206 Frank Carney 



COMPLEXITY OF THE PLEISTOCENE GLACIAL PERIOD 



In this discussion, so far, I have spoken of the glacial period 

 as a unit, as a simple affair. For a long time it was so considered; 

 but more careful study of glacial deposits has shown that all 

 drift is not of the same age. The age of glacial deposits is deter- 

 mined somewhat by the extent of weathering which they show. In 

 a given time, chemical agents will make certain changes in drift 

 of certain content. Multiples of that time unit would bring about 

 more marked changes in the same drift. Rainfall and resulting 

 stream courses gradually roughen all surfaces. If we compare two 

 drift plains, one of which is thoroughly creased by stream courses 

 and the other but slightly altered, the rainfall of the two areas 

 being the same, we conclude that the former has been subject 

 longer to sub-aerial weathering. These two distinctions are the 

 most obvious of the many that have led students to differentiate 

 glacial deposits on the basis of age.* 



Pleistocene stages. The drift in parts of northern North America 

 represents at least four and possibly five distinct ice advances. 

 It is thought that between each advance the glacier may not have 

 disappeared completely ; it probably receded towards the disper- 

 sion centers, and then readvanced. But the time interval between 

 a recessional and the next forward movement was sufficient for 

 some progress in the weathering of the drift last deposited, for 

 the development of stream courses over some of the drift area, and 

 for the reestablishment of flora and fauna. As a given ice sheet 

 receded, it is probable that plant life followed its margin closely. 

 The types of plants that kept nearest the ice were necessarily 

 arctic and subarctic ; other types came into the same zone as the 

 ice retreated further. When the glacier advanced again over that 

 territory, part of this vegetation was buried and later drift was 

 deposited on top of it, while much suffered degradation along with 

 other materials. Decayed plant remains, mingling with disinte- 

 grated rock, develops humus or the soil. To-day in some locali- 

 ties we find two till sheets separated by a soil zone, frequently 

 containing logs and other vegetation. Remains of animals are 

 also sometimes found in connection with the succeeding drift 



^ Leverett, American Journal of Science, vol. xxvii, (1909), "Weathering and 

 Erosion as Time Agents," pp. .349-368. 



