NEW EXPOSURES – NEW LESSONS IN GEOLOGY: THE EDUCATIONAL VALUE OF THE TAUM SAUK WASHOUT VALLEY

On December 14^th, 2005 a segment of the retaining wall for the Upper Taum Sauk Reservoir failed, releasing ~1.5 billion gallons of water in a torrent that swept down the western slope of Proffit Mountain (Reynolds County, MO). The resulting flood stripped vegetation and soil from the Proffit Mountain valley (PMV), partially filled the East Fork of the Black River with sediment debris, and damaged the Johnson’s Shut-Ins State Park campground. In contrast to the destruction of ecological and state park resources, the flood also exposed and/or produced many new geologic features in the PMV that can enhance geology field trips to the region. Many of these features mimic geologic exposures seen elsewhere in the St. Francois Mountains, reinforcing the Hutton concept of “The Present is the Key to the Past”.

The uppermost segment of the PMV was eroded to bedrock, exposing a checkerboard fracture pattern and differential weathering rates in adjacent Precambrian rhyolite, granitic, and mafic units. The fractures mimic the regional orthogonal pattern seen throughout the region, while the combined tectonic-differential weathering pattern can be used as an analog for explaining the larger-scale hummocky topography of the St. Francois Mountains. Exposure of a dissected paleoweathering field of Precambrian boulders allows students to examine the relationship between fractures and spherical weathering, in a similar manner to what they can observe at Elephant Rocks State Park. The concept of sea level eustasy can be demonstrated in Cambrian sandstones that initially transgress onto the boulder and associated conglomerate deposits, then regress basinward. The Cambrian sandstones were eventually overlain by shallow water carbonates complete with desiccation cracks, burrows, rhyolite-clast laden storm surges, and modern karst features.

The 2005 PMV flood deposits exhibit a sheet-like braided river morphology that is similar to gravel bars seen in the adjacent Black River, only at a larger scale. A high-energy cut bank section devoid of sediment is flanked by a point bar gravel accumulation zone, similar to features students may observe at Sutton Bluff. Reverse graded bedding, scour marks on bedrock surfaces, and an increase in downstream clast size are features that students will find unique to the PMV, and a challenge to explain.