Witch Rocks, near Echo Canyon, Utah
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Of these bridges the Edwin easily ranks first in graceful symmetry; its span is one hundred and ninety-four feet, its elevation one hundred and eight feet, its width thirty-five feet. Combining grace and massiveness, the Augusta stands pre-eminent. It rises in majestic proportions to the height of two hundred and twenty-two feet and has a span between abutments of two hundred and sixteen feet. The width of the road-bed is twenty-eight feet and the thickness of the arch at the keystone is forty-five feet. The height of the Carolyn Bridge is two hundred and five feet and the span one hundred and eighty-six feet.

All these bridges span canyons whose depths correspond to the height of the arched structures. In White Canyon, a few miles below Edwin Bridge, under its overhanging rock walls, are the ruins of numerous cliff-dwellings.

The largest natural bridge in the world yet discovered is the Nonnezoshi. It is in Nonnezoshiboko Canyon, Utah, not far from the place where the San Juan River enters the Colorado. This mammoth arch is more of a flying buttress spanning the canyon than a real bridge. Its height is three hundred and eight feet and its span two hundred and eighty-five feet.

To visit these bridges from the nearest railway station requires stage and horseback riding for upward of one hundred and twenty-five miles. The latter part of the journey is made over a faint trail through a rugged country; but the scenery amply repays one for the hardships endured.

The climatic changes during the ages have been such that this region is now almost inaccessible on account of the lack of water, except in the early spring when melting snows yield a temporary supply. Even the cattlemen pasturing their herds in that section keep them there but a few weeks during the year, so scarce is both water and vegetation.

In the main, natural bridges are the result of one or another of several causes. A limestone cavern may be partly destroyed by streams of water, leaving a portion of the cavern with its roof still in place; the part of the roof thus remaining becomes the arch of the bridge. A branch of the Southern Railway threads a natural tunnel near Anniston, Ala., and the tunnel is the remnant of an old limestone cavern.

In other cases a natural bridge is formed when bowlders, or a mass of rock, tumbling into a deep crevice is wedged and held in place. In still other instances a layer of hard or slowly weathering rock may rest upon a layer of rock which weathers rapidly. In such cases, if the rock layers form the face of a cliff, natural bridges, caverns, and overhangs are apt to result.