266 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



of the melting glaciers the land was strewn with erratics, with thick accumulations of 

 heterogeneous rocks deposited at the edge of ice-sheets and known as moraines, and with 

 great fans of boulder-clays and sands, all of this being the diluvium or deluge material of 

 the older philosophers and the drift or tills of modern students of earth science. 



Throughout more than a century of study we have learned how glaciers do their work 

 and what results are accomplished by their motion plus the action of temperature, air, and 

 water. The present geographic distribution of the glaciers, together with that of the 

 glacial deposits, shows us that during the Pleistocene or glacial period the temperature of 

 the entire earth was lowered. We also know that this cold period was not a uniformly 

 continuous one, but that during the Pleistocene there were no less than four intermediate 

 warmer cUmates, so warm indeed that during one of them Uons and hippopotamuses lived 

 in western Europe along with primitive man. We may now be living in another inter- 

 glacial warm period, though more probably we are just emerging from the Pleistocene ice 

 age. Figure 87 gives the known distribution of Pleistocene glacial materials. 



Fig. 87. — Map of Pleistocene Glaciation. 



With the reduction of temperature, great variations also took place in the local supply 

 of moisture, in the number of dark days, and in the air currents. How great these changes 

 were in Pleistocene time is now being revealed to us through the work of the geologists, 

 paleontologists, and ethnologists of Europe, where this record is far more detailed than in 

 North America. These observations picture a fierce struggle on the part of the hardier 

 organisms against the colder climates, a blotting out of those addicted to confirmed habits 

 and to warmer conditions, and a driving southward of certain elements of the flora and 

 fauna from the glaciated into the non-glaciated regions. The result was the disestablish- 



