Topography of the Western 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 others 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 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 the West, topography is intimately tied to weathering and erosion 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. With enough time, streams can cut deeply and develop wide flat floodplains on valley floors.

The pounding action of ocean waves on a coastline contributes to the erosion of coastal rocks and sediments. Ice also plays a major role in the weathering and erosion of the West’s landscape due to frequent episodes of freezing and thawing that occur at high elevations and high latitudes. On a small scale, as water trapped in fractures within the rock freezes and thaws, the fractures widen farther and farther (Figure 4.1). This alone can induce significant breakdown of large rock bodies. On a larger scale, ice in the form of alpine glaciers or continental ice sheets can reshape the surface of a continent through mechanical weathering.

Figure 4.1: Physical weathering from a freeze-thaw cycle.

Figure 4.1: Physical weathering from a freeze-thaw cycle.

See Chapter 2: Rocks to learn more about igneous, metamorphic, and sedimentary rocks.

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 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 found 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 California’s Jurassic and Cretaceous 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 Sierra Nevada. 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 and alluvial fans, are the least resistant to erosion. The limited degree of cement, compaction, and interlocking crystals found in alluvial fan sediments makes it difficult for these types of sediments to stand up to the effects of wind and water, which is why they tend to persist only in arid regions.

See Chapter 1: Geologic History for more information about the mountain-building events that helped to shape the West.

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. 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 them. 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 stick out and remain as ridges, while surrounding layers of less resistant rock erode away.

Ice sheets of the last glacial maximum, as well as extensive mountain glaciers, covered part of the West and had a dramatic effect on the area’s topography.

See Chapter 6: Glaciers for more about how glaciers influenced Western topography.

Glaciers carved away at the land’s surface as they made their way primarily southward, creating a number of U-shaped valleys such as Yosemite and depositional features such as moraines. Mountains were sculpted, leaving high peaks and bowl-like cirques. As the ice sheet and glaciers melted, other characteristic glacial features were left behind as evidence of the glaciers’ former presence, including glacial lakes and polished rock.

Just as we are able to make sense of the type of rocks in an area by knowing the geologic history of the West, we are able to make sense of its topography (Figure 4.2) based on the rocks and structures resulting from past geologic events.