540 PRE- WISCONSIN GLACIAL DRIFT IN MONTANA 



Kennedy Eidge; on Hudson Bay Divide, about 10 miles east by north 

 from Yellow Mountain ; on Swift Current Eidge and Boulder Eidge ; on 

 Saint Mary Eidge; 4 to 6 miles east of the mountain front, on the 

 highest parts of Milk Biver Eidge, as far as 6 miles out from the moun- 

 tain front, and on the highest parts of Cutbank and Two Medicine ridges ; 

 so that it is evident that there was a pre- Wisconsin extension of the 

 mountain glaciers well out beyond the mountain front. We are not, 

 however, prepared to prove that the ice extended so far as the remnant 

 of the highest plain in township 35 north, range 11 west — that is, 20 

 miles out on the plain — or that this extension occurred prior to the dis- 

 section of the highest plain surface, that of the Blackfoot peneplain. 



It is unfortunate that the exposure of the drift on this isolated mesa 

 (figure 2) is such that we were unable to determine whether it was water- 

 laid, such as a deposit of glacio-nuvial outwash gravels, or whether it is 

 the coarse residuum left in situ by the leaching and washing out of the 

 finer material of the matrix of unmodified glacial till. If it is glacial 

 outwash, it is evident that while the ice did not reach this far out on the 

 plain, the glacial extension occurred prior to the dissection of this part 

 of the highest or Blackfoot peneplain and the development of the next 

 lower plain, which forms most of the top of Milk Eiver Eidge. Gravels 

 could not be washed from the lower plain up onto one which is 50 to 100 

 feet or more higher. 



On the other hand, if this be regarded as the residuum of a deposit of 

 unmodified glacial till, it appears that while the ice must have extended 

 thus far — that is, 20 miles — from the mountain front in order to leave a 

 deposit of till in this position, such a glacier might have spread out on 

 the second plain already developed and have extended thence up over 

 remnants of the older and higher plain. It ma}^ be stated, however, that 

 the topographic relations of the highest drift-capped remnants to the 

 remnants of the second plain are just such as would be expected if it 

 were known that the earliest extension of the mountain glaciers occurred 

 prior to the development of the second plain. The most significant rem- 

 nants stand mesa-like (figure 2) above the levels of tracts of the second 

 plain between them and the mountains. The abrupt marginal slopes 

 facing the mountains have no drift banked against them leading down to 

 the lower level. The only drift on the slopes is a sprinkling of pebbles 

 let down by the recession of the slopes; the marginal slopes toward the 

 mountains are eroded and not coated by drift deposits. Bordering coulees 

 would hardly have been left unfilled with drift or have been excavated in 

 such positions were there drift slopes leading up from the second to the 

 highest plain. 



