This is a rock formation created by the passing of a glacier over underlying bedrock, resulting in asymmetrical erosional forms.

The upstream side (stoss) of the rock is smooth, often with striations/ridges, as a result of abrasion as the ice advances over the rock due to basal sliding and the influence of gravity.

The downstream side (lee) of the rock is rough due to plucking, where the ice attached itself to the rock and pulls rock fragments away as it moves down the valley.

Plucking could occur because as the ice moves up the stoss side there was a reduction in pressure, allowing liquid water to refreeze and attach the ice to the underlying rocks.

 

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