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Part of Hall of Planet Earth.
AMNH/R.Mickens
Slickensides are grooves and steps created on a fault surface by the slow, grinding movement of two bodies of rock against each other. When the surface is gently rubbed parallel to the grooves, it feels slightly rougher in one direction than in the other because of the presence of small steps on the surface. The movement on the fault was parallel to the grooves and toward the smooth-feeling direction.
AMNH collection