There are also deep canyons which have been cut by the rivers in their present courses, especially by the Snake river and its tributaries.
The two Cheyenne Canyons, with walls as high as 1000 ft.
Several smaller basins occur in Montana, all somewhat dissected and drained through narrow gorges and canyons by members of the Missouri system.
On its slope, which rises abruptly from the Bitterroot Basin, glaciers have cut canyons between high and often precipitous walls, and between these canyons are steep and rocky ridges having peaked or saw-toothed crest lines.
The canyons are largely the work of rivers, modified by glaciers that ran through them after the rivers had formed them.