section: Geology
question: How many types of bedrock are in Manhattan?
article: Central Park
candidates: There are four different types of bedrock in Manhattan. In Central Park, Manhattan schist and Hartland schist, which are both metamorphosed sedimentary rock, are exposed in various outcroppings. The other two types, Fordham gneiss (an older deeper layer) and Inwood marble (metamorphosed limestone which overlays the gneiss), do not surface in the park. Fordham gneiss, which consists of metamorphosed igneous rocks, was formed a billion years ago, during what is known as the Grenville orogeny that occurred during the creation of an ancient super-continent. It is the oldest rock in the Canadian Shield, the most ancient part of the North American tectonic plate. Manhattan schist and Hartland schist were formed in the Iapetus Ocean during the Taconic orogeny in the Paleozoic era, about 450 million years ago. During this period the tectonic plates began to move toward each other, which resulted in the creation of the supercontinent, Pangaea. Cameron's Line is a fault zone that traverses Central Park on an east-west axis. Various glaciers have covered the area of Central Park in the past, with the most recent being the Wisconsin glacier which receded about 12,000 years ago. Evidence of past glaciers are visible throughout the park in the form of "glacial erratics" (large boulders dropped by the receding glacier) and north-south glacial striations visible on stone outcroppings.