The northern Sawtooth Range formed from the Eocene Sawtooth batholith, while south of Alturas Lake the mountains formed from the Cretaceous granodiorite of the Idaho Batholith. The Sawtooth Range has a history of alpine glaciation, but while no surface glaciers exist today, perennial snow fields and rock glaciers remain, usually on north or east-facing slopes. There have been 202 perennial snow fields mapped in the Sawtooth Range. The Sawtooth Range was last extensively glaciated in the Pleistocene, but glaciers probably existed during the Little Ice Age, which ended around 1850 AD. Evidence of past glaciation given remnants of the glaciers such as glacial lakes, moraines, horns, hanging valleys, cirques, and arêtes.