Topography of the South Central US
Does your region have rolling hills? Mountainous areas? Flat land where you never have to bike up a hill? The answers to these questions can help you understand the basic topography of your region. 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 occur over 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 the South Central, topography is intimately tied to weathering as well as to the type and structure of the underlying bedrock, but it is also a story of plate tectonics and its associated folding, faulting, and uplift.
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 eroding their way down through bedrock to sea level, creating valleys in the process. Given sufficient time, streams can cut deeply and develop wide flat floodplains on valley floors. Streams, oceans, and ice also deposit the material they erode, creating new topographical features elsewhere.
The pounding action of ocean waves on a coastline contributes to the erosion of coastal rocks and sediments, while the emptying of a river can lead to the building of a delta. The deposition of fine silt that has been ground from rock by glaciers can lead to the formation of wind-blown deposits called loess, as seen in Missouri and Kansas. And though its effect is less pronounced in the South Central than in other areas, ice can change the landscape due to frequent episodes of freezing and thawing. On a small scale, as water trapped in fractures within the rock freezes and thaws, the fractures continue to widen (Figure 4.1). This alone can induce significant breakdown of large rock bodies.
Working in conjunction with mechanical 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, resulting in the breakup of rock. Weak acids, such as the carbonic acid found in rainwater, promote the disintegration of certain types of rocks. Limestone and marble may be chemically broken down as carbonic acid reacts with the carbonate mineral composition of these rocks, forming cavities and caverns. 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 Western Interior Seaway of the Cretaceous collected and preserved sediments that became sedimentary rocks, such as the chalk deposits of Kansas’ Great Plains region. Sedimentary rocks weather and erode differently than do crystalline (and generally harder) igneous and metamorphic rocks, such as those found in Texas’ Llano Uplift. 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 even 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 beaches, deltas, and alluvial fans, are the least resistant to erosion. The unconsolidated sediments of the Coastal Plain region along the Gulf of Mexico are not yet even considered rocks. The limited degree of cementation, compaction, and interlocking crystals found in these sediments makes it difficult for them to stand up to the effects of wind, chemical weathering, and water.
See Chapter 2: Rocks to learn more about the Niobrara Chalk and other Cretaceous deposits in Kansas.
The underlying structure of rock layers also plays an important role in surface topography. Sedimentary rocks are originally deposited in flat-lying layers that rest on top of one another. The movement of tectonic plates creates stress and tension within the crust, especially at plate boundaries. Intrusions and salt domes beneath the surface may also cause deformation of the crust. All these different sources of geological stress can deform the flat sediment layers through folding, faulting, 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 brings 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. Since tilted rocks expose underlying layers, resistant layers stick out and remain as ridges, while surrounding layers of less resistant rock erode away.
An area’s glacial history is also important to topography, and the glacial ice sheet of the most recent ice age covered the northernmost part of the South Central, leaving its mark on the landscape. The Loess Hills of northwestern Missouri formed from the blowing and deposition of fine-grained, glacier-pulverized rock fragments, and the Missouri River’s original valley was cut by the erosive action of melting glacial ice. As the ice age came to an end, sediment-laden meltwater flowed southward, sculpting river valleys and depositing sediment to form the Coastal Plain.
Just as we are able to make sense of the type of rocks in an area by knowing the geologic history of the South Central, we are able to make sense of its topography (Figure 4.2) based on rocks and structures resulting from past geologic events.
See Chapter 2: Rocks for more details about physiographic provinces within the regions of the South Central.
Topography is a central element of the broader concepts of geomorphology or physiography, which also include consideration of the shape (not just the height) of land forms, as well as the bedrock, soil, water, vegetation, and climate of an area, and how they interacted in the past to form the landscape we see today. A physiographic province is an area in which these features are similar, and are also significantly different from those of adjacent regions, and/or the region can be separated from adjacent regions by major geological features. The regions of the South Central US that we use in this book are examples of major physiographic provinces.