute quadrangles, Contra Costa and Alameda 
Counties, California: U.S. Geol. Survey Misc. 
Field Studies map MF-493, scale 1:62,500.
Nilsen, T.H., 1973b, Preliminary 
photointerpretation map of landslide and 
other surficial deposits of the Livermore and 
parts of the Hayward 15-minute quadrangles, 
Alameda and Contra Costa Counties, 
California: U.S. Geol. Survey Misc. Field 
Studies map MF-519, scale 1:62,500.
Nilsen, T.H., Bartow, J.A., Frizzell, V.A., and 
Sims, J.D., 1975, Preliminary 
photointerpretation maps of landslide and 
other surficial deposits of 56 7.5-minute 
quadrangles in the southeastern San Francisco 
Bay region, Alameda, Contra Costa, and Santa 
Clara Counties, California: U.S. Geological 
Survey Open-File Report No. 75-277, scale 
1:24,000.
Nilsen, T.H., Taylor, F.A., and Brabb, E.E., 1976, 
Recent landslides in Alameda County, 
California (1940-71)ÑAn estimate of economic 
losses and correlations with slope, rainfall, and 
ancient landslide deposits: U.S. Geologic 
Survey Bulletin 1398, 21 p.
Nilsen, T.H., Wright, R.H., Vlasic, T.C., and 
Spangle, W.E., 1979, Relative Slope Stability 
and Land-use Planning in the San Francisco Bay 
Region, California: U.S. Geologic Survey 
Professional Paper 944, 96 p.
Roberts, Sebastian, Barron, Andrew, Brabb, E.E., 
and Pike, R.J., 1998, Digital compilation of 
"Preliminary map of landslide deposits in 
Santa Cruz County, CA, by Cooper-Clark and 
Associates, 1975": U.S. Geol. Survey, Open-file 
Report 98-792, 22 p.
Wentworth, C.M., Graham, S.E., Pike, R.J., 
Beukelman, G.S., Ramsey, D.W., and Barron, 
A.D., 1997, Summary distribution of slides and 
earth flows in the San Francisco Bay region, 
CA: U.S. Geol. Survey, Open-file Report 97-745 
C, 10 p. plus 11 maps.  
<http:wrgis.wr.usgs.gov/open-file/of97-
745/index.html>.






I.

THE SOURCE MAP

The following excerpts from Nilsen (1972c) are reproduced verbatim.



INTRODUCTION

The nine San Francisco Bay region counties lie 
within a geologically active, young, and dynamic 
part of the central and northern Coast Ranges of 
California.  Significant movements of the earth's 
crust are occurring here at the present time, posing 
numerous problems to urbanization, including 
some of special concern.  Geological processes such 
as fault movements, earthquakes, land 
subsidence, landsliding, slow downslope 
movement of bedrock and surficial materials, 
coastal and stream erosion, flooding, and 
sedimentation are all potentially hazardous.  
Because of these factors, an understanding of the 
operation of physical processes in the Bay region 
is desirable for harmonious, efficient, and safe 
land-use planning, particularly now, with 
greatly expanded pressures for urban growth.

This map presents preliminary information about 
one aspect of the physical environment necessary 
to sound land-use planning; the nature and 
distribution of surficial deposits.  Because 
surficial deposits are common and well developed 
in much of the Bay region, it is useful to know 
how and why they have formed, as well as what 
properties they possess.  When maps like this are 
used in combination with other types of 
environmental information, such as data on soils, 
bedrock geology, slopes, vegetation, climatic 
variation, seismic response, and hydrology, it 
should be easier to arrive at sound decisions 
regarding the physical aspects of land use.  The 
U.S. Geological Survey is studying many of these 
factors in the Bay region and hopes to provide 
the community with much of the required 
information as part of its San Francisco Bay 
Region Study in cooperation with the 
Department of Housing and Urban Development.

The representation of surficial deposits on this 
map reflects the way in which a geologist, 
working exclusively with aerial photographs, 
interpreted the origin of various elements of the 
present landscape.  The deposits shown here 
have not been examined in the field.  However, 
by viewing overlapping vertical aerial 
photographs through a stereoscope, the geologist 
sees a three-dimensional relief model of the 
ground surface and can study and interpret the 
origins of landforms with considerable ease.  In 
fact, for mapping surficial deposits, particularly 
in reconnaissance-type studies, 
photointerpretation has advantages over both 
ground observations and laboratory studies of 
surficial materials.  Of course, better information 
can be provided when all aspects of the study are 
integrated.  These preliminary photo 
interpretation maps are only the first stage in a 
detailed study of surficial deposits in the Bay 
region, but they should provide land-use planners 
with immediately useful information about the 
regional distribution of landslide and other 
surficial deposits.

