mode of deposition of the till 419 



Deposition of the oveelying Till 



In the sections examined on the south and north sides of the excavation 

 near Birds Hill Station, descrihed in the foregoing pages, the extensive 

 but partially interrupted sheet of till above the main esker gravel deposit 

 has only the usual proportion of boulders ordinarily belonging to the till 

 of this region, nor are they of larger size and less worn and striated than 

 is their usual condition elsewhere in the surrounding general drift sheet. 

 At least two or three hundred large boulders, however, from 3 to 10 feet 

 in length, have been found during the progress of this excavation, in and 

 upon the overlying till. 



About the west end of the esker such large and unworn boulders are 

 more abundant, as is characteristic of marginal moraines, strewing the 

 surface plentifully where the west foot slope of the ridge is crossed by the 

 ?oad leading north from Birds Hill village, and also on the southwestern 

 slope where the road or trail descends from the crest to this village. They 

 similarly abound again a mile farther east, on the lower part of the 

 northeastern slope, within an eighth of a mile eastward from the deep 

 Winnipeg gravel pit, which is the end of the excavation. 



But along the crest of the esker, close above and south of the section 

 noted at the south side of the excavation, no boulders are seen, nor on the 

 greater part of its southward slope, the whole depth of the esker there 

 being probably gravel and sand. Nor was any boulder observed imbedded 

 in the sand and gravel beds in any part of the very extensive sections 

 shown on both sides of the excavation, either at my visit in 1887 or last 

 summer. 



The best explanation, as I believe, for the mode of deposition of the 

 till above the esker gravel is to refer it to a moderate re-advance of the 

 front or wall of the ice-sheet from the northern side of the esker chan- 

 nel, carrying some of its englacial and finally superglacial drift over the 

 northern fiank and west end of the esker ridge after that part of the. 

 gravel and sand deposit had been laid down in a somewhat wide channel 

 open above to the sky. While the till was thus being spread over the 

 north slope and west end, the continued deposition of stratified drift by 

 the esker stream may have been adding to the thickness of its axial por- 

 tion, raising its crestline above the till-covered northern slope. 



In the halting and partly readvancing and wavering stage of the ice- 

 border when any marginal moraine was accumulated, many of its kames 

 were probably more or less covered with till and boulders ; but the occur- 

 rence of a till envelope or mantle upon an esker is very rare. The 

 ob'^ervations here recorded for Birds Hill are perhaps the most remarkable 



i 



