464 ARTHUR BEVAN 



enabled the streams to carve out the present system of sharp valleys. 

 The movement was probably due to stresses that forced the over- 

 thrust mass still farther forward and upward along the fault surface. 

 Warping of the uplifted peneplain appears to have been sHght, as 

 the difference in altitude may be due in part to the original slope 

 of the plain. There is some suggestion in the stream profiles that 

 the central part of the range was elevated more than the lateral or 

 terminal portions. Westward tilting at this time is suggested by 

 the fact that the valleys on the northeast side of the axial divide are 

 cut much farther below the surface of the sub-summit plateau than 

 are those on the southwest slope. 



Another conspicuous result of the pronounced regional uphft 

 has been the development of the bold plains-ward front of the range 

 by differential erosion. The denudation of the highly resistant 

 massive granite and gneiss has been comparatively slight since the 

 uplift, whereas the much weaker Mesozoic and Tertiary ( ?) sedi- 

 mentary formations of the adjoining plains have been worn to much 

 lower levels. 



Several subsequent broad elevatory movements of less vertical 

 extent have successively rejuvenated the streams so that they have 

 produced the series of terraces that exist along the principal valleys 

 beyond the mountain front. The most recent important change of 

 level seems to have preceded the earlier glacial epoch, for in East 

 Rosebud Valley remnaiits of the right lateral moraine of this stage 

 descend the streamward face of the first conspicuous terrace above 

 the late glacial outwash. 



The last noteworthy event in the history of the Beartooth 

 Mountains was widespread glaciation during at least two distinct 

 epochs. The glaciers on the northeast side of the summit divide 

 were confined to the valleys whereas the plateau on the southwest 

 slope was covered by the Beartooth ice-cap. Apparently the sub- 

 summit plateau on the northeast slope has not been glaciated any- 

 where except at the heads of the valleys that rise within its borders, 

 but it experienced considerable nivation. Meager remnants of the 

 former extensive alpine glaciers still exist at the heads of several 

 valleys on the northeast side of the main divide. 



