What I did find is that you are talking about the Temblor Range.

On this page ( http://www.ca.blm.gov/bakersfield/georesources.html ) it says:

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.

Looks like your guess on the sands is probably right.

Canie

• Carrizo Plains