A glacier is an accumulation of snow and ice into a large mass that moves. The term used to describe the movement of glaciers is flow. Glacial flow can occur in two different ways, basal slip or plastic flow. In basal slip, gravity causes an entire glacier to slip downhill along the ground. In plastic flow, large amounts of pressure from overlying ice causes the underlying ice of a glacier to change shape and become distorted. This glacial flow usually begins approximately 50 meters below the surface of the ice. Unlike erosion that is caused by streams, when glaciers erode they carry all of the matter that they accumulate until the movement of the glaciers is halted and the glaciers melt.
Glaciers can cause two types of erosion, abrasion and plucking. In abrasion, the glacier and all of the matter that it contains migrates over the surface of bedrock. This scouring movement changes the appearance of the bedrock by leaving it smooth and polished. If the eroding glacier contains large boulders, these boulders may leave grooves in the bedrock as the glacier erodes. In plucking, melted water from the eroding glacier soaks into cracks in the rock that lies under the glacier. When the water refreezes it expands causing the surrounding rock to fracture. These pieces of rock are then picked up by the eroding glacier and carried downward.
There are many landforms that can result from eroding glaciers. A cirque is a depression that forms at the head of a glacial valley. It is surrounded on three sides by walls of rock and is formed when glaciers pluck rocks from the sides and bottom of a valley. When the glacier melts it may leave a lake in the cirque depression. A glacial trough is a U-shaped trough that is created when a large glacier moves down a valley that was once occupied by a flowing stream. The movement of the glacier causes the valley to rapidly widen and deepen. A hanging valley is a smaller valley that remains above the large glacial trough formed from the movement of smaller glaciers. These occur because the movement of smaller glaciers causes significantly less widening and deepening of the valley than does the movement of larger glaciers. Hanging valleys often result in waterfalls after the glaciers have melted and stream flow resumes. Arêtes are sharp mountain ridges that are carved by the erosion of glaciers through valleys.