Glacial erosion creates unique landforms through processes like abrasion and plucking that remain visible today. It shapes mountain ranges while leaving behind features such as cirques, U-shaped valleys, and glacial horns – some lasting even through modern times!
Abrasion occurs when rock particles embedded in ice wear away at bedrock, leaving grooves known as striations lines on it.
Cirques
Glaciers erode their bedrock through basal sliding, in which rock particles dragged by glacier are pulled along its path by gravity to scrape against it, producing features such as faceted clasts, striations grooves and glacial pavements – as well as producing fine dust known as rock flour.
Erosion of warm-based glaciers is particularly severe, as their ice remains at or below pressure melting points throughout each day, and can carry away large volumes of rocks across bedrock surfaces. On the other hand, some rock masses in cold-based glaciers remain undisturbed by basal sliding due to preexisting weaknesses that cannot be exploited through basal sliding.
A cirque is a circular basin with steep uphill sides that resemble a bowl from above, created either at the heads of glacial valleys or independently as indentations on mountainslopes by circular erosion. Other features created from circular erosion include aretes – sharp ridges between U-shaped valleys; cols, low points on aretes which create passes between valleys; and horns – steep pyramid-shaped peaks created when glaciers have worn away three or more sides of an arete glaciers from circular erosion;
U-shaped Valleys
As glaciers move across landscapes, they sculpt land through plucking and erosion, changing V-shaped river valleys into U-shaped valleys with flat bottoms and steep sides – providing stunning vistas, hallmarks of glacial activity.
Glaciers also erode the rocks they carry, reducing its surface area while smoothing it with abrasion. When these debris pile up on their moraines they deposit it there, or create hanging valleys when additional glaciers join from higher elevations – usually characterized by waterfalls cascading down its walls.
Glaciers possess much more power and can widen and deepen the valleys they carve, as well as form long, narrow lakes known as ribbon lakes. Ribbon lakes form where there is an uneven distribution of hard and soft rocks – the glacier erodes away only soft rocks while leaving behind any harder rocks unaffected. Thus forming ribbon lakes.
Drumlins
Glacier movement shapes the landscape by carving U-shaped valleys, smoothing and depositing sediments, transporting large boulders great distances and smoothing bedrock into drumlins, rock basins or cross stratified deposits known as varves.
Formation of these landforms remains a source of speculation and mystery. It seems implausible that an enormous sheet of ice could flow simultaneously over drift and bedrock without altering their patterns of stratification, nor that its edges could scrape over drift to produce hill-like features such as drumlins and roche moutonees.
The most widely held theory regarding drumlin formation suggests that they result from deforming soft bedrock or till. If certain parts of this till layer vary in strength, some areas might remain static or slow moving while other weaker sections will deform more readily; this explains why many drumlins feature cores composed of coarse-grained sediments well preserved within fluvially sorted sediments surrounded by more easily deformed till.
Erratics
Erratic rocks, created through glacial erosion, are rocks that were “plucked” away from their original site by glacial action and transported elsewhere for depositing or transport. Erratics typically consist of hard igneous material like granite and gneiss which has much greater resistance to erosion than soft sedimentary rock like limestone, while being significantly larger than average at reaching up to 10 feet across or even car size in some locations.
Erratics provide geologists with invaluable information about past glacial flows. Furthermore, they have also proven useful for locating ore deposits; several copper mines were even opened after tracking back an erratic to its source.
Many of us are familiar with erratics from their front yards. Each time the ground freezes and thaws, these rocks emerge to the surface again and again. North Dakota farmers in particular spend significant time picking up and transporting these rocks away after each harvest season has concluded.