TOPOGRAPHY OF THE PLAIN 691 



Inasmuch as the investigations of the past summer were confined 

 principally to observations in and adjacent to the mountains themselves, 

 a thorough examination was not made of all these elevated tracts on the 

 plain, and the deposits lying on them were not seen far from the moun- 

 tains. The results of the observations that were made, however, are so 

 significant that a preliminary statement concerning them seems war- 

 ranted at this time. 



Beginning at the south, a brief description will be presented of the 

 character and relations of the deposits on the several high ridges ex- 

 amined by Mr. Thomas and the writer in the belt near the mountains. 



Deposits on Two Medicine Ridge 



Issuing from the mountains about 7 miles northwest of the village of 

 Midvale, Two Medicine Eiver swings to the southeastward and traverses 

 the plain in a sharply cut, trenchlike valley. The lower lake lies about 

 800 feet below the level of the rolling drift-covered plain on the east. 

 Going northward from Midvale across this plain one finds well marked, 

 though mildly developed sag-and-swell morainal topography set with 

 numerous little ponds. This marks the marginal moraine of the Two 

 Medicine glacier of the Wisconsin stage (plate 39, figure 1). It ends 

 at the north along a fairly definite line near the foot of the south slope 

 of a long flat-topped sandstone ridge, part of (plate 38, figure 1) "Two 

 Medicine Ridge.^' To the east the ice extended through the gap in the 

 sandstone ridge, which is traversed by the Great Northern Eailroad and 

 spread over the lower land toward Browning. South of the railway the 

 glacier overtopped the ridge and left later glacial drift thereon. Through 

 a length of 3^^ miles northwest of the railway the ridge was not sur- 

 mounted by the later ice-sheet. That part of the ridge shown in plate 

 38, figure 1, has a relief of about 200 feet. Ascending the abrupt slope, 

 the top is found to be notably flat excepting where cut by a ravine which 

 partially divides the ridge. It rises gradually westward from 5,300 to 

 5,600 feet above the sea. This plateau surface, which is a remnant of 

 the Blackfoot peneplain, is beveled across the edges of the Cretaceous 

 sandstone, which at one point noted has a dip of 42 degrees toward the 

 southwest. Capping the ridge is a thin deposit consisting of gravel 

 ranging in size from fine to boulders II/2 f^G^ long. The stones are 

 partly well rounded, but mostly subangular. They are principally of 

 quartzite, white, yellowish, banded pink, and red, and with these are 

 pebbles of maroon argillite, green argillite, and diorite, named in the 

 order of relative abundance. All these rocks are from the pre-Cambrian 

 formations of the mountains to the west, and the same kinds of pebbles 