This map indicates the dominant surficial 
processes that have probably been operative by 
showing the distribution of different types of 
surficial deposits.  Processes such as weathering, 
erosion, and the slow as well as rapid downslope 
movement of earth materials have constantly 
reshaped the land surface in the past and will 
continue to in the future, although at varying 
rates.  The processes are interrelated to varying 
degrees.  For example, crustal uplift of the Coast 
Ranges will lead to increased erosion and 
downcutting by streams that in turn generally 
results in increased deposition of sediments in 
river valleys, lakes, and shoreline areas.  Older 
flood plains and river deposits may be eroded, 
leaving elevated terrace deposits. In addition, 
downcutting, by streams may cause adjacent 
slopes to become unstable, thereby increasing the 
possibility of slope failures.

Man's activities can alter natural physical 
processes in many ways.  Simple acts such as 
overwatering a lawn or placing a septic tank 
drainfield in ground that is marginally stable 
may weaken the bedrock and surficial materials 
enough to induce landsliding.  Relatively stable 
areas may be made unstable as a result of 
construction activities that involve cutting or 
oversteepening of natural slopes.  Engineers, 
builders, conservationists, and others concerned 
with land use must evaluate the potential effects 
of all types of development, and maps that show 
the nature and distribution of surficial deposits 
should provide much of the basic information 
they need.

This map, then, shows the cumulative effects of 
various processes that have yielded surficial 
deposits up to the time the photographs used for 
photointerpretation were taken.  It does not 
indicate directly areas where processes will be 
most active, nor does it show the rate at which 
they will operate.  However, knowledge of the 
history of geologic events is a key to 
understanding and predicting the evolution of an 
area, even where man's activities significantly 
change the character of the land.  Almost all new 
landslides, for example, occur in areas with a 
history of landslide activity.


EXPLANATION FOR LANDSLIDE 
DEPOSITS SHOWN ON THE MAP

Landslide deposits:  debris composed of fresh and 
weathered rock fragments, sediment, colluvial 
material, and artificial fill, or any combinations 
thereof, that has been transported downslope by 
falling, sliding, rotational slumping, or flowing.  
Landslide deposits smaller than approximately 
200 feet in longest dimension are not shown on the 
map.  

¥ (polygon symbol) - Landslide deposit 
larger than approximately 200 feet in 
longest dimension (except in the La 
Costa Valley quadrangle, where the 
size cutoff is 500 feet); arrows indicate 
general direction of downslope 
movement; queried where identification 
is uncertain.

¥ (arrow symbol; La Costa Valley 
quadrangle only) - Landslide deposit 
between approximately 200 feet and 500 
feet in longest dimension; arrow 
indicates general direction of downslope 
movement, and is positioned over 
location of deposit; queried where 
identification uncertain.

Complex landslide deposits, which result from 
combinations of different types of downslope 
movement, are perhaps the most common type of 
landslide deposit in the Bay region.  In 
particular, materials near the head of landslide 
deposits typically move in a different manner 
than materials at the toe.  The landslide 
deposits shown on this map have not been 
classified according to either type of movement or 
type of material of which the deposit is 
composed.  The deposits vary in appearance from 
clearly discernible, largely unweathered and 
uneroded topographic features to indistinct, 
highly weathered and eroded features 
recognizable only by their characteristic 
topographic configurations.  The time of 
formation of the mapped landslide deposits 
ranges from possibly a few hundred thousand 
years ago to 1966.  No landslide deposits that 
formed since 1966 are shown.  The thickness of the 
landslide deposits may vary from about 10 feet to 
several hundred feet.  The larger deposits are 
generally thickest; many small deposits may be 
very thin and may involve only surficial 
materials.


FACTORS AFFECTING MAP 
ACCURACY

Date of photography:  Modifications of the 
landscape that have occurred since the date the 
aerial photographs were taken are not shown on 
this map.  Thus, landslide deposits and large 
artificial fills that postdate the photography 
are not delineated, although some of the 
topographic base maps were photorevised in 1968 
and do show the extent of urbanization to that 
date.

Urbanization:  Surficial geologic features can be 
obscured in urbanized areas by (1) modification of 
the natural landscape by grading (leveling, 
cutting, filling, or terracing), and (2) man-made 
structures that cover the natural land surface.  
Less than 10 percent of the area included in this 
map has been extensively urbanized.

Forest cover:  Surficial deposits may be difficult 
to recognize in forested areas, so that such areas 
may be mapped less accurately than grass-
covered areas.  Many landslide deposits may be 
impossible to recognize on slopes covered with 
dense stands of tall trees.  Less than 15 percent of 
the area included in this map is densely forested.

Quality of photography:  The accuracy of the 
map varies directly with the clarity and contrast 
of the aerial photographs used.  Accordingly, 
haze, cloud cover, or poor sun angles make 
photointerpretation more difficult; also, the 
steepness of the topography and the location and 
extent of shaded areas-affect the usefulness of 
individual photographs.  In general, however, 
the photographs used to prepare this map are of 
excellent quality.

