546 



PRE-WISCOXSIN GLACIAL DRIFT IX MONTANA 





»<« 







siderable interval elapsed between the exposure of the mountain drift on 

 the recession of the fronts of the mountain glaciers and the deposition 

 of the overlapping drift by the continental ice-sheet, and he also stated 

 that the lack of modification by erosion of the surface of the moraines 

 corresponded as evidence of relative recency of deposition with that of 

 the moraines of the late Wisconsin drift of the Mississippi Valley. Cal- 

 houn and we also were therefore led to the conclusion that drift of the 

 mountain glaciers deposited in the valleys near and south of the Interna- 

 tional Boundary should be referred to the Wisconsin stage of glaciation, 



and that the same is true of the overlap- 

 ping drift of the continental ice-sheet. 

 AYe think this deduction applies also at 

 least as far north as the valley of Oldman 

 Creek, where the mountain drift is not 

 confined to the valleys, but spreads over 

 the intervening undulating areas, where 

 erosion has left no remnants of the Black- 

 foot peneplain. This overlap was observed 

 by us on South Fork of Oldman Eiver 

 west of Pincher, Alberta (township 7 

 north, range 21 west), and on Drywood 

 Fork of AVaterton Eiver (township 4 north, 

 range 30 west), and it was also observed 

 by Calhoun and by us both on Belly Eiver 

 (township 2 north, range 28 west) west 

 of Mountainview, Alberta, and on Saint 

 Mary Eiver near the International Boun- 

 dary and some miles to the northward. 



In the last two situations the two drifts 

 are exposed near the levels of the present 

 streams and at elevations of more than 

 1.000 feet lower than the nearest pre-TVis- 

 consin drift-capped remnants of the Blackfoot peneplain but a few miles 

 awav. From this relation it is clear that the deposition of the imbricated 

 drift in the valleys post-dated that of the drift on the high-level tracts 

 by a long interval, during which the valleys were deepened hundreds of 

 feet. It is to be expected on a priori ground that when climatic condi- 

 tions were such as to cause the development of a vast mer de glace, ex- 

 tending from the Keewatin plateau to the Boeky Mountain front, con- 

 siderable extensions of the mountain glaciers would also take place. 

 Having then good evidence in the relations of the deposits themselves that 



Figure 6. — Section on Saint Mary 

 River North of Sloans Ranch 



A. shale ; B. northeastern drift. 

 2 feet, containing many decom- 

 posed boulders : C. typical loess. 5 

 feet, containing concretions, hard 

 shells, and bones of small ani- 

 mals : D. soil. 10 feet, of black 

 clayey loam, consisting of the 

 weathered surface of the lower 

 loess bed ; E. loess. 6 feet in 

 thickness, sandy at the top : F. 

 northeastern till, containing crys- 

 talline boulders. 



