RETREATAL MORAINES ADJOINING LAKE AGASSIZ. 79 



Avith the formation of all its adjacent moraines, for any one of which 

 there would be, in proportion to its size, some ten, twenty, or fifty years. 

 Even the Leaf hills probably required for their accumulation no more 

 than half a century. To bring the morainic drift so rapidly and abun- 

 dantly to the margin of the ice-sheet, subgiacial dragging and pushing 

 seem to me quite inadequate. If this drift, however, was chiefly en- 

 glacial and at last superglacial, becoming several or many feet thick on 

 the outer part of the melting ice surface, a pause in the recession of the 

 glacial boundary, Avith a continual forward movement of the ice to its 

 moraine at a rate probably much faster than the motion of the glaciers 

 in the Alps, but slower than of arctic glaciers terminating in the sea, 

 would amass a common moraine in five or ten years, and a few decades 

 of years would suffice to form the Leaf hills. This view looks on the 

 morainic drift as brought down from the adjoining higher part of the 

 ice-sheet, where its currents were strong and rapid because the slope 

 was steep ; but if the drift was transported subglacially it must have 

 moved very slowly on account of retardation of the basal ice currents 

 by friction on the land and by the obstruction of the growing moraine, 

 upon which, in the highest hills, the vertical amount of upward pushing 

 would be 100 to 350 feet. 



Where the courses of continuation of these moraines traverse the area 

 of lake i\gassiz, no hills nor even hillocks are found, but the surface has 

 unusually plentiful bowlders. The drift corresponding to the moraines 

 is Si)read so evenly that one scarcely observes any addition above the 

 general sheet of till forming adjacent parts of the lake bed ; except that 

 one of these belts of smoothly levelled morainic till, a few miles wide, 

 stretches continuously across the Red River valley, rising very slightly 

 above its broad alluvial tract which otherwise is uninterrupted along a 

 distance of 250 miles. If the morainic drift was supplied from the sur- 

 face of the melting frontal slope of the ice-sheet, it would be washed 

 away and levelled in the lake by the great waves of storms ; but if it had 

 been subgiacial and thence was pushed out and first uncovered from 

 the retreating ice under the water, much of it would have escaped the 

 levelling action of the lake. 



The foregoing Examples regarded as Types of the general Manner 

 OF Transportation and Deposition of the Materials of Kames, 

 EsKERS, and Moraines. 



During my studies of kames, eskers, and moraines in many districts, 

 exploring new fields each year since 1874, the explanation of their origin 

 chiefly from englacial and finally superglacial drift has seemed to me 

 generally applicable everywhere. Because the validity of this explana- 



XI-BuLi-. Geot,. Soc. Am., Voi,. 5, 1893. 



