Chatter mark

In glacial geology, a chatter mark is a wedge-shaped mark (usually of a series of such marks) left by chipping of a bedrock surface by rock fragments carried in the base of a glacier (glacial plucking).

Marks tend to be crescent-shaped and oriented at right angles to the direction of ice movement.

The crescentic gouge is an upstream concave that is made by the removal of a piece of rock.

The crescentic fracture is a downstream concave that is also made by the removal of rock.

The lunate fracture is also a downstream concave made without the removal of rock.

Brown crescent-shaped chatter marks on a formation of gray sandstone.
Chatter marks on sandstone south of Lac Beauchamp , in Gatineau , Quebec, Canada