692 W. C. ALDEX rRE-AVISCONSIX GLACIAL DRIFT IN MONTANA 



and boulders comprise the coarse material of the later glacial drift on 

 the adjacent plain below. The only constituents of the later drift not 

 found on this part of the ridge were limestone and a quartzite con- 

 glomerate containing pebbles of black chert^ AYhich occurs principally in 

 the drift somewhat farther south and is from ledges exposed in the pass 

 traversed by the railway and provisionally referred by Doctor Stanton to 

 the Lower Cretaceous. Fragments of this conglomerate may never liave 

 been dej^osited on this upland tract, and the limestone if deposited may 

 have been removed by solution. Careful examination of the pebbles 

 failed to reveal any showing glacial striation. 



Going westward, the undulating drift plain gradually rises, and north- 

 east of Lower Two Medicine Lake for about a mile the later drift over- 

 laps the higher plain. Crossing this interval, the sandstone ridge rises 

 above the later drift plain, increasing rapidly in height, until one stands 

 on a flat-topped mesa with an elevation of 6,200 feet above the sea, a 

 height of 400 feet above the margin of the later drift, a relief of about 

 1,350 feet above the lake. Here there is a somewhat thicker deposit of 

 the same material and among the pebbles a few of limestone were noted. 

 This high tract has a length of about half a mile. Beyond this there is a 

 isharp drop in the crest of 100 feet or more. West of this sag the crest 

 rises again until the 6,200-foot level is attained. 



On the day following that on which this part of the ridge was exam- 

 ined, approaching thp ridge from the northeast, we climbed 1,200 feet 

 from the South Fork of Cut Bank Creek, over slopes of upturned and 

 folded sandstone, till we reached the flat top of the western part of Two 

 Medicine Eidge. On this top were found coarse gravels and boulders of 

 the same character as noted farther east. Many of the stones, however, 

 were much larger, some being boulders 8 feet in length. The deposit 

 seems to thicken toward Two Medicine Valley, and at the southwest ex- 

 tremity of the half-mile-wide mesa top a fresh scarp, due to slumping of 

 the underlying Cretaceous, exposed 150 feet of the material. Here the 

 deposit is typical glacial till (figure 2, plate 38). There is no assort- 

 ment or stratification, the stones, which range in size from small pebbles 

 to boulders 8 feet in lengtli, being heterogeneously mixed and imbedded 

 in a matrix of reddish clay. A large part of the deposit is cemented by 

 calcium carbonate to liard, tillite conglomerate. This stands with A^er- 

 tical face and as towers 15 to 25 feet in height. Some of the stones are 

 well rounded, but a large part of the pebbles and boulders are sub- 

 angular, facetted, and beautifully striated (figure 2, plate 39). The 

 material is wholly derived from the mountains, being of the same litho- 

 logic character as the gravels found on the ridge farther east. Lime- 



