Geography of South Dakota - Geology

Geology

South Dakota geologic formations and deposits range in age from several billion to several thousand years, and the age of the rocks generally decreases as one moves from west to east across the state. The oldest geologic formations in the state were created over two billion years ago during the Precambrian, and consist of metamorphic and igneous rocks. These form the central core of the Black Hills, but they can also be found in two isolated locations in eastern South Dakota near Milbank and Sioux Falls. Formations from the Paleozoic Era form the outer ring of the Black Hills; these were created between roughly 540 and 250 million years ago. This area features rocks such as limestone which were deposited here when the area formed the shoreline of an ancient inland sea.

Outside of the Black Hills, much of western South Dakota features rock formed during the Mesozoic Era, from 250 million to 65 million years ago. At the time, much of western and central South Dakota was again covered by a shallow inland sea. Marine skeletons from this ocean settled to the seafloor and were compacted to form the sedimentary rocks in the area today. During this period, the Black Hills, which had been pushed up to an elevation of around 15,000 feet (4,500 m), lost around 6,000 feet (1,800 m) worth of rock layers due to erosion. Many of these sediments ended up in the same area as the marine deposits from the inland sea, and both contribute to western South Dakota's present-day geological makeup. In several areas in western South Dakota, tertiary deposits may also be found. These were formed between 65 and 2.5 million years ago from eroded mountains to the west. There is also volcanic material deposited here that presumably came from the Yellowstone area. These layers of sedimentary rock are distinctly visible in the multi-colored rocks and cliffs of Badlands National Park.

Layers deposited during the Pleistocene epoch, starting around two million years ago, cover most of eastern South Dakota. These are the youngest rock and sediment layers in the state, and are the product of several successive periods of glaciation which deposited a large amount of rocks and soil, known as till, over the area. The thickness of the glacial till layer ranges between 100 and 900 feet (30 and 270 m).

Much of the human history of South Dakota was shaped by its geology. Gold seekers founded most of the larger cities around the Black Hills, and quarrying was an important economic activity in several areas in eastern South Dakota. Mines and quarries in present-day South Dakota produce gold, Sioux quartzite, Milbank granite, sand, gravel, limestone, mica, and uranium. The state also produces a very limited amount of oil and natural gas.