108 Ww. UPHAM—NIAGARA GORGE AND SAINT DAVIDS CHANNEL. 
outlets were occupied successively during only a brief time, or, more 
probably, that these outlets were obstructed by the receding ice-front 
until after the land there had risen, from its Champlain depression to 
such altitude that the Saint Clair and Detroit rivers continued to be con- 
stantly the outlet from the upper lake basins, sending their waters to 
the Niagara river and falls during all their history. 
GLACIAL LAKES BELOW NIAGARA RIVER 
Lakes Algonquin and Iroquois were contemporaneous, and the Ontario 
basin inclosing lake Iroquois was at the same time uplifted toward the 
northeast, with inclination of its earlier shorelines, and with gradual rise 
of the lake on the land westward because its outlet at Rome was raised 
much more than the western part of the basin. While these two glacial 
lakes were undergoing such changes, a lobe of the mainly retreating but 
wavering ice-sheet lingered on the highlands north of lake Ontario; and 
twice its moderate readvance was recorded by deposits of till intercalated 
with the stratified beds of a lacustrine delta in the extensive section of 
Searboro heights near Toronto. The uplift of the Iroquois basin, as well 
as that of the Algonquin basin, is thus shown to have been far advanced 
and nearly completed during the continuance of their ice barriers. 
Latest, the glacial lake Saint Lawrence, held by the final blockade of 
the waning ice-sheet on the Saint Lawrence valley below Montreal, ex- 
tended into the lake Ontario basin with a depth of about 150 feet above the 
Thousand islands, but with its water level beneath the present surface of 
the west part of this lake. In like manner with the earlier lake Iroquois 
the progressing northeastward uplift caused the level of the lake Saint 
Lawrence and afterward of lake Ontario to rise upon the land in the 
southwest part of the Ontario basin. It was during these late stages of 
the lacustrine history of this region that the deep channel of the Niagara 
river at the mouth of its gorge may have been eroded, the channel being 
subsequently partially refilled with water by the continuance of the north- 
eastward land elevation. 
EpPrErRoGENIC UPLIFTING CONTEMPORANEOUS WITH THE GLACIAL LAKES 
It has been already quite fully noted in the foregoing references to 
lake Agassiz and the glacial lakes of the Saint Lawrence basin that the 
area which had been ice-covered and depressed under the weight of the 
thick continental ice-sheet was gradually uplifted, and to a greater height 
at the north than at the south, during the removal of the ice burden. 
While lakes Agassiz and Warren still existed the northern parts of their 
areas were raised, in comparison with their southern outlets, 300 to 400 
feet or more. It is also found by the present inclinations and relation- 
