Great Smoky Mountains - Geology

Geology

Most of the rocks in the Great Smoky Mountains consist of Late Precambrian rocks that are part of a formation known as the Ocoee Supergroup. The Ocoee Supergroup consists primarily of slightly metamorphosed sandstones, phyllites, schists, and slate. Early Precambrian rocks, which include the oldest rocks in the Smokies, comprise the dominant rock type in the Raven Fork Valley (near Oconaluftee) and upper Tuckasegee River between Cherokee and Bryson City. They consist primarily of metamorphic gneiss, granite, and schist. Cambrian sedimentary rocks are found among the outer reaches of the Foothills to the northwest and in limestone coves such as Cades Cove.

The Precambrian gneiss and schists— the oldest rocks in the Smokies— formed over a billion years ago from the accumulation of marine sediments and igneous rock in a primordial ocean. In the Late Precambrian period, this ocean expanded, and the more recent Ocoee Supergroup rocks formed from accumulations of the eroding land mass onto the ocean's continental shelf. By the end of the Paleozoic era, the ancient ocean had deposited a thick layer of marine sediments which left behind sedimentary rocks such as limestone. During the Ordovician period, the North American and African plates collided, destroying the ancient ocean and initiating the Alleghenian orogeny— the mountain-building epoch that created the Appalachian range. The Mesozoic era saw the rapid erosion of the softer sedimentary rocks from the new mountains, re-exposing the older Ocoee Supergroup formations.

Around 20,000 years ago, subarctic glaciers advanced southward across North America, and although they never reached the Smokies, the advancing glaciers led to colder mean annual temperatures and an increase in precipitation throughout the range. Trees were unable to survive at the higher elevations, and were replaced by tundra vegetation. Spruce-fir forests occupied the valleys and slopes below approximately 4,950 feet (1,510 m). The persistent freezing and thawing during this period created the large blockfields that are often found at the base of large mountain slopes. Between 16,500 and 12,500 years ago, the glaciers to the north retreated and mean annual temperatures rose. The tundra vegetation disappeared, and the spruce-fir forests retreated to the highest elevations. Hardwood trees moved into the region from the coastal plains, replacing the spruce-fir forests in the lower elevations. The temperatures continued warming until around 6,000 years ago, when they began to gradually grow cooler.