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Wallace Creek did not always look the way it does today.  Its shape has been constantly changing over time.  If you study the present configuration of Wallace Creek and think about how the San Andreas fault behaves, you might be able to guess what the creek looked like in the past.

At Posts 1 and 2, you learned that when the modern channel was first cut, its path was straight across the fault.  Since then, the shape of the channel has been changed by motion along the fault.  What did Wallace Creek look like before the modern channel was cut?

Remnants of an older channel are still visible.  If you look across the modern channel from Post 2, you will see another channel in the distance, draining toward the plain to the southwest.  This is a much older downstream segment of Wallace Creek.  It has been completely separated from the upstream portion.  This is known as a beheaded channel.  About 10,000 years ago, it lined up with the upstream segment of the creek.

Earth scientists can see what the original, older channel looked like by going backwards in time along the fault.  By "sliding back" the fault, geologists imagine Wallace Creek evolving in this manner:

(a) (c)
(b) (d)

Wallace Creek (a) when the older channel was first cut; (b) after the older channel had been offset some distance, but before the modern channel was cut; (c) just after the modern channel was cut; and (d) in its present form.  The shaded channel is the active channel at the time depicted.

In the future, Wallace Creek will erode a new path straight across the fault, and the downstream portion of the modern channel will become another beheaded channel.