vi PREFACE TO FIFTH EDITION. 



E. H. Williams from the Atlantic Ocean to Ohio and found to 

 be from twenty to thirty miles south of their terminal moraine. 



The facts relating to this border, brought out by Professor 

 Williams, have a most important bearing upon the discus- 

 sion both of the cause and of the date of the Pleistocene glacial 

 epoch. 



There has also been a great accumulation of evidence, 

 collected pretty largely by Mr. Frank Leverett and the geolo- 

 gists of Iowa, Minnesota, Dakota, and Canada, concerning 

 the episodes of the Pleistocene glacial epoch, leading to the 

 division into the Kansan, Illinoisan, Iowan, and Wisconsin 

 periods of advance and retreat. The relative length of time 

 occupied by these episodes is still a most interesting subject of 

 investigation. 



The question of the date of the Pleistocene glacial epoch is 

 still a subject of hot discussion, but a great accumulation of 

 facts, relating to post-glacial erosion and sedimentation, are 

 rapidly establishing a very moderate glacial chronology. 



Likewise a great accumulation of facts is limiting the the- 

 ories concerning the cause of glaciation to changes in land 

 elevation and in the direction of oceanic currents. The chap- 

 ters upon the date and the cause of the epoch have been 

 greatly enlarged and rewritten. 



The final chapters upon the discovery of human relics in 

 deposits connected with the Glacial epoch in North America 

 have also been thoroughly revised and enlarged, to take into 

 consideration the more recent discoveries of facts bearing 

 both for and against man's existence here in glacial times. 



G. Frederick Wright. 



Oberlin, Ohio, December 22, 1910. 



