542 PKB-WISCONSLN GLACIAL DRIFT IX MONTANA 



tuting a clay matrix, in which the pebbles are irregularly embedded as in 

 till. Here and there are streaks of sand. 



The pebbles range in size from fine ones to boulders 1 foot in diameter, 

 and in shape from subangular to rounded, and on the whole they are 

 somewhat better rounded, as by water wear, than those on Horsethief 

 Ridge, but are not well assorted as to size nor well bedded, though evi- 

 dently water handled. The material as exposed resembles glacial mo- 

 rainal drift, but careful search by both of us failed to reveal a single 

 pebble which was undoubtedly glacially striated, though faint markings 

 on some of the pebbles suggested striae. 



Comparison with the Extension or Two Medicine Glacier of the 



Wisconsin Stage 



The areal distribution of all the patches of high-level pre-Wisconsin 

 glacial drift, excepting those on either side of Belly River, is on the plains 

 directly opposite the catchment basins of Saint Mary, Cutbank, and Lake 

 Creek glaciers of the Wisconsin stage, and this indicates that the ice 

 which spread eastward onto the Blackfoot peneplain headed in these com- 

 bined basins (plate 13). It appears, however, that either the bulk of the 

 ice at the earlier stage must have been considerably greater than at the 

 Wisconsin stage or that the topographic relations must have been mark- 

 edly different from what they are now. Cutbank and Lake Creek glaciers 

 at the Wisconsin stage were of comparatively small extent and extended 

 directly eastward down unobstructed valleys. During the same stage the 

 ice issuing from Kennedy Creek, Swift Current, Boulder, Saint Mary, 

 and Bed Eagle valleys, which combined to form Saint Mary Glacier, in- 

 stead of extending northeastward onto the plain, was deflected northward 

 down the Saint Mary Valley by the great ridge 1,000 to 1,600 feet high 

 on the east. Farther south, Two Medicine Glacier at the same stage 

 spread about 38 miles northeastward from the mountain front over an 

 undulating plain. 



The total area of the combined catchment basins of Saint Mary, Cut- 

 bank, and Lake Creek glaciers is approximately 310 square miles. The 

 area of the catchment basin of the main part of Two Medicine Glacier 

 (plate 13) is approximately 295 square miles — that is, excepting the 

 basins at the heads of TThitetail and Birch creeks, which probably did not 

 contribute largely to the main part of the Two Medicine piedmont glacier. 



The average elevation of the divide forming the rim of the northern 

 basin above sealevel is approximately 8,075 feet: that of the southern 

 basin 7,525 feet. 



