NATURAL BRIDGES BY DEPOSITION 



335 



horseback journey of three days. Not only is the bridge iiniqiie in re- 

 spect to its origin, but is, moreover, one of the most beautiful natural 

 bridges in the United States, and, with the exception of those in San 

 Juan County, Utah, one of the largest. As one journeys south from Flag- 

 staff through the forest reserve he passes for miles over a lava plateau on 

 which, with the exception of pine and cedar trees, there is little vegeta- 

 tion; when, then, the brilliant green of the irrigated, travertine-filled 

 valley above the natural bridge comes into view, its beauty seems unusual 

 (plate 27, figure 1). The vertical distance from the top of the bridge 

 (plate 27, figure 2) to the creek bed is about 128 feet on the north and 150 

 feet on the south end. The opening beneath the bridge averages about 

 140 feet in width, and the length, at the narrowest place, approximately 

 400 feet. The thickness of the arch is approximately 75 feet, leaving the 





Figure 12. — Diagrammatic Cross-section across Pine Creeh, Arizona 



To show the geological relationships. The underlying rock is red porphyry, that Im- 

 mediately overlying is sandstone and limestone, and the capping rock is lava. (See 

 also plates 27 and 28.) 



height of the opening beneath the arch between 60 and 70 feet. The alti- 

 tude of the bridge above the sealevel is approximately 4,700 feet. The 

 origin of the bridge is as follows : Several large springs tliat flow into the 

 valley from the east side contain lime in solution, which, upon evaporation 

 or loss of carbon dioxide, is deposited as travertine (figure 12). For 

 many years these springs have been depositing travertine in an old valley 

 of erosion cut into red porphyry. As a result of this, an almost level 

 floor of travertine of approximately the same height as the springs in- 

 creased in width toward the west, filling the valley until it has forced the 

 stream against the porphyry wall on the west side. In one place the 

 travertine was strong enough to support itself, until it was built over the 

 stream to the opposite side of the valley, thus forming a natural bridge. 

 As will be seen fl-om the illustration (plate 28, figure 2) , the rock of which 

 the bridge is composed is stalactitic in structure and quite compact. Be- 



