Date of this Version
People studying and collecting sedimentary rocks, minerals, and fossils in eastern Nebraska often find that locating rock exposures can be difficult. Most of the rolling hills in the eastern sixth of the state are underlain by thick, interlayered, glacial deposits, loess (windblown silt), and alluvium (stream-deposited sediments) of very young geologic age (fig. 1). These sediments cover the bedrock in most of that area.
Natural exposures and human excavations of bedrock are mostly confined to the sides and floors of stream valleys. However, in the southeasternmost counties of the state, the younger sediment cover is commonly thin or absent. Pawnee and Richardson counties are particularly good places to look at bedrock exposures because natural outcrops and those made by humans are so common.
Rocks of Late Paleozoic age, belonging to the Pennsylvanian and Permian systems, are exposed at the surface in Pawnee and Richardson counties (figs. 2, 3). These rocks include sandstone, siltstone, claystone, shale, limestone, and coal. They are usually in nearly horizontal layers, except where they have been deformed by folding or faulting along the Humboldt Fault Zone (fig. 2), which runs north to south in western Richardson County.
Massive limestones form ledges, which are somewhat more resistant to erosion than the other rock types just mentioned and often cap the tops of hills in this area, just as they do farther to the south in Kansas. In fact, if the younger deposits were removed, much of the area in eastern Nebraska would resemble the surface features of Nemaha and Brown counties in eastern Kansas.
Early in the study of the Pennsylvanian and Permian rocks of eastern Nebraska, geologists recognized that these strata included certain repetitions of sedimentary rocks. They also recognized that certain kinds of fossils were restricted to particular kinds of rock and that these fossils occurred in repetitive sequences when such rocks were present.
The objectives of this educational circular are: 1) to describe the repetitive sequences of Pennsylvanian and Permian rock types seen in Pawnee and Richardson counties; 2) to describe the kinds of fossils found in each kind of rock; 3) to explain how the fossils and rocks can be used to interpret the causes of these deposits and the ancient environments about 300 million years ago; and 4) to provide a guide to Significant surface exposures.
The rocks and fossils in the Pennsylvanian and Permian strata of Pawnee and Richardson counties can be examined at a number of localities. These places can be visited during a one- or two-day trip to the area. Most of these sites are on public land, but a few are on private land. You must get the landowner's permission before you visit a private locality. The route of the field trip is shown on figure 4. DRIVE CAREFULLY!
Also keep in mind that the appearance of rock outcrops can change because of erosion, slumping, and removal of material for construction.
Appendix I offers information on the rock units seen on this field trip. Appendix II is a road-log for the route you will travel. Appendix III is a glossary of terms used in this circular.
We hope that you will find this educational circular and the field trip as interesting as we did when we put it together. The questions addressed in the circular are those asked by geologists. Part of the fun of your experience will be to check our observations and perhaps produce alternative answers and other questions that we have not asked. Enjoy your trip.