568 PRE-WISCOXSIN GLACIAL DRIFT IN MONTANA 



Xunierous striated pebbles were also found on the ridge north of Goberts 

 Banch, between the two stage roads, in sections 25, 35, and 36, township 

 36 north, range 13 west, and in west y 2 , section 30, township 36 north, 

 range 12 west (plate 14, number 6). 



Benmants of this second plain in the northwestern part of the Black- 

 foot quadrangle, in township 3T north and ranges 9 and 10 west, carry a 

 similar gravelly material, evidently a continuation of the same deposits. 

 These, however, were not examined with sufficient care to determine 

 whether or not they contain striated pebbles. 



The flat top of that part of Horsethief Bidge in township 35 north, 

 range 8 west, is probably an isolated remnant of this second set of plains. 

 This ridge, which is about 35 miles east of the mountain front, is capped 

 with a gravelly deposit of the same character and composition as the 

 ridges to the west. The pebbles here are almost wholly of quartzite, with 

 but very little greenish argillite. They are mostly subangular, with more 

 or less flat faces and with edges smoothly worn; but only a small part of 

 them, even the smaller ones, are well rounded. Xo striated pebbles were 

 found where the deposit was examined in the slight exposures at the 

 crest of the west slope. 



These observations show that the deposits on the second set of plains 

 are quite as characteristically glacial, or glaoio-fluval, drift as are those 

 on the remnants of the highest set of plains at points equally distant from 

 the mountains. The relations are not such, however, as to show clearly 

 that "these deposits represent a second and distinct stage of glaciation. 

 As has been shown on pages 539 and 540, there are several possibilities : 



A. The drift on the highest set of plains may have been deposited by a 

 glacial extension prior to the development of the second set of plains. If 

 the drift on the isolated higher tracts and in township 35 north, range 11 

 west, and in section 12, township 35 north, range 12 west, is a glacio- 

 fluvial deposit, then its deposition and the first glaciation must have pre- 

 ceded the dissection of the peneplain. Depending on this interpretation 

 are two hypotheses concerning the drift on the second set of plains : 



(1) If this latter drift is in part at least unmodified glacial till, as we 

 are inclined to think from the exposures seen on that part of Milk Biver 

 Bidge south of Horse Lake, then we may regard it as evidence that at a 

 second stage of glaciation the ice extended out onto the second set of 

 plains. 



(2) If this latter deposit be regarded as consisting wholly of modified 

 drift, it may have been let down by erosional degradation of the deposits 

 on the higher plain. In this case it is not evidence of a second stage of 

 glaciation. 



