Slickensides are naturally polished rock surfaces that occur when the rocks along a fault rub against each other, making their surfaces smoothed, lineated, and grooved. Their 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. Slickensides appear to lie between the grinding of shallow rocks that makes fault gouge
) and the deep-seated friction that melts rock into pseudotachylites
Slickensides may be scattered surfaces as small as your hand or, in rare cases, thousands of square meters in extent. The corrugations show the direction of motion along the fault. Unusual minerals may occur given the combinations of fluids and pressures along slickensides. But even familiar rocks, as we will see, take on unusual features too.