Region 2: Topography of the Central Lowland

Nearly all of the bedrock in the Central Lowland is sedimentary and fairly easily eroded. During the last ice age, a series of huge ice sheets worked their way primarily southward and flattened most of whatever varied topography the region once had. Furthermore, as the glaciers retreated, they dumped sediment that formed the flatlands and the low, rolling hills that are characteristic of most of the region. There are several areas with present-day relief caused by glacial deposits, especially that created by hill-forming moraines and drumlins. While these formations stand out against the surrounding landscape, they do not usually rise more than 60 meters (200 feet) from base to peak.

Most of the topography of the Central Lowland is controlled by the rivers running through it. Since the ice last retreated from the region, rivers have had only 20,000 years, at the most, to shape the young terrain, so even the largest river valleys are not yet that deep.

See Chapter 6: Glaciers for more about the Driftless Area.

Within the Central Lowland, the Driftless Area may be viewed as a window into the region’s topographic past (Figure 4.3). The glaciers of the last several advances did not reach as far south as where the borders of Minnesota, Wisconsin, Iowa, and Illinois meet, leaving that area with bedrock similar to the surrounding landscape but with starkly different topography. Here, streams and rivers have had hundreds of thousands, and perhaps tens of millions, of years to carve steep relief into the same types of rocks that, just miles away, glaciers recently scraped flat. Both mechanical and chemical weathering here have created a karst topography, defined by bedrock that has been affected by dissolution in water to form features like sinkholes, caves, and cliffs. The highest points in Iowa and Illinois are both located in the Driftless Area. Given enough time, the rest of the Central Lowland might appear as the Driftless Area does today, after running water washes away the glacial sediment and cuts into the bedrock. West Blue Mound, with an elevation of 523 meters (1716 feet), is the highest point in the Driftless Area, while the Mississippi River is appreciably lower at 184 meters (603 feet). Ultimately, this does not result in huge changes in elevation, but the steep cliffs and valleys contrast dramatically with the nearby flatland.

Figure 4.3: Map showing location of the “Driftless Area” of the Central Lowland.

Figure 4.3: Map showing location of the “Driftless Area” of the Central Lowland.

Karst Topography

Karst topography refers to a region where the landscape’s features are largely the result of chemical weathering by water, resulting in caves, sinkholes, disappearing and reappearing streams, cliffs, and steep-sided hills called towers. These structures form when water picks up carbon dioxide from the atmosphere and ground to form carbonic acid. Even this fairly weak and dilute acid dissolves carbonate rocks (such as limestone) relatively easily, resulting in dramatic features while other rock is comparatively unaffected. Karst is found in every state except Hawaii, and it is the source of a significant amount of our drinking water, particularly in the Midwest. While common, karst is not always easily identifiable since it is often not expressed at the surface or its topography has been affected by other factors. Karst topography is a relatively mature type of landscape, taking many tens of thousands of years to develop, and it can indicate that a region has been free of other forms of erosion, or deposition, for an extended period. Karst topography in the Midwest is found in places that were not eroded by glaciers during the last ice age, including northern Michigan, Mitchell Plateau and Muscatatuck Plateau in Indiana, and the Driftless Area. (See discussion above in Topography of the Central Lowland and Chapter 6: Glaciers.)