Date of this Version
Robert F. Diffendal, Jr. 1997. Geology of the Ponca Creek and Keya Paha River Drainage Basins in Nebraska. Chapter 5 (pp. 16–19) in Steven R. Holen and Danial R. Watson (eds.), An Archaeological Survey of Ponca Creek and the Keya Paha River Drainages in Nebraska. Nebraska Archaeological Survey Technical Report 97-02, University of Nebraska State Museum, Lincoln.
The geology of the study area has been investigated many times over more than a century. Interested readers are directed to the report by Diffendal and Voorhies (1994), which gives a more detailed overview of the results of the investigations and lists most of the more important reports about the area’s geology.
The names of the sedimentary rock strata and unlithified sediments exposed in and underlying all or parts of the study area are shown in Table V.I. These formations range in age from more than 85 million years to sediments being deposited today by winds, water, and downslope movements (landslides).
The oldest sedimentary rocks belonging to the Niobrara Formation and younger Pierre Shale were deposited on the floor of a seaway that connected the Gulf of Mexico with the Arctic Ocean near the end of the Cretaceous Period. The Niobrara Formation is exposed only along the lower valley sides at the downstream end of Ponca Creek in Boyd County and along the lower valley sides along the Missouri River. The Niobrara Formation consists primarily of gray to white chalks, shaley chalks, and limestones formed mainly of the microscopic limey skeletal debris from marine plants. Larger fossils of clams, oysters, cephalopods, fish, and marine reptiles occur in this formation. The Pierre Shale is composed principally of gray to black platey shales and limey shales. Sulfur-bearing minerals in the shale often yield a rotten-egg odor in the air above exposures. Neither of these formations yields significant materials for tools.