THE WORK OF SNOW AND ICE 



137 



The Great Lakes of the present day were larger than now after 

 the ice had retreated north of their basins, but while it still covered 

 the lower part of the St. Lawrence Valley. A part of their history, 

 dating from the time when the last ice-sheet was waning, is sug- 

 gested by Figs. 144 to 147. These lakes did not exist, so far as 



I .. X^ J \ Js-^W: -~^/ ^JU. 



Fig. 144. The beginning of the Great Lakes. The ice still occupied the larger 

 parts of the present lake basins. (U. S. Geol. Surv.) 



now known, before the glacial period, but rivers probably flowed in 

 the direction of their longer diameters. Lake basins seem to have 

 been developed from these river valleys as a result of (1) the deep- 

 ening of the valleys by ice erosion, (2) the building up of the rims 

 of the basins by the deposition of drift, and (3) perhaps the down- 

 warping of the sites of the basins. 



6. Rivers. The disposition of the drift also deranged the rivers. 

 After the ice melted, the surface drainage followed the lowest lines 

 open to it; but these lines did not always correspond with the 

 former valleys, for some of them had been filled, and most of them 



