Crescentic gouges on a rock knob west of The Pond. The fracture surfaces dip towards the top of the picture, indicating ice movement in that direction. Faint striae can also be seen.
Flint's (1971) sketches of crescent fractures produced by glacial erosion. Arrows indicate ice flow. Crescentic gouges are convex in the direction of flow. Lunate fractures are concave in the direction of flow. Crescentic fractures are now more widely known as chatter marks. Figure GG-17 of Sanders and Merguerian (1994)
More on friction cracks
Crescentic gouge
Definition (USGS): a curved mark or fracture produced by chipping of the glacier bed. Larger than chatter marks, typically the horns of these gouges point up glacier.
Crescentic gouges are not easy to spot in Central Park. The best examples occur on the up-ice slopes of rock hills on polished and striated rock surfaces of homogeneous composition.
Crescentic gouges form
when blocks are pushed down with great force by ice into the underlying
brittle rock surface, creating a conical fracture. The block may become jammed
against a small rock projection and sufficient force quickly builds up to
fracture bedrock.