358 HISTORICAL GEOLOGY 



extensive kame-moraine areas, thus hindering agriculture, trans- 

 portation, and travel. 



In many places, as in parts of New England, New York, and 

 eastern Canada, the cultivation of the soil has been made difficult 

 because of the numerous glacial boulders it contains. In these 

 same regions many of the old lake or other deposits are too sandy 

 or gravelly to be very fertile. 



Large areas now covered by lake waters would make valuable 

 farming land. This is particularly true of the Great Lakes. 



All things considered, it seems certain that the advantages due 

 to glaciation are notably greater than the disadvantages. 



Duration of the Glacial Epoch 



According to Chamberlin and Salisbury, the most important 

 criteria for estimating the duration of the Glacial epoch include: 

 " (1) the amount of erosion of the drift; (2) the depth of leaching, 

 weathering, and decomposition of its materials; (3) the amount of 

 vegetable growth in interglacial intervals; (4) the climatic changes 

 indicated by interglacial and glacial floras and faunas; (5) the 

 times needful for the migration of faunas and floras, particularly 

 certain plants whose means of migration are very limited; (6) the 

 time required for advances and retreats of the ice; and some 

 others." A few of these, as the first, are subject to direct measure- 

 ment, but most of them are matters of judgment. 



The average of the estimates of five glacial geologists who 

 have most studied the data is shown in the following table : 



From the Late Wisconsin to the present 1 time unit. 



From the Iowan to the present 3 to 5 time units. 



From the IUinoian to the present 7 to 9 time units. 



From the Kansan to the present 15 to 17 time units. 



From the Sub-Aftonian (Jerseyan) to the present .... X time units. 1 



" After carefully considering many points, these same authors 

 (Chamberlin and Salisbury) offer the following table accompanied 



Climax of the (Late) Wisconsin 20,000 to 80,000 years ago. 



Climax of the Iowan 60,000 to 400,000 years ago. 



Climax of the IUinoian 140,000 to 720,000 years ago. 



Climax of the Kansan 300,000 to 1,360,000 years ago. 



Climax of the Sub-Aftonian (Jerseyan) Y to Z years ago. 



1 Chamberlin and Salisbury: College Geology, pp. 890-891. 



