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Post 1.  Look northeast.  Wallace Creek drains through the channel you see (the channel being the bed of the creek, which is dry most of the year), downhill from the Temblor Range toward the plain to the southwest.  The creek is fairly straight until Posts 1 and 2, where you can see that it sharply bends.  What causes these bends?  Why doesn't the upstream half of the channel line up with its downstream half?

Observe that the San Andreas fault crosses Wallace Creek almost exactly through the two tight bends.  The fault cuts across the surface of the Earth from northwest to southeast.  Everything that lies southwest of the fault, including the low Carrizo Plain and the downstream half of Wallace Creek, has been sliding slowly to the northwest (toward San Francisco).  Everything that lies northeast of the fault, including the Temblor Range and the upstream half of Wallace Creek, has been sliding to the southeast (toward Los Angeles).

When the "modern" channel of Wallace Creek first formed about 3800 years ago (see Science in Action), it cut a path directly across the fault.  There were no bends.  Repeated motion along the fault, due to large earthquakes every few hundred years, caused the upstream half of Wallace Creek to break away from its downstream half.  Geologists call this feature an offset channel.

(a)              (b)  

Evolution of the modern channel of Wallace Creek:
the modern channel when it was first cut across the fault, before it was offset by motion (earthquakes) along the fault;  (b) the modern channel today, after being offset 420 feet.

There is more to see here than the modern channel.  Before the modern downstream leg of Wallace Creek was cut, Wallace Creek occupied another channel.  As you walk back down the hill from Post 1, try to pick out the older channel.  (Look carefully -- it may not be that easy to see!)  Can you connect what you can see to the San Andreas fault's history?