ventifact - large display specimen of a rock faceted by wind-blown sand
Ventifacts are formed when a rock remains in one position on the desert floor for a long period without being disturbed and is faceted by wind-blown sand. Typically, ventifacts have two or more flat faces - facets - with a sharp ridge separating one facet from another. The "textbook" example is a dreikanter, three cornered and symmetrical, though these are not often encountered.
This ventifact was on the way to becoming a dreikanter. If the thin tail (at the top of the photo) had been worn away, a third faceted face would likely have formed there.
Many ventifacts form in Banning Pass, California, where this was collected. The pass separates the Los Angeles Basin from the Mojave Desert to the east, and is a funnel for strong winds, with wind turbine farms taking advantage of the blast to generate electricity. There is an ample supply of sand, as the pass separates the two highest peaks in Southern California, Mt. San Jacinto on the south, where the top is accessible via the Palm Springs tram, and Mt. San Gorgonio on the north. The San Andreas Fault runs roughly east-west through the pass. At the east end, Garnet Hill is covered in spectacular ventifacts. Many are large, granitic, and well sand blasted. Since Garnet Hill is a regular geology teaching, research and field trip stop, we don't collect there.
In the field photo of the large Garnet Hill ventifact, the wind direction is from left to right. Wind-blown sand has drilled the holes.
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