ACADEMY OF SdENCES] NIAGARA RIVER 157 



double publication; first in the Sixth Annual Report of the Commissioners of the [New York] 

 State Reservation at Niagara, and again in the Forty-fifth Annual Report of the Smithsonian 

 Institution of Washington. 



The history of Niagara River, as presented in the lecture, involves an understanding of 

 three main factors: The general relief of the region, the progressive retreat of the ice from it, 

 and the upheaval of the land in the northeast during that retreat. The manner in which 

 Gilbert analyzed the interaction of these factors excited universal admiration. As to the first, 

 the region concerned owes its larger features to long-enduring subaerial erosion in preglacial 

 time, as a result of which the Laurentian uplands of ancient crystalline rocks on the north were 

 stripped of the sedimentary strata that stretched a certain distance over them, and thus came 

 to be flanked by a series of broad and shallow lowlands excavated along belts of relatively 

 weak, gently south-dipping strata, alternating with broad uplands that survive along belts of 

 more resistant strata. One of the upland belts is maintained by the so-called Niagara limestone, 

 which falls off in a well-defined north-facing escarpment in western New York and the adjacent 

 part of Ontario; a second upland of greater altitude but not so closely related to the Niagara 

 problem is the Appalachian plateau, which begins in the southern part of New York and the 

 adjoining part of eastern Ohio and extends far southward. Of the two lowland belts, the one 

 north of the Niagara upland belt is now occupied in part by Lake Ontario and by the Georgian 

 Bay extension of Lake Huron; the one to the south is in part occupied by Lake Erie. 



Gilbert does not appear to have expressed any opinion as to the preglacial form of these 

 belts, other than that noted in the preceding section, probably because that problem still re- 

 mained largely indeterminate; but he made it clear that, as already explained, there was a time 

 in the retreat of the ice sheet when it obstructed the Mohawk Valley between the Adirondack 

 Mountains and the Appalachian plateau on the south, as well as the St. Lawrence Valley be- 

 tween the Adirondacks and the Laurentian highlands on the north; and that a great proglacial 

 lake then had a southwestward outlet which flowed to the Mississippi. The lake surface then 

 stood at so high a level that not only were the two lowland belts submerged as far as they had 

 been evacuated by the retreating ice, but even the Niagara upland between them had a hun- 

 dred feet of water over it. But after a time the continued retreat of the ice margin opened 

 a passage between the Appalachian plateau and the Adirondacks — a later chapter will tell of 

 the dramatic incidents of this opening as other of Gdbert's discoveries — and as this passage 

 was lower than the southwestward outlet that led to the Mississippi, that outlet was abandoned 

 and an eastward discharge was adopted in its place. The lowering of the great proglacial lake 

 that followed was sufficient to lay bare the Niagara upland; thereupon the single proglacial 

 lake was divided into two smaller ones; a higher one occupying the basin of Lake Erie on the 

 southwest of the upland; and a lower one, the Lake Iroquois already noted, the precursor of 

 Lake Ontario, on the northeast of the upland. The higher lake selected the lowest points of 

 the upland for its point of discharge to the lower Lake Iroquois — the manner of selection was 

 detailed in another of Gfibert's later discoveries — and thus the Niagara River came into exist- 

 ence. How far this sequence of events had been established by others need not be here inquired 

 into; it was never so clearly set forth as in Gilbert's lecture at Toronto. 



It must be remembered that when Niagara River was thus born the land to the north- 

 east was lower than now, for at present the shore lines of all the extinct lakes gradually ascend 

 in that direction. The river was of large size, for the upper Great Lakes appear then, as now, 

 to have drained by the roundabout Detroit channel into the Lake Erie basin. But because of 

 the lower stand of the land in the northeast, the Lake Erie of that time did not extend so far 

 to the southwest as that lake does now; hence the outlet of Lake Huron southward and east- 

 ward was not only a longer river than the Detroit River of to-day, but eroded its channel with 

 respect to a lower baselevel than that of the present Erie. It is the flooding of this deeply 

 eroded channel by the backward extension of Lake Erie to its present size, as the land rose in 

 the northeast, that has made the Detroit River navigable, as above noted. 



