354 



HISTORICAL GEOLOGY 



history, those lakes discharged through the Mohawk Valley and 

 across the Little Falls divide. It was the passage of this great 



volume of water over the di- 

 vide which caused the cutting 

 of most of the gorge as we now 

 see it, except for the narrow 

 trench in the hard, low-lying 

 rock, which is no doubt due 

 to post-Glacial erosion. Dur- 

 ing the gorge cutting, ag- 

 gradation (building up by 

 sediments) of the valley bot- 

 tom took place westward from 

 Little Falls, so that the drain- 

 age from Rome, N. Y., was 

 able to continue eastward 

 even after the disappearance 

 of Lake Iroquois. Thus we 

 have here an excellent illus- 

 tration of exact reversal of 

 drainage due to glaciation, 

 and by this means the upper 

 waters of the present Mohawk 

 River were added to what was 

 the pre-Glacial Mohawk. 



In the southeastern Adi- 

 rondack Mountains certain 

 important principles of drain- 

 age changes due to glaciation 

 are illustrated by the upper 

 waters of the Hudson River. 

 The accompanjdng sketch map 

 (Fig. 222) gives an idea of the 

 changes. Near Warrensburg 

 the Hudson River was de- 

 flected westward from its pre- 

 Glacial channel because of the 

 presence of a lobe of the wan- 

 ing ice sheet in the Lake 

 George depression. At Corinth 



Fig. 223 

 Sketch map of the Niagara River gorge. 

 (Modified after Gilbert, from Nor- 

 ton's "Elements of Geology," by 

 permission of Ginn and Company, 

 Publishers.) 



