South Sister volcano's northeast face with stacks of andesite lava flows and oxidized (red colored) scoria.
Prouty Glacier headwall on the northeast face of South Sister. Dipping stacks of lava and scoria were excavated by Prouty Glacier, exposing cross-sectional views of the last 33,000 years of summit activity. The 350-m-thick (11,500-ft) lava stacks are truncated at about 3000 m (9,800 ft) elevation, forming the ragged rim of a broad paleocrater. Gray, stratified, dacitic phreatomagmatic deposit 150 m thick (500 ft) makes up upper slopes of the edifice, capped only by a thin ledge of mafic agglutinate on the skyline. Beneath Hodge Crest (left shoulder of edifice) is another thick, gray phreatomagmatic deposit, this one cut by many dikes that altered host fallout, which erodes into colorful spires. Little Ice Age moraines in foreground are barren and unstable.
Hildreth et al., 2013