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Why do we see a bipartite cliff,i.e., a vertical top portion and a slanted bottom portion at the edge of a mesa or butte? I came to know from the internet that the top portion is the hard caprock And the bottom is composed of softer Rock which is more easily eroded by the wind. But why is the cap Rock have vertical edges but the underlying portion have a slanted edge? Why can't the wind erode that cone of soft rock? Is the outer cone belt entirely made up of caprock material with the original soft rock lying inside? Is it protected by the talus deposits of the caprock cliff? I later found that the caprock protects the underlying soft rock from being eroded away. But how so? Same for hoodoos(called 'demoiselles' in my textbook). The wind can erode from the side, right? I suspect eroding the soft rock from the top is more effective, but can't find the logic behind it. Please note that I am a high school student(grade 10) trying to understand this for an hour. So a simple yet satisfying answer would be very useful.

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Over long periods of time, the slope tends towards having the same net erosion rate everywhere so that the shape remains constant (equilibrium). For a uniform material that's not excessively steep, that gives you the stereotypical rounded hill, since erosion at lower slopes is roughly proportional to the slope, so the slope has to steadily increase to keep $erosion_{net}=erosion_{here} - erosion_{above}$ constant (where erosion from above provides new soil here).

In the case of a mesa, the harder rock above has a much lower erosion rate than the softer rock below, so a consistent shape is maintained if the lower slopes are much shallower than the upper slopes. Consider two alternative starting points:

  • The whole thing is very steep. Fast erosion of the softer material below quickly levels out the slopes there (stuff falls to the bottom) while undermining the caprock, which leads to slope failure above and a cliff. You now have a mesa.
  • The whole thing is shallowly-sloped. Very little erosion happens from the caprock, while the softer material below is eroded more quickly until it undermines the caprock, resulting in slope failure and a cliff. You now have a mesa.

I'm not too familiar with hoodoos, but it appears that some of the "protection" is because the column is under relatively high compression. Rock/soil material that's under high compression is more resistant to shear stress.

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