Glacial erosion is when soil or rocks are worn down or moved by a glacier, creating many impressive and unique landscape features. Glaciers are bodies of ice that originate in mountains and tend to move slowly downhill. This downhill movement is a significant way in which glaciers cause erosion. There are three types of glacial erosion: plucking, abrasion and freeze thawing. The most significant, recent glacial erosion occurred during the Pleistocene epoch, 100 thousand to 1.6 million years ago, in the Quaternary period. During that time, over about 2 to 3 million years, the polar icecaps repeatedly melted and froze.
The types of glacial erosion are all interrelated, but may also erode on their own. Plucking and abrasion are the most noteworthy. Plucking occurs when cracked and broken rocks are frozen into glaciers. When the ice moves downhill the rocks are moved, or plucked, from their positions. Abrasion is when rock that is frozen to a glacier, usually from plucking, scrapes on other rock as the glacier moves. The debris in glaciers is called englacial moraine. Freeze thawing is sometimes a precursor to plucking and happens when melted water from a glacier seeps into cracks in rock and refreezes. When the water freezes, it expands the cracks and loosens pieces of rock. The loose rock not only begins moving downhill on its own, but it is more easily plucked.
Among the unique landscape formations caused by glacial erosion are troughs, arëtes, corries and pyramidal peaks. Glacial troughs are large U-shaped valleys that are distinguished from the V-shaped valleys caused by flowing water. Glacier National Park in Montana displays prominent glacial troughs as well as other glacier formations. Arëtes are steep ridges and corries are enlarged hollow areas. The Red Tarn of the Lake District in England is a corrie in which the glacier that formed the neighboring Helvellyn Mountain as well as the corrie itself melted in the corrie. The Matterhorn in Switzerland is a prime example of a pyramidal peak, clearly featuring a point and steep, flat sides.
In addition to the landscape features that are mainly the result of rock being taken away, glacial erosion also deposits rock in new places, creating depositional formations. As a glacier melts, it leaves behind parts of its moraine, like a large rock in a forest, called a glacial erratic. The largest known glacial erratic is the Big Rock in Alberta, Canada. The rocks left behind by ice are usually very different from the bedrock in the area and have a wide range of sizes.
Water deposits only feature fine sediments because water cannot move large rocks. Drumlins are asymmetrical hills made of glacial sediments. They are long and narrow with a gentle slope pointing in the direction the ice moved. Rock flour is another by-product of glacial erosion, specifically of extreme abrasion. It is created when rock is ground up so finely it becomes a powder and can contaminate melted glacial water.
Geological records have preserved evidence of more ancient periods of glacial erosion in the middle of the Precambrian era and also during the Permo-carboniferous. The latter was the most extensive glacial erosion that has been discovered. Today glaciers only make up 10 percent of the earth, mainly in very high mountains and the poles, but in other time periods they composed as much as 30 percent of the surface. Geologists study the formations caused by glacial erosion to see where ice once was and where it moved. This helps them to understand what the surface of the earth was previously like.
