We know that wind, water, and freezing and thawing cause rock to be slowly worn down over time. There are about 25 millimeters in an inch. If we assume that it takes a year to weather down one-quarter of a millimeter (one-hundredth of an inch) of rock, then it only takes 4 million years to wear down a kilometer of rock, or 6.4 million years to get rid of a mile of rock. At this rate, we could rasp Mount Everest off the face of the Earth in less than 40 million years. Remember that the Earth is 4,567 million years old, more than enough time to get rid of Mount Everest a hundred times, and you begin to realize that mountain ranges can come and go.

— Cruisin' the Fossil Freeway by Kirk Johnson, Ray Troll (Page 24)