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Glacial Erosion

Glacial erosion refers to a group of processes that wear down land surfaces from the action of glacial ice or glacial meltwater. Particles are picked up and transported away, denuding underlying surfaces and often leaving a variety of recognizable erosional features. Glacial erosion can be subdivided into three main categories: glacial abrasion, glacial plucking, and glacial meltwater erosion. All three forms of erosion are evident in Saskatchewan, although they are especially well developed on exposed bedrock surfaces in northern Saskatchewan.

Glacial abrasion refers to the ability of debris-rich glacial ice to act like a sheet of sand paper as it moves over underlying surfaces. The size, concentration and hardness of entrained debris in the ice, the hardness of the underlying substrate, and the velocity of the moving ice control the type of erosional feature produced on the underlying surface. High concentrations of sand-sized particles in the ice can polish bedrock, leaving a shiny surface, while pebbles or boulders embedded in the moving ice can create fine scratches called striae (singular: striation), or larger features called glacial grooves on bedrock surfaces. The significance of striae and grooves is that they record the direction of glacial ice movement. Materials in the ice can also chip or fracture the underlying bedrock to form features like crescentic gouges, or leave a line of nested crescentic fractures called chattermarks.

Glacial plucking occurs when portions of bedrock become frozen in the overlying glacier. As the glacier advances, these blocks can be plucked or lifted out of the bedrock as with a car jack. A roche moutonée is a bullet-shaped erosional feature that shows both abrasion and plucking processes. The up-ice side is smoothed and abraded by the advancing ice, while the down-ice side is ragged in appearance from blocks plucked from the bedrock.

From time to time, substantial amounts of meltwater can be released catastrophically from a glacier. The combination of water flowing at high velocity and large loads of sediment being carried along makes glacial meltwater highly erosive. Various landforms have been recognized in Saskatchewan, including potholes and winding channels eroded into bedrock.

A combination of glacial erosion processes results in features from a few centimetres to hundreds of metres in size. Areal scouring and whalebacks are common erosional features in Saskatchewan's Precambrian Shield, where exposed bedrock has been eroded into a variety of smoothed shapes that form islands and ridges aligned in the direction of ice action. There is less evidence of glacial erosional landforms in southern Saskatchewan, as most bedrock surfaces have been covered with glacial sediments, although striae can be observed on rocks in these deposits.

Janis Dale

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