874 GEOLOGY 



The Succession of Ice Invasions 



The glaciation of North America did not consist of a single ice 

 invasion, but of a series of invasions separated by long intervals of 

 time. It is not yet known how far the ice retreated in the intervals 

 between the advances, but some of the interglacial intervals were 

 much longer than the time since the last ice-sheet disappeared. 

 There is also good evidence that in some of them the climatic 

 conditions became at least as mild as they are to-day. 



The proofs of the interglacial intervals and the evidences of 

 their duration are found (1) in the erosion effected by streams 

 after the deposition of one sheet of drift, and before the deposition 

 of the next, (2) in the depths to which earlier sheets of drift were 

 leached and oxidized by weathering before the deposition of later 

 ones upon them, (3) in the accumulations of peat, soil, etc., now 

 found between different sheets of drift, and (4) in the changes of 

 topographic attitude which intervened between the deployment of 

 successive ice-sheets. 1 



The following are the American stages of the glacial period 

 commonly recognized in the interior of North America, numbered 

 in the order of their age: 



XI. The Champlain substage (marine). 



X. The glacio-lacustrine substage. 



IX. The Wisconsin, the fifth invasion, sometimes divided into 



two, an Early Wisconsin and a Late Wisconsin. 

 VIII. The Peorian, the fourth interglacial interval. 

 VII. The low an, 2 the fourth invasion. 



VI. The Sangamon, the third interglacial interval. 



V. The Illinois, the third invasion. 



IV. The Yarmouth, or Buchanan, 3 second interglacial interval. 



III. The Kansan, or second invasion now recognized. 



II. The Aftonian, the first known interglacial interval. 

 I. The sub-Aftonian, or Jerseyan, the earliest known invasion. 



1 Distinct glacial epochs and the criteria for their recognition, Jour. ( ;<>!.. 

 Vol. I, pp. 61-84. 



2 Question has recently been raised as to the reality of this drift-sheet . 



3 The Buchanan gravels lie between the K:ms:m :nul lo\v:in drift-shcrtv. 

 in localities where the Illinois is not present, and hence their age is not <|uitr 

 certain. 



