In Innocents Abroad, Mark Twain described how he once camped on a glacier, hoping it would transport him miles while he slept. He wanted to hitch a ride on an ice stream, a term that refers to the movement of a whole glacier down a valley. Twain was disappointed; the imperceptible creep of such a glacier might be better compared to the slow, plastic ﬂow of warm putty than to the swift ﬂow of water. If it adheres to the bed and walls of a valley, an ice stream moves by slow deformation. Other ice streams slide forward on sheets of meltwater. When glaciers from side valleys push into a valley glacier, the result is a compound glacier, made of two or more ice streams “ﬂowing” side by side. These parallel streams are visibly delineated, separated by long, sinuous black stripes called medial moraines, as the encroaching glaciers deposit trails of rock debris scraped from the valley walls onto the trunk glacier’s surface. Ice stream also refers to another phenomenon. Within ice sheets and ice caps, ice streams—narrow zones of faster-moving ice— are bounded by broader regions of slower-moving ice. The border between fast and slow ice is marked by spectacular crevasses. A bird’seye view reveals ice streams as bands within the ice that “overﬂow” the ice margin. The forces that cause ice streams are not fully understood, though landscape features under the ice sheet may be factors. For instance, ice ﬂowing over buried valleys increases in velocity in the same way that a ﬂooding river ﬂows faster in its deep channel than it does over its ﬂoodplain.