As glaciers move slowly through mountain valleys, they erode rocks and soil away, leaving behind various landforms in their wake.
Some of these landforms can be both destructive and constructive. Glacial erosion creates several features such as faceted clasts, striations and grooves in rock, glacial pavements and rock flour that form as it erodes.
Cirques
A cirque is an amphitheater-shaped depression at the head of a glacier valley that typically contains steep-sided mountain walls on all sides, often having flat floors, which may contain lakes (tarns) when glaciers retreat; otherwise they contain steep-sided mountainsides enclosing sharp aretes, cols, and horns due to erosion by glacial waters.
Glacial erosion creates its unique signature on Earth by loosening and transporting rock particles over long distances, as well as scouraging and eroding bedrock of the ground surface. When these rocks are eventually dumped by melting glaciers into an unsorted deposit called moraine.
Landforms produced by glacial erosion include U-shaped valleys, U-shaped cirques, aretes, horns and moraine; most people recognize these features as evidence of former glaciers. Furthermore, erosion may create striations patterns on rocks steps and even create fjords reflecting various interactions such as climate conditions and geologic history of an area.
U-shaped valleys
Glaciers’ massive weight erodes the landscape, leaving behind U-shaped valleys with flat bottoms and steep, straight sides – distinct from V-shaped valleys carved by rivers.
Glaciers moving down valleys concentrate their erosive actions, widening the floor and oversteepening walls, before melting away to leave behind distinct steep-walled valleys known as fjords or hanging valleys that remain.
Glacial erosion is a slow process in which rock particles rub together, leaving features such as facets (abrasion marks on rocks), striations patterns and grooves as it erodes away. Rock flour produced when rocks abrade against each other during glacier movement becomes part of this fine powder known as rock flour; while V-shaped river valleys become U-shaped glacial valleys through this phenomenon known as “scouring.”
Horns
Glaciers erode rock by plucking, the process whereby bits of embedded rock in a glacier are pulled apart by its weight, leading to deep scratches in its undersurface, creating long parallel grooves known as striations that may reach centimetres deep and several meters long.
Ice is not an effective agent of erosion due to its soft nature (Mohs hardness of 1.5 at 0degC), instead rock fragments embedded in it are the main agents responsible. For this reason, continental glaciers erode differently from alpine ones.
Horns have long been used as tools by humans for various purposes, from storing gun powder to decoration and ceremonial purposes. Furthermore, animals have used them in courtship rituals; male blue wildebeests for instance use their horns during mating ceremonies as displays of courtship behaviour.
Moraine
Moraines are long mounds composed of glacial till – boulders, pebbles, sand and mud – deposited by glaciers during melting periods. Moraines form as moving glaciers pick up rocks and soil that they deposit with each pass over its path as its plowing effect deposits them along its route.
Moraines typically form in front of or along the sides of glaciers. A moraine formed at the front is known as a terminal moraine while those along either side can be classified as either lateral moraines or medial moraines.
Glacial erosion transforms landscapes by wearing away hilltops, polishing bedrock into polished surfaces and softening valley bottoms. Furthermore, glacial erosion widens valleys generated by streams into U-shaped cross profiles; its presence indicating glacial influence. Horns, moraines and U-shaped valleys are telltale signs that an area has been altered by glacial erosion.