138 X. H. WINCH ELL WAS MAN IN AMKRICA IN GLACIAL PERIOD 



boniferous more rapidly than the Archean. We observe here that these 

 softer rocks occupy large areas in that part of the interior of the conti- 

 nent where the superficial decay has been greatest. Large amounts of 

 this have been carried to the gulf of Mexico by the Missouri, the Mis- 

 sissippi, and their tributaries, but the western countr}^, especially in the 

 region of the plains, is still covered with a thick sheet of this residuum. 

 This sheet becomes thinner toward the north or toward any of the crys- 

 talline rocks. It was not only thinner toward the outset in areas of the 

 harder rocks, but has been reduced by glaciation and the powerful drain- 

 age and erosion engendered by it. 



Advent of the Tce-sheets 



With a covering of this kind, which may be said to have been uni- 

 versal over the land areas, the ice invasions came over the northern por- 

 tion of the United States. It was at first supposed that the Ice age'was 

 marked by a single invasion. About 1870 it was discovered in Ohio 

 that there were two, and that they were separated by an epoch of tem- 

 perate climate sufficiently long to produce a thick soil and a forest 

 growth. This has been recognized very generally in America, and it has 

 been discovered farther that other fluctuations of the ice-sheet were 

 equally marked and other interglacial epochs, marked by similar proofs 

 of mild climate and forest growths, have been detected from time to 

 time. According to the latest statement of Professor S. Calvin, the State 

 Geologist of Iowa, where by far the greater part of the deciphering of 

 the history of the Pleistocene has been done, there were five ice epochs 

 separated by four interglacial epochs. These have been detected and 

 more or less defined in the state of Iowa, namely : 



First glacial stage : pre-Kansan, or sub-Aftonian. 

 First interglacial stage : Aftonian. 



Second glacial stage : Kansan. 



Second interglacial stage : Yarmouth. In Iowa, Buchanan. 



Third glacial stage : Illinoian. In Iowa, Buchanan. 



Third interglacial stage : Sangamon. In Iowa, Buchanan. 



Fourth glacial stage : lowan, which formed the main loess deposit. 

 Fourth interglacial stage : Peorian. 



Fifth glacial stage : Wisconsin. 



The Wisconsin drift, which is that which prevails in Wisconsin and 

 Minnesota and is supposed to be that which covers New England, is 

 marked by the great moraines which have been traced across the United 

 States. Professor Calvin says it is 



" very nuicli younger than the Kansan or tlie pre-Kansan. There is an enormous 

 interval between the earliest and the latest of the ice invasions. The earlier gla- 



