Page:Popular Science Monthly Volume 78.djvu/435

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NORTH AMERICAN NATURAL BRIDGES
PSM V78 D435 Natural bridge formation of an entrenched meander 1.png
Fig. 10. Block Diagram to show the Formation of a Natural Bridge by the Perforation of an Entrenched Meander. (See Fig. 9.)
PSM V78 D435 Natural bridge formation of an entrenched meander 2.png
Fig. 11. Block Diagram to show a Bridge formed by the Tunneling at a Meander Bend of a Narrow Divide between the Main and Tributary Stream.

deposit (travertine) but also the whole of the cultivated valley of about 25 acres. The bridge has a span of 140 feet, a height of more than 125 feet and a width of about 400 feet. In fact the bridge is so large that the visitor is likely to walk over it, as the writer did, without knowing he is on it. The top of the bridge is under irrigation and produces crops of alfalfa.

The formation of the bridge is simple and the process can sometimes be seen when moist, drifting snow forms a bridge across a small valley. Several large springs that empty into the valley on the east contain lime in solution which, upon evaporation or loss of carbonic acid gas, is deposited. For many years this deposit has been accumulating and with sufficient rapidity to force the stream (Pine Creek) to the west side of the valley. In one place the deposition was rapid enough