Sol 4426, July 6, 2016. Two Navcam frames show the rock feature on the floor of Marathon Valley that is dubbed informally as “the Mesa” or “the Stump.” It is seen also here.
The end of the Mesa nearest the rover appears to have linear striations similar to the rock pavement nearby. This may come from the internal structure of the rocky material or it could come from etching by wind-blown particles funneling through the valley. (Click image to enlarge it.)