Crescentic gouges occur when glaciers move over a rock surface.
Crescentic gouges are part of a larger geological phenomenon known as glacial erosion. Glacial erosion occurs when a glacier moves across a landform. Through this process of erosion, glaciers can move, scratch or gouge land or rock. Crescentic gouges are breaks that are made when glaciers move across boulders or bedrock.
Glacial Erosion
The two major occurrences of glacial erosion are the movement of large amounts of rock and sediment by the glacier and the abrasive effects that the glacier has on the landforms that it moves across. Glacial abrasion is comprised of scouring and friction cracks. Scouring leaves a number of different types of abrasions known as striations. Striations are scratches that occur from the continuous movement of the glacier across landforms -- especially rocks. Friction cracks are caused by a heavy glacial rock's irregular contact with bedrock.
Friction Cracks
Friction cracks are a variety of cracks, gouges, marks and breaks in bedrock and large boulders. They are caused by rocks that are carried along by the force of a glacier. Crescent fractures, crescent gouges and chatter marks are three major types of friction cracks. Using the direction of the concave shape of the crescent gouge, these marks can reveal the direction that the glacier was moving.
Crescentic Gouges and Fractures
Crescentic gouges are conical-shaped fractures in rocks that have been otherwise smoothed by the passing of a glacier. These gouges occur when large rocks are pressed down by the weight of the glacier with a force great enough to break the bedrock. These rocks are compressed into the surface of the landmass. When the force is sufficient, this pressure causes small pieces of the bedrock to break -- often in a crescent moon shape. Crescent fractures occur when there is not enough force to break pieces off the bedrock, but enough only to crack it.
Chatter Marks
Chatter marks are small, curved, irregular pressure fractures that -- contrary to crescent fractures -- have no distinct shape and do not readily show the direction of a glacier's movement. They often occur in groups and are usually 1 to 5 cm in length, although they can be even smaller. Chatter marks are seen on granite or other hard, brittle bedrock. Like other friction cracks, they are caused by the impact and pressure of glacial rocks.
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