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.
