Directly above the whirlpool the Niagara gorge is first contracted, but almost immediately swells out into the form of a sausage, which under the name of the Eddy Basin extends to the constricted channel occupied by the Whirlpool Rapids. This Gorge of the Whirlpool Rapids extends to and a little above the railroad bridges, where it again suddenly widens and deepens and with surprisingly uniform cross section now continues as far as the cataract. This uppermost section is known as the Upper Great Gorge. About a mile below the whirlpool is that remarkable projection into the gorge from the Canadian wall which is known as Wintergreen Flats, below which and nearer the river are Fosters Flats. Almost throughout its entire length the Niagara gorge is bordered on either side by a narrow and gently incurving terrace eroded below the general level of the plain and meeting the gorge in a sharp angle (Fig. 377). The features immediately about the cataract show that the Falls are to-day in a condition which, so far as we know, has occurred but once before in their entire history—the waters of the river are divided unequally by an island, and for this reason, as we shall see, the cataract enters over the _side wall_ of the gorge instead of at its _end_ (Fig. 381), although the turning of the channel from this cause is combined with a bend of the river. [Illustration: FIG. 378.—View of the bed of the Niagara River above the cataract, where water has been drained off in installing a power plant. Some separated blocks of limestone are still in place (after J. W. Spencer).] =The drilling of the gorge.=—There appear to be two important processes which are responsible for the recession of the Falls, the rate of which is determined largely by the resistance of the limestone capping and the tenacity of the looser shale beneath it. One of the eroding processes operates from below and undermines the cap until the unsupported cornice falls in blocks to the bottom of the gorge; the other makes its attack directly from above, selecting for the purpose the lines of jointing of the rock which it widens by solution and corrasion until the included blocks are in so far separated that they are torn out and go over the brink of the Falls (Fig. 378). This process of overhead attack in the powerful currents just above a cataract is even better illustrated by the Falls of St. Anthony near Minneapolis, which have had a similar history of recession to that of the Niagara Falls (Fig. 379). [Illustration: FIG. 379.—Falls of St. Anthony, looking westward from Hennepin Island in 1851 (after N. H. Winchell, daguerreotype by Hessler of Chicago).]