Passage III

Uncovered at Johnson’s Shut-Ins

In Reynolds County, Missouri, a one-billion-gallon blast of water caused by a breach of the Taum Sauk reservoir roared down Proffit Mountain into the east fork of the Black River on December 14, 2005. They ripped a channel through Johnson’s Shut-Ins, one of Missouri’s most popular state parks. Though flood damage marred the parks beauty for a time, the scar the raging water left in its wake specifically revealed over a billion years’ worth of Earth’s geologic history.

The area known today as Johnson’s Shut-Ins State Park had began to develop 1.5 billion years ago. When the volcanoes that created the St. Francois Mountains exploded. Slow-moving magma cooled down its temperature and crystallized to form silica-rich rhyolite rock. Over time sedimentary rock such as limestone and shale, formed from material deposited by shallow inland seas, buried the rhyolite. After the seas had receded, gravel-rich rivers and streams eventually chipped away the soft sedimentary rock in some areas, exposing the erosion-resistant rhyolite rock and creating pockets and pits. In low places, the Black River was confined (or "shut in") by the rhyolite and creating the natural waterslides and canyon-like gorges that have become a summer playground for thousands of visitors.

Although the flood left the shut-ins unscathed, the surge of water that tore through the park in 2005 stripped away all trees, soil, and sedimentary rock in its path. Left behind is a channel that is composed of granite—and previously unexposed rhyolite rock—and contain rocks from at least three other geological eras. The menacing floodwaters also revealed a half-billion-year-old beach made of both sand and gravel.

Five years of work has restored most of the park surrounding the shut-ins. Some have returned back. Geologists from around the world visit to get a close look at the ancient volcanic rock along what has been named the “Scour Channel.” The “Scour Channel” now rivals the park’s other geologic curiosities for most frequently visited site.