LAWRENCE MARTIN. [Post-glacial Modifications in and Around the Great Lakes], Mon. 52, U. S. Geol. Surv., 1911, pp. 455-459. Uptilt in northern Europe:— G. DE GEER. Quaternary Changes of Level in Scandinavia, Bull. Geol. Soc. Am., vol. 3, 1892, pp. 65-68, pl. 2. H. MUNTHE. Studies in the Late Quaternary History of Southern Sweden, paper No. 25, Livret Guide, Cong. Géol. Intern., 1910, pp. 96, many plates and maps. CHAPTER XXV NIAGARA FALLS A CLOCK OF RECENT GEOLOGICAL TIME =Features in and about the Niagara gorge.=—A striking example of those permanent alterations of drainage which have resulted from the presence of the late continental glacier in North America is to be found in the Niagara gorge between Lakes Erie and Ontario. With the aid of borings many of the now buried channels of the region have been followed out, and in a later paragraph we shall refer to some of the stronger lines of the earlier drainage system. Before undertaking the study of Niagara history, it is essential that one become somewhat familiar with the present topography in and about the Niagara gorge. Below the present cataract the river flows through a deep gorge for about seven miles before issuing at the Lewiston Escarpment (Fig. 381, p. 355). This gorge has been cut in beds of rock sediments which dip at a gentle angle southward toward Lake Erie. The capping of the rock series is a compact and relatively resistant limestone which is known as the Niagara limestone, beneath which there are alternating beds of shale with thinner limestone and sandstone. The plain formed by the upper surface of the limestone capping terminates in the Lewiston Escarpment, which is transverse to the direction of the gorge and seven miles distant below the Falls. The depth of the gorge varies markedly, the above-water portion being represented at the upper end by the height of the cataract, one hundred and sixty-five feet, while at its lower end near Lewiston it is twice that amount. Halfway down the gorge a sharp turn is made at an angle of more than ninety degrees, and the upstream arm is extended to form a basin which contains the famous whirlpool. This visible extension of the upper gorge is continued in a buried channel, the St. Davids Gorge, which extends to the escarpment, broadening as it does so in the form of a trumpet. The materials which fill this earlier channel are notably coarse glacial deposits (Fig. 389). [Illustration: FIG. 377.—Ideal cross section of the Niagara gorge to show the marginal terrace.]