The ancient gneiss and glittering schist that underlies most of Manhattan Island is folded and foliated in multiple directions, but the grooves running across this outcrop in Central Park are not part of the rock itself. They were slowly gouged into the tough stone 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 polish makes the top of this outcrop look wet, but it's dry.
For other views of Central Park, see the walking tour of trees in Central Park North and South by Forestry Guide Steve Nix or the Central Park Movie Locations by New York City Travel Guide Heather Cross.
