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.
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). Debris flows are absent from the
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.
Process_Description: Creation of original metadata record
Contact_Person: Jennifer Lenz
Contact_Organization: US Geological Survey
Address: 12201 Surise Valley Drive, Mail Stop 918
Originator: Nilsen, T.H.
Originator: Bartow, J.A.
Originator: Frizzell, V.A.
Originator: Sims, J.D.
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
Series_Name: U.S. Geological Survey Open-File Report
Source_Citation_Abbreviation: Nilsen and others, 1975
Source_Currentness_Reference: publication date
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. The area included by the database was mapped by Nilsen only.