Scale of maps and photography:  Landslide and 
other surficial deposits less than about 200 feet 
long are not shown because they are too small to 
be clearly identified on the photographs or 
clearly portrayed on the topographic base map.  
In addition, no attempt has been made to show 
the numerous small areas covered by artificial 
fill along highways, railroads and airstrips, in 
cemeteries, in populated and farming areas, or 
near quarries and mines, even though some are 
more than 200 feet in longest dimension.

Problems in interpretation:  Mapping of surficial 
deposits by photointerpretation alone presents a 
number of difficult problems, some of which can 
be resolved only through field checking.  
Problems that are especially difficult include: 
(1) distinguishing terrace-shaped slump-type 
landslide deposits from alluvial terrace 
deposits where both are located adjacent to 
stream courses; 
(2) recognizing bedrock cropping out beneath 
surficial deposits, especially where a creek 
or stream has cut down through the 
overlying surficial deposits to expose 
bedrock along the stream bed; 
(3) determining boundaries between adjacent 
surficial deposits that laterally grade into 
or interfinger with one another without 
leaving any easily discernible topographic 
boundaries, e.g., the downstream gradation 
of alluvial terrace deposits into alluvial 
deposits; 
(4) recognizing landslide deposit boundaries--
whereas the upslope boundary is commonly 
defined by an easily recognized scarp, the 
toe or downslope boundary is seldom well 
defined and is difficult to locate exactly; 
(5) recognizing stable masses of bedrock within 
landslide deposits, especially where the 
bedrock may appear only as a large block 
within the surrounding landslide deposit; 
and 
(6) distinguishing between irregular or 
hummocky topography caused either by 
variations in the erosional resistance of 
bedrock or by the erosion of landslide 
deposits.


CHARACTERISTICS OF SURFICIAL 
DEPOSITS RELEVANT TO LAND-USE 
PLANNING

General background:  The physical properties and 
engineering characteristics of the mapped 
surficial deposits can be inferred from knowledge 
of the geologic processes that formed them.  Thus, 
with the information provided by this map, 
preliminary evaluations of the significance of 
the materials and processes with regard to land-
use decisions can be made.

Landslide deposits:  Landslides occur when the 
pull of gravity on earth materials overcomes 
their frictional resistance to downslope 
movement.  Slope stability is affected by 
(1) type of earth materials - unconsolidated, soft 
sediments or surficial deposits will move 
downslope easier than consolidated, hard 
bedrock; 
(2) structural properties of earth materials - the 
orientation of the layering of some rocks and 
sediments relative to slope directions, as 
well as the extent and type of fracturing and 
crushing of the materials, will affect 
landslide potential; 
(3) steepness of slopes - landslides occur more 
readily on steeper slopes; 
(4) water - landsliding is generally more 
frequent in areas of seasonally high 
rainfall, because the addition of water to 
earth materials commonly decreases their 
resistance to sliding; water decreases 
internal friction between particles, decreases 
cohesive forces that bind clay minerals 
together, lubricates surfaces along which 
slippage may occur, adds weight to surficial 
deposits and bedrock, reacts with some clay 
minerals, causing volume changes in the 
material, and mixes with fine-grained 
unconsolidated materials to produce wet, 
unstable slurries; 
(5) ground shaking - strong shaking during 
earthquakes can jar and loosen bedrock and 
surficial materials, thus making them less 
stable; 
(6) type of vegetation - trees with deep 
penetrating roots tend to hold bedrock and 
surficial deposits together, thereby 
increasing ground stability; 
(7) proximity to areas undergoing active erosion - 
rapid undercutting and downcutting along 
stream courses and shorelines makes slopes 
in these areas particularly susceptible to 
landsliding.

All the natural factors that promote landsliding 
are present in the Bay region.  In addition, man 
has at times decreased the potential for slope 
failures by leveling slopes, building retaining 
walls at the base of slopes, planting trees or 
seeding forests, as well as practicing soil 
conservation.  However, other of his activities 
have increased the potential for slope failures, 
including increasing slope angles for road or 
building construction; adding water to marginally 
stable slopes by watering lawns, improperly 
handling rain-water runoff and choosing poor 
sites for septic tank drainfields; adding to the 
weight of marginally stable slopes by building 
structures as well as by adding fill for 
foundations; and removing natural vegetation.  
Thus, slope failure, a natural phenomenon that 
has occurred throughout the Bay region in the 
past, may be aggravated by improper land use.

