CHAPTER VI. 



SIGNS OF FORMER GLACIATION. 



Before attempting to delineate the exact southern 

 boundary of the ice during the height of the Glacial period 

 in North America, it will be necessary briefly to discuss the 

 evidence upon which the inferences concerning the Glacial 

 period are based. The reader will ask : How is it possible 

 to determine, with any reasonable degree of accuracy, the 

 3xtent of the region formerly covered by glacial ice, but 

 which has been free from such covering for many thousand 

 years, and during all that time has been subjected to the 

 disintegrating and modifying influences connected with the 

 ceaseless operation of the ordinary forces of Nature ? 



The consideration of this question will introduce us not 

 only to some of the most interesting problems of this par- 

 ticular subject, but to some of the fundamental principles 

 underlying all inductive reasoning. The study of the great 

 Ice age, like all other branches of geology, deals with the 

 effects of past causes. From the marks which have been 

 left upon the surface of the earth, we endeavor by scientific 

 processes to reproduce to our imagination the condition of 

 things which would account for these marks. As reasonable 

 beings, we are compelled to bring into the field of thought 

 a past cause sufficient to produce all the results observed, 

 both positive and negative ; and when our imagination has 

 found an adequate cause, true science compels us to rest with 

 that. 



From observation upon living glaciers, and from the 

 known nature of ice, we may learn to recognize the track 



