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Earth Geology Rock Layers Formations

Rock Formations

Exposed rock formations in the Bryant Watershed are, from oldest to youngest (bottom to top): the Gasconade Formation, the Roubidoux Formation, the Jefferson City/Cotter Formation, the Northview Shale, and the Pierson Limestone. The Gasconade, Roubidoux, and Jefferson City Formations are of Ordovician age whereas the Northview and Pierson Formations are of Mississippian age. 

The boundary between the Jefferson City/Cotter Formation and the Northview Shale represents a significant time when no sediments were formed and/or a period of erosion. This boundary, which represents missing time in the geologic record, is called an unconformity. This unconformity tells us that the Ozark region was above sea level and subject to erosion during this long time interval. 

Below the oldest rocks exposed down in the Bryant valley are more rock layers. These buried layers are important as aquifers, the source of our well water. The lower part of the Gasconade contains a sandstone layer known as the Gunter. Below the Gunter Sandstone are several formations of dolomite, which are of Cambrian age. This sequence of dolomite formations is several hundred feet thick. At the base is another sandstone called the LaMotte Sandstone. 

Way Below and Way Old

Below the Lamotte is the Precambrian basement complex. These Precambrian rocks are found at a depth of 1,500 to 2,000 feet. The rock types are granite in the southern part of the Bryant area and metamorphic gneiss (a volcanic rock) in the northern part. These rocks are known only from a few locations where they have been penetrated by deep drilling. Similar Precambrian granites and volcanic rocks are exposed at the surface in the St. Francis Mountains of southeast Missouri. These Precambrian rocks are approximately 1.5 billion years old whereas the Lamotte Sandstone that lies on top of them is of Cambrian age and therefore only about 550 million years old. A major unconformity separates the two, one of a billion years in length. This was a very significant period of erosion and non-deposition.