ACCUMULATION OF DRUMLINS FROM ENGLACIAL DRIFT. 21 



examined that area, mapping its marginal moraines and other drift de- 

 posits. 



Next northwestward, however, drumlins are again encountered in great 

 abundance and variety in the eastern part of Wisconsin. Professor T. C. 

 Chamberlin estimates their number in that district and the adjoining 

 northern peninsula of Michigan to be not less than 5,000.* 



Throughout a large region extending thence northwestward, compris- 

 ing Minnesota, northern and central Iowa, South and North Dakota, and 

 southern Manitoba, Professor N. H. Winchell's and my own exploration 

 and mapping of the drift and its marginal moraines have failed to dis- 

 cover any of the peculiarly moulded masses of till classed as drumlins. 



Beyond this region drumlins have been reported only by Mr J. B. 

 Tyrrell in lake Winnipegosis, where they form groups of lenticular and 

 elongated low islands,! and similarly in Cree lake and the country south- 

 east of lake Athabasca, from 400 to 500 miles still farther northwest. J 



Accumulation of Drumlins from englacial Drift. 



Whenever the warm climate terminating the Glacial period extended 

 unchecked through many years, the depth of the ablation or superficial 

 melting of the outer part of the ice-sheet was probably not less than 15 

 to '25 feet each summer, as has been observed on the Muir glacier, in 

 Alaska, and on the Mer de Glace, in Switzerland. At such rates of melt- 

 ing any district enveloped by ice 2,000 to 4,000 feet thick, as was true of 

 the central portions of New England and doubtless also of a broad belt 

 thence west to the Laurentian lakes and to Minnesota and southern Man- 

 itoba, would be uncovered in one or two centuries, and the recession of 

 the Glacial boundary would average probably a half mile or more yearly. 



During any long series of years when the ice-sheet was being thus rap- 

 idly melted, its outer portion to a distance of probably five or ten miles 

 from its boundary, being reduced by ablation to a thickness ranging from 

 100 or 200 feet near the edge upward to 1,000 feet or more, would bear 

 on its surface, especially in the valleys and hydrographic basins of its 

 melting, much drift which had been before contained in the higher part 

 of the ice.§ Only scanty englacial drift, mainly consisting of boulders 

 borne away from hills and mountains, appears to have existed at alti- 

 tudes exceeding 1,000 or 1,500 feet; but all the lower ice probably con- 

 tained an increasing proportion of detritus and boulders which had been 



* Proc. Am. Assoc. Adv. Sci., vol. xxxv, for 1886, p. 20t. 



fGeol. and Nat. Hist. Survey of Canada, Ann. Rep., new series, vol. iv, for 1888-89, p. 22A. Bull. 

 Geol Soc. Am., vol. 1, 1890, p. 402. 



JGeol. Survey of Canada, Ann. Rep., new series, vol. vi, for 1892-93, p. 15A (1892). Am. Geologist, 

 vol. xi, pp. 132, 175, Feb. and March, 1893. 



gBull. Geol. Soc. Am., vol. iii, 1892, pp. 134-148; vol, v, 1894, pp. 71-86, Am. Geologist, vol. viii, 

 pp. 376-385, Dec, 1891 ; vol. x, pp. 339-362, Dec, 1892; vol. xii, pp. 36-43, July, 1893. 



