A tremendous accumulation of snow, compacted by weight and turned to ice, often carrying rock and sediment debris with sometimes a meltwater river below, a glacier moves with gravity due to its own mass. Glaciers may flow—and always in a distinct direction—from a few feet to several miles per year. A glacial system can be as large as a continent, or it can fill a small valley between mountains—an alpine or valley glacier. From the high cirque where snow recrystallizes into firn, then ice, and begins to move, down valleys that shape and are shaped by the moving mass of ice, to the terminal snout where low-altitude temperature ends the ice journey, the alpine glacier reveals geologic processes accessible to perception in human time. Glaciers, past and present, cause massive changes in our landscape; more change would be wrought if, for example, the Greenland glaciers were to melt and flood the world’s seacoasts. Glacial ice cores are texts for ancient pollen and atmospheric conditions. The weight of the great continental glaciers not only shaped in passage but also physically depressed whole landscapes so severely that the land continues to rise (“isostatic rebound”) thousands of years after the melting of the glacial ice. In motion, glaciers shape and polish bedrock features, convey and deposit erratic rocks and massive moraines, grind rock flour that turns meltwater streams milky, and in their accumulation and ablation exhibit evidence of climatic change.

Kim Stafford