From the Pleistocene to the Present

A cooling climate triggered the start of a series of glacial advances shortly before the Pleistocene began. The most recent glacial period, prior to the present interglacial period, began 65,000 years ago and affected the Midwest until about 10,000 years ago. Initially, the ice that covered the Midwest spread from a single dome located in northern Canada over the Hudson Bay. Approximately 20,000 years ago, this ice sheet reached its maximum extent, reaching as far south as northernmost Kansas and Kentucky (Figure 6.9).

The formation of glaciers comes from the precipitation of water that originates from the evaporation of ocean water. Thus, significant glacial build-up ties up water in ice sheets, causing a sea level drop. During the Pleistocene glacial advances, the sea level dropped an estimated 110 meters (360 feet)!

Figure 6.9: Glaciers flowed south during glacial advances;; each advance differed slightly and left different sets of deposits.

By 18,000 years ago, the ice sheet was in retreat because of a slight warming of the climate. Though the ice sheet alternately moved forward and melted backward, overall it was on the retreat. Even during glacial advance, the glacier was always melting at its fringes. During times of glacial retreat, the ice sheet was not actually flowing backwards: the glacier continued to flow forward, but it was melting faster than it was advancing.

Rebounding of the Crust

The ice sheets could exceed two kilometers (1.2 miles) in thickness. The enormous weight of all of that ice over the continent depressed the crust into the asthenosphere (the uppermost part of the mantle) just as the weight of a person in a canoe causes the boat to ride lower in the water. When the person steps out of the canoe, the buoyancy of the canoe allows it to once again rise. As the ice sheet retreated from the Midwest during the current interglacial period, the crust rebounded, and it continues to do so today. The equilibrium achieved between the crust and mantle is known as isostasy (Figure 6.11). This rebound is thought to be a cause of infrequent, but occasionally severe, seismic activity in the Midwest.

Figure 6.10: In addition to other deposits, glaciers resulted in moraine deposits through the Midwest, in some cases adding topographic relief to otherwise flat terrain. The moraines help define the extent of glacial advances.

The Iowa Pleistocene snail was abundant throughout the Midwest during much of the Pleistocene epoch. At one time it was only known from the fossil record and thought to have gone extinct after the last glacial maximum, but it was discovered alive and well in 1955. It survives today scattered among a few dozen algific talus slopes in five counties in Iowa and one in Illinois. It is currently classified as an endangered species.

Iowa Pleistocene snail (Discus macclintocki), found only in specific types of karst habitats. Adult shell width about 7 mm (0.25 inch).

Figure 6.11: Through isostacy, glaciers push the crust down, and, when they melt, the crust rebounds upward.

The Great Lakes

The Great Lakes are a prominent geologic feature of the Midwest and include three of the five largest lakes in the world. In fact, they contain 21% of the world’s fresh water. And 20,000 years ago, they did not exist. At that time, the ice sheets extended well past where the lakes would come to be prior to glaciation. The Great Lakes were actually river valleys that had been scoured and deepened repeatedly by the numerous ice advances during the Quaternary period. Many sizable glacial lakes were formed at the edge of the melting glacier, yet they no longer exist today or have significantly shrunk in size. As the glacier retreated and the basins filled with glacial runoff, the still-forming lakes drained southward, eventually into the Mississippi River (Figure 6.12).

Figure 6.12: Glaciers over the Midwest retreated over the course of 10,000 years after the Last Glacial Maximum, leaving behind the Great Lakes, among other features.

The Loess Hills

The Loess Hills of extreme western Iowa, extending into northwesternmost Missouri, are named after a glacial deposit formed of windblown rock flour: loess. This type of glacial feature is found in only a few places on Earth. Loess deposits are found in parts of Wisconsin, Minnesota, Illinois, and Iowa, but the loess hills in Iowa represent the most prominent of all the loess deposits. While these hills form only a very narrow, 320-kilometer (200-mile) long, north-south band immediately east of the Missouri River floodplain, they are an important part of the story of the glaciation of the Midwest (Figure 6.13). The hills stand more than 60 meters (200 feet) above the surrounding low farmland and often display very sharp profiles, having been cut away into steep bluffs. They were formed during several glacial/interglacial cycles when glaciers ground down the bedrock. Then, as the ice retreated, meltwater deposited the fine sediments in expansive mudflats. When the mudflats dried, strong westerly winds blew the sand into great dunes, and the finest material (silt and clay) was carried farther in massive dust clouds. The dunes were eventually stabilized by vegetation and matured into the hills, but the loose material of the hills can still be easily eroded and carved. Slumping, mudslides, and undercutting caused by wind and water have produced steep slopes and a landscape of narrow ridges.

Fig 6.13: Thickness of loess deposits in Midwest.

Much of the soil throughout the Midwest is composed, in part, of sediment blown from the huge mudflats on the banks of the ancient Missouri River, which was a major channel for floods of glacial meltwater.

Driftless Area

The Driftless Area, also known as the Paleozoic Plateau, is located around the upper Mississippi River valley where Minnesota, Wisconsin, Iowa, and Illinois meet. It is a place that beautifully contrasts with the surrounding glacial landscape because the Driftless Area was completely missed by the advancing ice during the Pleistocene. Its deep valleys, steep bluffs, and high hills stand in stark relief to the flat plains surrounding it, and it is suggestive of what the rest of the Midwest might have looked like before glaciers leveled it. The Driftless Area’s topography is largely controlled by the water flowing through it, which has been carving into its rock for millions of years. Other creeks and rivers in the Midwest have been following their current courses for a few tens of thousands of years at most—orders of magnitude less time in which to shape their landscapes. As they scraped over the area, the ice sheets covered river valleys and filled them with sediment, forcing water to flow around their margins, effectively shoving the rivers out ahead of them. Much of the Missouri, Ohio, and Mississippi Rivers outline the farthest extent of ice sheet advances. This underscores the importance of glaciers to the Midwest: the borders of several states, defined by these rivers, are the direct result of glacial action!

The Driftless Area is an example of karst topography, a kind of landscape defined by bedrock that has been weathered by dissolution in water, forming features like sinkholes, caves, and cliffs (Figure 6.14). Here, cold underground lakes create a kind of natural air conditioning that cycles air into cracks and caves in the rock where it is cooled by the water before returning to the surface. This unique geologic phenomenon, coupled with cliffs that block much of the sunlight, make microhabitats that are tens of degrees cooler than the surrounding area just yards away. These habitats, called algific talus slopes, tend to occur only on the northern slopes of hills where they receive very little sunlight, and they are scattered across the Driftless Area. Furthermore, they do not occur anywhere else on Earth. They are home to unique flora and fauna, many members of which are usually only found much farther north, and some of which are no longer found anywhere else. In short, in many ways the Driftless Area is like a time capsule of the ancient Midwest.

Figure 6.14: Cave of the Mounds near Blue Mounds, Wisconsin.