136 THE GLACIAL LAKE AGASSIZ. 



The chief characters of the euglacial upper portion of the till, as 

 compared with the subglacial lower poilion, are its looser texture, its 

 more plentiful and larger bowlders, the prevailingly angular and suban- 

 gular shapes of its bowlders and smaller rock fragments, whereas they are 

 mostly worn smooth by glaciation in the lower till, and the usually more 

 gravelly and sandy and less clayey composition of the englacial till, owing 

 to the washing away of much of its finer material by superglacial di'ainage. 

 To these originally inherent characters we must add the very noticeable 

 postglacial change of color of the upper till already mentioned. This 

 change has generally extended through the englacial till, stopping at the 

 more impervious subglacial deposit. Between the two there is also fre- 

 quently a layer of subglacial stratified, gravel and sand, from a few inches 

 to several feet thick. The extremes of thickness of the englacial till 

 appear to range from almost nothing or only a few feet for minima to 

 40 feet or more for its maxima near massive terminal moraines and where 

 great currents of the ice-sheet converged.^ 



Rock fragments and other drift inclosed in the ice at a considerable 

 height above the ground were borne forward without attrition. The 

 higher part of the englacial drift is thought by the present wi'iter to have 

 supplied most of the material forming the terminal moraines, which, there- 

 fore, have a remarkable profusion of bowlders and angular gravel. When 

 the ic.e-sheet Avas finally melted, its inclosed bowlders were droj^ped, and 

 they now lie frequently as conspicuous objects on both the lower and 

 higher parts of the land. Scattered here and there in solitude on an 

 expanse of prairie, or perched on the sides and tops of hills and moun- 

 tains, they at first suggest transportation and stranding by icebergs or 

 floe ice. 



Bowlders and gravel from Arcliean and Paleozoic formations. — Bowlders 

 are frequent or plentiful in the till throughout the area of Lake Agassiz, 

 their abundance being nearly the same as in the least rocky parts of the 

 till of New England, New York, and the country surrounding the Lauren- 

 tian lakes. Their usual range in size extends up to a diameter of 4 or 5 

 feet; but in a few localities, especially in the course of morainic belts, they 



' Biill«tiu, G. S. A., Vol. Ill, pp. 134-148. Am. Geologist, Vol. VIII, pp. 376-385, Dec, 1891. 



