It was finally warm enough yesterday that there were even people out doing some rock climbing in Central Park (without shirts even!) There are quite a lot of rock formations in Central Park, even though most of them aren't that big.
The type of rock you can see here is called manhattan schist. According to what I read on http://www.greenswardparks.org,/ "although there are three major rock types underlying Manhattan Island, only one of them, Manhattan Schist, is exposed in Central Park. This is a layered rock in which mica-bearing bands alternate with layers composed primarily of quartz and feldspar."