RAPID ACOUMULATIOJf OF MORAINES. 243 



Alaska, 30 to 100 feet per day.^ Doubtless the continental ice-sheet moved 

 faster than the glaciers of the Alps, but the waste from its border hx melt- 

 ing must evidently have been less than the discharge of ice from these 

 Arctic glaciers where they terminate in tlie sea and are broken into bergs 

 and floated away. 



The two factors on which the accumulation of the terminal moraines 

 depends are the rate of motion of the ice-sheet and the amount of the en- 

 glacial drift which was thus brought forward to its margin. In the region 

 of Lakes Benton, Shaokatan, and Hendricks, in southwestern Minnesota, Ave 

 have- evidence that the drift contained within the ice amounted to a sheet 

 at least 40 feet thick.- As great volume of englacial drift is also indicated 

 in Manitoba by the relation of the Birds Hill esker to the adjoining sheet of 

 till.^ The inequalities in the aggi-egate mass of the di'ift forming diiferent 

 portions of the morainic belts, causing these to rise in great prominence 

 upon some areas, while in other places they are low and scanty, seem due 

 to unequal distribution of this drift within the basal part of the ice-sheet.* 

 It was most abundant in those portions where glacial currents had con- 

 verged between the great lobes of the ice border during the time of maxi- 

 mum area of this ice-sheet, as from the vicinity of Minneapolis and Lake 

 Minnetonka northwestward to the Leaf Hills, to Lake Itasca, and to Bu-ds 

 Hill, and in the countrv west and northwest of Cooperstown, N. Dak., to 

 the Washington Lakes, Devils Lake, the Big Butte, Broken Bone Lake, and 

 to Turtle Mountain. Upon these areas the morainic belts show a close 

 relationship, not only by their parallelism and the similar positions of 



' Helland, iu Quart. Jour. Geol. Soc, Vol. XXXIII, 1877, p. 149. Nature, Dec. 29, 1887. Prestwich's 

 Geology, Vol. II, 1888, pp. 530-533. Prof. G. F. Wright, on the Muir Glacier, Am. .lour. Sci. (3), Vol. 

 XXXIII, pp. 1-18, Jan., 1887, aud Ice Age iu North America, Chapter III. The daily motion of the cen- 

 tral portion of the Muir glacier in 1886 was reported hy Professor Wright, according to a series of 

 measurements, to he from 40 to 70 feet ; but in 1890, when the front of this glacier had fallen back a 

 hnlf mile to two-thirds of a mile from its place four years earlier, more reliable measurements of its 

 motion by H. F. Reid (National Geographic Magazine, Vol. IV, pp. 19-84) aud H. P. Gushing (Am. 

 Geologist, Vol. VIII, pp. 207-230) show a maximum of only about 7 feet per day. In 1886 the ice front 

 projected into the Muir Inlet as a promontory, but iu 1890 it was nearly straight. At each date the 

 length of the ice front was almost 2 miles and its height about 250 feetiabove the water of the inlet, 

 which is 600 feet deep. See a discussion of "Recent changes in the Muir glacier," by S. Prentiss 

 Baldwin, Am. Geologist, Vol. XI, pp. 366-375, June, 1893. 



■Geology of Minnesota, Ninth Annual Report, for 1880, pp. 322-326; Final Report, Vol. 1, 1884. pp. 

 603, 604. 



■'Chapter IV, pp. 183-187. 



^Bulletin, G. S. A., Vol. Ill, 1S92, pp. 134-148 ; id., Vol. V, 1894, pp. 71-86. Am. Geologist, Vol. VIII, 

 pp. 376-385, Dec, 1891; id.. Vol. Xll, pp. 36-43, July, 1893. 



