The San Andreas Fault and the Geology of the
Carrizo Plain National Monument
Nowhere does the Carrizo Plain flaunt its geologic past as it does on the eastern edge of the plain where the San Andreas Fault cuts along the base of the Temblor Range. Stream channels, such as Wallace Creek, course suddenly northward as they cross the fault line, and ridges rise sharply from the plain to form the Panorama and Elkhorn Hills. Salt encrusted sag ponds also appear along the fault, trapping rainwater that harbor fairy and brine shrimp. This complex and corrugated topography, the most spectacular along the fault's 800-mile long corridor, is best viewed in the long, soft shadows of early morning and late afternoon. About 9 million years ago boulders, cobbles, and sands began to be washed from mountains west of Carrizo Plain eastward across the San Andreas fault into the shallows of the ocean basin that is now the southern San Joaquin Valley and Temblor Range. In Pleistocene Time, about 2.5 million years ago, with the uplift of the Temblor Range, this drainage was blocked, trapping runoff within the Carrizo Plain. Runoff within the central Carrizo Plain still drains internally - resulting in Soda Lake, but erosion southward by tributaries of the Salinas River has captured much of the former drainage on the northern end of Carrizo Plain. Unlike other Pleistocene lakes throughout the Great Basin, the shore line of Soda Lake is not characterized by strand lines. Strand lines resemble a bath tub ring of aligned cobbles, pebbles and sand, at various lake levels. However  a higher lake level can be deduced from the presence of clay dunes and "slickspots" - barren shallow depressions common to sodic soils. Soda Lake, the centerpiece of the plain, is one of the largest undisturbed alkali wetlands in California. The 3,000 acre ephemeral lake provides important habitat for migratory birds and is surrounded by a rare plant community. With no outlet, the water that pools in the lake during the winter evaporates, leaving behind a glistening expanse of sulphate and carbonate salts that appear to ripple and sway in the heat waves of summer. Throughout the dry season, the wind creates white, majestic spires of salt and dust that whirl their way upward into the sky.
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