No. 4.] CLAYPOLE — PRE-GLACIAL GEOGRAPHY. 205 



The scenery along such a river presented alternately the low- 

 wide landscape of the open plain and the deep contracted view 

 of the narrow pass, each passing into the other, as the under- 

 lying rocks change from soft to hard or from hard to soft. The 

 spread of drift-material over the face of the country left the 

 narrow gorges completely choked. As the rivers began again to 

 flow after the ice-age had passed away they were unable to find 

 their ancient channels along these narrow chasms, and conse- 

 quently the water accumulated in the valley until it rose suffi- 

 ciently high to flow over the barrier at its lowest point, wheu it 

 commenced anew the task of cutting a gorge through the same 

 limestone ridge, first in the drift material on the surface, and 

 then in the solid rock below. This process may now be seen 

 going on at Niagara Falls. 



The great American lakes therefore are nothing but mere 

 drift-dammed pools, filling the wide portions of the channels of 

 pre-glacial rivers, while the narrow chasms connecting them are 

 concealed by superficial deposits of clay and sand. Should the 

 present condition of things continue long enough, the rocky bar- 

 rier between Erie and Ontario will be again cut down, and the 

 present lake above the Falls converted again into the broad open 

 plain of the later Tertiary age. New falls or rapids will be 

 developed near Detroit as the excavation of the Erie basin pro- 

 ceeds, and the levels of lakes Huron and Michigan correspond- 

 ingly lowered ; w T hile by the gradual wearing down of the rocky 

 bars now forming the rapids on the St. Lawrence, as much of 

 the water in Lake Ontario will be carried away as the relative 

 levels of that lake and the Atlantic will allow. But neither 

 Michigan, Huron, nor Ontario can ever be laid dry by this pro- 

 cess, and their end cau only come, catastrophes excepted, by the 

 slow but steady process of silting up. The same process to some 

 extent must occur in Lake Superior. The wearing down at the 

 falls of St. Mary will lower its level, but the deposit from its 

 tributary streams alone can entirely obliterate it. 



One result of the Quaternary age has therefore been to transfer 

 a great part of the basin of the pre-glacial Mohawk to the basin 

 of the St. Lawrence, a younger and Quaternary river. But no 

 great alteration in level would be required to change again the 

 course of these northern waters. The sewers of Chicago now 

 carry the water of Lake Michigan into the Mississippi valley, 

 the watershed between the two being only li) feet above the 



