FIG. 388.—Bird’s-eye view of the cuestas south of Lakes Ontario and Erie (after Gilbert).] =Episodes of Niagara’s history and their correlation with those of the Glacial Lakes.=—Of the early episodes of Niagara’s history, our knowledge is not as perfect as we could desire, but the later events are fully and trustworthily recorded. The birth of the Falls is to be dated at the time when the ice front had here first retired into what is now Canadian territory, thus for the first time allowing the waters from the Erie basin to discharge over the Lewiston Escarpment into the basin of the newly formed Lake Iroquois (Fig. 364, p. 334). Since the level of Lake Iroquois was far above that of the present Lake Ontario, the new-born cataract was not the equivalent in height of the escarpment to-day. The Iroquois waters then bathed all the lower portion of the escarpment, so that the foot of the Fall was upon the borders of the Lake. In order to interpret the history of the Niagara gorge, we must remember that the effective drilling of this gorge was in each stage dependent mainly upon the volume of water discharged from Lake Erie, a large discharge being recorded by a channel drilled both wide and deep, while that produced by the discharge of a smaller volume was correspondingly narrow and shallow. To-day the gorges of large cross section have, moreover, a relatively placid surface, whereas through the constricted sections the water of the river is unable to pass without first raising its level at the upper end and under the head thus produced rushing through under an increased velocity. The best illustration of such a constricted section is the Gorge of the Whirlpool Rapids. [Illustration: FIG. 389.—Sketch map of the greater portion of the Niagara Gorge to show the changes in cross section in their relations to Niagara history (based upon a map by Taylor).] Our reading of the history should begin at the site of the present cataract, since the records of later events are so much the more complete and legible, and it should ever be our plan to proceed from the clearly written pages to those half effaced and illegible.