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. Ages of the Bridges Several phases of bridge-making may be seen at Natural Bridges National Monument. Kachina Bridge is a new bridge. It is huge and bulky, and has a comparatively small opening. White River is still actively enlarging the opening beneath the span. Sipapu Bridge is mature. It is a graceful, symmetrical span, and its abutments now lie far enough from the stream bed that the river has little or no cutting action on the rock. [Illustration: _Sipapu Bridge, looking up White Canyon._] Owachomo Bridge is in its old age. It suffers no erosion from the stream; only the slight erosion from rains, frost action, or wind-blown sands now attack its surface. The life expectancy of Owachomo is short compared with that of the other two; it may span the canyon for centuries yet, or the crack which will ultimately cause its collapse may already have started. The fate of all the bridges may be seen a short distance up White Canyon from Sipapu Bridge where faint scars and damaged abutments on the canyon walls indicate the point where a fourth bridge once spanned the canyon. Comparison With Other Types of Natural Bridges 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. Wildlife