22 



GUIDEBOOK OP THE WESTERN UNITED STATES. 



plains is less uneven and slightly lower, and the superficial deposits 

 consist of fragments of rock brought from the Rockj Mountains. 

 Those differ from the glacial drift in containing rounded pebbles, 

 none of which bear evidence of glacial origin. They seem to have 

 been brought from the mountains by streams which through long 

 ages wore engaged in leveling the Great Plains, much as Platte River 



cutting away the higher 



is now grading its broad bottom lands, 

 placas and building up the lower ones. 



deposits, deeply \reathered and in places 

 cemented to hard conglomerate by lime 

 or iron oxide, OA^erlie the pre-Kansan 

 glacial till at several places in the river 



The late Prof. Samuel Calvin identified 

 the remains of horses, camels, stags, ele- 

 phants, mastodons, mammoths, and sloths. 

 When these animals lived in western Iowa 



FiGUBE 4,-Map of North Amerka showing the area covered by the Plelstoceae Ice sheet at Its maxlmuTn 



exteoi^ion md the three main centers of ice accuniulation. 



blnffa, A remarkable aaBemLIage of ani- 

 mals invaded the region after the ic^ had 

 disappe^ed, and the bones and teeth 

 of many of these animals have been found 

 in the Aftonian depoeits of western Iowa. 



the climate there must have been com- 



mx 



dant. Prof. CaMn gays: *'To supply 

 the^e great herbivores vrith. food required 

 an abundance of vegetation such as could 



