Nebraska during the Ordovician Period

	485 Ma Lower Ordovician Epoch Shallow sea carbonate deposits of the late Cambrian continue into the early Ordovician (Oneota dolostone). Near the end of the Lower Ordovician Epoch, part of southeastern Nebraska was uplifted and subsequently eroded, causing an EROSIONAL GAP in the rock record.
	470 Ma Middle Ordovician Epoch In the Middle Ordovician, the seas advance across the low-relief landscape of Cambrian and Lower Ordovician carbonate rocks, leaving behind a thin beach sand. The St. Peter Sandstone (which is present in many Midwestern states) is deposited at this time. It's a very pure sandstone, consisting of uniform-sized quartz grains with little cement. This sandstone is porous and permeable, making it a good reservoir for petroleum or natural gas.
	450 Ma Upper Ordovician Epoch As the waters became deeper, the sands were buried by another carbonate unit called the Viola Formation. Both the St. Peter Sandstone and Viola Formation have produced some oil within the southern portion of the state.

The images above are Paleozoic paleogeographic maps of the central United States (from Dr. Ron Blakey- http://jan.ucc.nau.edu/~rcb7/ ) that are based on observations in the geologic record. Nebraska is outlined in yellow. Click HERE to visit Dr. Blakey's website to learn more about paleogeography and how the continents have changed position throughout geologic time.