174 STUDIES FOR STUDENTS 



ages of each, corresponded with the two epochs of glaciation which 

 were pronounced in Utah and Nevada and the two periods of expan- 

 sion of Lakes Bonneville and Lahontan. This taken in consideration 

 with the place of deposit of the gravels, in the upper portions of the 

 river valleys, leads to the view that the Gila conglomerate originated 

 from an increase in the ratio of erosion to transportation, due to the 

 severe cold and consequent frost action of the glacial times, without 

 a correspondingly large increase, in this arid region, of precipitation. 

 The ultimate cause of the accumulation under this view was greater 

 cold and not a desiccation, since the precipitation was doubtless 

 somewhat increased as shown by the mammalian bones. Any 

 conclusion in regard to the exact cause and correlation is, however, 

 of minor importance in comparison with the broader one that the 

 deposit is due to climatic causes rather than those of a local or 

 regional tectonic nature. If this conclusion be well founded it is 

 seen that in this desert region with mountainous topography climatic 

 changes have been a sufl&cient cause to result in the laying-down over 

 wide areas adjacent to the higher mountains of a conglomerate 

 formation largely over a thousand feet in thickness, justifying the 

 statement that climatic changes may result in sedimentary formations 

 as important as those due to tectonic or oceanic causes. 



Effects of increased cold on snowfall and erosion. — In regions where 

 the increased cold results in the precipitation of snow which previously 

 had fallen as rain, frost action and also chemical action may not be 

 increased, and the chief results of the spring floods resulting from the 

 melting snow niay be an increase of transportative power. The 

 protecting power of snow against both disintegrating and decom- 

 posing agencies has been cited by Salisbury as probably contributing 

 to the fresh and unweathered appearance of the Wisconsin drift of 

 the Bighorn Mountains when compared with that of the continental 

 interior.^ 



In mountainous regions such as the Sierras, where the snowfall 

 is markedly greater at the higher elevations, the floods produced by 

 the spring melting are not proportionately augmented upon reaching 

 the lowlands, and deposition of the excess load is to be expected 



I Geology oj the Bighorn Mountains, Professional Paper 51, U. S. Geological Sur- 

 vey, 1906, p. 87. 



