Glacial Grooves & Polish: Central Park, NY

Grooves gouged into the tough rock by the
continental glacier that once covered the area.

Ice won't scratch rock, of course; the sediment
picked up by the glacier does the work. Stones
and boulders in the ice leave scratches while
sand and grit polish things smooth. The top
of this outcrop looks wet, but instead it's
glacial polish.