Glaciers are powerful landform-shapers, and through erosion they can transform certain kinds of landscape. We will discuss two of the more prominent glacial erosion processes: plucking and abrasion.
Glacial erosion rates vary across once glaciated areas and appear to be driven primarily by climate and glacier thermal regime.
Corries
Glacial erosion can produce some fascinating features, such as striations (grooves in rocks), glacial pavements and rock flour (finely ground up rock). But for it to work effectively it must take place in the right places at the appropriate times.
Snowfall on mountain slopes can create a corrie, an area with low ground. When compressed into ice, this begins eroding away at its surrounding mountains slowly while moving along its route; along the way it can also scrape away rocks or stones that have become frozen to its bottom – known as plucking.
When two or more corries erode simultaneously, they can form an arete ridge. As the glacier melts away, this will reveal a lake known as a tarn. These features can often be found in mountainous regions like Scotland, Alps or Rocky Mountains where glaciers once flowed.
Aretes
Aretes are steep, spiky mountain ridges that form between glacial valleys. They originate when two corries form either side of a mountain – over time the ridge between them becomes increasingly sharp and steep as two corries merge together into one sharp peak. If a third corrie forms along side an arete it will further sharpen into a pyramidal peak if left to its own devices; should its base melt away into a lake (known as a tarn), these mountains can eventually collapse back down to form round lakes (known as cirques).
Glaciers erode bedrock via two processes called plucking and abrasion. Plucking occurs when ice rubs against rock surface to scrape it away; while abrasion occurs when rocks embedded within ice grind against bedrock surface and create long parallel grooves (glacial striae). Both continental and alpine glaciers may produce this landform.
Pyramidal Peaks
Pyramidal peaks, also referred to as glacial horns, are mountaintop features formed by glaciers’ erosional power and are formed when their ice wears away at the mountain’s summit via processes like plucking and abrasion.
When two corries erode back together they form an arete (think Helvellyn in the Lake District for an iconic example), which over time can form steep knife-edge ridges which, eventually meeting at their ends and meeting each other again as sharp pyramidal peaks, eventually meet each other and create steep knife-edge ridges known as an arete.
If two or more pyramidal peaks are located on adjacent mountains, they can form a sharp pass called a col. Additionally, other valley landforms including ribbon lakes, truncated spurs and roches moutonnees (also referred to as “crag and tails”) may result from glacier erosion along the sides of the valley.
Cirques
A cirque is defined as a doubly concave hollow with two concave surfaces connected by steep headwalls, separated by more gently sloped floors showing signs of glacial erosion3.
They start off as snowy depressions on mountaintops that, over time, transform into glacier ice. From there they move on to eroding the surrounding rock by plucking and abrasion; finally leaving behind an oval-shaped depression known as a bowl or, occasionally even higher altitude lakes known as tarns.
Cirques can be transformed into horns, the dramatic and iconic icon of glacial erosion resembling pyramidal mountain peaks such as Switzerland’s Matterhorn. Horns form when multiple cirques whittle away at one mountain from different directions, intersecting and carving out an iconic pointed summit2. Other evidences of glacial erosion can include rock flour from bits dragged along by glaciers as well as striations or grooves found within rocks themselves, rock flour from bits that get carried along on glacier journeys as well as grooves within rocks themselves which all point towards glacial erosion as evidences.
U-Shaped Valleys
Ice is a potent force that can erode rock in many different ways, from wearing away its surface to scraping it against itself or plucking off pieces altogether. Ice erosion often results in corries, aretes or pyramidal peaks being formed as well as U-shaped valleys.
U-shaped valleys can be found worldwide, particularly near high mountains where glaciers may form. An outstanding example is Nant Ffrancon Valley in Wales.
Glaciers can also produce hanging valleys – smaller valleys suspended above a larger U-shaped valley – caused by smaller glaciers with less erosion power compared to their parent glacier, or lesser glaciers that lack its impact. Due to this disparity between their erosional abilities, smaller valleys often remain perched above larger ones.