144 PROCEEDINGS OF WASHINGTON MEETING. 



on an area ten miles long and one and a half miles wide being equal to that <>f 

 Monadnock mountain in New Hampshire.* In the deep north to south valleys of 

 southern New York the morainic deposits; according to ( lhamherlin, have probably 

 sometimes a depth of 500 or 600 feet.t The Leaf hills, which are the most conspicu- 

 ous moraine of Minnesota, rise LOO to 350 feet above the surrounding drift-covered 

 country. In Manitoba the moraine that forms the western part of the Tiger hills and 

 the Brandon and Arrow hills is piled up 100 to 250 feet at its highest points; and 

 equally prominent morainic hills, according to Mr. J. B. Tyrrell, lie on the top of 

 Duck mountain, rising to altitudes of 2,500 to 2,700 feet above the sea. j 



All these great moraines, and the less conspicuous portions of the same belts con- 

 sisting of small hills or having only a moderately rolling contour not more than 20 

 to 50 feet ahove the country on either side, were accumulated from englacial drift. 

 Let us consider, therefore, the probable rate of motion of the ice and the amount 

 of its englacial drift, to ohtain therefrom some estimate of the length of time 

 occupied in the formation of the moraines. The How of the glaciers of the Alps, 

 as is well known, varies from one to two or three feet per day: hut the daily 

 advance of the central parts of the thick and wide glaciers of Greenland and 

 Alaska, where they enter the sea. is found to be from 30 to 100 feet. Doubtless the 

 continental ice-sheet moved faster than the Swiss glaciers ; hut the waste from its 

 border by melting must evidently have been far less than the discharge of ice from 

 arctic glaciers that terminate in the sea and arc broken into bergs and floated 

 away. If the average amount of englacial drift supplied by the ice-sheet where its 

 moraines are largest he assumed equal to a thickness of twenty feet or even ten or 

 five feet, thus supposing half or a much larger part of the whole volume of ice-held 

 drift to he very near the ground where its onward movement was retarded by 

 friction and it was prevented from contributing very rapidly to the marginal mo- 

 raine, and if we assume a rate of motion in the higher part of the ice somewhat 

 greater than that of Alpine glaciers, a short computation will show that a few 

 decades of years, or at the longest no more than a century, would suffice for the 

 accumulation of even the largest of our terminal moraine-. 



Forms in which the Englacial Drift was deposited. 



Forms of Drift. — Four classes of drift may he discriminated, differing in their 

 place and manner of deposition, namely: (] i Subglacial till, which was accumu- 

 lated beneath the ice-sheet ; (2) Marginal till, constituting generally the principal 

 mas- of the terminal moraines ; (3) Englacial till, which, during the departure of 

 the ice. became superglacial and finally was dropped on the land when the glacial 

 melting was completed: and (4) Modified drift, comprising the glacial sediments 

 that were derived directly from the ice-sheet, hut were assorted, transported, and 

 deposited by water. The last-named class occurs in many diverse forms. Some 

 of its beds were subglacial and others marginal, a- to their place of deposition ; hut 

 far the greater part of the modified drift was englacial at the time of the final 

 melting, and was then washed away from the ice surface by the streams of its abla- 

 tion and by rain-. 



Our enumeration of the various forms in .which the englacial drift was deposited 

 during the Champlain epoch, that is, the time when the ice-sheet was melted away, 



*U. - Geol Survey, Seventh annual report, for l885-'86, p. ■■\± 

 tU. S. Geol. Survey, Third annual report, for :881-'82, pp. 351-358. 

 } Am. I ieologist, vol. viii. p. 22, July, 1891, 



