Index map
California's Central Valley resembles a
great elongate bath tub. Its present, remarkably flat surface
consists largely of material eroded from the rising Sierra
Nevada and Coast Ranges to the east and west, respectively,
and deposited in low alluvial fans. On more than one occasion
the valley impounded a large lake, which left behind a veneer
of muddy deposits. About 650,000 years ago, rising waters of the
most recent lake carved a gap through the mountain range to the
west and drained into the Pacific Ocean through a low pass just
south of the city of San Francisco.
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