THE QUATERNARY PERIOD 



347 



land of the Helderberg escarpment for wall on the south. Many 

 of these glacial stream channels, often high up on the hills of 

 central to western New York, are still plainly visible. 



By successive stages, due to complete removal of ice from 

 central New York, and a draining of the glacial lake in the Mohawk 

 Valley, the waters dropped to below Warren level until Lake Iro- 



Miles 



If iV If 



Fig. 219 



The Algonquin-Iroquois stage of the Great Lakes, with outlet through the 

 Mohawk-Hudson Valleys of New York. (After Taylor, courtesy of the 

 New York State Museum.) 



quois was formed (see Fig. 219). The old beach line of this lake 

 is still plainly visible in New York. Lake Iroquois covered some- 

 what more than the present area of Lake Ontario, and the dis- 

 tinctly lower water level here than in the Erie Basin allowed the 

 modern Niagara River to begin its history by flowing northward 

 over the limestone plain near Buffalo. Meantime the waters of 

 the upper lake basins had merged to form Lake Algonquin, which 

 at first probably discharged past Detroit through the Erie Basin 

 and into Lake Iroquois by way of Niagara River. Later, however, 



