When erosional forces break down the rims and sidewalls of a canyon, disintegration proceeds more quickly in the softer layers of rock. Where the softer layers alternate in some regular fashion with harder layers, erosion creates a pattern of scree slopes (the softer rocks) and vertical cliff walls (the harder rock), suggesting a staircase of risers and (tilted) treads. Without this sidewall erosion, geological uplifting and down-cutting by the canyon’s streamflow system would produce a narrow, U-shaped cut in the earth. It’s erosion of the sidewalls that creates the classic V-shape of a canyon. The higher one points on the canyon wall, the longer, at that spot, erosion has been going on. The rock is eroded directly by wind and rain, by cycles of freezing and thawing that spall rock and force open cracks, and by the pull of gravity. The process goes all the faster, oddly, in landscapes with little rainfall, which lack a deflecting cover of vegetation. The orderly stacking of extended sheets of sedimentary rock, all laid down parallel to one another, like a pile of thick blankets of different weaves on a slightly rumpled bed, is what makes stair-step erosion so stark and readable.