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603284-001 <br />2.0 GEOLOGIC CONDITIONS <br />2.1 Regional Geoloqv and Tectonics <br />The subject site is located in the Downey Plain within the southeastern margin of <br />the Los Angeles Basin. The Basin is a large structural depression within the <br />Peninsular Ranges geomorphic province of California. In general, the Downey <br />Plain is bordered by the Coyote and Peralta hills on the north, the Santa Ana <br />Mountains and Tustin Plain to the east, the Pacific Ocean to the south, and Los <br />Angeles County to the west. Several broadly warped coastal mesas represent <br />uplifted areas along the Newport-Inglewood structural zone. These mesas are <br />separated by erosional gaps which were created by historic routes of the Santa <br />Ana River. <br />The site lies near the lower reaches of the Santa Ana River. This lower alluvial <br />fan reach is a low-gradient, broad sandy fan that merges with marine deposits <br />near the coast. Differentiation of deposits of the alluvial fan, now largely covered <br />by asphalt and concrete, is based in part on the soil map of Eckmann and others <br />(1919). The surface distribution of Holocene sediments, as recorded in early <br />editions of regional soil survey maps (Eckmann and others, 1919) suggests that <br />the Santa Ana River has recently wandered back and forth across the Downey <br />Plain from Alamitos Bay to Newport Bay. Historical accounts and documents <br />further support the process of widespread sheet flooding as the dominant <br />depositional process associated with the Santa Ana River prior to the <br />construction of Prado Dam in 1941 (California Department of Water Resources, <br />1959). <br />2.2 Site-Specific Geoloqv <br />The Quaternary age fluvial soils that cover the floor of the Downey Plain are <br />composed primarily of unconsolidated recent floodplain and channel deposits <br />consisting predominately of sands derived from within the watersheds that drain <br />the surrounding mountains and hills with finer clays and silts deposited over the <br />broad floodplain of the Santa Ana River (Figure 3, Regional Geology Map). Brief <br />descriptions of these units, as observed on the site, are presented below. <br />-6- <br />Leighton