at the upper ends of denali's glaciers are steep-walled semicircular basins called cirques. cirques form from freeze-thaw cycles of meltwater in the rocks above the glacier, and by glacial erosion and mass wasting occurring under the glacier. as cirques on the opposite sides of a ridge are cut deeper into the divide, they form a narrow, sharp, serrated ridge called an arête. as the arête wears away from glacial ice breaking it down, the low point between cirques is called a col (or if it is large, a pass). cols are saddle-shaped depressions in the ridge between cirques. a spire-like sharp peak, called a horn, is formed when cirques cut back into a mountaintop from three or four sides.