Topography of the Midwestern US
Does your region have rolling hills? Mountainous areas? Flat land where you never have to bike up a hill? The term topography is used to describe the changes in elevation over a particular area and is, generally speaking, the result of two processes: deposition and erosion. These processes can happen on an enormous range of timescales. For example, a flash flood can erode away tons of rock in a matter of hours, yet which rock is broken down and which remains can depend on how it was formed hundreds of millions of years ago. In addition to these processes, the topography of the Midwest is intimately tied to weathering and erosional forces, along with the type and structure of the underlying bedrock.
Weathering includes both the mechanical and chemical processes that break down a rock. Wind, water, and ice are the media by which physical weathering and erosion occur. Streams are constantly trying to erode the bedrock down to sea level, creating valleys in the process. With sufficient time, streams can cut deeply and create wide flat floodplains on the valley floor.
Wave action on the shores of the Great Lakes contributes to the erosion of rocks and sediments. Ice plays a major role in the weathering and erosion of the Midwest landscape because of the frequent episodes of freezing and thawing in temperate latitudes. On a small scale, as water trapped in fractures within the rock freezes and thaws, the fractures widen farther and farther. This alone can induce significant breakdown of large rock bodies. On a larger scale, ice in the form of glaciers in mountain valleys and continental ice sheets can reshape the surface of a continent through physical weathering.
See Chapter 2: Rocks to learn more about igneous, metamorphic, and sedimentary rocks.
Working in conjunction with physical weathering, chemical weathering also helps to break down rocks. Some minerals contained in igneous and metamorphic rocks that are formed at high temperatures and pressures (far below the surface of the Earth) become unstable when they are exposed at the surface where the temperature and pressure are considerably lower, especially when placed in contact with water. Unstable minerals transition into more stable minerals, which results in the breakup of rock. Weak acids, such as carbonic acid found in rainwater, promote the disintegration of certain types of rocks. Limestone and marble may be rapidly broken down chemically as carbonic acid reacts with the carbonate mineral composition of these rocks, forming cavities and caverns in the rock. Other sedimentary rocks held together by carbonate cement are also particularly susceptible to chemical weathering.
The specific rock type at the surface has an important influence on the topography of a region. Certain rocks are able to resist weathering and erosion more easily than are others;; resistant rocks that overlie weaker layers act as caps and form ridges. The inland ocean basins of the Ordovician Taconic and the Devonian Acadian mountain-building events collected and preserved sediments that eventually became sedimentary rocks. Sedimentary rocks weather and erode differently than do the crystalline, and generally harder, igneous and metamorphic rocks that are more common in the Superior Upland province. Silica-rich igneous rocks have a crystalline nature and mineral composition that resists weathering far better than do the cemented grains of a sedimentary rock. The metamorphic equivalents of sedimentary and igneous rocks are often more resistant due to recrystallization. There are exceptions, however, such as schist, which is much weaker than its pre-metamorphic limestone or sandstone state. Landscapes of unconsolidated sediments like soil or glacial till are the least resistant to erosion.
See Chapter 1: Geologic History for more information about the mountain-building events that helped to shape the Midwest.
The underlying structure of the rock layers also plays an important role in the topography at the surface. Sedimentary rocks are originally deposited in flat-lying layers that rest on top of one another. Movement of tectonic plates creates stress and tension within the crust, especially at plate boundaries, which often deform the flat layers by folding, faulting, intruding, or overturning. These terms are collectively used to describe rock structure, and they can also be used to determine which forces have affected rocks in the past. The folding of horizontal rock beds followed by erosion and uplift exposes layers of rock to the surface. Faulting likewise exposes layers at the surface to erosion, due to the movement and tilting of blocks of crust along the fault plane. Tilted rocks expose underlying layers. Resistant layers erode relatively slowly and remain as ridges, while surrounding layers of less resistant rock erode away.
See Chapter 6: Glaciers for more about glacial depositional features.
Glacial ice sheets of the most recent ice age covered most of the Midwest and had a dramatic effect on the topography of the area. Glaciers carved away at the land’s surface as they made their way generally southward, creating characteristic glacial depositional features such as drumlins, eskers, and moraines. Hills were worn and valleys widened.
Just as we were able to make sense of the type of rocks in an area by knowing the geologic history of the Midwest, we are able to make sense of its topography (Figure 4.1) based on the rocks and structures resulting from past geologic events.