Glacial Grooves and Polish
(c) 2004 Andrew Alden, licensed to About.com, Inc. (fair use policy)
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 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.
For another view of Central Park, see the walking tour of trees in Central Park North and South by Forestry Guide Steve Nix.
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society
