26 W. UPHAM — DRUMLINS AND MARGINAL MORAINES 



the ice-sheet. On the land bed, where it was impeded by friction, the 

 rate was very small, thence gradually increasing upward. In the second 

 mile the ice would retain its height unchanged under this ablation by an 

 average onflow of 4.3 feet daily for the stratum of ice 200 feet thick termi- 

 nating in that mile ; the third mile would require for its stratum of 150 

 feet a daily current of 5.8 feet, and the fourth and fifth miles would re- 

 quire currents, respectively, of 7.3 and 8.8 feet. Between nine and ten 

 miles from the ice front, at an altitude of 1,257 to 1,312 feet, the ablation 

 could be offset only by a current of 16 feet daily. By such currents, 

 urged forward by the great weight of the more central and increasingly 

 thicker part of the ice-slieet, the superficial wasting of the ice border 

 would be evenly balanced, holding, therefore, the nearly steady frontal 

 line indispensable for abundant marginal drift deposition. The gradients 

 thus assumed for the ice surface near its boundary are probably twice as 

 steep as they were during the earlier stages of predominant ice accumu- 

 lation. Hence, wdth the greatly increased Champlain temperature, the 

 rates of glacial movement were perha])s five or even ten times faster than 

 during the maximum stage of glaciation. 



If the outermost five miles of the ice, having the conditions here as- 

 sumed, remained in essentially unchanged position thirty years, the total 

 volume of drift there becoming suj^erglacial would be equivalent to about 

 50 feet on a width of one mile. With the previously superglacial drift of 

 the same outer belt of the ice, which, like the foregoing, must have been 

 carried forward to the boundary, there would be a thickness of about 85 

 feet ; and with all received in the same tinie from the more distant part 

 of the ice surface, up to ten miles from the margin, the total terminal 

 mass of drift would equal at least an average of 100 feet on a belt one 

 mile wide. This amount, amassed by the small frontal oscillations of 

 the ice so as to form irregularly grouped hills and ridges, separated, as 

 those of the moraines usually are, by deep and wide hollows, would con- 

 stitute a morainic belt probably unsurpassed either in North America or 

 Europe. Under the same conditions, a small but distinct moraine might 

 be formed in only five or ten years; or, where the ice-sheet had less en- 

 glacial drift, as a quarter or only a tenth as much, the smaller parts of a 

 moraine belt would be made during the same thirty 3"ears in which else- 

 wdiere its most prominent portions were being deposited. 



Drumlins and Moraines both referable chiefly to the Champlain 



Epoch. 



From this discussion of the origin of drumlins and marginal moraines 

 it will be seen that their accumulation belonged chiefly to the Champlain 

 epoch of land depression, restored warmth and mainly rapid glacial re- 

 treat, interrupted by times when the ice-sheet for several years or decades 



