THE QUATERNARY PERIOD 



355 



and Northampton, respectively, the Hudson and Sacandaga rivers 

 show remarkable eastward deflections instead of following broad, 

 deep pre-Glacial valleys southward into the Mohawk Valley. 

 These deflections were caused 

 by heavy morainic deposits 

 acting as dams across the 

 valleys south of Corinth and 

 Northampton. 



The world-famous Niagara 

 Falls and gorge are wholly 

 post-Glacial in origin. After 

 plunging 167 feet at the falls, 

 the river rushes for 7 miles 

 through the gorge, whose depth 

 is between 200 and 300 feet. 

 When the glacial waters in the 

 eastern Great Lakes region 

 had dropped to the Iroquois 

 level, the Niagara limestone 

 terrace in the vicinity of 

 Buffalo and with steep escarp- 

 ment or northern front at 

 Lewiston and Queenston, 

 ceased to be covered with lake 

 water, and the Niagara River 

 came into existence by flowing 

 northward over this limestone 

 plain. The river first plunged 

 over the escarpment at Lewis- 

 ton, thus inaugurating the falls 

 there. Since that time the 

 falls have receded the 7 miles 

 up stream to their present 

 position. Soft shales underlie 

 the layer of harder Niagara 

 limestone, and the recession of 

 the falls has clearly been caused by the breaking off of blocks of 

 limestone due to undermining of the soft shales. A glance at the 

 map (Fig. 223) will show that the gorge development is really 

 taking place on the Horseshoe Falls side, where the volume of 



Fig. 224 

 Pre-Glacial drainage of the upper 

 Ohio River Basin. (After Chamber- 

 lin and Leverett, from Norton's 

 "Elements of Geology," by permis- 

 sion of Ginn and Company, Pub- 

 lishers.) 



