554 pre-wisconsin glacial drift in montana 



Extent of the upper and lower Drift of the Keewatin Ice-sheet 



Regarding the relative extent of the upper and the lower till exposed 

 at Lethbridge, Doctor Dawson makes this statement i 11 



"As already mentioned, it is not certainly known how far the lower and upper 

 boulder-clays of the plains or either of them extend to the west. Both are 

 found at Lethbridge, 60 miles from the mountains, and if the line observed in 

 sections on Highwood River corresponds with this division, both are there 

 present to within about 15 miles of the base of the mountains and at an actual 

 elevation of 3,700 feet. One or the other of these boulder-clays, however, ex- 

 tends westward along the Oldman River beyond the longitude of the Porcupine 

 hills and at least as far west as Calgary, on Bow River, and there is some 

 reason to believe that it is the upper boulder-clay which is thus most widely 

 spread." 



The uppermost glacial drift observed by the writers on Dry wood Fork, 

 in township 4 north, range 30 west, about 15 miles south of Pincher, 

 Alberta, and but a few miles from the mountain front, is till containing 

 pebbles of Laurentian granite and hence considered a deposit of the 

 Keewatin ice-sheet. In this vicinity the surface is marked by sags and 

 swells and many lakelets and marshy tracts, such as are characteristic 

 only of morainal belts of the Wisconsin drift in the Missouri and Mis- 

 sissippi valleys. Similar topography characterizes the marginal belt of 

 continental drift, thence southeastward to the International Boundary 

 in the vicinity of Saint Mary River, and also in the drainage basins of 

 Milk and Marias rivers. 



In addition to the youthfulness of this morainal topography, 'the oc- 

 currence of the morainal drift within short distances of, but at levels 

 hundreds of feet lower than, the pre- Wisconsin mountain drift on the 

 remnants of the high-level plains near Belly and Saint Mary rivers shows 

 unmistakably that the Keewatin ice-sheet which extended to the limit of 

 continental glaciation in the region was that of a late stage, evidently the 

 Wisconsin stage. 



Without much more thorough examination than we have had the op- 

 portunity of making, we would not care to hazard an opinion as to the 

 maximum extent of the earlier drift of the Keewatin ice in southern 

 Alberta. The differences between the lower and the upper drift of the 

 Keewatin Glacier where seen by us are not so marked as to enable us to 

 be certain of the identification where only one was exposed. The lower 

 part of much of the till seen in the bluffs along Belly River is very similar 

 to the lower till at Lethbridge, even approximating its semi-indurated 



11 Op. clt, p. 60. 



