Glaciers form where more snow falls than melts. A glacier's accumulation area, located at higher elevations, accrues a wealth of snow and ice. The ablation area, located at lower elevations, loses ice through melting (downwasting) or calving. A glacier's terminus or face advances when more snow and ice amass than melt, and it retreats when melt exceeds accumulation. When melt equals accumulation, a glacier achieves equilibrium and its face remains stationary. Whether the glacier's face is advancing or retreating, glacial ice persistently glides down-valley.
Coerced by gravity, ice pursues the path of least resistance. Ice depth and bedrock angle influence the rate of glacial flow. Glaciers contain two zones of ice flow. The zone of plastic flow, ice closest to the bedrock, experiences extreme pressure from the weight of the ice above and conforms to the anomalies in the bedrock. The zone of brittle flow, the upper 150 feet of glacial ice, lacks this pressure and reacts in-elastically to the bedrock features, forming elongated cracks called crevasses which fluctuate with the glacier's flow. Tubular chutes or moulins drain surface meltwater, and formidable spires of ice called seracs reach skyward. Ice plummets over particularly steep terrain creating ice falls. One theory suggests that differences in seasonal flow rates over an icefall create the convex bands called ogives at the base of the falls, which undulate down glacier. The erosive power of glacial flow changes the landscape and scrapes much of the soil and rock from the valley walls that channel its irrepressible flow.