Englacial and Snpcvglacial Drift- 42^ 



glacial currents moved and the ice surface sloped toward the south- 

 east. 



My studies of the prominent kame called the Devil's Heart hill,* 

 on the south side of Devil's lake in North Dakota, and of Bird's Hill,t 

 near Winnipeg, convince me that much drift was carried upward into 

 the ice-sheet of this region, to heights of 1,000 to 1,500 feet or more 

 above the ground. The distance from Bird's Hill to the boundary of the 

 glacial drift is about 700 miles to the south and 300 miles to the south- 

 west. It may be estimated, from altitudes of the drift on the White 

 niountains, the Catskills, and the Adirondacks, that the ice-sheet simi- 

 larly rising over Manitoba attained a maximum thickness of at least 

 one mile, or more probably one and a half miles, about 8,000 feet. 

 The gradients of its surface were similar to the slowly ascending 

 slopes by which the ice-sheets of Greenland and the Antarctic conti- 

 nent rise to altitudes of about two miles above the sea. In the lower 

 quarter or sixth part of the ice covering Manitoba, that is, to a height 

 oi probably 1,500 feet, much drift had been carried by its variable and 

 partly rising currents. 



Near the border of the ice-sheet during its time of accumulation, 

 little drift could thus be carried into it, and therefore in the melting 

 and recession of that outer part the englacial and finally super- 

 glacial drift was generally inconspicuous; but at any considerable dis- 

 tance inside the glaciated area as a score of miles or more, the final 

 melting set free much formerly englacial till and modified drift. 

 The processes of drift transportation and deposition here empha- 

 sized were well stated by Prof. N. H. Winchell in 1873,* by 

 Prof. C. H. Hitchcock in lS78,t and by me in 1876 and 1878 and in 

 numerous later papers and reports.! At the present day these processes 

 are exemplified by the Malaspina glacier or piedmont ice-sheet in 

 Alaska, which during the last century has been much reduced in area 

 and thickness; but the Greenland and Antarctic ice-sheets, which are 

 now constant or increasing by snowfall, have no superglacial drift. 



* The Glacial Lake Agassiz, U. S. Geol. Survey, Monograph XXV, 1895. 

 pp. 156, 157. 



t Ibid., pp. 183-188; also a paper presented to the Geological Society of 

 America, December, 1909, vol. XXI, pp. 407-432. 



* T*he Drift Deposits of the Northwest, Popular Science Monthly, vol. 

 Ill, pT). 202-210, 286-297 (especially page 294, relating to superglacial drift). 



t Geology of New Hampshire, vol. Ill, pp. 282. 283, 309, 326, 333-8. 



tProc A. A. A. S., vol. XXV, for 1876, p. 218; vol. XXVII, for 1878, 

 pp. 299-310. Geol of N. H., vol III, 1878, pp. 9, 10. 175-6, 285-309. Geol. 

 of Minnesota, Final Report, vol. I, 1884, pp. 440, 603-4; vol. II, 1888, pp. 

 252, 254-6. 409-417. Am. Geologict, vol. X, 1892, pp. 339-362; vol. XII, 

 1893, pp. 36-43; vol. XIV, 1894. pp. 69-83; vol. XVI, 1895, pp. 100-113; vol. 

 XIX, 1897. pp. 411-417; vol. XX. 1897, pp. 383-7; vol. XXIII. 1899, pp. 

 369-374; vol. XXV, 1900, pp. 273-299. 