The landslide deposits shown on the map may or 
may not be continuously or intermittently moving 
at the present time.  The potential for continued 
movement varies greatly and depends on many 
factors, including the age of the deposits and 
their previous histories of activity.  Some 
deposits may pose no problems for many types of 
development, while development on others may 
offer serious problems.  Most landsliding takes 
place in areas where landsliding has occurred 
before, and old landslide deposits are commonly 
reactivated by either natural or artificial means.  
The materials that form landslide deposits may 
be so broken up and disturbed that landsliding 
may easily recur, especially if slope angles or 
moisture contents are changed.  Landslide 
deposits are characterized by 
(1) small isolated ponds, lakes, and other closed 
depressions; 
(2) abundant natural springs; 
(3) abrupt and irregular changes in slope and 
drainage pattern; 
(4) hummocky irregular surfaces; 
(5) smaller landslide deposits that are 
commonly younger and form within older and 
larger landslide deposits; 
(6) steep, arcuate scarps at the upper edge of the 
deposit; 
(7) irregular soil and vegetation patterns; 
(8) disturbed vegetation; and 
(9) abundant flat areas that might appear 
suitable as construction sites. 
In general, fewer of these characteristics will be 
noted in the smaller deposits.  Detailed ground 
studies, of course, are required for predicting the 
future behavior of landslide deposits under 
changing conditions.


SUGGESTIONS FOR MAP USERS

Planning departments and developers:  The 
density of landslide deposits is a crude measure of 
the importance of slope failure as an erosional 
process and, therefore, a measure of the overall 
slope stability of an area.  However, this map 
cannot be used to determine the probability of 
future landsliding, primarily because geologic 
and climatic changes during the past few hundred 
thousand years have altered slope stability and 
because the map does not provide detailed 
information regarding the composition and type 
of movement of individual landslide deposits.  
Therefore, the map should not be used as a 
substitute for detailed site investigations by 
engineering geologists and soils engineers; areas 
susceptible to landslide activity should be 
carefully studied before any development.

Geologists and engineers:  This map has been 
prepared to provide a regional context for 
interpreting detailed site investigations and 
should be used in conjunction with slope maps, 
bedrock geology maps, soils maps, and other 
available information. It is not intended as a 
substitute for site investigations, and its 
limitations should be clear.  Comments regarding 
its usefulness and accuracy would be appreciated.

Home buyers:  Areas with relatively low 
densities of landslide deposits probably have 
good slope stability compared with areas with 
high densities of landslide deposits.  However, 
landslide deposits less than 200 feet long have 
not been mapped, and the scale of this map is 
such that individual buildings cannot be 
precisely located.  In fact, areas mapped as 
landslide deposits are not necessarily less stable 
than adjacent areas.  The map, therefore, should 
not be used as a substitute for a report by an 
engineering geologist or soils engineer, because 
detailed site investigations are necessary for 
judgments about the slope stability of individual 
areas.  In addition, other types of surficial 
deposits may pose construction problems and 
require investigation.





REFERENCES


Eckel, E.B., ed., 1958, Landslide and engineering 
practice: Highway Research Board Spec. Rept. 
29, NAS/NRC 544, Washington, D.C., 232 p.
Flawn, P.T., 1970, Environmental geology; 
conservation, landuse planning, and resource 
management: New York, Harper & Row, 313 p.
Goldman, H. B., ed., 1969, Geologic and 
engineering aspects of San Francisco Bay fill:, 
California Div. Mines and Geology Spec. Rept. 
97, 130 p.
Leighton, F.B., 1966, Landslides and hillside 
development, in Engineering geology in 
southern California: Assoc. Eng. Geologists, Los 
Angeles Section, Spec. Publ., p. 149-193.
Mitchell, J. K., 1963, Engineering properties and 
problems of the San Francisco Bay mud: 
California Div. Mines and Geology Spec. Rept. 
82, p. 25-32.
Nichols, D. R., and Wright, N. A., 1971, 
Preliminary map of historic margins of 
marshland, San Francisco Bay, California: U.S. 
Geol. Survey open-file map, scale 1:125 000.
Sharpe, C.F.S., 1960, Landslides and related 
phenomena: Paterson, N.J., Pageant Books, 137 
p.
Terzaghi, Karl, 1950, Mechanism of landslides, 
in Paige, S.M., chm., Application of geology to 
engineering practice (Berkey volume): New 
York, Geol. Soc. America, p. 83-123.
Trask, P.D., and Rolston, J.W., 1951, Engineering 
geology of San Francisco Bay, California: Geol. 
Soc. America Bull., v. 62, no. 9, p. 1079-1110.
Zaruba, Quido, and Mencl, Vojtech, 1969, 
Landslides and their control: Amsterdam, 
Elsevier, 205 p.





II.

