The Blue Ridge Mountains used to be a much taller mountain range, as you’ll read about soon. Over the past millions of years, these mountains have been eroding. Since continental crust is essentially floating on the ocean of the earth’s mantle, as the surface of the mountains erode, the buoyancy of the continental crust causes it to rise some amount to make up for the loss of mass on top. Think of a cube of ice floating in water–cut off the top part of the ice cube, and what will it do? It will not stay where it is, it will rise further out of the water because of its buoyancy.
Monadnocks
As this process of isostatic rebound happens and as more of the continental crust erodes, harder minerals become exposed as they reach the surface, because they erode more slowly than the surfaces around them. This exposes large deposits of minerals which creates features like monadnocks (isolated mountains or hills) in the Piedmont.