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!