Geologic History (Precambrian to Ordovician Periods)
The earliest geological record remaining in Wisconsin dates back 3.5 billion years, when the region that is now the Midwestern United States and north central Canada was the site of intense volcanic activity. By 2.5 billion years ago, deformation and metamorphosis of these volcanic substrates into gneisses, with additional intrusions of igneous granitic magmas, resulted in the formation of the Superior Continent, which would eventually merge with other embryonic continents and form the present‐day continent of North America (Dott and Attig 2004). Between 1.8 and 1.9 billion years ago, additional plate tectonic activity intruded igneous rocks (principally granite and quartzite) into the present‐day Baraboo Hills region located to the north‐northwest of the Preserve. The Baraboo Hills are the remnants of what were once the Penokean Mountains, a Precambrian mountain range that extended from Michigan to Minnesota. Although the area occupied by the Preserve was at the foothills of this mountain chain and there are no igneous intrusive features in the site’s regolith (apart from any unconsolidated igneous material brought in to the site by erosion and glacial activity), erosion and sedimentation of these mountains over the course of hundreds of millions of years has likely influenced the Preserve’s present‐day bedrock geology and soil characteristics.
The majority of southern Wisconsin is underlain with Paleozoic‐age bedrock deposited during the Cambrian and Ordovician geologic time periods. The bedrock geology of the Preserve consists of Cambrian sandstones underlying early‐Ordovician dolomite, with late‐Ordovician sandstone caps present at certain locations. The oldest stratum in this series consists of Jordan sandstone, which was deposited as the quartzite of the Penokean mountains was weathered and transported south by wind and water approximately 545 million years ago during the Cambrian geologic period. During that time period, Wisconsin and much of the Midwest was inundated by a large inland sea with an estimated average depth of 100 feet. Approximately 485 million years ago, during the early Ordovician period, sea levels dropped to an estimated 20 – 30 feet, and the next layer, consisting of Prairie du Chien dolomite, was gradually deposited (Dott and Attig 2004). The Prairie du Chien layer was initially deposited as marine limestone (calcium carbonate) but was geochemically transformed to dolomite (calcium‐ magnesium carbonate). Some weathering products of this dolomite still bear discernable fossils of small marine organisms, but fossils are uncommon at the Preserve. The inland sea that covered southern Wisconsin became shallower and periodically intertidal during the late Ordovician period (approximately 450 million years ago). Weathering of exposed segments of Jordan sandstone, followed by wind and water erosion, led to deposition of a relatively thin layer of St. Peter sandstone atop the Prairie du Chien dolomite. The St. Peter layer, occurring as sandstone caps on the taller portions of the northeast and southwest ridges (Figure 3) provides substrate for remnant dry prairie that supports a unique assemblage of several rare plant species.
Glaciation (Quaternary Period)
During the Quaternary Period (beginning 1.8 million years ago), glaciers periodically covered much of northern United States. In a broad sense, glaciers form when the amount of snowfall during winter exceeds the amount of snowmelt in spring and summer (though the detailed mechanisms underlying glacial epochs are still debated by geologists). The gradual accumulation of snow and ice gives rise to large continental glaciers that move slowly in response to gravity, scouring and modifying the landscape as they advance. The continental glaciers that covered Wisconsin originated in the Hudson Bay area of Canada. The channeling of ice flow to the east and west by the deep Lake Superior and Lake Michigan basins protected a region covering portions southwestern and central Wisconsin, northeastern Iowa, northwestern Illinois, and southeastern Minnesota from glaciation (Dott and Attig 2004).
Glacial Retreat (Holocene Epoch)
Approximately 12,000 years ago, northern latitudes began experiencing climactic warming and the glaciers began to recede. Rivers of glacial meltwater flowed through depressions in the east and west valleys of the Preserve, scouring out sediment and widening them as they did so, a demonstration of the tremendous erosive power of water. As the glacier receded from the area, meltwater flow rates and discharge decreased and these outwash rivers deposited at least ten feet (and up to 50 feet in some areas) of unconsolidated glacial outwash material in the valley bottoms of the Preserve. As the continental glaciers that covered western portions of North America melted, meltwater streams and rivers carried sediment into the western Great Plains Region of the United States. When these meltwater streams dried up, the highly‐sorted sediment that was deposited in vast outwash plains dried to the point that lighter fractions could be picked up and transported great distances by wind. Strong easterly winds that predominated Wisconsin’s climate at the closing of the Pleistocene transported this fine sediment and deposited it throughout the Midwest as loess, a silt loam with high water holding capacity ideally suited to agriculture but also to the development of a suite of grassland vegetation communities (Pielou 1991). Loess deposits in western Dane County are estimated to range from one to three feet thick, and can be found in varying degrees of thickness throughout the preserve.
Throughout the Quaternary Ice Age, subarctic temperatures during periods of glacial advance resulted in the development of a permafrost layer, which penetrated deeply into the Prairie du Chien dolomite stratum. As the glaciers advanced and receded, repeated freeze‐thaw cycles fractured the dolomite into a series of large blocks, some of which were tilted and skewed as the permafrost melted. Today, these blocks provide habitat structural elements for a variety of animal species. For example, bullsnakes use them for dens and for thermoregulation.
Beginning in 2011, John Attig and Eric Carson of the Wisconsin Natural History Survey have been collecting sediment cores from the south marsh (management unit 18) to determine the geological and botanical history of the Preserve and surrounding landscape during the previous 30,000 years. These data are still being analyzed, and their findings will be incorporated into future editions of the management plan as they become available.