Action of Streams and Glaciers.
In the layers forming the crater wall there is evidence of the action of water. In some places this is shown by the cutting of valleys; in others, by the accumulation of water-carried gravel and boulders.
Glacial ice, carrying sand, pebbles, and boulders, scratches and polishes rock surfaces over which it moves. Glacial polish and thick beds of glacial debris are common around the mountain. They occur on the surface rock and between earlier layers, showing that glaciers existed at various stages in the history of the mountain.
U-shaped valleys, such as Kerr Notch, Sun Notch, and Munson Valley on the southeast slope of Mount Mazama, are evidence of glaciation. The lava flow which formed Llao Rock filled an ancient glacial notch.