360 HISTORICAL GEOLOGY 



years. While closer estimates are practically impossible, it is at 

 least certain that the time since the Ice age is far less than its 

 duration, and that, for the region of the northern United States, 

 the final ice retreat occurred only a very short (geological) time 

 ago. 



When we consider the slight amount of weathering and erosion 

 of the latest glacial drift, we are also forced to conclude that the 

 time since the close of the Ice age in the United States is to be 

 measured by only some thousands of years. Thus kames, drumlins, 

 extinct lake deltas, and moraines with their kettle-holes, have 

 generally been very little affected by erosion since their formation. 



In order to determine the number of years ago since the last 

 (Wisconsin) ice sheet reached its climax, it is further necessary 

 to know how long it took this ice sheet to recede from its southern- 

 most limit to Niagara Falls, or about 600 miles. 1 If we allow for a 

 rate of retreat of 100 to 200 feet per day, it required about 15,000 

 to 30,000 years for the retreat to Niagara. Combining these 

 figures with those above given for the time since the inauguration 

 of Niagara Falls, we get some idea of the time since the last ice 

 sheet reached its climax, or about 22,000 to 80,000 years ago. 



Cause of the Glaciation 



The cause of the glaciation has been a very perplexing problem. 

 Various hypotheses, often of widely different character, have been 

 offered by way of explanation, but there is nothing like general 

 agreement on the subject. We have here a fine illustration of the 

 difference between "fact" and "hypothesis" which the student of 

 natural science must always keep clearly in mind. Thus, the fact 

 of the Glacial epoch (including much of its history) is conclusively 

 established, but the cause of the glaciation is a matter concerning 

 which we have only hypotheses or speculations. 



In this elementary work we can do no more than suggest several 

 of the leading hypotheses. Those further interested in the subject 

 are referred to special articles and larger general works, particu- 

 larly Chamberlin and Salisbury's "Geology," Vol. 3. One point to 

 be borne in mind is that no hypothesis is required to account for 

 an average yearly temperature of more than 10 or possibly 15 de- 



1 It should be remembered that the latest or Wisconsin ice did not extend 

 as far south as certain earlier ice sheets. 



