To make a natural bridge Mother Nature must have several ingredients: a
proper stone that 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. It is of Permian age and similar in
appearance and structure to the more recent Navajo sandstone in which so
many caves and arches have been formed.
When the land began to rise slowly from its ancient sea bed, two small
streams formed on the western slopes of the Elk Ridge. The streams are
known today as White River and Armstrong Creek, its tributary. 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.
The tortuous streams in their deep rock channels were constantly trying
to straighten their courses. During floods the silt-laden waters were
thrown with great force against the walls of the meanders. In several
places the fins of rock around which the windings of the streams passed
were so thin that during the course of many centuries of buffeting the
rock gradually wore away and a hole was bored through the fin. The
waters poured through the gap and the bridge was born. After the initial
breach was made, the stream continued to enlarge the opening and to cut
its channel still deeper. Eventually, the old meander was left high and
dry as a "fossil" stream bed.