South Dakota's rocks are a carpet of Cretaceous seabed deposits, punctuated by areas of extremely old rock on east and west. (more below)
Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman (fair use policy)
Click the map for a larger version
South Dakota occupies a large area of the North American craton or continental core; this map shows the younger sedimentary rocks that are draped upon its ancient flattened surface. Cratonal rocks appear uncovered at both ends of the state. In the east, the Sioux Quartzite of Proterozoic age in the south corner and the Milbank Granite of Archean age in the north corner. In the west is the Black Hills uplift, which began rising late in Cretaceous time (about 70 million years ago) and was eroded to expose its Precambrian core. It is ringed with younger marine sedimentary rocks of Paleozoic (blue) and Triassic (blue-green) age that were laid down when ocean lay to the west.
Soon afterward the ancestor of today's Rockies erased that sea. During the Cretaceous the ocean was so high that this part of the mid-continent was flooded with a great seaway, and that's when the swath of sedimentary rocks shown in green was laid down. Afterward in Tertiary time, the Rockies rose again, shedding thick aprons of debris upon the plains. Within the last 10 million years much of that apron was eroded away leaving remnants shown in yellow and tan.
The thick green line marks the western limit of the ice age continental glaciers. If you visit eastern South Dakota, the surface is almost totally covered with glacial deposits. So a map of South Dakota's surface geology, like the clickable map from the South Dakota Geological Survey, looks rather different from this bedrock map.
More about South Dakota Geology
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