Paleontology and geology
A sea covered much of South Dakota during the Jurassic. Shallow-water and near-shore sediments contain fossils of clams, ammonites, crinoids, and starfish. As the seas retreated, erosion removed many of these marine sediments and deposited them in lakes, swamps, and streams. These deposits contain abundant plant fossils and the remains of giant sauropod dinosaurs such as Camarasaurus.
Although only a small portion of the Jurassic rocks are shown on this map, they occur around the entire circle of undifferentiated rocks known as the “red racetrack” that surround the Black Hills in the western part of the state.