Ice is a powerful agent of erosion, leaving behind beautiful landforms in its wake such as ribbon lakes, mountain peaks, aretes, pyramidal peaks, crag and tails and striations.
Glacial erosion transforms V-shaped valleys created by rivers into U-shaped valleys by widening and deepening them, as a result of plucking and abrasion.
1. Abrasion
Glaciers’ immense bodies of rock cause erosion by rubbing against rocks and sediment below, known as abrasion; this process is key in creating the distinctive landforms associated with glaciated regions.
Glacial abrasion becomes particularly significant where scattered rock fragments at the bedrock-glacier interface prevent sliding of glacier, and friction at its surface is high. Abrasion rates vary with how many clasts dragged along by glaciers and their concentration on bedrock surfaces, which directly correlate to glacial rates of advance.
Glacial abrasion is unlikely to be significant beneath thick deforming till (Cuffey and Alley 1996). The latter provides a cushion, smooths the bed to reduce stress concentrations, damps water-pressure fluctuations that would otherwise amplify stresses at points of contact between ice and bedrock, helps control erosion rates, and contributes to creating long straight lines etched by moving glaciers – called “striations.”
2. Plucking
Plucking refers to the process by which glaciers remove and incorporate bedrock fragments, specifically large “joint blocks”, into their moving ice flow. Plucking has played an essential part in the creation of distinctive landforms like cirques and aretes.
Frost action, a type of freeze-thaw weathering, helps to cause pluckeding by loosening rock fragments from bedrock and transporting them into glaciers. It works best when cracks and joints in rocks have already been compromised through prior cycles of freezing and thawing.
Plucking can also be affected by relationships between ice velocity, bedrock deformability, subglacial cavities and conditions on the glacier margin. Evidence from granite bedrock in Dee Valley indicates a late surge of plucking near its margin towards the end of each glacial cycle – suggesting a late pulse of plucking and boulder transport occurred beneath thin ice close to its margin at some point during each cycle.
3. Weathering
Glaciers, giant masses of ice that move along mountain valleys like rivers, can dislodge rocks and soil from mountain tops as they travel downhill, taking this material with them when they melt – this process is known as erosion. Gravity also contributes to this form of destruction as rocks and soil fall from higher up, especially when wet.
Glaciers create spectacular landforms when they erode rocks, such as ribbon lakes, mountain-ridged rock faces, crag-and-tail landforms and striations patterns that stand out on the landscape. These are known as glacial landforms.
Glacier weathering methods determine their ability to erode landscapes. Basal sliding is most often found on warm-based glaciers where pressure melting ice reaches pressure melting point quickly, eroding bedrock more effectively than usual. Freeze-thaw weathering also plays an essential part in glacial erosion as water expanding when frozen expands and breaks apart rocks into crumbly pieces that crumble or break away altogether.
4. Quarrying
Geologists understand abrasion and plucking, which leave clear evidence in rocks, more readily than quarrying; yet glacial erosion’s influence on bed slope and effective pressure suggest it must occur on some scales, making quarrying inevitable.
Quarrying typically entails drilling and blasting rock to form a ledge of debris, large enough for loading into off-highway trucks or belt conveyors. Depending on the size of boulders, secondary breaking may also be required using excavator-mounted hydraulic hammers.
Quarrying and weathering create various geomorphic features, including striations. Striations are long lines that appear scratched on rock surfaces and tell geologists a great deal about how glaciers shaped it and its effects. Other geomorphic features produced by glacial erosion include horns, U-shaped valleys, corries, ribbon lakes and roche moutonnee. Glacial erosion also sculpts mountain tops into pointed shapes known as pyramidal peaks.