326 H. F. CLELAND NORTH AMERICAN NATURAL BRIDGES 



this place has cut through several layers of conglomerates and marls before 

 reaching its present position^ it is probable that this process has been 

 repeated several times, perhaps with the formation at different levels of 

 several natural bridges that have since disappeared. 



h. Big Bad Lands of South Dakota. — This bridge (plate 23, figure 2) 

 spans the valley of a temporary stream and was evidently formed as de- 

 scribed above. It can not, however, be considered a typical example. 

 "The rock consists of a coarse sandstone, rather soft, and apparently with 

 siliceous cement.'^^^ 



c. Earth bridges. — To the above should be added small earth bridges. 

 Such earth bridges occur where a muddy or alluvial valley bottom cov- 

 ered with turf is in process of trenching by head erosion. The low water 

 drainage becomes subterranean, following cracks or joints, along which 

 it develops channels. Such bridges are necessarily very temporary and 

 the streams very small. 



B. Bridges initiated by Wave Action 



One could cite scores of examples of arches which have been produced 

 by wave action, but few of these should strictly be considered as natural 

 bridges. The natural bridge at Santa Cruz, California (plate 24, figure 

 1), however, should be so considered, since it does span a valley of ero- 

 sion. This bridge was described by Shaler^* as the portion of an arch 

 that has fallen in, the space being widened by the waves which roll be- 

 neath the bridge. He states further that the horizontal strata are of 

 limestone and are thus easily soluble by the waves. This latter statement 

 is an error, since the upper part of the bridge consists of Pleistocene 

 deposits a few feet thick resting on horizontal diatomaceous shales of 

 Miocene age. The cavity was excavated by the mechanical action of the 

 waves and not by solution. "This bridge occurs on the edge of a marine 

 terrace and was formed by the waves eating in under the nearly hori- 

 zontal shale beds forming a cave, the roof of which dropped in behind 

 the bridge, leaving the arch across the entrance.'^^^ Several of these 

 bridges have been formed at different times along this terrace, but they 

 usually last but a short time. Photographs of two or three are still 

 preserved.^^ 



It will be seen that such a structure could be termed either a natural 

 arch or a bridge without going far astray. It has, however, been in- 



" N. H. Darton in a letter to the writer, February, 1909. 

 " Shaler : Sea and Land, p. 18. 

 ^ Letter from Prof. J. C. Branner. 

 " Letter from Dr. Ralph Arnold. 



