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 500 million years ago, southern Minnesota, Wisconsin, and Iowa became a shallow sea with islands in southwestern Minnesota, near Taylors Falls, and at Baraboo, Wisconsin (Fig. 1). The sea was bordered on the northeast side by higher ground called the Wisconsin Dome. Over the next 130 million years, sediments accumulated as more or less flat layers in this sea. These sediments were later buried and cemented, eventually forming layers of rock. Fossils contained in this rock record life that existed in this ancient sea. They are described in Minnesota at a Glance: Fossil Collecting in the Twin Cities Area.
If you examine the rock 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-grained sandstone layers are composed almost entirely of quartz grains, a mineral consisting of 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 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 State Park.
The Paleozoic rock layers are more than 1,500 feet thick in some places, and were deposited over a span of 130 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
- Devonian time—when the depositional environment was dominated by carbonate deposited in a wider variety of conditions.
Deposition on a texturally graded shelf
To best envision what southeastern Minnesota may have looked like in the Cambrian and much of the Ordovician Periods, picture a sandy coast such as 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. 3). The coarsest-grained sand was laid down in a shallow marine environment known as the shoreface. The shoreface includes the beach and shallow water nearshore where oceanic waves and currents were relatively strong. At the same time, finer-grained 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 Era formation, was deposited during the initial Cambrian Period flooding of Minnesota, during which the shoreface migrated northward when southeastern Minnesota was covered with water. The younger Wonewoc and Jordan Sandstones were deposited during subsequent major sea-level changes. When the sea level fell, the sandy shoreface retreated southward into Iowa, leaving behind a "trail" of quartz sand (Fig. 5). When the 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-age St. Peter Sandstone. The St. Peter Sandstone 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 (Fig. 5). At these times, most of southeastern Minnesota was a large offshore shelf under relatively deep water where clay, silt, and fine-grained sand accumulated. Such offshore shelf deposits form the layers now called the Eau Claire Formation and the 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 Formation, Glenwood Formation, and Decorah Shale (the lowest part of the Galena Group), 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. Part of the lower Prairie du Chien Group, the Platteville Formation, and the remainder of the Galena Group, were deposited in such deep water. The upper part of the Prairie du Chien Group is an exception; it was deposited in shallow water shoals much like carbonate sediment accumulating in the Bahamas today. The Makoqueta Formation is the last deposit of the texturally graded shelf system. Afterward, sea level fell enough that the shoreline was mostly south of Minnesota for about 60 million years.
Deposition in carbonate-dominated systems
The seas returned to Minnesota in the middle Devonian Period, about 370 million years ago. The shoreline remained at or near extreme south-central Minnesota, rarely, if ever, extending as far north as the Twin Cities area. The uppermost bedrock layers, the Cedar Valley and Wapsipinicon Groups, were laid down at this time. 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, or by carbonate rock layers such as the Galena Group. Such conditions inhibited erosion across the Wisconsin Dome and nearly eliminated the supply of sand to the shoreline. As a result, deposition was dominated by calcareous sediments at all water depths. Communities of shelled organisms, such as corals, locally accumulated into tropical reefs.
There are no rocks in Minnesota representing the remainder of the Paleozoic Era and much of the early Mesozoic Era (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 southwest Minnesota, but in southeast Minnesota, only thin, patchy remnants of Cretaceous strata are present.
Erosion of Paleozoic rocks
Paleozoic Era rock formations are no longer the continuous layers they were when first deposited 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 most 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 deeper into their valleys. Thus, the bluffs visible today along the major rivers in southeastern Minnesota are not mountains as early surveyors thought. Instead, they are more or less horizontal layers of Paleozoic rocks that have been carved by water over tens of thousands of years (Fig. 2). Relatively soft formations, such as the Jordan Sandstone, are easily eroded and commonly form the valley floors. Harder, more resistant rocks, such as the Oneota Dolomite and Platteville Formation, stand as cliffs along the valley walls.