Natural bridges are made by the same agency that forms the intermittent springs--the dissolving power of water--and, like the springs, are characteristic of limestone regions because limestone is readily dissolved in water. In the little model of a limestone region "a" and "a" are "sink-holes"--saucer-shaped hollows dissolved and washed into funnels through which the surface water joins underground streams such as you see flowing beneath the two "bs," which are natural bridges in the making. The lower picture shows just how one of the bridge-builders looks while at work, dissolving and wearing down the rock. The next two pictures will help tell you two other ways in which rivers make their own bridges. ] If you have ever been in a machine-shop you must have noticed how a planing-mill works away on a job it has been set to do, without anybody watching it at all; and when it gets done with its job it stops, all by itself. Such machinery is called "automatic," because, to a certain extent, it runs its own affairs. A river, in planing down and reshaping valley scenery, has an automatic stop. When it has cut its valley down to sea level it stops, because, being then no higher than the sea, it can no longer flow toward it. [Illustration: AFTER A FEW CUPS OF TEA When winding rivers get a few cups of tea--that is, are in flood--they rush straight ahead and, while much of the water may for a time still go on around the bend, some of it is forced through openings in the rock and in time carves out a bridge. How they do this is shown in the upper diagram on page 83. ] But before this automatic stop shuts off their machinery the work that rivers do is immense. The Mississippi River carries enough solid matter to the Gulf every year to make a mountain a mile square and 268 feet high. [Illustration: 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