Geological Survey Program
Proffit Mountain Geology Revealed as a result of the Taum Sauk Reservoir failure
The Dec. 14, 2005 breach of the Taum Sauk Reservoir sent more than one billion gallons of water down Proffit Mountain and into Johnson’s Shut-Ins State Park near Lesterville. Thankfully, no lives were lost. However, the park superintendent’s house was washed away and the popular park sustained extensive damage.
The release of more than a billion gallons of water in approximately 12 minutes exposed an incredible amount of rock, evidence of a beach, volcanic activity and to top it off, 900 million years of rock are missing! To say that the geologists at the department’s Division of Geology and Land Survey had a field day at the site would truly be an understatement.
Cheryl Seeger, a department geologist who has spent a considerable time at the site to better understand the character of the bedrock, said, “It is absolutely astounding to see all of these relationships exposed. The most intriguing aspect is that we rarely see an outcrop tell such a fascinating story.”
The bedrock on top of Proffit Mountain is purple rhyolite and pale pink granite, indicative of early volcanic activity in the mid-continent region.
Continuing down the scar and cutting crossways is evidence of a weathered dike. This cut between the rhyolite and granite is very unusual as it separates the fine-grained volcanic rhyolite and the harder, more durable coarse granite.
Further down is a conglomerate "beach" deposit made of sand-sized granite and rhyolite grains that grade up to the size of boulders. Grayish-white Cambrian-era dolomite follows. The Cambrian Period marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. Next is tan residuum from the weathered bedrock and last is the newest material deposited by the flood.
Seeger said, “We rarely see all of these rocks and features in one location and on such a large scale. Add to that, more than 900 million years of the geologic record are missing between the exposed granite at the top of the scar and dolomite bedrock at the bottom.” This timeframe encompasses almost one-fourth of the history of the earth. Seeger concluded, “A field day, indeed.”