55 ^ow ^'^^y 9°^ *hat way 



Nevertheless, and still later, toward the end of the Mcso- 

 zoic era, or say a hundred million years ago, most of what 

 is now the Rocky Mountain region was part of a great 

 seaway five hundred to a thousand miles wide, stretching 

 from the Gulf of Mexico to the arctic, and covering parts 

 of New Mexico, Texas and eastern Arizona, so that much 

 of what is now arid was drowned in salt water then. 



Presently the Rockies, the Sierra Nevadas, and the Cas- 

 cade Mountains appeared, cutting ofiF, as they were lifted, 

 the moist winds blowing in from the Pacific. Hence the 

 Great Basin area of Utah was soon very dry and the 

 Great Plains semi-arid. But until the mountain ranges 

 reached their greatest height, their eastern flanks were 

 probably warmer and moister than they are now. And as 

 for the Sonoran Desert, it is probably relatively recent, 

 though the Sonoran has been something like what it now 

 is since the Miocene, which means for approximately ten 

 million years. 



The evidence indicates that the eastern or dry flanks of 

 the Andes were also very dry by then, and because close 

 relatives of some of our most characteristic desert plants 

 are found in South America, it is thought that they prob- 

 ably originated there. Moreover, the soil of the Sonoran 

 Desert floor is much younger than the mountains which 

 surround it, because it is a thick layer of detritus washed 

 down from these same mountains as they have weathered 

 away and it now fills the once deeper valleys. In places 

 it is at least 1800 feet thick, as we know from the fact 

 that well drillers in the Tucson area have failed to find 

 rock even at that depth. Even now the desert is chang- 

 ing — in some respects rapidly, because of what man has 

 done to it since he set cattle and sheep to grazing and 



