Geology of the Appalachians
[2][3] The Appalachian Mountains formed through a series of mountain-building events over the last 1.2 billion years:[4][5] The first mountain-building tectonic plate collision that initiated the construction of what are today the Appalachian Mountains occurred during the Mesoproterozoic era at least one billion years ago when the pre-North-American craton called Laurentia collided with other continental segments, notably Amazonia.
The mountains formed during the Grenvillian era underwent erosion from weathering, glaciation, and other natural processes, resulting in the leveling of the landscape.
The basin continued to subside, and over a long period of time, probably millions of years, a great thickness of sediment accumulated.
[6] Eventually, the tectonic forces pulling the two continents apart became so strong that the Iapetus Ocean formed off the eastern coast of the Laurentian margin.
Shells and other hard parts of ancient marine plants and animals accumulated to form limey deposits that later became limestone.
The weathering of limestone exposed at the land surface produces the lime-rich soils that are so prevalent in the fertile farmland of the Valley and Ridge province.
For instance, mafic rocks have been found along the Fries Fault in the central Blue Ridge area of Montgomery County, Virginia.
[10] During the middle Ordovician (about 458-470 million years ago), a change in plate motions set the stage for the first Paleozoic mountain building event (Taconic orogeny) in North America.
The once quiet Appalachian passive margin changed to a very active plate boundary when a neighboring oceanic crust, the Iapetus, collided with and began sinking beneath the North American craton.
[10] Mountain building continued periodically throughout the next 250 million years (the Caledonian, Acadian, Ouachita, Hercynian, and Alleghanian orogenies).
The massive bulk of Pangea was completed near the end of the Paleozoic era (the Permian period) when Africa (Gondwana) plowed into the continental agglomeration, with the Appalachian-Ouachita mountains near the middle.
As Pangea rifted apart a new passive tectonic margin was born, and the forces that created the Appalachian, Ouachita, and Marathon Mountains were stilled.
By the late 1880s and early 1900s, the trend extended both north and south across east-central Ohio and included several counties in western New York where gas was discovered in lower Silurian Medina Group sandstones.
[12] Another drilling boom occurred in the 1960s in Morrow County, Ohio, where oil was discovered in the Upper Cambrian part of the Knox Dolomite.
[12] The Blue Ridge, Piedmont, Adirondack, and New England Provinces are collectively known as the Crystalline Appalachians because they consist of Precambrian and Cambrian igneous and metamorphic rocks.
[14] The Blue Ridge Thrust Belt Province underlies parts of eight states from central Alabama to southern Pennsylvania.
The province is bounded on the north and west by the Paleozoic strata of the Appalachian Basin and on the south by Cretaceous and younger sedimentary rocks of the Gulf Coastal Plain.
