Paleontology and geology
In the Early Carboniferous (Mississippian), plate collisions far to the east produced the Appalachian Mountains, as well as gentle uplifts in the interior of the state. As the land rose, the seas drained away, leaving the state exposed to erosion.
Later in the Carboniferous (Early Pennsylvanian),the seas returned, and rivers carried sediment from the eroding Appalachian Mountains to the seas, forming swampy areas along the shorelines in eastern Nebraska. These marshes and swamps eventually became coal beds, which contain fossils of ferns and giant horsetails. Farther away from shore, the water was clear and warm. Water depths fluctuated between about 200 meters and 20 meters at regular intervals. The black shales that formed in the deeper, oxygen-depleted waters contain very well-preserved fossils of shrimps, conodonts, bony fish, sharks, and other cartilaginous fish. Limestone reefs formed when the water was shallower, and these deposits contain fossils of stromatolites, fusilinids, brachiopods, bryozoans, molluscs, corals, trilobites, eurypterids, and crinoids.
By the end of the Carboniferous, western Nebraska was emerging above sea level again. Vast coastal plains formed from sediments eroding off the Ancestral Rockies to the west. These sediments contain fossil soil zones (paleosols) marked by oxidized iron and root fossils.