Ok, so the technical name for this is “slickenside,” but I find “slickenslide” easier to remember.
(I loved the concept of slip n’ slides as a child.)
Anyway, a slickenside is created as the two sections of rock alongside a fault move past each other, slowly polishing the surfaces of the rock. This creates smooth, slick rock faces along a fault line. Sometimes you can find “slickenlines” on the slickenside, which are small, directional scrap marks left by the movement.
In the above picture, the slickenside (and fault) cross at about 35 degrees from the left to the right. In the below picture, the slickenside is more difficult to see, but is at about 60 degrees.
That, it should be noted, is the definition of slickenside that I learned on a field trip – if anyone has a more technical definition, I would be really interested to hear it!