This geologic map database for the El Mirage Lake area describes geologic materials for the dry lake, parts of the adjacent Shadow Mountains and Adobe Mountain, and much of the piedmont extending south from the lake upward toward the San Gabriel Mountains. This area lies within the western Mojave Desert of San Bernardino and Los Angeles Counties, southeastern California (see Fig. 1). The area is traversed by a few paved highways that service the community of El Mirage, and by numerous dirt roads that lead to outlying properties. An off-highway vehicle area established by the Bureau of Land Management encompasses the dry lake and much of the land north and east of the lake. The physiography of the area consists of the dry lake, flanking mud and sand flats and alluvial piedmonts, and a few sharp craggy mountains. This digital geologic map database, intended for use at 1:24,000-scale, describes and portrays the rock units and surficial deposits of the El Mirage Lake area. The map database was prepared to aid in a water-resource assessment of the area by providing surface geologic information with which deepergroundwater-bearing units may be understood. The area mapped covers the Shadow Mountains SE and parts of the Shadow Mountains, Adobe Mountain, and El Mirage 7.5-minute quadrangles (see Fig. 2). The map includes detailed geology of surface and bedrock deposits, which represent a significant update from previous bedrock geologic maps by Dibblee (1960) and Troxel and Gunderson (1970), and the surficial geologic map of Ponti and Burke (1980), it incorporates a fringe of the detailed bedrock mapping in the Shadow Mountains by Martin (1992). The map data were assembled as a digital database using ARC/INFO to enable wider applications than traditional paper-product geologic maps and to provide for efficient meshing with other digital data bases prepared by the U.S. Geological Survey's Southern California Areal Mapping Project. Dibble, T.W. (1960), Preliminary geologic map of the Shadow Mountains quadrangle, Los Angeles and San Bernardino Counties, California, Troxel, B.W. Gunderson, J.N. (1970), Geology of the Shadow Mountains and northern part of the Shadow Mountains SE quadrangles, western San Bernardino County, California, Ponti, D.J. Burke, D.B (1980), Map showing Quaternary geology of the eastern Antelope Valley and vicinity, California: U.S., Martin, M.W. (1992), Stratigraphic and structural evolution of the Shadow Mountains, western Mojave Desert, California: Implications for the tectonic development of the central and western Mojave Desert, 196, University of Kansas, Unpublished Ph.D. dissertation Source.

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