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



that the highest terraces occur at elevations of from 5,500 to 5,300 feet ; 

 that below this there is a remarkable paucity of terraces down to about 

 4,450 feet, between which and a height of 4,300 feet another well marked 

 group of old water-lines appears. These facts are fully described in my 

 forthcoming report on the Kamloops map-sheet. The circumstance 

 may not be more than a coincidence, but it is certainly a striking one 

 and one worthy of further investigation. 



As it has already been stated that no certain evidence has been found 

 such as to show that the lower boulder-clay may not be that extending 

 farthest west and in toward the base of the mountains, it may be ap- 

 propriate now to mention the hypotheses which present themselves on 

 that assumption. If the lower boulder-clay holds this position and was 

 deposited contemporaneously with the high-level erratics and gravels, 

 the upper boulder-clay may very well have been laid down in the body 

 of water standing later at the inferior levels of from 4,500 to 4,200 feet 

 and indicated by the well marked terraces and gravel plains already 

 alluded to. This hypothesis, of course, assumes that a boulder-clay 

 may be deposited from floating ice, and to me it a])pears probable that a 

 material of this nature may have been formed in au}^ one of three ways, 

 namely, beneath a glacier, about the edge of a glacier as a fluvio-glacial 

 deposit, or below a body of water charged with floating ice. 



According to still another possible hypothesis, it may be supposed that 

 while the lower boulder-clay is that stretching farthest west and spread- 

 ing around the base of the Porcupine hills, the high terraces may be due 

 to a subsc(j[uent flooding about the time of the ui)per boulder-clay. This, 

 however, does not appear to accord well with the facts, for in this case 

 there is no recognizable deposit in the lower parts of the flooded district 

 near the Porcupine hills to represent this period of submergence. 



Respecting the actual western limit of eastern erratics, the investiga- 

 tion here reported upon seems to show that the line marked upon the 

 map accompanying the report of 1882-'84 nearly corresponds with ob- 

 served drift of this origin in tlie l)()ulder-clays proper, slightly exceeding 

 this to the south of the Porcupines and falling a little short of it to the 

 north, but that scattered erratics occur in places considerably forther to 

 the west. These are found upon the higher ridges and hills, and if 

 present equally in the valleys have there been concealed by a later wash 

 from the mountains. Behind the Porcupines, the occurrence of such 

 erratics is in inverse proportion to the amount of shelter atibrded on the 

 east by the higher parts of these hills — a fact equally explicable under 

 an}'' hypothesis of their deposition; but the occurrence of such sporadic 

 erratics renders it difficult to draw any precise western line, and it is 

 possible that renewed investigation of the higher foothills may in some 



