Region 2: Soils of the Central Lowland

The Central Lowland is home to some of the richest agricultural land in the US. Although the soil here has been affected for a considerable amount of time by the climate, life, and plate tectonics, most of the topography and soil found here is the result of glacial activity during the Pleistocene. Multiple episodes of formation and melting of glaciers over the last 2.5 million years have shifted massive amounts of sediment and rock, carved gorges with their advance, and produced caves, lakes, river beds, and streams with their melt.

During this stretch of time, the glaciers advanced, receded, melted, and reformed repeatedly as the climate cooled during the ice age and then warmed somewhat during interval periods. The farther south one goes in the Midwest, the fewer the advances that were experienced. The glaciers brought with them rocks, sands, silts, and clays as they traversed the Canadian terrain. The more obvious glacier depositional features can be seen north of Illinois and Indiana, such as drumlins, kames, eskers, and moraines (Figure 8.9). The surfaces in the southern parts of the region are covered mostly by outwash and loess deposits that developed near the terminus of the ice sheets. These deposits are made up of predominantly silt and clay-sized material. While the parent material for much of the northern area is ground moraine, the sediment was plastered down by the advancing ice. Normally, the composition of the ground moraine is at least partly determined by the type of bedrock encountered by the advancing ice. Not surprisingly, much of central Wisconsin and Minnesota has sandy ground because the bedrock is Cambrian sandstone, whereas eastern Wisconsin has very clay-rich soils because the glaciers advanced over exposures of the Maquoketa Shale, a very soft and clay-rich shale. Prominent glacial features in Wisconsin are drumlins, elongated hills that form parallel to the flow of a glacier and are made of compacted glacial sediments, frequently till. They make for dramatic-looking farm fields, such as those seen in Figure 8.10.

Figure 8.9: Glacial deposits.

Figure 8.9: Glacial deposits.

Figure 8.10: Drumlins formed by glacial drifts near Cross Plains, Wisconsin.

Figure 8.10: Drumlins formed by glacial drifts near Cross Plains, Wisconsin.

The Driftless Area, found in parts of southwestern Wisconsin, eastern Minnesota, and northeastern Illinois and Iowa, did not experience glaciation. This locale is known as the Driftless Area since it lacks glacial deposits, which are collectively called drift. Glaciers are known to have reached all sides of the Driftless Area at various times throughout the Quaternary Ice Age, but are not known to have completely encompassed the area at any time.

See Chapter 6: Glaciers to learn more about the Driftless Area.

This resulted in an island of much older soils in a sea of younger soils. It also is one of the few areas of the Central Lowland with highly dissected river valleys, known locally as the Coulee Country.

Parts of Wisconsin also have remnants of drained glacial lakes. The most prominent example is Glacial Lake Wisconsin that drained rapidly as it broke through a terminal moraine of the Green Bay Lobe near the site of the Wisconsin Dells. The draining water carved the Wisconsin River Valley at the Dells and left behind a glacial lakebed near Tomah, Wisconsin where cranberries are grown today. Other noted glacial lakebeds that contributed to the agriculture of the state are Glacial Lake Oshkosh in the northeast part of the state, and Glacial Lake Yahara in the south central part of the state.

Western and northern Ohio along Lake Erie also have a distinct glacial history because much of the area is composed of glacial lakebed sediments generated when the ice retreated into the Lake Erie Basin, producing a marginal lake known as Glacial Lake Warren. This area is known for its vineyards, orchards, and fields of farm vegetables.

The dominant soil types for the Central Lowlands are Alfisols, of suborders Udalfs, and Aqualfs, with a small section in northwestern Minnesota of Boralfs. The first two are more common in warm humid regions, whereas the last is found in cold regions. They are not as weathered as some soils, so they are still rich in nutrients. The soils to the south of the region are thicker than those to the north.

The second most common soil types are Mollisols, which can be found in the Central Lowlands where loess deposits are dominant and the vegetation was originally grassland. These soils are prominent in western Minnesota, Iowa, and much of Illinois and are of the suborder Udolls.

There are small sections with Entisols (suborder Psamments) in central Wisconsin associated with Glacial Lake Wisconsin. Northern Minnesota has a section of Histosols, and northeast Michigan has a small area of Inceptisols.