Many are confused as to why glacial erosion is hard to see first-hand, as there are various reasons behind it:
Glaciers erode bedrock through three primary mechanisms: quarrying (often producing sediment in cobble-sized increments), abrasion producing finer-grained material (often silt-sized in nature) and subglacial fluvial action.
Cirques are mountain valley heads shaped into deep hollows by glacier erosion, eventually filling up with water after their melt. When these glaciers melt away, their remaining snowfall fills in these cirques to become what are known as cirque lakes.
The morphology of these cirques differs across regions. They typically orient towards the north or northeast and feature steep western headwalls with gently sloping eastern headwalls due to climatic and nonclimatic factors, including rock type/tectonics.
Mountain glaciers typically travel downhill from mountain peaks into valleys where there is exposure of bedrock with differing resistances to erosion, eroding away weaker rocks while covering stronger ones with ease. As they pass, erosion reduces weaker ones while flowing over stronger ones forming steps made of more resistant bedrock which often become filled with water to form staircase lakes.
These circular mountain ranges can be found all around the world and typically cover an area approximately one kilometer long and wide.
In the Alps, for instance, some cirques can span as wide as 4 kilometers and serve as an indicator of long periods of Alpine glaciation.
The Zastler Loch in Germany provides an excellent example of cirque stairways; it lies below Feldberg Mountain, the highest peak in the Black Forest. Here, an upper cirque stands at an elevation of 1500 meters.
Cirques can be found throughout US National Parks of North Cascades and Canadian Arctic. These serve as examples of how glacial erosion has transformed landscapes into breathtaking environments.
These cirques make great places for hiking. Their trails winding throughout them offer spectacular scenery.
A cirque is a hollow in the shape of an amphitheater with three walls featuring steep cliff-like headwalls that have been worn away over time by plucking, abrasion, and ice segregation. Should these processes continue unchecked, a cirque could become an enormous bowl shaped depression in the side of a mountain.
During the last glacial period, Matterhorn and Mont Blanc in the Alps were completely covered with glaciers; this stands as an apt demonstration of how glacier erosion can alter landscapes dramatically.
Valleys are an integral feature of Earth’s landscape, particularly upland areas. Valleys form wherever there is an irregular surface that attracts running water; thus creating highly durable landforms which have survived long periods.
U-shaped valleys are unique forms of valleys created by glaciers moving downslope and scouring of the ground with friction, which wear away at rocks over time. As opposed to V-shaped valleys which typically feature narrow bottoms with steep walls, U-shaped cross sections tend to have flat bases and wide sidewalls compared to their V counterparts.
To form a U-shaped valley, ice must first erode away at rock along both the base and upper sides of a V-shaped valley through freeze-thaw weathering; this weakens rock structures while relieving pressure in cracks.
Once ice has penetrated deeply enough, it begins to scrape across rocks along its route, creating moraines or erratics (small boulders carried along by the ice) along its way. These moraines may remain behind when the ice retreats or melts altogether, leaving behind an U-shaped valley with flat bottom and steep sides.
U-shaped valleys are a signature feature of upland regions like Europe’s Alps, England’s Lake District and New Zealand’s Southern Alps. Under the shadow of Matterhorn is an example of such valleys; an impressive 100 meter-deep U-shaped groove was cut into its valley floor by millennia of tectonic uplift and river erosion has resulted in this pattern in this particular spot of valley floor erosion.
Glacial valleys come in various forms, yet all share an inherently parabolic or U-shaped cross section with relatively wide and flat bottoms and steep, even vertical sideswalls. Furthermore, glacial valleys may exhibit irregular bedrock features with short steep steps separated by longer flatter sections of bedrock.
Steps are likely the result of differential erosion of bedrock that varies in hardness; harder bedrock tends to resist ice flow better while softer or less fractured rocks may be more vulnerable.
These differences in bedrock characteristics are one of the main causes of glacial erosion along U-shaped valleys being difficult to observe. Slopes of beds of rocky material are usually steep and narrow, making it hard for observers to get a clear view of what’s taking place at ground level; furthermore, snow often covers them so as to hide their surface features.
There are various glacial landforms that can document the extent, behavior and impacts of past glaciations events. One such landform is the periglacial trim line: these features lining valley edges demonstrate height of ice during erosion periods while also marking the boundary between smooth bedrock below it and frost-scattered regolith formed through weathering processes on its upper surfaces.
Reconstructing glacial landscapes using satellite images is also an invaluable means of reconstructing glacial terrain as they allow us to see how much ice has receded and thinned from surface areas over time. They are an invaluable way of measuring climate change impacts on ice surfaces and slope dynamics while being an indispensable asset in risk evaluation and assessment.
Erosion rates depend on how quickly ice melts. This depends on both temperature and amount of water flowing into it; warm-based ice, often frozen slowly at its base, tends to experience greater erosion rates.
Glacial erosion also results in the formation of “hillocks”, or stoss-and-lee topography, or “hillocks,” of rock. These typically exhibit an asymmetrical shape with smooth-rounded upstream sides and steep, striated downstream ones; scale can range anywhere from meters to hundreds of meters in plan view (Fig 7-40).
Glacial valleys often display this sign, which indicates extensive erosion. Rock basins where glaciers buried deposits of hard, soft rock deposits can more easily be quarried than bedrock highs with harder and brittle rock layers.
These fine-grained abrasives, commonly referred to as fine grit abrasives, may be carried upward by the flow of ice or washed out with meltwater; the latter process provides an explanation for why cold-based ice allows abrasion to take place.
Studies have demonstrated how glacial and periglacial erosion effectively control mountain range elevation by creating a “snow buzz saw”, in which only limited crustal material rises above an equilibrium-line altitude (ELA). This hypothesis has also been applied to ice-free peaks, nunataks and other features found in temperate environments.
Ice is an immense force in nature and large areas of it have had an outsized influence in shaping many landscapes across various regions. Ice wears away at rocks beneath it through erosion caused by both abrasion and plucking processes, leaving behind only its imprint on their surfaces.
Abrasion occurs when rocks and boulders at the base of a glacier act as an enormous file, scraping away at rock surfaces with gouges, lines or even long striations lines left as evidence.
Glaciers also erode rock by freezing water into cracks in its bedrock – this process, known as freeze-thaw weathering or frost-shattering, is common in cold climates.
Freezing water into cracks of rocks can be caused by cold temperatures or by an oscillation between warmer and cooler temperatures, and causes the liquid to enter cracks in the rock, freeze back into it, exerting pressure upon it and eventually causing pieces to separate and be carried away by its weight.
Plucking is a form of glacial erosion found in valleys. When glaciers move down a valley, friction causes their basal ice to melt into cracks in bedrock, creating further openings or widening them with each freezing-thawing cycle (a form of hydraulic wedging).
Once a glacier reaches a section of rock with many fissures, it may pick out pieces and deposit them on the ground as it moves further, in what is commonly referred to as quarrying.
These rocks are carried down glaciers and end up in lakes or other rocky formations, including lakes. Some cirques – like Glacier National Park’s bowl-shaped one – are formed through plucking while others form through other forms of glacial erosion.
Sheeting is another major form of glacial erosion. This form occurs when flat-lying glaciers spread out and move across valley floors. As they push over bedrock, sheets break off and are carried away by the glacier’s advancement – sometimes up to several meters thick with ridged sections or waves that look similar.