INTERMEDIATE STAGE OF CORDILLERAN GLACIATION 569 



B. If the drift on the second plains be considered as partly -unmodified 

 till and partly the coarse residuum from the leaching of till, and if the 

 drift on the isolated high tracts also be regarded as of the same kind, then 

 it may be concluded that the first advance of the mountain ice occurred 

 subsequent to the development of the second set of plains; that the ice 

 deployed over these and extended thence up onto the higher tracts as far 

 as 20 miles from the mountain front. Under these conditions we have 

 evidence of but one pre-Wisconsin stage of glaciation. 



More detailed work will be required to determine whether there was or 

 was not a second pre-Wisconsin stage of glaciation. 



Summary 



1. With the exception of one not yet examined, all the high ridges ex- 

 tending up to the mountain front are capped with pre-Wisconsin glacial 

 till. 



2. The Blackfoot peneplain is represented by the top of Belly Eiver 

 Ridge and that of the ridge east of Waterton lakes. These are the only 

 remnants of the high-level plains seen in southern Alberta, unless the top 

 of Dawson's Milk River Ridge is such. All others have suffered degrada- 



-tion. 



3. There are finely preserved remnants of these high plains in the 

 Browning and Blackfoot quadrangle south of the International Boundary. 



4. The difference in topographic development between the areas north 

 and south of the boundary is believed to . be due to the ascendancy of 

 Hudson Bay drainage over that of Missouri River. 



5. The highest of the high-plain remnants represents the Blackfoot 

 peneplain. Second and third lower sets represent later cycles of erosion. 



6. The third set of plains was overrun by mountain glaciers of the 

 Wisconsin stage. 



7. G-laciated pebbles were found in the "quartzite gravels" on all the 

 remnants of the highest plain examined within 20 miles of the mountain 

 front. 



8. There is ground for the opinion that at the time of the earliest ex- 

 tension of the mountain glaciers that part of the Blackfoot peneplain in 

 the Browning quadrangle had been but little dissected, if at all, although 

 farther north degradation had reached an advanced stage. 



9. Gravelly material on Horsethief Ridge and Landslide Butte, 35 to 

 40 miles from the mountain front, is probably part of the same glacial or 

 glacio-fluvial deposit, although no striated pebbles were round. 



10. Comparison shows that when Saint Mary Valley was about 1.000 

 feet less deep than at present the conditions for extension of the coni- 



XXXIX— Bull. Gbol. Soc. Am., Vol. 24, 1012 



