South Dakota ranks 17th in size among the states. It covers 199,732 sq km (77,117 sq mi), including 3,191 sq km (1,232 sq mi) of inland water. The state’s extreme dimensions are about 395 km (about 245 mi) from north to south and about 610 km (about 380 mi) from east to west. The mean elevation is 670 m (2,200 ft).
Portions of two major natural regions of the western United States cover South Dakota: the Central Lowland and the Great Plains. In South Dakota, the Great Plains cover the western and central sections of the state and include the Black Hills. The Central Lowland covers the eastern part of the state. The boundary between these two natural regions follows the eastern edge of the glaciated sections of the Missouri Plateau, a belt of low hills, known as the Coteau du Missouri, that extends in a north to south direction across South Dakota about midway between the Missouri and the James rivers.
The Central Lowlands in South Dakota, like adjoining sections of Iowa and Minnesota, were covered by extensive ice sheets during the course of the last Ice Age, which ended about 10,000 years ago. Its present physical features are the result of repeated glaciation during the Ice Age and the subsequent changes produced by wind and water erosion. The Central Lowlands comprise the eastern one-third of South Dakota, and are made up of seven subregions. In the northeastern corner is the Minnesota River-Red River Lowland, a broad shallow river valley, once part of the great glacial Lake Agassiz. This subregion contains lakes Traverse and Big Stone, and at 294 m (966 ft), has the lowest elevation in the state.
The Coteau des Prairies, also on the eastern state border, is a massive highland area drained by the Big Sioux River and covered with glacial drift. West of the coteau is the James River Lowland, a broad area carved by ancient streams, then glaciated and now drained by the James River. The James is the longest nonnavigable river in North America. At the northern end of the lowland is a subregion known as the Lake Dakota Plain. This plain was once the bed of ancient glacial Lake Dakota. At the southern end of the lowlands is the James River Highlands, a group of three ridges of drift-covered bedrock known as Turkey, James, and Yankton ridges. In the extreme southeastern corner is the Eastern Section of the Southern Plateau. This is a stream-dissected highland made of a thick mantle of loessial (wind blown) soils that extends into neighboring Iowa. Finally, the southern boundary of the Central Lowland province is the southern section of the deeply cut Missouri River Trench, a flat, wide river trench. The western two-thirds of South Dakota is part of the Great Plains.
It is divided into two regions, the Missouri Plateau and the Black Hills. The Missouri Plateau is comprised of six subregions. The easternmost subregion is the Coteau du Missouri. It is located directly west of the James River Lowland. The Coteau du Missouri is an unevenly dissected highland covered with glacial drift, and contains several massive ridges and broad abandoned stream valleys. It marks the western extent of glaciation. West of that subregion is the Missouri River Trench, a narrow, steep river valley now occupied by four large reservoirs. West of the Missouri River are four other Missouri Plateau subregions. From north to south they are: the Northern Plateaus, a series of step-like terraces that rise westward from the Missouri River and are marked by butte formations; the Pierre Hills, an area of smooth rounded hills of Pierre Shale or gumbo soil; the Southern Plateau, a zone of young rocks, mesas, and buttes, where streams have cut deep gorges, and water and wind have carved the famous White River Badlands; and on the extreme south, the Sandhills, a 1,000-sq km (400-sq mi) extension of the Nebraska Sandhills.
The Black Hills are a region that was formed by mountain building forces at the same time the Rocky Mountains were formed. Originally, the Black Hills were three times their present height. The region is comprised of four subregions, three of which encircle a central core. The Great Hogbacks form the outer wall or ring of the Black Hills. It is a residual hogback ridge with steep inside cliffs of sandstone. Inside the Hogbacks lies the Red Valley (or racecourse) a broad open valley that gets its name from the red soil layer, and circles the hills. The next circular subregion is the Limestone Plateau. The Plateau is the highest part of the hills, with deep cut stream canyons, including the beautiful Spearfish Canyon and numerous caves. The core of the Black Hills is comprised of the Central Crystalline Basin. This basin is the heart of the hills. It is a highland area with mountain peaks and gulches, carved from crystalline rock. Harney Peak, reaching an elevation of 2,207 m (7,242 ft) and the state’s highest point, is in this subregion. "South Dakota" © Emmanuel BUCHOT, Encarta, Wikipedia
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