United States. National Park Service

“ They made meandering channels across the flat land and gradually entrenched themselves into tortuous canyons. As the land continued to rise, the streams cut ever deeper canyons.
The main purpose, or the driving force, of a stream is to make the shortest distance between two points, that is, a straight line. Every creek and river attempts to make a straight channel with an even grade from its source to its mouth. Hills, ridges, blocks of rock, or any other obstacles which a stream must bypass are gradually worn away as the stream makes a channel more to its choice. ”

“ 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.
Wildlife
Deer are numerous in the pinyon and juniper forest which surrounds the monument, and mountain sheep, which spend their summers on the Elk Ridge, winter in the canyons. Smaller mammals are numerous, as are the coyotes and wildcats. An occasional lion works through the monument. ”

“ The dimensions of the bridges are:
How Natural Bridges Are Made
To make a natural bridge Mother Nature must have several ingredients: a proper stone that 3 will shape well (a cross-bedded sandstone is best) , a slowly rising landscape, and a desert-type stream that occasionally will scour its bed with a tremendous head of water and sand. All these were present in southern Utah.
The stone of the Natural Bridges area is a cross-bedded grey sandstone, known as the Cedar Mesa sandstone. ”