300 N. S. SHALER — BROAD VALLEYS OF THE CORDILLERAS 



there is at present an annual rainfall of about 100 inches, there exist 

 very extensive deposits of rock-salt beds, which indicate an arid climate, 

 with a precipitation of probably less than 10 inches, and a very dr}^ air. 

 Although the age of these beds is not determined, it is, on the whole, 

 likely that they are of Mesozoic age. These facts, while they do not 

 suffice to correlate any particular climatal condition in the eastern face 

 of the Rocky mountains with a particular accident in the form of the 

 gulf of Mexico, clearly indicate the existence of actions sufficient to ac- 

 count for a wide range in the precipitation in tliat part of the Cordil- 

 leras. If it were necessary further to show that the eastern border of 

 the Cordilleras had been subjected to one or more humid periods before 

 the last Glacial epoch, the evidence could be adduced from the condi- 

 tions of the valleys of the rivers which drain from that district toward 

 the Mississippi. These valleys generally indicate the existence of a 

 greater flow, and this at a time anterior to the accumulation of the gla- 

 cial drift. 



Conclusions 



The foregoing considerations lead to the conclusion that the valleys 

 of the eastern face of the Cordilleras were in most, if not all, cases greatly 

 affected by a long-continued arid climate, which at some time probably 

 in the Tertiaries was interrupted by one or more continuousl}'^ humid 

 periods, with a return in the present epoch to dry conditions. During the 

 Tertiary periods, when aridity prevailed, the originally deep and rather 

 narrow valleys probably were filled with debris and widened under the 

 conditions of a constantly rising baselevel formed b}^ the thickening de- 

 posit, the conditions of this action being the same as those traced in the 

 interior valleys of the Cordilleran field. At the same time like accumu- 

 lations of detritus formed on the eastern border of the mountains — that 

 which faces the great plains. In later times there came a greater meas- 

 ure of rainfall, and, as a consequence, the removal of a large part of the 

 filling of these eastern valleys, as well as the greater portion of the talus 

 deposits on that border of the mountains. In this period the ui)per part 

 of the bed-rock slopes of the valle3's were bared and worn into foothills. 



At present the conditions of the eastern section of tlie Cordilleras 

 indicate a recent return to the arid climate which has for ages been 

 normal in that area. The taluses are evidently again in process of in- 

 crease, vigorous erosion being limited to the higher levels, where the 

 steepness of the torrent beds enable their scanty waters to sweep along 

 the debris which comes to their beds. Unless the gulf of Mexico should 

 again be brought over a considerable part of the southern lowlands, 

 there seems to be no reason to expect that there will be any increase of 

 rainfall in this area. 



