324 H. F. CLELAND NORTH AMERICAN NATURAL BRIDGES ] 



log of agatized wood has been imdermined by stream erosion until it now 

 spans a true valley of erosion. The canyon or gulch has a north direc- 

 tion and is very precipitous, beginning only 200 yards above the bridge 

 and rapidly broadening in its descent. At the point where the bridge 

 crosses the canyon it is about 30 feet wide, but the trunk lies diagonally 

 across and measures 44 feet between the points at which it rests on the 

 sides of the canyon. The canyon is here about 20 feet deep. The total 

 length exposed is 111 feet, so that more than 60 feet of the upper part of 

 the tree lie out on the left bank of the canyon. At about the middle of 

 the canyon the tree measures 10 feet in circumference, which implies a 

 diameter of over 3 feet. The rock in which the log was imbedded is 

 very soft and easily eroded. The Santa Fe Railroad Company has placed 

 two piers (shown in the illustration) under the bridge to support it. 

 These supports are, however, of little use and are very unsightly.^^ 



6. BY THE HEADWARD CUTTING OF TWO STREAMS 



Certain natural bridges near the station Natural Bridge, in Powell 

 County, Kentuck}^, near the middle fork of Red River, should be dis- 

 cussed here because of their peculiar erosional origin. The country in 

 which they occur is a high plateau region, in a mature stage of develop- 

 ment (plate 22, figure 2), composed of heavy bedded strata of carbonifer- 

 ous sandstones and limestones, which have been deeply dissected by stream 

 erosion, forming deep valleys with divides, which in many places are only 

 a few feet wide. The bridge from which the station Natural Bridge re- 

 ceived its name (plate 22, figure 2) is one of these and is perhaps the most 

 imposing of them, the arch itself being 32 feet high and the width of the 

 cavity at the bottom being about Q6 feet. The width of the span is 20 

 feet and its thickness is about 12 feet. This arch and two others in this 

 region were formed by the headward cutting of two streams the head- 

 waters of which have almost met (see map, figure 7), leaving, a divide 

 which, in the case cited, is but 20 feet in width. The cavity now spanned 

 by the bridge was produced by the combined action on the divide of water, 

 wind and frost. Two other "bridges" occur near the above. 



These arches were first described by A. M. Miller,^^^ who gave the cor- 

 rect explanation of their origin. 



7. BY REMOVAL OF UNCONSOLIDATED MATERIAL UNDERLYING A RESISTANT 



STRATUM 



a. Switzerland. — Unconsolidated material underlying a resistant 

 stratum may be carried away in suspension by water which first seeped 



11 For a discussion of tlie petrified forest, see "The petrified forests of Arizona." Li. F. 

 Ward, Smithsonian Report, 1899, pp. 289-307. 



lift A. M. Miller : Natural arches of Kentucky, Science, vol. 7, June 24, 1898, p. 845. 