DIGITAL COMPILATION


INTRODUCTION


We compiled the landslide-deposit database at 
a scale of 1:24,000 in version 7.1.1 of ARC/INFO, 
a commercial geographic information system, or 
GIS (Environmental Systems Research Institute 
[ESRI], Redlands, California), on a UNIX 
computer using the menu interface ALACARTE 
(versions 1 through 3.1: Fitzgibbon and 
Wentworth, 1991; Fitzgibbon, 1991; Wentworth 
and Fitzgibbon, 1991). 

The digital compilation was derived from 
linework inked directly by T.H. Nilsen on 25 
USGS Mylar greenline 7.5-minute (1:24,000-scale) 
quadrangles that cover Alameda County and 
were used to photographically reproduce the 
lines on his 1975 and 1976 maps.  The linework for 
each greenline was scanned (400 dots per inch), 
converted from raster to vector form, imported 
into ARC/INFO, hand edited to remove all 
information save landslides, and combined into a 
single digital file.  Landslide outlines were 
adjusted as needed to align across quadrangle 
boundaries.  The base map for the landslide data 
comprises topography, drainage, and culture from 
1:125,000-scale USGS Bay Region Topographic 
Sheets (Aitken 1997).  A 1:62,500-scale map 
image was derived from the digital database.

The boundary of Alameda County (from 
Wentworth, 1997) was added to the database to 
delineate the county from the surrounding area.  
Landslides outside the county boundary are 
excluded from this database.  The 25 constituent 
USGS quadrangles are: Richmond, Briones 
Valley, Oakland East, Las Trampas Ridge, 
Diablo, Tassajara, Byron Hot Springs, Clifton 
Court Forebay (formerly Bethany), San Leandro, 
Hayward, Dublin, Livermore, Altamont, 
Midway, Newark, Niles, La Costa Valley, 
Mendenhall Springs, Cedar Mountain, Lone Tree 
Creek, Milpitas, Calaveras Reservoir, Mt. Day, 
Eylar Mountain, and Mt. Boardman.  Nilsen 
mapped no landslides in four additional 
quadrangles in western Alameda 
CountyÑOakland West, Hunters Point, Redwood 
Point, and Mountain View.



LANDSLIDE-DEPOSIT UNITS


In all quadrangles but one, landslides larger than 
200 feet in maximum dimension are represented in 
the spatial database by polygons enclosing their 
approximate boundaries.  In the La Costa Valley 
quadrangle, Nilsen denoted landslides 200-500 
feet across by a small arrow.  In the digital 
database, small landslides for that quadrangle 
are represented by an arrow; they are plotted on 
the landslide-deposit map from the Postscript 
and PDF plot files included with the database.  
The arrow points in the direction of downslope 
movement and is centered on the location of the 
deposit.  Although smaller landslides in the La 
Costa Valley quadrangle range  from 200 to 500 
feet, all arrow symbols are the same size, about 
200 feet long.  The primary landslide identifier in 
the La Costa Valley quadrangle is a character 
string in the field PTYPE (polygon type) for the 
large landslide data layer and PTTYPE (point 
type) for the small landslide data layer.

Values in the PTYPE and PTTYPE fields give the 
two levels of certainty Nilsen ascribed to his 
interpretations.  The values for this field are 
'Landslide deposit' and 'Landslide deposit, 
identification uncertain'.  Comparatively few 
deposits are 'uncertain'.  In both the Postscript 
and PDF plot files, large landslide polygons are 
color coded accordingly; for the small landslides 
in the La Costa Valley quadrangle, the 
classification is printed at the head of each 
arrow as annotation.


Table 1.  General PTYPE and PTTYPE Categories

Descriptions of landslide categories are modified from Landslide Map Annotation

ls
- landslide deposit

lsu
- landslide deposit, identification uncertain



SPATIAL RESOLUTION


Uses of this digital map should not violate the 
spatial resolution of the data.  Although the 
digital form of the data removes the physical 
constraint imposed by the scale of a paper map, 
the detail and accuracy inherent in map scale are 
also present in the digital data.  Because this 
database was extracted from maps at a scale of 
1:24,000 and compiled at that same scale, higher-
resolution information is not present.  
Enlargement of the database to scales larger than 
1:24,000 will not yield greater real detail, 
although it may reveal fine-scale irregularities 
below the intended resolution of the database.  
Similarly, where this database is used in 
combination with other data of higher 
resolution, the resolution of the combined output 
will be limited by the lower resolution of these 
data.


SPECIFICS OF THE LANDSLIDE-DEPOSIT DATABASE


The landslide-deposit spatial database itself 
consists of two different data layers each with 
the naming convention <pf>-extn.  The prefix 
abbreviation <pf> is either <al> for Alameda 
County or <lc> for the La Costa Valley 
Quadrangle and the abbreviation <extn> is a 
data layer extension (suffix).  The two data 
layers are described in Table 2 below.  The map 
layer is stored in UTM projection (zone 10) (see 
Table 3), and projection files are included to 
convert between that and the state plane 
projection.  Digital tics define a 7.5 minute grid of 
latitude and longitude (see 7.5 MINUTE 
QUADRANGLE GRID, below).  The contents of 
the map database are described in terms of the 
lines, areas, and points that compose it.  Terms in 
Table 4 describe the database fields.




