MOST EASTERLY REMNANTS OE HIGHEST PLAIN 541 



Eegardless of the exact mode of the deposition of the drift on these 

 highest outliers, we think there is some ground for the opinion that the 

 earliest glaciation occurred prior to the development of the second set of 

 plains. 



Deposits on Horsethief Eidge and Landslide Butte 



The most easterly remnants of the highest plain known to the writers 

 are found in the highest parts of Horsethief Eidge, southeast 14? section 

 14, township 35 north, range 9 west, and Landslide Butte, township 36 

 north, range 8 west, distant about 35 and 40 miles respectively from the 

 mountain front. Both these tops, on which are triangulation stations, 

 are approximately 4,650 feet above tide water, or 350 feet lower than the 

 mesa top just described. The highest part of Horsethief Eidge, which is 

 a few square rods in extent, is capped with a deposit of coarse gravel 

 similar to that on the high plains to the west. The pebbles, which range 

 in size from a fraction of an inch to 1 foot in diameter, are composed 

 exclusively of mountain rocks — pink, gray, and white quartzites, maroon 

 and greenish argillite. In shape the pebbles are subangular, with more 

 or less flat faces, edges largely smoothly rounded, but not very well 

 rounded even among the smaller pebbles. No pebbles were found with 

 glacially striated surfaces. The only difference in character between this 

 and the deposit on the ridge 350 feet higher and 15 miles farther west is 

 the apparent absence of striated pebbles on Horsethief Eidge. If they 

 are parts of the same deposit, as seems probable, and if the deposit was 

 made by outwash from glacial drift, it would be more surprising to find 

 that pebbles rolled 15 miles or more by a stream had retained glacial 

 stria? on their surfaces than to find them worn away. On the other hand, 

 however, as was noted above, the pebbles are not particularly well 

 rounded. 



The deposit capping Landslide Butte is particularly well exposed at 

 the west end in a clean scarp, the result of slumping on the Bearpaw 

 shale. There is an undisturbed curved section 15 feet high and several 

 hundred feet long besides the exposures in the faces of the masses which 

 have slid down. Immediately beneath the black soil in part of the sec- 

 tion is a deposit resembling till, consisting of gray clay having a maxi- 

 mum thickness of 3 feet and containing rather few pebbles. The main 

 deposit consisls of coarse gravel of the same lithologic composition as 

 that on Horsethief Ridge and the high-level plains farther wrest, with the 

 interstices between the stones filled with gray clav. In one pari below 

 this gravelly bed then; is a Larger percentage of the liner material conati- 



