Where a river encounters a steep cliff, it descends as a waterfall. Likewise, where a glacier encounters a steep cliff, it descends as an icefall. To an observer, the icefall appears as immobile as a sculpture; however, compared to the glacier’s normal velocity, the ice actually “pours” down the cliff at speeds of hundreds of yards per year. Ice on the underside of the curving glacier moves over a shorter distance than ice at the top, causing the surface to fracture into a fantastical ice scape of crevasses, seracs, ogives, septa, and pinnacles. John Muir and his dog Stickeen, on a hike across a glacier in Alaska’s Glacier Bay in 1880, came upon an icefall that “in the form of a magniﬁcent ice-cascade two miles wide, was pouring over the rim of the main basin . . . its surface broken into wave-shaped blades and shattered blocks, suggesting the wildest updashing, heaving, plunging motion of a great river cataract.” At an icefall’s base, glacial movement is arrested. The ice thickens, compressed now, restoring a smoother surface to the glacier’s ﬂow. With valley glaciers, an icefall often separates a glacial plateau from a glacial basin. The chaotic, collapsing walls of icefalls intimidate even great mountaineers, as suggested by the name of one on Alaska’s Mendenhall Glacier: the Suicide Icefall.