This map is the first of several in the San Francisco Bay region showing the distribution and differentiation of the late Cenozoic alluvial, estuarine, and volcanic deposits. The sedimentary deposits of gravel, sand, silt, and clay were separated into geologic map units on the basis of their post-depositional soil development, texture, and geomorphology. Some of the geologic units are associated with different landforms having recognizable topographic expression such as alluvial fans, natural stream terraces, levees, and interfluvial basins. The relative ages of these unites were established on the basis of intensity of soil profile development, stratigraphic position, and geomorphic expression. The older deposits exhibit strongly developed soil profiles with strong horizon differentiation whereas younger deposits display minimal soil profile development, consisting primarily of organic matter accumulations near the land surface. Geomorphic expression and degree of erosion and dissection were additional criteria used to aid in the age determinations. For example, younger deposits form well-defined morphologic features such as levees, terraces, and broad, undissected alluvial fans along the margin of the bay basin and are related to present drainage patterns. The oldest deposits shown on this map (QTs) are structurally deformed by folding and faulting and therefore exhibit no original depositional geomorphic features. These deposits are not related to present drainage patterns but suggest earlier patterns much different from those existing today.
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USGS Numbered Series
Geologic map of Late Cenozoic deposits, Santa Clara County, California