Cracked pieces of rock, plucked or torn from the bedrock, are carried with other debris in and on the glacier. This debris scrapes the valley walls and floors, leaving grooves and striations. Rock debris is crushed and ground into fine grains, called rock flour. The depth of ice and steepness of slope are primary factors affecting the flow. Alaskan glaciers vary in thickness from several hundred feet to 2,000 feet.
Movement creates elongated cracks, called crevasses, in the more brittle upper crust of the glacier.
Below depths of 100 to 150 feet, the ice, under more pressure, behaves more plastically. Wildlife Mountain goats use the rocky, high elevations of Horn Cliffs . The terrain protects the goats from predators who need more secure footing. The sparse vegetation is high quality, enriched by the nutrient-laden soil left by the glaciers. The Stikine River Delta, a haven for migrating shorebirds and waterfowl, is formed from glacial silt carried downstream by the river.