886 N. H. WINCHELL ON THE RECESSION 



51 . The Secession of the Falls of St. Anthony. By N. H. "Winchell, 

 Esq., State Geologist of Minnesota. (Read April 3, 1878.) 



[Communicated by James Greikie, LL.D., F.B.S.] 



In studying the superficial accumulations of the North-west, the 

 phenomena of the drift in the vicinity of the Falls of St. Anthony 

 have come under review, and it has been found that the intimate 

 connexion between the history of that drift and the recession of the 

 Falls has afforded another datum from which to compute the date 

 of the last cold phase of the glacial epoch *. The consideration of 

 this question naturally divides itself into three parts : — 



First. The Drift in the vicinity of the Falls of St. Anthony. 



Second. The Gorge formed by the recession of the Falls. 



Third. The Recession of the Falls. 



T. Drift in the vicinity of the Falls of St. Anthony. 



In describing the Drift in the vicinity of the Falls of St. Anthony 

 it will be necessary to state, in the first place, that the surrounding 

 country presents the same general character as that of most of the 

 North-west. It lies in the midst of that great plain known as the 

 " Mississippi Basin," which extends from the spurs and foot-hills 

 of the Rocky Mountains to the Adirondacks. The surface for hun- 

 dreds of miles is flat or gently flowing, the slight elevations and 

 depressions being chiefly duo to corresponding undulations in the 

 nearly horizontal rocky strata underneath, while the short abrupt 

 hills which give sometimes a rapidly rolling contour are made up 

 of drift deposits. The principal component of the drift is a stony 

 clay, which is the unmodified deposit of the glaciers. Its thickness, 

 outside the river-valley, seems to be about 150 feet. It lies gene- 

 rally in an unbroken sheet, but shows an undulating or rolling 

 upper surface. This accumulation, which is well known under the 

 names of "hardpan" and "boulder-clay" in America, and of "till" 

 in Great Britain, is of two kinds, the differences between which 

 show that it is made up of two distinct deposits ; they are 



The Grey Hardpan and 

 The Red Hardpan. 



The principal differences between these deposits are these : — The 

 red underlies the grey and gradually runs deeper and deeper below 

 an increasing thickness of the latter, towards the west, while 



* [Professor Ramsay has noticed a somewhat similar connexion between the 

 history of the drift and the recession of the Falls of Niagara. He thinks the 

 Falls commenced during the deposition of the Leda-cl&y, or a little before the 

 close of the drift period. " If, with accumulated data," he remarks, "the rate 

 of the past recession of the Falls be actually determinable, we shall then be in 

 a condition approximately to show the actual number of years that have 

 elapsed since the close of the North- American drift." — Quart. Journ. Geol. Soc. 

 vol. xv. (1859), p. 212.— J. G.] 