Table 2.  Data Layers

DATA LAYER NAME
DESCRIPTION

al-slid_um
polygons for landslides, internal quadrangle 
boundaries, and Alameda County boundary

lc-smls_um
point features for small landslides for the La Costa 
Valley Quadrangle


Table 3.  Map Projection

Projection
UTM (universal transverse mercator)
Units
meters
Zone
10
Datum
NAD27
spheroid
Clarke 1866



Table 4.  Field Definition Terms

ITEM NAME
NAME of the DATABASE FIELD (item)

WIDTH
maximum number of digits or characters stored
OUTPUT
output width
TYPE
B - binary integer, F - binary floating point number, 
N - ASCII floating point number, I - ASCII 
integer, C - ASCII character string
NDEC
number of decimal places maintained for floating 
point numbers



¥ Lines  Ñ  Database lines (arcs) are recorded as strings of vectors with characteristics that are 
described in the arc attribute table (see Table 5).  In the al-slid_um map layer they define the 
boundaries of the landslide units, the quadrangle boundaries, and the Alameda County Boundary.  
These distinctions are recorded in the LTYPE database field according to the line types listed in 
Table 6.



Table 5.  Content of the Arc Attribute Table  (AL-SLID.AAT)

ITEM NAME
WIDTH
OUTPUT
TYPE
N.DEC
DESCRIPTION

FNODE#
4
5
B
-
starting node of arc (from node)
TNODE#
4
5
B
-
ending node of arc (to node)
LPOLY#
4
5
B
-
polygon to the left of the arc
RPOLY#
4
5
B
-
polygon to the right of the arc
LENGTH
4
12
F
3
length of arc in meters
PF-SLID#
4
5
B
-
unique internal control number
PF-SLID-ID
4
5
B
-
unique identification number
LTYPE
35
35
C
-
line type



Table 6.  Line Types Recorded in the LTYPE Field

LINE TYPE
DESCRIPTION

contact, certain
arcs delineating large landslide polygons
county boundary
Alameda County boundary
map boundary
quadrangle boundary



¥ Areas  Ñ  Landslide units (except those smaller than 500 feet in the La Costa Valley quadrangle) 
are recorded as vector polygons with characteristics described in the polygon attribute table (see 
Table 7).  The primary unit identifier is PTYPE. 



Table 7.  Content of the Polygon Attribute Table  (AL-SLID.PAT)

ITEM NAME
WIDTH
OUTPUT
TYPE
N.DEC
DESCRIPTION

AREA
4
12
F
3
area of polygon in square meters
PERIMETER
4
12
F
3
length of perimeter in meters
AL-SLID#
4
5
B
-
unique internal control number
AL-SLID-ID
4
5
B
-
unique identification number
PTYPE
35
35
C
-
landslide category



¥ Points  Ñ  Small landslide units in the La Costa Valley quadrangle (maximum dimension 200 to 500 
feet) are recorded as point features in ARC/INFO.  These features are plotted as arrows showing 
the direction of downslope movement and are centered on the location of the slide.  Although the 
slides range in length from 200 to 500 feet, all arrows are about 200 feet long as plotted.  In both 
Postscript and PDF plot files, the value for PTTYPE is printed as an annotation at the head of the 
landslide arrow for slides attributed as ÒquestionableÓ.



Table 8.  Content of the Point Attribute Table  (LC-SMLS.PAT)

ITEM NAME
WIDTH
OUTPUT
TYPE
N.DEC
DESCRIPTION

AREA
4
12
F
3
area of polygon in square meters
PERIMETER
4
12
F
3
length of perimeter in meters
PF-SMLS#
4
5
B
-
unique internal control number
PF-SLID-ID
4
5
B
-
unique identification number
PTTYPE
35
35
C
-
landslide category
STRIKE
3
3
I
-
direction of downslope movement, in 
degrees
REFERENCES CITED


Aitken, D.S., 1997, U.S. Geological Survey Bay 
Region topographic Sheets 1:125,000 1970: U.S. 
Geological Survey, Open-File Report 97-500.
Fitzgibbon, T.T., 1991, ALACARTE installation 
and system manual (version 1.0): U.S. 
Geological Survey, Open-File Report 91-587 B.
Fitzgibbon, T.T., and Wentworth, C.M., 1991, 
ALACARTE user interface - AML code and 
demonstration maps (version 1.0): U.S. 
Geological Survey, Open-File Report 91-587 A.
Wentworth, C.M., 1997, General Distribution of 
Geologic Materials in the San Francisco Bay 
Region, California 1: 125,000. A Digital Map 
Database: U.S. Geological Survey, Open-File 
Report 97-744.
Wentworth, C. M., and Fitzgibbon, T.T., 1991, 
ALACARTE version 1 through 3.1: U.S. 
Geological Survey, Open-File Report 91-587 C.








