Natural History of the Mount Vernon Area
by Chuck DeMets
Professor of Geology and Geophysics,U. W. Madison, Wisconsin
Throughout the four seasons, while Mt. Vernon residents go about their day-to-day business, the slow geologic processes of erosion and weathering silently sculpt the rolling hills and valleys of the Mt. Vernon countryside, as they have without interruption for the past 450 million years. Unlike much of the remainder of Dane County and Wisconsin, whose landscapes and soils were profoundly altered by continental ice sheets (glaciers) that periodically advanced south from the Hudson Bay region over the past 2.5 million years, the Mt. Vernon area has been shaped by largely non glacial processes. Mt. Vernon lies near the edge of the "Driftless Area," which is a ~10,000 square mile island in southern Wisconsin that has by chance never been overrun by glaciers. As a result, the Mt. Vernon area lacks the deep deposits of glacially-transported sediments and glacial landforms so characteristic of other areas in the state. The geologic evolution of the Mt. Vernon countryside must instead be studied in a non-glacial context, one whose origins date back to the Cambrian period approximately 500 million years ago.
Between 550 and 450 million years ago, shallow continental seas advanced and retreated five times across much of the central United States, including southern Wisconsin. After each regression of the sea, flat lying marine sediments such as sandstones and dolomites were left behind. When seas retreated for the final time, over 100 feet of sandstones, dolomites, and shales (mud-based rock) had been deposited in the Mt. Vernon area. The final retreat of the seas left behind a largely flat continental landscape that gradually took shape through the everyday processes of water and wind erosion. Stream valleys and ridges in southern Wisconsin began evolving into their present state, and soils formed through the mechanical and chemical weathering of the rock surfaces. Deer Creek, Fryes Feeder, and Mt. Vernon creek all eroded down through the upper, younger rock layers, the Platteville and Galena dolomites into the older St. Peters Sandstone beneath. Donald Rock, near the intersection of Town Hall Road and Highway 92, is an erosion resistant block of St. Peters Sandstone that became isolated as more erodable surrounding sandstone was carried away by water action over many millions of years.
Since 2.5 million years ago, when the edges of large continental glaciers approached the Mt. Vernon area, the two most profound influences on the landscape of the Mt. Vernon area have been humans and glaciers, in that order. Before ~13,500 years ago, the local environment was nearly identical to that in northern Alaska today. The ground was permanently frozen several feet down. The upper soil layer thawed only for a few weeks each year and tended to slide down slopes rapidly due to its muddy character. The local ice sheet dammed the outlets of nearby streams, leading to small glacial lakes that backed up into the valley of Mt. Vernon creek and its tributaries. In some instances a several inch thick layer of light-grey clay was deposited at the bottoms of these lakes and can still be found several feet beneath the valley floors. Plant pollen samples recovered from lake sediments in Dane County indicate that plant life consisted mainly of a treeless tundra with low shrubs and grasses. Since ~13,500 years ago, the local vegetation evolved to a spruce forest as the ice sheet retreated. The bones of woolly mammoths, mastodons, giant beavers, and other large mammals indicate that the local environment supported a rich wildlife population.
After man arrived in central North America some 11,500 years ago, large mammals became extinct, presumably due to overly efficient hunting techniques. The local vegetation evolved to a mixture of prairie and oak woodlands by 10,000 years ago, at which point the continental ice sheet had retreated to the southern edge of Lake Superior. The Mt. Vernon landscape had almost certainly assumed its present form by this time.
The influence of modern humans on the local landscape has overwhelmed the more gradual natural processes described above. Agricultural and construction activities that strip vegetation from slopes have increased soil erosion rates at near 100 times above their natural levels. Water and wind erosion now remove soil from fields far more quickly than natural processes can replace it, although modern farming practices and stricter government regulation of construction sites have reduced the unsustainable loss of soil to levels only several times higher than the replacement rate. Contamination of the sub surface water that feeds streams, ponds, and wells in the area and is stored in the thick underlying rock layers is a continual concern, with the vast majority of such contamination occurring from agricultural chemicals, fertilizers, and manure. Finally, mankind's most profound influence on the landscape, suburban-style tract housing that subjugates the entire landscape to lawns, houses, and driveways, is creeping toward Mt. Vernon as inexorably as glaciers once did.