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Erosion of the Land
AMOUNT OF EROSION
How much erosion has taken place in North Dakota since the current erosion cycle began, 15 million years ago? A lot, but it's not really possible to know how much material has been removed from an area where the original amount is unknown. We can make some estimates by considering some of the highest elevation features, such as the Killdeer Mountains, Chalky Buttes and the tops of various other buttes.
Both the Killdeer Mountains and Chalky Buttes are capped by layers of resistant limestone, material deposited in lakes a little more than 20 million years ago. Since that time, a complete reversal of topography has taken place. What were lowlands then are highlands now. While they existed, during Miocene time, lakes were situated in some of the lowlands. Surrounding areas nearby were at higher elevations than the lakes themselves (that is, the lakes were surrounded by uplands). The same is true of the materials on top of many other western North Dakota buttes; they began as riverbeds or lakes, existing in settings lower than their surroundings. These same materials, once lowlands, are now found on uplands rising as much as 800 to 1,000 feet above nearby areas. Might we infer, then, that since Miocene time, at least 800 to 1,000 feet of material (the height of the buttes) has been removed from areas surrounding the uplands?
Yes, we might infer it, but that does not mean we would necessarily be correct in doing so. The amount of erosion that has taken place cannot be determined simply by subtracting the elevation of the lower area surrounding an upland from the elevation at the top of the upland and then assuming that the difference equals the amount of erosion (the tops of the Killdeer Mountains are at 3,100 feet above sea level, and the surrounding area is at 2,500 feet, so 600 feet of material must have been removed by erosion, etc.). It is not that simple.
At the time materials that now form the caprock on top of the Killdeer Mountains were being deposited in lakes, we do not know how much higher the nearby uplands were. It may indeed be that 600 feet of sediment, or perhaps more, were removed in the immediate area of the Killdeer Mountains, but that figure cannot be projected away from there for any great distance. At the same time erosion was taking place, the Earth's crust in that area was rising in response to the weight of the sediment being removed. Just as the crust sank under the weight of the more than three miles of sediment that had earlier been deposited in the Williston Basin, the land rose when some of that sediment was eroded away (more on the concept of isostatic rebound in my discussion of crustal rebound in Chapter Seven on the Red River Valley and glacial Lake Agassiz).
I attempted a variety of techniques, trying to determine how much the amount of erosion in western North Dakota might have been. The results ranged from an estimated average of 300 feet to more than 1,000 feet of material removed from the area.6 The various methods I used gave such differing results that I prefer to simply say that extensive erosion occurred over western North Dakota during the past 15 million years.7
Less erosion took place in the eastern part of the state than in the west. The Red River Valley, for example, saw perhaps 50 to 100 feet of material removed, but much of that material was later replaced by glacial and lake sediment, deposited during the Ice Age. The Ice Age ended the 15 million-year erosion event, at least in places reached by the glaciers (erosion continues, uninterrupted, in much of the western part of the state).The glaciation that began in North Dakota, perhaps as long ago as 3 million years ago, provided its own version of erosion; erosion by glacial ice rather than running water. Over parts of eastern North Dakota, the glaciers probably brought in more material than had been eroded away during the millions of years preceding their arrival.
What was the North Dakota landscape like during the 15 million years of erosion? The western part of the state was probably a broad, level surface covered by gravel, much like the landscape in eastern Montana today. Buttes and valleys formed, were worn down and eroded away, a sequence that was repeated several times, and continues today.
The eastern part of the state during the 15 million-year erosion event was a level to gently rolling plain, less hilly than the west. I do not know when the Red River Valley first formed, but it has existed as a lowland for several million years, and a river has flowed north through it ever since it was formed. In places, the river carved a valley through granite and limestone, now buried beneath 100 to 500 feet of glacial deposits. Before it was glaciated, relief in the area that is now the Red River Valley was likely greater than in the rest of eastern North Dakota. Intricate valleys were carved from a variety of much older materials, ranging from Precambrian granite to layers of limestone and sandstone. All of this relief is today buried beneath several hundred feet of glacial and lake deposits, under the flat floor of the Red River Valley.
