STUDIES OF SPECIAL LAUUSTUAL HISTORY. 97 



observers have also contributed to tliis study, but not in such a methodical 

 manner as those whose names have just been mentioned. Some of the 

 probl-ems that have presented themselves during this investigation liave 

 not yet been satisfactorily explainetl, but at least an outline of the Pleis- 

 tocene history of the Laurentian l)asin may be presented with the under- 

 standing that it is to be modified as additional facts are obtained. 



The most dramatic episode in the geological history of North America 

 was the formation during Pleistocene time, of glaciers many hundreds of 

 feet in thickness over the northern part of the continent. The ice advanced 

 from the north and not only covered the Laurentian l);vsin, but spread 

 southward beyond the southern border of its watershed. The ice covered 

 this region with various advances and retreats for thousands of years, and 

 when it finally withdrew, the immediate ancestors of the present Great 

 Lakes were born. There are severel observations tending to the conclu- 

 sion that during an interglacial time when the ice receded far north of its 

 niaxiniuni limit, lakes were formed in the same l)asin. l)ut in this connec- 

 tion there is little evidence to claim popular attention. 



Previous to the Glacial epoch or the Great Ice age, as it is frequently 

 termed, the region under review was an old land surface with rivers flow- 

 ing across it to the sea. Its drainage system was well developed and the 

 streams meandered through broad valleys, bounded in i)art by steep escarp- 

 ments. In general relief, it must have resembled the upper portion of the 

 ^lississippi valley as it exists to-daj^, where the top()gra})hy has not been 

 modified b}* glacial action. 



The conclusion that the Laurentian region was exposed to erosion foi- 

 a long period previous to the Glacial e})Och, is based on the character of the 

 relief of the hard rock surface now covered in [)art l)y glacial dej)osits and 

 on the fact that no sediments of younger date than the Carboniferous 

 period, with the possible exceptions of terranes of Cretaceous age in por- 

 tions of Minnesota, occur within its borders. 



It may be suggested as a tentative hypothesis, tliat jnevious to the 

 Tilacial epoch the greater part of the Laurentian l>asin discharged its 

 \\aters southward to the Mississippi, and tliat during the first advance of the 

 ice from the noilh, the drainage was not ol)structed so as to form impurtanl 

 lakes. This suggestion rests in part on the fact that no lake deposits 

 have yet been found l)eneath the lowest sheet of glnciid del>ris lining the 

 basin, — this negative evidence is of litth' wciglit. however, as such 

 deposits, if they exist, wouhl ])e mostly bcneatli the jn'csent lakes and 

 tlierefore exceedinq-lv diriiciilt to <bsro\-cr. and on tlie cliaraclei' of an 



