Slickensided serpentine
When a fault moves, the rocks on each side will usually have some texture to them. Some little parts will stick out and press into the opposite plane. When the fault is not moving, this roughness contributes to the static friction that keeps the fault locked, but when the stress on the fault overcomes this friction these grains will rub into the opposite plane of the fault. They grind into the opposing rock face, the exact same effect you might make using sandpaper to grind into a solid surface.
Since faults typically move in a single direction, the faces of the fault are ground in lines that match the fault motion direction. These lines are called slickensides or slickenlines, and they can be used to both distinguish fault planes from other cracks in rocks and to illustrate the direction faults moved. A trained hand running across a plane like this could likely even tell which side of the fault moved up and which side moved down as the plane will be ground much smoother in the direction the fault actually moved than in the opposite direction.
This rock is a serpentinite, the type of rock that forms when minerals of the mantle are exposed to water and fluids near the surface. These often are found where ocean basins closed through subduction as the violent process of closing an ocean can trap slivers of mantle rocks in-between. This fault is found in Central Washington State, in the middle of rocks that were violently accreted to North America as island arcs over the last 100 million years.
-JBB
Image credit: Richard Droker https://flic.kr/p/5zoBnP
