QUATERNARY 651 



from the last ice sheet. The effect of the last recrudescence of glacia- 

 tion in the Wisconsin stage is best known, since its deposits cover 

 the earlier drifts. It seems probable, moreover, that the surface of 

 the last drift had a relief stronger, originally, than that of any of the 

 former ice sheets. One marked feature is the lobate form of the mo- 

 raines which occur in a succession of crescentic belts (Fig. 567). 



History of the Great Lakes 



There is general agreement that the Great Lakes were not in exist- 

 ence immediately previous to glacial times. This belief is based 

 upon the fact that the region now occupied by them had been sub- 

 jected to erosion so long that any lakes which might at some time 

 have been in existence must have been destroyed by filling or the 

 cutting down of their outlets. It is probable that in preglacial times 

 the region in the vicinity of the Great Lakes was not unlike that 

 of central and eastern Tennessee, Kentucky, and southern Indiana, 

 where the weak rock formations are marked by lowlands, and the 

 more resistant by highlands. 



Preglacial Drainage. — The precise course of the preglacial drain- 

 age of this region is yet to be determined, but the evidence indicates 

 that the St. Lawrence River was not as now the channel through 

 which it flowed to the ocean. At that time, indeed, the head of the 

 St. Lawrence River may have been in the vicinity of the Thousand 

 Islands. Well borings in Michigan and Indiana have revealed the 

 fact that ancient, drift-filled valleys of great depth lead towards the 

 south, giving strong support to the supposition that the preglacial 

 drainage was southward to the Ohio and Mississippi, instead of to 

 the east. In fact, at the present time a lowering of the land a few 

 feet at the southern end of Lake Michigan would turn the drainage 

 to the Mississippi Valley. 



Origin of the Basins. — The basins of the Great Lakes are lowlands 

 which have been modified in several ways: (1) by drift deposition 

 which has not only blocked up the valleys leading south, but has 

 also increased the height of the divides by terminal moraines ; (2) by 

 glacial erosion, though whether the glaciers accomplished more than 

 the removal of the weathered rock and soil is a mooted question ; 

 all will concede, however, a deepening to this extent at least; (3) by 

 a depression of the region at the north as a result of the weight of the 

 ice. Since the disappearance of the ice sheets the land at the north 



