crescentic gouge

groove

striation

 

 

Image by Steve Dutch

Chatter mark

Definition: sets of small curved fractures on brittle rock surfaces by the stick-slip motion of stones across the glacier bed

Rows of small crescent-shaped cracks are called glacial chatter marks - a reference to the marks made when a chisel slips across a wood or stone surface. In contrast to crescentic gouges, the horns of the crescents point in the direction of ice movement.

By analogy with steel rolling, chatter marks appear to form when basal ice sticks to the bed and then tension is released in a shuddering movement after build up of basal water pressure..

I have yet to track down chatter marks in Central Park - the generally coarse-grained metamorphic rocks are perhaps not suited to chatter mark formation? Let me know if you find any!