536 PRE-WTSCOXSIX GLACIAL DRIFT IX MONTANA 



spillway afforded by the sag in the crest of Milk Eiver Eidge encroached 

 on the plain in the head of the valley of South Fork of Milk Eiver. The 

 ice heading in the upper part of Two Medicine Valley, in Marias Pass, 

 and in the valleys to the south united to form Two Medicine Glacier, 

 spread eastward about 38 miles from the mountain front, and encroached 

 on Carlow Flat. TVaters from the melting of these glaciers and from the 

 subsequent drainage of the area cut into these plains of the third set, 

 eroding the inner valleys occupied by the present streams, some of which, 

 notably that of North Fork of Milk Eiver, are very sharp. The relations 

 of the glaciers to these topographic features thus definitely determine 

 that the third set of plains was developed prior to the last great extension 

 of the mountain glaciers. 



To the north of the Browning and F>lackf oot quadrangles the dissection 

 of the plains was carried so far at these several stages of erosion that no 

 remnants of the higher levels have been recognized excepting those de- 

 scribed on the east sides of vVaterton lakes and Belly Eiver Valley. To 

 the south the only remnants of the higher plains noted are those seen by 

 Mr. Stebinger between Birch and Badger creeks. 



Pre- Wisconsin Drift ox the Blacetoot Pexeplaix 



Observations in 1911 and 1912 determined that the flat top of each of 

 the high ridges near the mountain front was underlain by a deposit of 

 glacial drift (plate 14, figures 2 and 4: plate 15. figure 5 ). in most places 

 unmodified till, varying in thickness up to a maximum of about 250 feet 

 on Saint Mary Eidge, which was clearly much older than the drift left 

 by the mountain glaciers of the last great extension. 



In this connection it may be noted that one of the finds of the past 

 season was an excellent exposure of the drift on the east side of Saint 

 Mary Eidge. In the woods on the north slope of the valley of one of the 

 branches of Livermore Creek which cuts the east slope of the big ridge 

 6 miles due west of Horse Lake, in southeast 1 '^. section 6, township 34 

 north, range 13 west, at an elevation between 5, TOO and 5.800 feet above 

 the sea, is a clean scarp due to slumping on the underlying Cretaceous 

 shale. The scarp is about 75 yards long and has a maximum height of 

 about 25 feet. At the west end a mass of the drift has crept out on the 

 shale, leaving a rift 5 to 10 feet wide and 15 feet deep above the filling in 

 the bottom. In the western half of the exposure the upper 10 to 12 feet 

 consist of sand and gravel interstratified with glacial till : below this are 

 15 feet of coarse, bouldery till (plate 14, figure 5 8 one of the finely 

 striated boulders in this are 5 to 6 feet in length. The pebbles and 



