Mesas get smaller as their cliffs erode and become small, flat-topped hills called BUTTES. Buttes are formed by erosion when hard rock covers a layer of softer rock that is easily worn away. The hard rock avoids erosion while the soft rock around it wears down.
A CANYON is a deep valley between cliffs often carved from the landscape by a river. This erosion takes a very long time. The canyon is formed by the water flowing through the river valley eroding the earth beneath it and carrying away loose rock and soil. The rock and soil being carried away weathers the ground beneath it by one object rubbing against another. Canyon walls are often formed of strong sandstones or granite. The Colorado River and the Snake River run through canyons. Over time, a canyon valley can get deeper and deeper.
VALLEYS are formed by a rivers or glaciers. A valley formed by flowing water is V-shaped. If the water is moving fast, there will be a narrow valley at the bottom. Slower moving waters create wider valleys. Valleys created by glaciers are U-shaped and they are created when the glacier warms up and disappears.