Ground Water Atlas of the United States -
Segment 1 California Nevada
Geologic Setting
The intermontane basins in the coastal mountains of California are structural
troughs or depressions that parallel the coastline and formed as a result of
folding and faulting (fig. 103). Most of the folds and faults trend
northwestward and result from the deformation of older rocks by the intense
pressures of colliding continental plates. The rocks that underlie the basins
and form the surrounding mountains are primarily marine sediments and
metamorphic and igneous rocks, all of which are of Mesozoic age but locally
include rocks of Cenozoic age.
The basins are partly filled with unconsolidated and semiconsolidated marine
sedimentary rocks that were deposited during periodic encroachment of the sea
and with un-consolidated continental deposits that consist of weathered igneous
and sedimentary rock material which was transported into the basins primarily by
mountain streams. These marine sediments and continental deposits are tens of
thousands of feet thick in some basins. In the basins just north of San Francisco
Bay, permeable basalt and tuff compose a portion of the materials
overlying the older consolidated rocks. In most basins, however, almost all of
the permeable material consists of unconsolidated continental deposits,
primarily sand and gravel (fig. 103).
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