Behold the Power of Ice

Glacial Erosion

Usually the phrase 'glacial erosion' refers to the process in which a glacier moves over the land, and some of the land is taken away by the glacier. Since a spectacular amount of our planet has been shaped by glaciers, it would be a pity to stop with the minutia of rocks eroding. I will also describe the hills, mountains, and lakes that glaciers leave behind. But first...

Abrasion: also called scouring and glacial milling, this is just what it sounds like; a mountain of ice scraping over a mountain of rock, and grinding bits off of it when it does. This causes a number of side effects, the most obvious of which is rocks that show signs of either glacial polish or striations (scrape marks; also called stria). These striations can be just light scratching along a rock face, or it can be deep gouges, largely depending on how frozen the glacier was -- harder ice means a smoother ride.

Abrasion produces stones, gravel, sand, and rock flour (AKA glacier flour) -- a fine rock silt that often causes meltwater streams to have a milky appearance, giving rise to the nickname glacier milk. Windblown rock flour is called loess.

Plucking: also called quarrying, this is when the glacier freezes around a rock, the glacier moves, and the rock comes along with it. Of course, it's a little more complicated than this. When a glacier flows over the stone of the valley floor it will hit bumps of various sizes. Glaciers are big, meaning that they have a lot of force behind their movement, slow though it may be. When 1,000,000 tons of ice lean against a bump, there is a lot of pressure, and pressure means heat; additionally, added pressure reduces the melting point of ice. If you get enough heat/pressure to melt the ice, the resultant water seeps into cracks in the bedrock -- where it will refreeze, expanding and cracking the rock. And the glacier creeps off, taking some new stones along with it.


Erosional landforms

Well, enough about how the erosion happens. Here's what you get after the glaciers drive by.

After passing, glaciers turn V-shaped valleys into U-shaped valleys ("glaciated valleys") with a wide, flat floor and steep sides. Fjords are one example of glacier valleys. Other examples can be found in most mountain ranges the world over.When a smaller glacier meets a larger one, the larger one will erode its valley's bottom more than will the smaller one, leaving the smaller one as a hanging valley, with a sharp drop from its outlet to the floor of the larger valley (which can make for some spectacular waterfalls, after the glaciers are gone).

A Cirque, also known as a corrie or a cwm, is a steep-walled semicircular basin formed by a small 'pocket glacier', which often feeds into a larger glacier. Aretes are narrow, jagged ridges formed when two cirques abut each other, eroding a ridge from two sides. A horn is a sharp point formed by three or more cirques eroding around a mountain peak, giving it very steep, sharp cliffs. The Matterhorn in Switzerland is an example of a horn. A tarn lake is refers to water that has collected in a cirque.

The continental ice sheets of ancient ice ages also scooped out large shallow basins in the land, which have often filled and become lakes. Many of the lakes on the Canadian Shield, including the ever popular Great Lakes, were created this way.

Roches moutonnée form when plucking and abrasion work together; the glacier drives up a hill, polishing the slope smooth, and then it does some serious plucking on the downslope. The result is a smooth, low up-slope, and a sharp, bumpy down-slope. Roches moutonnée translates as 'fleecy rocks' in French, on the theory that they look like a field of fluffy sheep from a distance. They are also sometimes called sheepbacks in English.

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