SUCCESSION OF ICE-SHEETS 633 



their duration are found (i) 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 stages of the glacial period recognized in 

 North America numbered in the order of their age: 

 VIII. The Glacio-lacustrine (including the Champlain). 

 VII. The Wisconsin or Wisconian, the last important invasion. 

 VI. The Sangamon-Peorian, or third interglacial interval. 

 JVB. The lowan, the third invasion in the Keewatin field. 

 IVA. The Illinoian, the third invasion from the Labradorean field. 

 IV. The Yarmouth or Buchanan, 2 the second interglacial interval. 

 III. The Kansan, or second ice invasion. 

 II. The Aftonian, or first interglacial interval. 

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

 nized. 



I. Jerseyan or Sub-Aftonian glacial stage. The oldest drift 

 which appears in New Jersey is but the frayed edge of a once con- 

 tinuous sheet, and is very old. On the Allegheny and upper Ohio 

 rivers, the great age of the oldest drift is shown by the deep erosion 

 of the valleys since the first ice invasion turned the streams into new 

 channels. Farther westward the corresponding old drift is covered 

 by later drift. In the Keewatin area in Iowa, a very old drift (sub- 

 Aftonian), probably the equivalent of the Jerseyan, lies below the 

 Aftonian and Kansan. Very old mountain drift has recently been 

 found (Atwood) high on the mesas near the San Juan Mountains in 

 Colorado and also on the high mesas in front of the Rocky Moun- 

 tains in Montana (Alden). The evidences of age of all these seem 

 to be of the same order, and they are thought to represent the 

 earliest ice invasions in the Labradorean, Keewatin, and Cordil- 

 leran fields. 



II. Aftonian interglacial interval. Overlying the oldest till in 



1 Distinct glacial epochs and criteria for their recognition, Jour. Geol., Vol. I, 

 pp. 61-84. 



The Buchanan gravels lie between the Kansan and lowan drift-sheets where 

 the Illinoian is not present, and hence their age is not quite certain. 



