For most of its existence, the Tower lay buried and protected from weathering beneath a thick cover of sedimentary rock layers. Uplift, both local (the Black Hills/Bear Lodge mts.) and regional (the entire Rocky Mountain area) and the resulting increase in the rate of erosion, exposed the Tower at the surface. Since that time, the Tower itself has been a victim of weathering and erosion.
About the time the rocks of the Tower cooled, the Rocky Mountains had already formed. They formed during a mountain building event called the Laramide Orogeny. It created the various large mountain ranges in the area that we are familiar with today. These ranges include the Bighorn and Wind River Mountains, the Colorado Front Range and Utah’s Uinta Mountains. The Laramide Orogeny also created the local Black Hills and Bear Lodge Mountains. The Powder River Basin that lies to the west between Devils Tower and the Bighorn Mountains was dropped down to form a lower lying area. Click here to see a map of these features.
The uplift of the Black Hills and Bear Lodge Mountains area raised the rock layers, including the Tower, to a higher elevation. Erosion began to take place, wearing away the sedimentary layers over the crests of the mountains. The debris was dumped into the adjacent basins such as the Powder River Basin and the Badlands area of the Great Plains to the east. However, the area was still much closer to sea level than it is today.
About five million years ago, the mountains and basins of the Rocky Mountain region began to rise upward. This uplift carried the region to its present altitude of about 5000 to 6000 feet above sea level. The increase in elevation also increased the gradient of rivers in the area. As a result, the Belle Fourche River which flows through the monument, began to flow faster. It cut down into the sedimentary layers beneath its bed, exposing the older rocks as well as the Tower.
As the Belle Fourche River cut down into the soft sedimentary rocks, the harder rocks of the upper part of the Tower were exposed first. Initially, the Tower would have appeared rather short and stubby.
It may have been similar in appearance to the present-day Missouri Buttes that lie about four miles west of the Tower.
However, the Tower lies nearer the Belle Fourche River than the nearby buttes. Further downcutting by the river exposed the lower part of the Tower and made it appear more or less the way we see it today.
The upper part of the Tower was exposed to the elements for a longer period of time. The weathering caused the upper portion be more cracked than the lower part of the Tower. The more rugged appearance of the upper third of the Tower may also be due to how it cooled. The upper part lay at a shallower depth and cooled more rapidly than the lower part of the Tower.
The rocks of the Tower are very hard compared to the surrounding sedimentary rocks, but they are not completely resistant to erosion. The cracks and columnar joints are the Tower’s weak spots. During the winter months, water from rain and snowmelt enters the cracks between the columns. When the temperatures fall below freezing, the resulting ice expands and pries the columns apart, enlarging the cracks.
Other agents of erosion include the lichens, trees and other plants that grow on the Tower. These plants attack the rocks of the Tower, both chemically and a physically. Organic acids secreted by the plants dissolve the rock chemically. The roots of the plants wedge themselves into the cracks in the rock and, as the plant grows and the roots enlarge, they slowly pry the cracks apart.
Eventually, the columns become so weak that they fall forming the massive talus pile of broken columns at the base of the Tower.
However, no columns have fallen off the Tower in over 100 years! So this is a very slow process.
The Belle Fourche River was the major cause of erosion of the sedimentary rocks around the Tower. But the river sometimes deposits sediment as well.
The river is a meandering river which, when seen from above, consists of a series of horseshoe-shaped meander bends.
The river erodes sediment on the outer curve of the meander bends—the cut bank side. It deposits loose sediments (sand and gravel) as point bars on the inside curve of the meander bends. Click here to see the cut bank and point bar areas of a meander bend.
In the 1930’s, cement tetrahedrons were installed along the cut bank side of a meander to reinforce the river channel. They were also designed to prevent the river from eroding its banks so that the river channel would always remain in the same location. Do you think this worked?
Notice how the tetrahedrons start on the stream terrace, then tumble over the bank and into the river? The tetrahedrons tumble into the river when the river washes away the ground from beneath them. The tetrahedrons may even increase erosion by causing turbulence in the flowing stream.
Sometimes the gravel and sand carried by the stream builds up along the banks of the Belle Fourche River faster than it can be removed by the stream. These wide flat areas underlain by sand and gravel are called stream terraces. They mark the location of older river floodplains. One of these terraces hosts the monument’s prairie dog town. The prairie dogs prefer the terrace gravels because they are easier to dig in than the harder, older sedimentary rocks. The river sometimes cuts down through these terraces, giving the land around the river a step-like appearance.
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