And as soon as the mountains were built, they began, just as ineluctably, to wear away. For all their seeming permanence, mountains are exceedingly transitory features. . . . Right now the Appalachians are shrinking on average by 0.03 millimeters per year. They have gone through this cycle at least twice, possibly more—rising to awesome heights, eroding away to nothingness, rising again . . .
–Bill Bryson, A Walk in the Woods