78 W. UPHAM — DERIVATION OF KAMES, ESKERS, AND MORAINES. 



feet in thickness. Imbedded in the gravel of the ballast excavation I 

 noted a mass of ordinary till, unstratified bowlder-clay inclosing gravel 

 and bowlders in a solid matrix of somewhat sandy clay, wholly bounded 

 by definite but irregular outlines, its dimension vertically being about 

 10 feet and its length 20 feet. No other mass of till, either of small or 

 large size, was observed in this entire section, which had an extent of 

 three-fourths of a mile and a depth varying from 10 to 30 feet. This till 

 mass probably was derived from the drift that finally overspread the 

 surface of the ice-sheet when the greater part of its thickness was melted, 

 for it seems to have fallen into the channel of the glacial river from the 

 brink of one of its enclosing high ice-walls. 



Retreatal Moraines adjoining Lake Agassiz. 



Conspicuous moraines record four halts or stages of slight re-advance 

 of the ice-border, interrupting its general retreat upon the country at 

 each side of the glacial lake Agassiz, which was held by this receding 

 barrier in the basin of the Red river of the North and of lake Winnipeg. 

 These are the Fergus Falls, Leaf Hills, Itasca, and Mesabi moraines, being 

 the most northern four of the series already mapped in Minnesota and 

 the region extending thence west and northwest ; but they all belong to 

 the southern third or half of the entire extent of lake Agassiz. When 

 the forest-covered and almost uninhabited country northward shall be 

 fully explored, probably the number of moraines ascertained to have 

 been formed contemporaneously with the existence of this glacial lake 

 will be at least doubled. Each of the four moraines noted comprises 

 portions which rise in hills 150 to 200.feet high ; and the Leaf hills, the 

 most prominent morainic accumulations ever seen by me, are scattered 

 upon a width of three to five miles along an extent of fifty miles, and 

 lift their crests 100 to 350 feet above the intervening hollows and the 

 general drift expanse. 



Inquiring how these high hills of marginal drift were amassed, we 

 must bring ous explanations into accord with a very surprising rapidity 

 of accumulation and brevity of time during which the ice-front was 

 nearly stationary for the formation of each moraine. Comparison of 

 the shore erosion and resulting beach gravel and sand deposits of lake 

 Agassiz with those of lake Michigan shows that the duration of the 

 glacial lake was no longer than a tenth of the postglacial epoch. The 

 entire term of existence of lake Agassiz, therefore, was about a thousand 

 years.* Such a geologically short time witnessed the recession of the 

 ice-sheet from the lake area, nearly 700 miles long from south to north, 



*Geol. and Nat. Hist. Survey of Canada, An, Rep., new series, vol. iv, for 1888-89, pp. 50, 51 E. 