III.

THE DIGITAL DATABASE


INTRODUCTION


This report consists of a 1:24,000-scale digital 
database, a 1:62,500-scale digital map image 
derived from it entitled "Landslides in Alameda 
County, California, a digital database extracted 
from preliminary photointerpretation maps of 
surficial deposits by T.H. Nilsen in USGS Open-
file Report 75-277", and supporting files.  The 
report is stored as several digital files, for the 
spatial data both ARC export (uncompressed) 
and ARCVIEW shape formats, and for the map 
image both Postscript and PDF formats.  The 
exported ARC coverages lie in UTM zone 10 
projection and the shape versions are coded in 
decimal degrees.  This pamphlet, which only 
describes the content and character of the digital 
map database, is included as Postscript, PDF, and 
ASCII text files and is also available on paper as 
USGS Open-File Report 99-504.  A plotted copy of 
the 1:62,500-scale map can be ordered from a 
USGS Earth Science Information Center or by 
phone at 1-800-USMAPS.  Any or all of the 
digital files can be obtained over the Internet or 
by magnetic tape copy, as described at the end of 
this section. 

The full versatility of the 1:24,000-scale spatial 
database is realized by importing the ARC 
export files into ARC/INFO or an equivalent GIS.  
Other GIS packages, including MapInfo and 
ARCVIEW, can use either the ARC export or 
shape files.  The Postscript map image can be 
used for viewing or plotting in computer systems 
with sufficient capacity, and the considerably 
smaller PDF image files can be viewed or plotted 
in full or in part from Adobe ACROBAT software 
running on Macintosh, PC, or UNIX platforms.  
Please note that the PDF version contains two 
plot files (sheets 1 and 2) because of limits to the 
dimensions of PDF plots.


DATABASE CONTENTS

The eight sets of digital files comprising the database are encoded in more than one 
format.  Names of the files are unique designators based on the report identifier, of99-
504, followed by part numbers (i.e. 1 through 8 below) and an extension indicating file 
type.  Some files have been bundled using the tar (UNIX Tape Archive) utility (.tar 
extension).  All larger files have been compressed with the gzip utility, indicated by 
the .gz extension.  The files and their identities are as follows:

1. Revision List:  A list of the parts of the report and at what version number of the 
report each was last revised (if at all) followed by a chronological list that describes 
any revisions (see REVISIONS, below).

			of99-504revs_1a.txt	ASCII file


2. Open File Text:  The open-file pamphlet (this text), which describes the database and 
how to obtain it. 

of99-504_2a.txt		ASCII file
of99-504_2b.ps		Postscript file
of99-504_2c.pdf		PDF file


3. Database for Landslide Deposits:  ARC export coverage containing both lines and 
polygons.  Import aml will name this coverages al-slid_um.

of99-504_3.e00.gz	Arc export coverage


4. Database for Small Landslide Deposits:  ARC export coverage for La Costa Valley 
quadrangle containing points.  Import aml will name this coverage lc-smls_um.

of99-504_4.e00.gz		Arc export coverage


5. Line and polygon ARCVIEW shape files bundled as one tar file.  When opened, the 
tar file yields:

-line files
al-arc.dbf
al-arc.shp
and al-arc.shx
-polygon files
al-poly.dbf
al-poly.shp
and al-poly.shx
-point files
lc-smls.dbf
lc-smls.shp
and lc-smls.shx

Note:  Point files exist only for the La Costa Valley quadrangle.

of99-504_5.tar.gz			ARCVIEW shape files

6. Supporting files for ARC/INFO use, bundled as one tar file.  
    When opened, the tar file yields:

utm2sp.prj and sp2utm.prj:    These are projection files to convert between the UTM zone 10 
projection of the database and state plane projection.

import.aml:    This is an ASCII script written in Arc Macro Language for converting the ARC 
export files into usable coverages and INFO files that are assigned standard names (see 
IMPORTING THE ARC EXPORT FILES). 

of99-504_6.tar


7. Quadrangle Index Database:  The data files representing lines and polygons of the 
quadrangle index (ARC export and ARCVIEW shape format).  The ARC version also 
includes quadrangle names as annotation.

of99-504_7a.e00.gz	ARC export coverage containing lines, polygons, and 
annotation.  Import.aml will name this coverage al-
index_um.

of99-504_7b.tar.gz	Line and polygon ARCVIEW shape files bundled as one tar 
file.  When opened, the tar file yields:

line files:		grdlns.dbf, 	grdlns.shp, 	grdlns.shx
polygon files:	grdpys.dbf, 	grdpys.shp, 	grdpys.shx


