Region 1: Inland Basin
Sedimentary rocks are very abundant in the Inland Basin. During the Silurian and Devonian, approximately 400 million years ago, much of this area was a shallow inland sea, the perfect environment for the deposition of thick layers of sand, silt and clays to form sandstone, siltstone and shale. The shells of abundant sea life were deposited to form limestone. Sediments eroded from the Taconic and Acadian highlands which lay to the east, and formed thick deltaic sequences stretching toward the west across the basin. Precambrian igneous and metamorphic Grenville rocks lie exposed in the Adirondack region because of uplift and erosion of the overlying cover of younger sedimentary rocks.
Region 2: Appalachian/Piedmont
Exposed in the Appalachian and Piedmont region are the remains of ancient mountains that preceded the Appalachians (the Taconic, Acadian and underlying Grenville Mountains), sedimentary rocks formed by erosion of the highlands, and igneous and metamorphic rocks formed during the crunch caused by the Taconic, Acadian and Alleghanian mountain building events. In the Appalachian/Piedmont area, Triassic Rift Basins filled with sedimentary rock (red beds) and basalt flows mark the break up of Pangea.
Region 3: Coastal Plain
Loose sediments that have not solidified to become rock dominate the geology of the Coastal Plain. Gravel, sand, silt and clay transported from inland form a wedge that thickens oceanward towards the continental slope at the edge of the continent.
Region 4: Exotic Terrane
Igneous and metamorphic rocks dominate the Exotic Terrane region. New England was formed by the addition of small slices of land (exotic terranes) that collided with North America millions of years ago during the Taconic and Acadian orogenies. The intense heat and pressure of the collisions created large igneous intrusions and metamorphosed the exotic terranes and adjacent crust.