The quasi-periodic oscillations between normal and fast motion exhibited by surge-type glaciers provide the best observational opportunity to determine limiting conditions that allow fast motion. The measurements from Variegated Glacier prove that its surge motion is caused by rapid sliding induced by high water pressure. This arises from a major restructing of the basal hydraulic system, which impedes water discharge prior to and during surge. Although the evolving glacier geometry and stress distribution play a principal enabling role, the seasonal timing of two distinct surge pulses, each initiated in winter and terminated in summer, indicates a major influence from variable external water inputs. This influence is not considered in existing surge models and should promote caution in the use of data from temperate and subpolar surge-type glaciers to deduce surge potential in polar ice masses. The spatial spreading opf surge motion from a zone of local initiation occurs by stress redistribution, which may spread the surging zone rapidly upglacier or downglacier inside a region of active ice, and by mass redistribution with compressional thickening at the surge front, which enables down-glacier propagation into less active ice. The data from other surge-type glaciers, including the extensive data from Medvezhiy Glacier, are not inconsistent with the above processes but are inadequate to establish whether completely different mechanisms operate in some surges. The way by which water accumulates and produces fast sliding is not established in detail for any surge-type glacier and may be different on different glaciers depending, for example, on the presence or absence of unconsolidated debris between the ice and rock. ¿ American Geophysical Union 1987