Page:Popular Science Monthly Volume 78.djvu/427

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THE

POPULAR SCIENCE

MONTHLY

 

MAY, 1911




THE FORMATION OF NORTH AMERICAN NATURAL BRIDGES
By Professor HERDMAN F. CLELAND

WILLIAMS COLLEGE

ALTHOUGH there are more than fifty natural bridges of considerable size in North America, comparatively few persons have ever seen one, the reason being that, with the exception of the Virginia bridge and the natural bridge in North Adams, Mass., most of them are more or less inaccessible.

A bridge, according to the usual definition, is a structure that permits one to pass from one side of a depression to another, whether that depression be a railroad cut, a street or a river. As used in this article a natural bridge is a natural stone arch that spans a valley made by running water; a natural arch being a structure that does not span a valley of erosion.

Although a number of descriptive articles on natural bridges have

PSM V78 D427 Natural limestone bridge formation in partial tunnel roof caving.png
Fig. 1. Block Diagram illustrating the Formation of a Natural Bridge in Limestone by the Partial Caving in of the Roofs of Tunnels.