Middle Sister volcano's east face with shingled stacks of thin lava...

The east face of Middle Sister is 350 m (1150 ft) high. Ice-sculpted pile of lavas at right, variously called "Black Hump," "Prouty Point," or "Step Sister," has 190 m of relief and consists of at least five flows. Hayden Glacier is below both summits; 25-m-thick cliff in front of right side of glacier is terminal lobe of fissure-fed andesitic scoria, spatter, and fountain-fed lava flow that drapes south flank of North Sister; dated at 21 ±6 ka, this flow (unit anh) is much younger than North Sister but about same age as Middle Sister and "Black Hump." Well-formed moraine in foreground was left by Hayden Glacier in Little Ice Age time.