YOU KNOW THIS BRIDGE, OF COURSE
The Natural Bridge of Virginia is an example of still another style of river bridge-building. This bridge used to be part of the roof of a cave and remained after the rest of the roof fell in.
When ordinary people want to cross a mountain they have to climb over it. But do you know what a river does? It cuts its way right through and makes what is called a water-gap—a great gate of stone that is always open and through which the stream forever flows. All the river used was tools and time. The tools were the sand and pebbles it swept along. So in the course of ages, running like a band saw, the Potomac made the water-gap at Harper's Ferry, the Delaware River the Delaware Water-Gap.
HOW MOUNTAINS HELP MAKE THE WATER GATES
But how could a river do this? It couldn't flow up one side of the mountain and down the other, could it? No, certainly not. What then? Wherever you find a river cutting through a mountain range you may be sure the river was there before the mountains rose, and that the mountains rose so slowly the river kept right on in its old channel and wore down the rock under that channel as fast as the mountains rose; while, on either side, they could rise as high as they wanted to for all the river cared!
GROWING MOUNTAINS AND THE EARTHQUAKES
But suppose, before I had explained how water-gaps are made I had told you I could show you a mountain growing. You wouldn't have believed it. Regions in which mountains are still rising, as on our Pacific Coast, are liable to earthquakes. The reason is that as mountains rise the rock layers of which they are made are strained dreadfully. Every once in a while they crack and the rocks on either side of this crack grind against each other. This makes the earth shake, much as the house shakes when a heavy table is pushed across a bare floor.
If you want to see a job of river engineering that will make you catch your breath, look over into some of the river canyons and gorges of the West.
THE GREAT CUMBERLAND WATER-GAP