ENGLACIAL DEBRIS. 275 



At various points on the hills that border the valley I found heaps of till 

 of various shapes, but they have the forms of accumulations of englacial 

 till. The only deposit having distinctly the form of a lateral moraine that 

 I found in the valley is situated on the north side of the river and about a 

 mile east of the Lead Mine bridge in Shelburne. This is one-third of a 

 mile or less in length, and its origin is somewhat uncertain. The hills on 

 each side rise often steeply to a lieight of 500 to 2,500 feet, and surface 

 moraines were as likely to form, in this valley as in any in New England 

 except a few in the heai't of the White Mouiitains. 



At all the other terminal moraines found in Maine the absence of 

 lateral moraines emphasizes the conclusion that there was but little morainal 

 matter borne on the surface of the ice that was derived, like the moraines 

 of glaciers of the Alpine type, from avalanches and debris sliding from 

 above onto the ice. 



Agassiz long ago reached the conclusion that the ice-sheet covered all 

 the land, and hence the only way for morainal debris to get into the ice 

 was from below. The above-stated facts prove that late in the ice epoch, 

 after the time when the higher hills began to rise above the ice, not much 

 deljris fell from above onto the ice, even in valleys bordered by steep hills. 

 Up to the very last the drift was almost wholly basal. 



QUANTITY OF ENGLACIAL DEBRIS. 



The depth of the upper or englacial till does not necessarily give an 

 estimate of tlie quantity of morainal matter contained in tlie ice at one 

 time. If we conceive the ice-sheet suddenly divested of all motion, the 

 scattered mass or sheet of englacial till left when the ice melted will repre- 

 sent the amount of, debris in the ice at that time. But if the ice is in 

 motion the case will be far different. Whenever the forward flow of the 

 ice equals the terminal melting the ice front is stationary and a terminal 

 moraine gathers as a frontal ridge. As the ice advances, the dcebris con- 

 tained in a zone of ice perhaps a half mile or more in breadth is brought 

 forward and dropi^ed on the narrow moraine. In other words, the thickness 

 of the moraine may represent the englacial matter not only of an area of 

 ice equal to that of the moraine but many times this area. If now the 

 melting comes to exceed the rate of advance, a series of j^arallel moraines 



