A slickenside is a smooth, often shiny surface of a fault plane, caused by the movement of rocks along the fault.
Slickensides are important for geologists studying the movement of the Earth's crust.
Examples:
The geologist studied the slickenside on the fault plane.
The slickenside was shiny and smooth.
The slickenside showed where the rocks moved.
The slickenside looked like a mirror on the fault plane.
The slickenside was so shiny, it looked like a diamond.
The slickenside was the most interesting part of the rock for the geologist.