The gorge through which the Mississippi flows, below the Falls of St. Anthony, has been carefully studied by Professor N. H. Winchell, the state geologist of Minnesota, who thinks he can pretty closely approximate to the truth concerning its antiquity.1

During the glacial age the old channel of the Mississippi, leading past Minneapolis to the larger valley of the Minnesota River, was so filled with glacial débris as to turn the course of the water, after the glacial age, into the new channel which it now occupies. This post-glacial gorge extends. from Minneapolis to Fort Snelling, a distance of not far from eight miles; and is our dividend. This is the whole work done there by the Mississippi since its course was changed by the deposits of "till" left by the retreating ice of the glacial epoch.

The next thing is to find a divisor. The gorge is through limestone strata of pretty uniform composition. It remained for Professor Winchell to ascertain the annual rate at which the waterfall recedes. The falls were discovered by the Jesuit Hennepin in 1680, and visited by Carver in 1766. Fortunately, both these zealous propagandists and explorers took such copious notes and made such

1 Fifth Annual Report of the Minnesota Geological Survey, pp. 156-189, first published in the Quarterly Journal of the Geological Society of London for 1878, pp. 886-901.

accurate drawings that the exact position of the falls at the time of their visits can be determined. The distance which the falls have receded since 1680, a period of two hundred years, is very nearly one thousand feet, or five feet per year. At this rate the river cannot have been much over 9,000 years in wearing the gorge from Fort Snelling to the present falls at Minneapolis. Other things, as the shortness of the tributary gorges, like that below the Falls of Minnehaha, confirm the approximate accuracy of this estimate. This brings the date of the close of the glacial period down to very moderate limits.

Plate V. illustrates Professor Winchell's calculation about the falls of St. Anthony at Minneapolis. The shading along the banks of the rivers shows the comparative width of their valleys. The preglacial channel from above Minneapolis to the Minnesota is marked by a succession of lakes, and is determined by facts brought to light in digging wells and by boring. The gorge from Fort Snelling to Minneapolis is about one hundred feet deep, and the rocks here show much less weathering on the Mississippi than below Fort Snelling or on the Minnesota. The bluffs along the Minnesota are from twenty-five to sixty feet higher than on the Mississippi above Fort Snelling.

Another source from which something may be expected to shed light upon the date of the with

PLATE V.

drawal of the continental ice-sheet lies in the abundant opportunities to estimate the amount of superficial erosion of gravel and till, and of the disintegration of rock, which have taken place since that period.

Professor Chamberlin, the state geologist of Wisconsin, remarks that no sensible denudation has taken place there since glacial times. Even Mr. Croll, whose views concerning the antiquity of the glacial period are, as we have seen, extreme, expresses surprise (as well he may) at the small amount of erosion which has taken place since the kames of Scotland were deposited. Both in Europe and in America these peculiar gravel ridges which we connect with the glacial period, retain a sharpness of outline which it is difficult to believe could have survived the protracted period of 100,000, or even of 40,000 years. When, also, he considers the chemical agencies at work to decompose the rocks even where protected by a covering of till, the freshness of the glaciated surfaces never ceases to be a cause of astonishment to the reflective observer. Material is accumulating for more definite estimates from such data; but it is as yet too early to formulate the conclusions.

Closely connected with the preceding class of facts are the observations made upon the extent to which lakes dating from the glacial period have 1 Geology of Wisconsin, Vol. ii. p. 632. 1877

filled with sediment. Little reflection is required to make it evident that our present lake-basins could not always have existed; for in due time the "wash of the hills will fill to the brim all (except where counteracting agencies are at work) enclosed areas of depression. Mr. Upham of the Minnesota Geological Survey, is impressed with the small extent to which the lakes dotting the surface of that state have been filled by the solid matter continually washing into them. "The lapse of time since the ice age has been insufficient for rains and streams to fill these basins with sediment, or to cut outlets low enough to drain them; though in many instances we can see such changes slowly going forward." 1

Dr. Andrews of Chicago has made calculations, deserving of attention, concerning the rate at which the waters of Lake Michigan are eating into the shores, and washing the sediment into deeper water or towards the southern end of the lake. The United States Coast Survey have carefully sounded the lake in all its parts, and have ascertained the width of the area of shallow water extending inwards from the shores. It is well known that waves are limited in their downward action, so that there will be a surrounding shelf, or shoulder of shallow water, in cases where the waves of a deep lake are eroding its banks. This fringe of

1 Minnesota Geological Report for 1879, p. 72.