Slickenside

In geology, a slickenside is a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. This surface is normally striated in the direction of movement. The plane may be coated by mineral fibres that grew during the fault movement, known as slickenfibres, which also show the direction of displacement. Due to irregularities in the fault plane exposed slickenfibres typically have a stepped appearance that can be used to determine the sense of movement across the fault. The surface feels smoother when the hand is moved in the same direction that the eroded side of the fault moved (see diagram for explanation), as the surface steps down in that direction, like the scales on a fish when stroked from the head.