be seen by a study of a map of the region. Another Kentucky bridge formed by the lateral erosion of a tributary stream has produced a perforation (Figs. 11 and 12). The water flows through the opening into the tributary stream when the river is high and from the tributary to the river when the water in that is high. One at least of the Utah bridges may have been formed in this way.
On the coast of California is a natural arch (Fig. 13) made by the beating of the waves against a cliff of soft shale. The top of the arch is so level that a team of horses can be driven across it in safety. It was formed by the partial falling in of the roof of a sea cave. Openings of a similar nature are not uncommon on rough and stormy coasts but it is seldom that a structure so perfect as this is formed.
It will be seen from the above that natural bridges are formed in many ways; that they are not confined to any particular kind of rock, nor are they restricted to any particular region but are found alike in deserts and fertile lands, in mountains and on plateaus.
Natural bridges are short lived, geologically considered. The marble natural bridge in North Adams, Mass., for example, was formed many years after glacial times, but already a portion of the bridge has fallen in. Since the great ice sheet is believed to have disappeared from Massachusetts between 20,000 and 80,000 years ago and since this bridge was not formed until long after it had vanished, it will be seen that the life is not long as time goes. Nevertheless, short lived as they are geologically, some of them probably were in existence when the human race was very young.