Page:Popular Science Monthly Volume 49.djvu/26

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14
THE POPULAR SCIENCE MONTHLY.

lake, but four miles farther. Again, the waters of Ontario were raised and the descent of the river was reduced to the present amount. Thus it is apparent that at the birth of Niagara Falls

PSM V49 D026 Map of the gorge at fosters flats.jpg
Fig. 14. — Map of the Gorge at Foster's Flats. F, Location of the cross-section (Fig. 15).

the cataract was much smaller than now, and very much resembled the size of the American Falls as shown in Fig. 13. With the discovery of the history as here set forth, we had to wait several years before any clew was obtained as to how far the falls receded during each of the episodes. But at last the inquiry was partially successful; for at Foster's Flats (Fig. 14, see also Fig. 9), about two miles and a half above the mouth of the gorge, the remains of an old terrace occur (Fig. 15) at the height which shows that the falls had receded to that point before the descent of the river was greatly increased or the volume of the water enlarged to the drainage of all the upper lakes.

In this determination of the distance to which the falls had probably receded before Lake Huron drained into Lake Erie, Prof. Gilbert has followed the writer. Now, by applying the laws of the variability of erosion to the observed modern rate of recession of the falls, an approximate determination of their antiquity became possible.

PSM V49 D026 Section of the gorge at fosters flat.jpg
Fig. 15. — Section of the Gorge at Foster's Flats (FT, Fig. 9). Platform (F) of the old river floor projecting into the cañon. Its section is shown in broken shading, but with ravines descending from both sides of it; T, rock terrace surmounted by huge blocks of Niagara limestones; b, original river terrace; r, surface of river; L O, surface of Lake Ontario. Bottom of river about eighty feet below the surface of the lake.

Causes of Fluctuation in the Volume and Descent of Niagara River. — In the survey of the deserted shores it was found that since they were formed as old water-lines they have been tilted upward toward the north and east at variable rates, from a few inches in a mile at the southwest to four or even seven feet per mile in the opposite direction. The phenomenon belongs to the consideration of the history of the lakes, but its effect was to tilt the lake basins so that the water ran over the southern rim of Lake Huron into the Niagara drainage. So, also, the tilting of the Ontario basin raised the barrier at the outlet and caused the waters to rise and flood the lower lands at the head of the lake, and shorten the Niagara River by four miles