62 G. M. DAWSON — GLACIAL DEPOSITS OF SOUTHWESTERN ALBERTA. 



local differences to be worked out in detail, but others already recorded 

 have a similar meaning. 



When the liighest terraces and shingle beds were formed upon the 

 Porcupines there is further evidence to show that in the body of water of 

 which these formed the shores a prett}'' definite current must have existed. 

 Some distance to the eastward, this probabh^ flowed southward or south- 

 westward, but where it reached the Rocky Springs plateau the appear- 

 ances indicate that it was moving nearly parallel to the border of the 

 glaciated region in Montana,''^ west or to the north of west ; thence it im- 

 pinged upon tlie l)ase of tlie Rocky mountains and was deflected to a 

 northeasterly direction, a circumstance shown by the occurrence, else- 

 where referred to, of pebbles of the locally developed greenstone of the 

 mountains in some abundance on the higher parts of the Porcupine hills. 

 Such a current may reasonabl}^ be accounted for by the prevailing direc- 

 tion of the winds at the time and season of the driftage of the ice. 



In the case of these high-level drifts of the Porcupines the deposit of 

 eastern and western material must have been contemporaneous. Both 

 find their upper level at the same plane, and there are no antecedent 

 deposits at such a height from which either can have been derived. At 

 this time, moreover, some deposit must have been in course of formation 

 beneath the surrounding deeper waters across which the debris-bearing 

 ice floated, and, because of the melting of the ice and other accidents, 

 this could not have been otherwise than a nota])ly stonj^ one. As already 

 stated, this is believed to be represented by the upper boulder-clay, the 

 silts overlying it, or in part by both. 



The terracing of the Porcupines is not so pronounced as to require the 

 long presence of the water-margin at any of the higher levels, ])ut the 

 well rounded character of most of the stones, ])articularly those from the 

 mountains, is such as to imply prolonged attrition. The same character 

 is notable in the vast majority of the stones included in the boulder- 

 clays. It seems, in fact, probable that during the winter months at this 

 period a massive ice-foot formed along the abrupt base of the mountains, 

 upon which, in the spring, gravels from flooded streams were often dis- 

 charged, while large angular limestone blocks from clifl'-falls also lodged 

 upon it in some localities. When in summer this ice broke away it 

 would carry with it the load thus acquired. 



That the glaciers which at the period of the Saskatchewan gravels pro- 

 truded from the mountains nmst at this time have shrunk back within 

 the range, in the southern part of the district at least, is shown by the 

 stranding of Laurentian boulders upon the old moraines of these glaciers 

 close up to the foot of the mountains. It is possible that the Bow Valley 



* Report of Progress, Geol. Survey of Canada, 1882-84, p. 148 C. 



