Sioux quartzite, granite, and schistose metamorphic rocks have been recognized in the pre-Cambrian. The present irregularities, the “basins and highs,” on the pre-Cambrian surface are the result of erosion and a long structural history. In general succeedingly younger rocks rest unconformably by overlap against the pre-Cambrian “highs.” The principal erosional and structural “highs” are: the “Nemaha mountains,” the Cambridge anticline, the Chadron dome, and the Sioux Falls area. “Basins,” or saddle-like depressions, occur on the pre-Cambrian surface between the “highs.” The largest of these trends from southeast to northwest across the central part of Nebraska. The history of each ridge or “high” is more or less individualistic, but it seems certain that the structural framework of Nebraska came into existence in late pre-Cambrian time and has dominated the structural and depositional history of the state ever since.
Every Paleozoic system below the Pennsylvanian is represented by identifiable rocks in the subsurface section of Nebraska, and all are present at least east of the buried Nemaha ridge. The exact subsurface distribution of each of the Paleozoic systems is not yet known with certainty. Apparently all formations are the same and are continuous with the pre-Pennsylvanian rocks of Iowa. The Cambrian and Ordovician rocks of the southern part of the state are also quite similar to correlative formations in Oklahoma. The fact that most of the deep test wells so far drilled in the state have been located on structural or on monadnock-like pre-Cambrian “highs” has precluded the discovery of many pre-Pennsylvanian Paleozoic rocks, which do occur in the “basins.” This has led to the misconception that Nebraska was not invaded by all of the Paleozoic inundations.