92 W. UPHAM — THE SUCCESSION OF PLEISTOCENE FORMATIONS. 



before the Ice age, but in part was contemporaneous with the earliest 

 stages of the Pleistocene glaciation : 



The bowlder-clay throughout the greater part of the valley is overlain by heavy 

 deposits of stratified sands, clays and gravels, probably lacustral in their origin, 

 and is underlain by a gravel terrane somewhat similar to and occupying the same 

 relative position as the Saskatchewan gravels of the plains of Alberta and Assini- 

 boia, but differing in containing a much greater proportion of Laurentian pebbles. 

 These beds have a thickness in some cases of fully 150 feet, and contain well- 

 rounded pebbles, ranging in size up to eight or ten inches in diameter. They are 

 intimately connected with the bowlder-clay, and in one place were observed to 

 alternate with it, and they seem to show that the bowlder-clay period in this region 

 was preceded as well as followed by what may be termed a lacustrine epoch, during 

 which the surface depressions were filled with extensive lakes. "^ 



The manner of deposition of the preglacial beds described here and of 

 those in the Saskatchewan region and in the Mississippi valley seems to 

 me most probably to have been in each case by river floods, rather than 

 in the still waters of lakes or of the sea. In the Saskatchewan region 

 they lie 2,000 to 2,500 feet or more above the ocean, and in none of the 

 districts noted have any shorelines, marine fossils, or other proofs of 

 the presence of the sea been discovered. 



Glacial Drift and marginal Moraines. 



The till or bowlder-clay extends either continuously or in patches to 

 the extreme limits reached by the ice-sheet in its maximum advance ; 

 but generally on the outer portion of the drift-bearing area glacial plana- 

 tion and striation of the bed-rocks is absent or rare. Upon the greater 

 part of the area of the earlier drift, outside the prominent marginal 

 moraines, a thin remnant of the preglacial residuary clay. and decaying 

 rock is left beneath the till, whose material was derived mostly not from 

 the area over which it is spread, but from districts farther north. A 

 broad peripheral belt of the ice-covered country was thus characterized 

 by the deposition of drift, which was supplied from inner portions of 

 this area where the predominant action of the ice-sheet was erosion. 



At least twice the continental glacier advanced upon the region of 

 early drift in the Mississippi basin, depositing its till first on the mostly 

 or partly uneroded preglacial surface, and in its second advance similarly 

 sparing large tracts of a forest bed, whose trees and peat swamps had 

 grown during the time between these ice incursions. Russell's observa- 

 tions of the Malaspina glacier show that large trees may flourish close 



*Geol. and Nat. Hist. Survey of Canada, An. Rep., new series, vol. iv, for 1888-'89, p. 2G D, 



