Sag Pond
(c) 2004 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Sag ponds are depressions that mark transcurrent, or strike-slip, fault zones (see the three types of fault). In this photo, taken from Soda Lake Road at the southern end of Carrizo Plain National Monument, the San Andreas fault zone runs along a linear valley. The fault zone is actually a set of nearly parallel, overlapping fault segments that respond sensitively to the subtly changing stresses that affect the region. Where fault segments diverge, the ground between them may subside in a sag basin. In two places here the subsidence has carried the ground below the water table, resulting in sag ponds.
Other sag ponds are shown in the San Andreas fault tour, the Hayward fault gallery and the Oakland geology tour.
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