Paleontology and geology
The Grenville Mountains formed during the Precambrian as North America collided with an ancient supercontinent and the sandstone, shale, and limestone deposited earlier were squeezed and pushed up onto the margin of the early North American continent. The intensity of the collision metamorphosed the rocks: sandstone became quartzite, gneiss, or schist; limestone became marble; and shale became gneiss and schist. These are the oldest rocks found at the surface in the Northeast. In New York, Precambrian rock is exposed primarily in the Adirondack region in the northern part of the state.