There are probably many small streams that are spanned by bridges of this sort, but few of them have been reported. Two such occur in Vermont.
In Kentucky are several arches with an unusual origin which should perhaps be included under the term natural bridge. They were formed in a plateau composed of horizontal sandstone and limestone by the cutting back of the heads of two streams flowing in opposite directions in deep valleys. The streams continued to cut back until only a narrow ridge or divide separated their basins. This divide was in time perforated by the action of water, wind and frost until at length a fine bridge resulted. One of these (Fig. 5) near the station of Natural Bridge on the Lexington and Eastern Railroad, is 32 feet high and 66 feet wide. There are three bridges, or arches, of this origin within a radius of three or four miles.
In narrow mountain valleys natural bridges are sometimes formed by a large rock falling down the mountainside and wedging into the valley. In Switzerland two bridges of this sort are actually in use by pedestrians, but none has been reported in this country, though many doubtless exist. An unusual bridge formed by gravity (Fig. 6) is one consisting of a large slab which was separated from one side of a valley and fell to the other side. When the crack was filled with debris a usable bridge resulted.
In the Yellowstone National Park a natural bridge (Fig. 7) composed of a lava made up of vertical plates of compact and porous rock spans Bridge Creek near Yellowstone Lake. The bridge, although only forty feet high, is very interesting, both because of its rugged beauty and of its unique origin. An examination shows that the bridge is made of two vertical slabs of lava, one two feet and the other four feet thick, separated by an opening two feet wide. The bridge was formed as follows: