Basal sapping occurs as water flowing around the rock layers of the mesa erodes the underlying soft shale layers, either as surface runoff from the mesa top or from groundwater moving through permeable overlying layers, which leads to slumping and flowage of the shale. As the underlying shale erodes away, it can no longer support the overlying cliff layers, which collapse and retreat. When the caprock has caved away to the point where only a little remains, it is known as a butte.