Heavily eroded by glaciers, which no longer exist on Diamond Peak, the mountain has several cirques from glacial erosion, some of which have talus deposits due to Holocene glacial motion. Glaciers also placed both lateral and ground moraines along the lower to middle elevations of the wilderness area, unlayered pebble deposits interbedded with sand and rock. These deposits have very few weathering rinds, corresponding to their production during the late Pleistocene. Diamond Peak lies directly on the Cascade Crest and therefore receives a heavy snowpack. Snowfields on the northern side may have been glaciers as recently as 100 years ago.