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Page:Popular Science Monthly Volume 49.djvu/27

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or more, and reduce the descent of the river by eighty feet or more.

Episodes of the Niagara River and their Duration.—At first the river flowed without falls, as shown by the old banks and terraces, for a period estimated at a thousand years. Then the waters of the lower lake began to subside, whereupon the Niagara Falls had their birth. The new cataract slowly grew in height, although characterized by temporary pauses, until the river cascaded two hundred feet from the tableland into the edge of the gulf or lake which occupied the Ontario basin, as shown in Fig. 16.

PSM V49 D027 Longitudinal section of the falls showing the geology.jpg

Fig. 16.—Longitudinal Section, showing the Retreat of the Falls and the Geological structure. A. Brow of escarpment and original site of falls; I, Iroquois beach and level of water; B b I, chasm at the end of the first episode; C c h g, falls retreating in three cascades, but from h to g the slope was extended over a distance of twelve miles beyond the escarpment; D d m g, position of cataracts at the end of the second episode; E e g, development of gorge at the end of the third episode; F, present site of the falls, and F m g, the modern cañon; g, level of the lowest stage of water in the river history; L, Lundy beach capping the drift; J, Johnson's ridge. Broken shading about whirlpool shows occurrence of drift on west bank only with rock on the eastern; block shading represents limestone; dotted, sandstone; broken lines and unshaded portion, shales. Bottom of river eighty feet below present surface of Lake Ontario, as shown in figure.

The volume of the water of Lake Erie is about one fourth that of all the upper lakes, and only this proportion of the discharge of the modern Niagara River formed the abrading agent of the falls at that early date. This general condition lasted for seventeen thousand two hundred years.[1] After this episode, the descent of the river was increased to four hundred and twenty feet, and the lake receded twelve miles from the foot of the mountain, and then there was a series of three cascades, the lower always gaining upon the upper on account of the softer rocks. Yet the increased amount of work to be done, even though easier than the recession

  1. For the methods of computation of the duration of the episodes of Niagara Falls see Duration of Niagara Falls and the History of the Great Lakes, pp. 1-126, 1895. Also see Duration of Niagara Falls, American Journal of Science, December, 1894, pp. 455-472—both works being by the writer.