Slickenside
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Slickensides occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved. Slickenside formation may involve simple friction, or if the fault surface was once deeply buried, actual growth of oriented mineral grains may respond to the forces on the fault.
Fault motion, except at depth, does not seem to heat the rocks appreciably. The ground-up rocks along a fault may turn into a rock called cataclasite. In rare circumstances true melts called pseudotachylytes can form, but that occurs on superfaults or major subduction zones, not the ordinary strike-slip fault shown here.
See a gallery of slickensides that includes several more photos from this spectacular site in San Francisco's Corona Heights.
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