The beginning of Triassic time in South Dakota is not too different from the end of Paleozoic time, a time of erosion in South Dakota. Late in the Jurassic, shallow continental seas again submerged parts of western South Dakota.

During most of Cretaceous time, North America contained a central north-south seaway that accumulated thick deposits of marine sediments. The end of the Cretaceous marks the final retreat of the continental seas from South Dakota. It also marks the birth of the Black Hills in western South Dakota. As the Black Hills area was domed upwards, erosion actively attacked the soft shales and finally the harder and older sediments. Erosion removed more than 6,000 feet of sedimentary rock layers from the central part of the Black Hills. During the later stages of uplift, great volumes of molten rock were forced upwards, forming many of today’s valuable mineral deposits (i.e., gold) in the northern Black Hills.