DRIFT ON THE BLACKFOOT PENEPLAIN 539 



pebbles at this place are subangular to well rounded and range in size 

 from small pebbles to boulders IV2 f ee t in length. A brief but careful 

 examination near the west end of the north slope, northeast of Davis 

 Ranch, where the turf was broken by small scars due to slumping, re- 

 vealed pebbles of greenish argillite clearly marked with finely graven 

 striae characteristic of glaciation (plate 15, figure 2). 



On the ridge south of Dry Fork and Middle Fork of Milk Eiver is the 

 next remnant of this highest plain. That part west of the stage road, 

 representing this highest level, is very narrow, most of the top of the 

 ridge having been lowered to the second plain. On the slope at the west 

 end, in northeast %, section 27, township 35 north, range 13 west, striated 

 pebbles were plentiful, and also at the scarp, due to slumping in northeast 

 14, section 13, of the same townships, where 3 feet of weathered gravelly 

 drift is exposed (plate 14, figure 3). The deposit on the main highest 

 part of the ridge, east of the stage road, was not examined, but on a small, 

 isolated remnant of this highest plain, in section 12, township 35 north, 

 range 12 west, gravelly drift was found containing well striated pebbles 

 of greenish argillite (plate 15, figure 3). This drift-covered remnant of 

 the highest plain is between 16 and 18 miles east of Flattop Mountain, 

 and at the west end stands 65 to 75 feet higher than the top of the ridge 

 to the west, which represents the second plain. 



Going 4 miles farther in a direction somewhat south of east across the 

 South Fork Valley, one reaches the most easterly remnant of the highest 

 plain on which striated pebbles were found, that in township 35 north, 

 range 11 west. This has a maximum width of about IVi miles and a 

 length from north to south of 3^2 miles. At points 3 miles apart gla- 

 cially striated pebbles were found, although here as elsewhere only peb- 

 bles of greenish argillite have so far resisted surficial etching as to retain 

 the striae (plate 15, figure 4), and from a large part of these the finely 

 graven scratches have been removed. The drift is coarse and gravelly, 

 with pebbles ranging in size from fine to 1 foot in length and in shape 

 from subangular to well rounded. The deposit is exposed in the upper 

 part of the abrupt marginal slope, not in a clean, undisturbed section, so 

 that it was not satisfactorily determined whether it constitutes a strati- 

 fied, water-laid deposit, such as glacial outwash, or whether it is the 

 coarse residuum of a deposit of unmodified glacial till. 



Ttme and Extent of ttik first Mountain Qlaciation 



As has been indicated, unmodified glacial till occurs /'// situ on Belly 

 River Ridge, nearly 10 miles north by west from Chief Mountain ; on 



