Most of the natural bridges of the United States are in the Four Corners
region of the Southwest where favorable materials and conditions for
bridge-making are to be found. There are two other types of bridges
which are entirely dissimilar. Tonto Natural Bridge, in central Arizona,
is a "built-up" bridge created by travertine deposited by springs.
Natural Bridge, in Virginia--another well-known bridge--apparently was
formed when most of the roof of a cavern collapsed.