Glacial Erratic

Central Park, besides being a world-class urban resource, is a showcase of New York City geology. The beautifully exposed outcrops of schist and gneiss bear traces of the ice ages, when continental glaciers scraped their way across the region leaving grooves and polish on the tough bedrock. When the glaciers melted, they dropped whatever they were carrying, including some large boulders like this. The rock has a different composition from the ground it sits on and clearly comes from elsewhere. Boulders like this are called erratics. Below is another erratic from Alaska.

_{U.S. Geological Survey photo by Bruce Molnia (fair use policy)}

Glacial erratics are not the only precariously balanced rocks: those also occur under other circumstances, especially in desert settings (here's more on how those arise). In some areas they are even useful as indicators of earthquakes, or their long-term absence.

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