Although the climate was probably tropical, land plants had not yet evolved so the land surface was barren except for some primitive algae and bacteria. Sea level began to rise much higher than it is today, and eventually most of North America was covered by the ocean. As a result, about 520 million years ago, southern Minnesota, Wisconsin, and Iowa became a shallow sea with islands at Baraboo, Wisconsin, in southwestern Minnesota, and near Taylors Falls (Fig. 1). The sea was bordered on the northeast side by higher ground called the Wisconsin Dome. Over the next 200 million years, sediments eroded from highlands to the north accumulated in more or less flat layers in this sea. These sediments were later buried and cemented eventually forming the layers of bedrock. Fossils contained in this bedrock record life that existed in this ancient sea.
If you examine the bedrock exposed in quarry walls, road cuts or steep hillsides in southeastern Minnesota, you can see that it consists of more than one kind of rock—sandstone, shale, dolostone, and limestone (Fig. 2). The coarsest sandstone layers are composed almost entirely of quartz grains, a mineral rich in silica—similar in composition to window glass.Other layers are mostly limestone or dolostone—an altered limestone made up of calcium, magnesium and carbon dioxide. Still other layers are mostly shale, or a mixture of fine-grained sand, shale and carbonate (limestone and dolostone) rock. Geologists have assigned names to these individual rock layers. The names are from places where the rocks are, or were at one time, well exposed. For example, the Jordan Sandstone is named for the city of Jordan, Minnesota; and the St. Peter Sandstone, for the St. Peter (now Minnesota) River near Fort Snelling.
The Paleozoic rock layers are more than 1,500 feet thick in some places, and were deposited over a span of some 200 million years during the three geologic time periods known as the Cambrian, Ordovician, and Devonian (Fig. 2). The manner in which they were deposited varied through time, and is described in greater detail in two parts:
- Late Cambrian to Late Ordovician time—when the rock layers were deposited as part of a texturally graded shelf; and
- Late Ordovician and Devonian time—when the depositional environment was dominated by carbonate.
Deposition in a texturally graded shelf
To best envision what southeastern Minnesota may have looked like in the Cambrian and most of the Ordovician, picture the sandy coast of the Gulf of Mexico, but with a barren, mostly lifeless land surface. Sediments deposited in Minnesota at this time consisted mostly of sand, silt and clay-sized particles that were carried by streams from the Wisconsin Dome to the shoreline. Shallow ocean currents subsequently sorted and deposited these sediments forming a "texturally graded shelf" (Fig. 4). The coarsest sand was laid down in a shallow marine environment known as the shoreface. The shoreface includes the beach and shallow water nearshore where oceanic currents were relatively strong. At the same time, finer sand, silt and clay-sized particles carried seaward by storm currents were deposited in deeper water on the offshore shelf. In the deeper parts of the offshore shelf, hundreds of miles from the shoreface, silt and clay-sized particles and carbonate grains slowly settled out of suspension.
The different layers of Paleozoic rocks that stretch across southeastern Minnesota were formed when sea level, and therefore the depth of the ocean, changed through time (Figs. 4 and 5). Large changes in sea level led to drastic changes in the position of the sandy shoreface, moving it off of the Wisconsin Dome, and back and forth across southeastern Minnesota. Each time the shoreface passed across southeastern Minnesota, it left behind sandy deposits. The Mt. Simon Sandstone, the oldest Paleozoic formation, was deposited during the initial Cambrian flooding of Minnesota during which the shoreface migrated northward as southeastern Minnesota was covered with water. The younger Wonewoc and Jordan sandstones were deposited during subsequent major sea level changes. When sea level fell, the sandy shoreface retreated southward into Iowa, leaving behind a "trail" of quartz sand (Figs. 4 and 5). When sea level rose again, the shoreface moved northward across Minnesota, also leaving behind a trail of sand. The last major shoreface sandstone accumulation is represented by the Ordovician St. Peter Sandstone. The St. Peter was deposited during a slow rise in sea level that followed an extended period of low sea level and erosion across much of Minnesota.
