3 - Erosion

Devil Canyon Overlook viewed from south along the canyon rim, illustrates the erosion of the canyon

NPS

 

Water Erosion
When we see a muddy river we are watching erosion. If we take a jar full of the muddy water and watch what happens, we will see the heavier rock particles settle to the bottom first. This illustrates that it is the energy of the moving water that gives it the power to carry the sediments downstream. As the finer sediments settle to the bottom we should realize that it takes less energy to carry the finer sediments.

If we experiment and take measurements we can learn much more about the powers of erosion. When we measure how much sediment the water holds and how much water is moving downstream, we can calculate how much sediment the river is carrying, and thus how much the river can erode over a set period of time. But if we increase the amount of water or increase the steepness of the river, we will also increase the erosive powers of the river.

Rocks and Resistance
Other factors may complicate the situation such as how resistant to erosion the rocks are. If the rocks are very resistant, a water fall will occur and erosion upstream will slow until the river can erode through the rocks creating the waterfall. If we take all the factors into consideration, we could visualize the canyon getting bigger as time passes.

If we can reverse all the processes, we can get a good idea of what the land looked like further and further back in time. The problem is calculating all the factors. The river is like a never ending saw that cuts down through the rock and erodes the rock away. Every time it rains or snows upstream, our “saw” gets longer and longer.

Rock Falls and Other Forms of Erosion
The canyon walls will gradually wear away when exposed to all the elements of the weather. When a section of the canyon wall overcomes its support, it will come crashing down into the canyon and the canyon in that spot becomes wider. A rockfall may be just a few rocks or a huge slab of canyon wall.

When the land was uplifted, the stress of that uplift cracked the rock. Water flowing into those cracks in the winter can freeze and when it does it expands and cracks the rock apart further. This frost wedging is also part of the erosion. Wind erosion is also a factor here as anyone who has tent camped at Horseshoe Bend can attest.

Thousands of Feet Already Eroded
With all the different kinds of erosion going on here and having done so for millions of years, huge amounts of erosion have occurred. We can look at the rocks on one side of the canyon and match them up with the rocks on the other side to get some idea of how much erosion there has been.

What we really should do is look at the rocks at one end of the canyon at Horseshoe Bend and match them up with the same rock layers at the other end of the canyon fifty miles to the north at Fort Smith. Then we realize that thousands of feet of younger rock layers have been eroded off from above the top of the canyon for the whole length of the canyon.

Finally... Chapter 4: The Future