8. Plot Files for the Map:  Landslide Deposits in Alameda County, which measures 36 
by 53 inches when plotted.  

of99-504_8a.ps.gz	Postscript file (12 MB compressed to 3 MB)
of99-504_8b.pdf.gz	PDF file (4.5 MB) 
Available on Adobe Acrobat Version 4.0 only



REVISIONS

Changes to any parts of the report (the numbered items described above and listed in 
the revision list of99-504revs_a.txt) may be made in the future if needed.  These could 
involve, for example, fixing files that don't work, correcting or adding landslide 
details, or adding new file formats or other components.  Major revision of the basic 
landslide information would result in a new report.


The report begins at version 1.00.  Any revisions will be noted in the revision list and 
will result in the recording of a new version number for the report.  Small changes will 
be indicated by decimal increments and larger changes by integer increments in the 
version number.  Revisions will be announced and maintained on the Web page for this 
report on the Western Region Geologic Information Server (see next section).


OBTAINING THE DATA FILES

The database may be obtained in three ways.

1.  The simplest way to obtain the database is to download it over the World Wide Web 
from the USGS Western Region Geologic Information Server:

http://wrgis.wr.usgs.gov

From the main page, click on 'Geologic map databases' under the heading 'Data On-
line'; next click on 'California'.  Scroll down to the listing for this database (Open File 
Report 99-504) and click on the Open-File button, which takes you to the page for this 
publication.  You can also go directly to that final page at: 

http://wrgis.wr.usgs.gov/open-file/of99-504

On this page, the several parts of the report in their different file types are separately 
available.  Set your Web browser to save to a local disk and click on the appropriate 
links to download the desired files.


2.  To download the files from the Internet via anonymous ftp:

¥ ftp wrgis.wr.usgs.gov		- make ftp connection with USGS computer wrgis.
¥ Name:  anonymous		- enter "anonymous" as your user name.
¥ Password:  [your address]		- enter your own email address as password.
¥ pub/open-file/of99-504		- subdirectory where files from this report are stored


3.  To obtain files from the database on magnetic tape, send a blank tape with your 
request specifying the desired files and your return address to:

		San Francisco Bay Geologic Materials Database
		c/o Database Coordinator 
		U.S. Geological Survey
		345 Middlefield Road   MS 975
		Menlo Park, CA  94025

The specified files bundled in a compressed tar file will be returned to you on 
the tape.  The acceptable tape types are:  2.3 or 5.0 GB, 8 mm Exabyte tape.



OPENING THE DATABASE FILES

Some of the files are packaged as tar files, and the larger files containing 
the databases and images have been compressed with gzip.  Thus, gzip is 
required to uncompress the files, and a tar utility is required to open the 
tar files.  The necessary utilities are available on-line:



Compressed Gzip Files

Files compressed with gzip (those with a .gz extension) can be 
uncompressed with gzip.  The gzip utility converts the compressed file 
name.gz to its uncompressed equivalent name.  The compressed file is 
replaced by the uncompressed file.  This utility is free of charge over the 
Internet at:

http://w3.teaser.fr/*jlgailly/gzip



Tar Files (UNIX Tape Archive)

To extract the contents of a tar file, first uncompress it with gzip if the 
extension is .tar.gz.  Once the tar extension is exposed, extract the contents 
with a tar utility.  This utility is included in most UNIX systems.  Tar 
utilities for PC and Macintosh can be obtained free of charge via the 
Internet from Internet Literacy's Common Internet File Formats Web 
Page:  

http://www.matisse.net/files/formats.htm



WinZip

This commercial package runs on PCs and can deal with both gzip and tar 
files.  An evaluation copy of WinZip for Windows 3.1, 95 and NT can be 
downloaded from:

http://www.winzip.com/winzip/:




IMPORTING THE ARC EXPORT FILES 

The ARC export files with the extension .e00 can be converted to 
ARC/INFO vector maps (coverages) and INFO files by running the 
import.aml that is included in the database.  This will import the export 
files and assign standard names (see below).  Be sure to uncompress the 
of99-504_7a.e00.gz file with the gzip utility before running the import aml.  
Run import.aml from the ARC prompt in the directory containing the 
export files:

		ARC: &run import.aml		- run import.aml 

Note that the arc coverages and separate INFO files will be given standard names:

of99-504_3.e00		(Landslide Deposit Database)	is named	al-slid_um

of99-504_4.e00		(Small Landslide Deposit
				    Database for La Costa
				    Valley quadrangle only)		is named	lc-smls_um

of99-504_7a.e00.gz	(Quadrangle Index Database)	is named	al-index_um


1
USGS OF 99-XXX
20
USGS OF 99-504