SUMMARY AND DISCUSSION. 61 



pines, or a slope of about 4 J feet to the mile. But if it be assumed that 

 this level marks that of the surface of a mer de glace, an extension of the 

 Laurentide glacier (as has been done by Mr Upham), a similar westward 

 depression must likewise be admitted. In so far as such a surface might 

 have departed from horizontality, it must have done so by sloping down 

 toward its termination in the west. Ice standing at a level of 4,400 feet 

 at the Cypress hills could under no conceivable conditions have been 

 pushed up to a height of 5,300 feet at the Porcupines, 200 miles further 

 in the general direction of its flow.* Thus, under this hypothesis, we 

 would require to add the amount of slope of the surface to that neces- 

 sary under the first mentioned assumption.f 



As to the period to which this great western depression may be as- 

 signed, it is pretty clear that it must accord with one or the other of the 

 glacial formations not already accounted for. In other words, it must 

 have been synchronous with the lower or upper boulder-clays or with 

 the silty deposits subordinate to them. I have elsewhere given reasons 

 for the belief that both these boulder-clays of the western plains are 

 attributable to the agency of floating ice, J but this hypothesis need not 

 here be specially insisted on. Important bedded silty deposits are found 

 to blend with the upper part of the upper boulder-clay, and the fact that 

 large erratics are most abundant on the plains at the top of or overlying 

 the upper boulder-clay, with the similarity of these to those found on 

 and about the Porcupine hills and foothills farthest in toward the moun- 

 tains, leads me to suggest that this period of greatest depression corre- 

 sponded with that of the upper boulder-clay or immediately followed it. 



A closer comparison of the highest levels of erratics in different parts 

 of the field shows that the area of greatest depression, and that of greatest 

 subsequent uplift, touches the southern part of the Porcupines and ex- 

 tends thence in an east-southeasterly direction, and that to this direction 

 a series of " isobasic " lines of decreasing amount must have been roughly 

 parallel for some distance to the northeastward. The changes in eleva- 

 tion seem, however, to have been accompanied by deformation of some 

 importance, for the highest level of drift upon West butte is found to be 

 considerably below what it should be had the difference in level been dis- 

 tributed uniformly in proportion to distance between the foothills and 

 the Cypress hills, although all three of the localities are approximately 

 in an east-and-west line. The facts are as yet too few to enable these 



*The maximum depth of iee or water covering the adjacent low counti-y must have been about 

 2,000 feet near the Cypress hills and 2,100 feet near the Porcupines. 



t A similar relative change of level would, of course, be equally implied on the supposition of a 

 great western glacier-dammed lake. 



I On the Physiographical Geology of the Rocky Mountain Region in Canada. Trans. Roy. Soc. 

 Can., vol. viii, sec. 4, p. 63 et seq. 