When sea level was relatively high, the sandy shoreface was on higher ground to the northeast outside of Minnesota (e.g., Fig. 5). At these times, most of southeastern Minnesota was a large offshore shelf under relatively deep water where clay, silt and fine sand accumulated. Such offshore shelf deposits form the layers now called the Eau Claire Formation and Tunnel City Group. Even higher sea level and deeper water led to offshore conditions where only silt, clay and carbonate particles accumulated. The layers called the St. Lawrence, Glenwood and Decorah formations are composed of variable proportions of shale, siltstone, and carbonate sediments deposited in such a setting. When the sea was at its highest levels, nearly all of Minnesota and surrounding areas were flooded, and carbonate deposition occurred in the deep water that covered southeastern Minnesota. The part of the Priaire du Chien Group, the Platteville Formation, and the Galena Group were deposited in such very deep water. At about the same time, the small patch of Paleozoic rocks in northwestern Minnesota was also deposited. The Galena Group represents the last deposit of the "texturally graded shelf" system.
Deposition in carbonate dominated systems
Depositional conditions during the latest Ordovician and Devonian were dominated by calcareous sediments at all water depths. Sand was largely absent, even in the shallowest water environments at this time, indicating that Minnesota may have been very low-lying, and perhaps much of its surface was covered with newly evolved vegetation. Such conditions inhibited erosion across the Wisconsin Dome and nearly eliminated the supply of sand to the shoreline.
The latest part of the Ordovician Period came to a close with the deposition of the Dubuque and Maquoketa formations (Fig. 2), which together record a change from deep water deposition to shallow tidal-flat deposition as sea level fell and the shoreline retreated out of Minnesota. A tidal flat is a broad shoreline area where limy muds accumulated as water advanced and retreated during high and low tide cycles.
After a long period of erosion during Silurian and Early Devonian time, the seas returned to Minnesota in the Middle Devonian. Sea level was relatively low, and the shoreline remained at or near extreme south-central Minnesota, rarely if ever extending as far north as the Twin Cities area. The Cedar Valley and Wapsipinicon groups were deposited at this time (Fig. 2). Conditions were similar to those in latest Ordovician time—shallow water deposition dominated by carbonate sediments. Communities of shelled organisms, such as corals, locally built up into tropical reefs.
Post Devonian history
The are no rocks in Minnesota representing the remainder of the Paleozoic Era and much of the early Mesozoic (350 to 100 million years ago). For most of this time, the region was above sea level, and the land surface was eroded by wind and water. The sea never became high enough for the shoreline to advance further north than Iowa. The sea returned to Minnesota for the last time about 100 million years ago during the Cretaceous Period, a time when dinosaurs roamed the Earth. Deposits laid down at this time are common beneath the surface of southwestern Minnesota, but in southeastern Minnesota, only thin, patchy remnants of Cretaceous strata are present.
Erosion of Paleozoic rocks
Paleozoic rock formations are no longer the continuous layers they were when they were first laid down in the ocean. Instead, they have been eroded in places by relatively recent geologic activities, particularly during the Ice Age that began about two million years ago. At times, glaciers covered much of the state. The Paleozoic bedrock of southeastern Minnesota was deeply eroded when large amounts of water from melting glaciers to the north caused the ancient Mississippi, St. Croix, and Minnesota rivers and their tributaries to erode more deeply into their valleys. Thus, the bluffs you see today along the major rivers in southeastern Minnesota are not mountains as early explorers thought. Instead, they are more or less horizontal layers of Paleozoic rocks that have been carved by streams over tens of thousands of years (Fig. 2). Relatively soft formations, like the Jordan Sandstone, are easily eroded and commonly form the valley floors. Harder, more resistant rocks, like the Oneota Dolomite and Platteville Formation, stand as cliffs along the valley walls.
Written by A.C. Runkel, 2000, Minnesota at a Glance: Ancient Tropical Seas - Paleozoic History of Southeastern Minnesota: Minnesota Geological Survey; revised by A.C. Runkel, June 2002; modified for web by A.J. Retzler, June 2020.