GLACIAL LAKES 



321 



Temporary lakes due to ice blocking. Whenever, on the con- 

 trary, the advancing ice front entered a valley from the direction 

 of its mouth, or a re- 

 treating ice front retired 

 down the valley, quite 

 different results fol- 

 lowed, since the waters 

 were now impounded 

 by the ice front serving 

 as a dam. Though the 

 histories of such block- 

 ing of rivers are often 

 quite complex, the prin- 

 ciples which underlie 

 them are in reality sim- 

 ple enough. Of the 

 lakes formed during ad- 

 vancing hemicycles of 

 glaciation, and of all 

 save the latest reced- 

 ing hemicycle, no satis- 

 factory records are pre- 

 served, for the reason 

 that the lake beaches and the lake deposits were later disturbed 

 and buried by the overriding ice sheets. We have, however, every 



FIG. 347. Broadly terraced valleys outside the 

 divide of the St. Lawrence basin, which remain to 

 mark the floods that issued from the latest con- 

 tinental glacier during its retreat (after Leverett). 



FIG. 348. Border drainage about the retreating ice front south of Lake Erie. 

 The stippled areas are the morainal ridges and the hachured bands the valleys 

 of border drainage (after Leverett). 



reason to suppose that the histories of each of these hemicycles 

 were in every way as complex and interesting as that of the one 

 which we are permitted to study. 



