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© Copyright 1999, Jim Loy
In some desert places, you can see rocks
balanced precariously on top of other rocks. The lower rock has eroded away, as
the harder upper rock has eroded to a lesser degree. These seem to defy gravity
and common sense. Shouldn't they just topple over?
In the picture above left, we see such a balancing act. The upper rock is tipping to the right. This compresses the rock on that side. Some kinds of rock become harder when compressed. In this case the right side of the contact area becomes harder (and erodes very slowly), while the left side of the contact area becomes softer (and erodes much faster). And so, our upper rock slowly rights itself. It rotates to a level position. Over a period of years, the upper rock slowly descends, as the lower rock erodes away. But it remains balanced.