Ice as a Geomorphic Agent
Glaciers are among the most powerful erosive agents on Earth. A valley glacier hundreds of meters thick, sliding over its bed at tens of meters per year, sculpts landscapes at rates far exceeding those of rivers in the same setting. The transformation from V-shaped fluvial valleys to U-shaped glacial troughs is one of the most dramatic landscape changes in geomorphology, typically accomplished over 10,000-100,000 years of glacial occupation.
Abrasion & Quarrying
Glacial erosion operates through two complementary mechanisms. Abrasion occurs when rock fragments embedded in basal ice scrape across bedrock, producing polished surfaces, striations, and fine rock flour that gives glacial rivers their milky color. Quarrying — the more effective process — occurs when subglacial water pressure fluctuations cause ice to freeze onto bedrock blocks and pluck them from the bed, especially on the downstream faces of bedrock bumps (roches moutonnees).
The Glacial Erosion Law
Empirical and theoretical work suggests glacial erosion rate scales with basal sliding velocity and effective normal stress: E = K·u_b^l·τ_b^m. This means erosion is concentrated where ice is thick (high pressure) and fast-moving (high sliding). Since ice thickness and velocity are greatest at the valley center, the floor erodes faster than the walls, progressively widening and deepening the valley into its characteristic U-shape.
Glacial Landscape Legacy
Glaciated landscapes bear unmistakable signatures: U-valleys, cirques (armchair-shaped hollows where glaciers originate), aretes (knife-edge ridges), horns (pyramidal peaks like the Matterhorn), hanging valleys (tributaries left stranded above the main valley floor), and fjords (drowned glacial troughs). These landforms, sculpted during Pleistocene ice ages, dominate the scenery of Scandinavia, the Alps, Patagonia, and New Zealand.