Despite its relatively young age, every part of South Sister other than its peak has undergone significant erosion due to Pleistocene and Holocene glaciation. Between 30,000 and 15,000 years ago, South Sister's southern flanks were covered with ice streams, and a small amount of ice extended below . On the volcano's northern flank, below the summit peak, erosion from these glaciers exposed a headwall about high. During the Holocene, smaller glaciers formed, alternating between advance and retreat, depositing moraines and till between on the mountain. The Lewis and |Clark glaciers have cirques, or glacial valleys, that made the outer walls of the crater rim significantly steeper. The slopes of South Sister contain small glaciers, including the Lost Creek and Prouty glaciers.