GEOLOGY OF THE BEARTOOTH MOUNTAINS, MONTANA 463 



thrust is post-middle Eocene, the peneplanation of the deformed 

 mass most probably was not accompHshed before the Oligocene, and 

 perhaps not until the Miocene. Several lines of evidence, which 

 are discussed in the forthcoming paper, point to the conclusion that 

 this peneplain is not older than the Miocene, and may possibly be 

 of Pliocene age. 



After the completion of the summit peneplain the range was 

 uplifted about 2,000 feet, with slight longitudinal warping and 

 gentle tilting of the surface toward the southwest. This elevation 

 seems to have resulted from renewed movement along the over- 

 thrust fault surface. The new cycle of erosion thus initiated con- 

 tinued until a large portion of the range was again reduced to an 

 old age lowland — the present sub-summit plateau. The central 

 part of the range remained above this extensive plain as unreduced 

 remnants of the earlier erosion surface. 



The age of this sub-summit peneplain can not be closely deter- 

 mined until the place of the summit peneplain in the erosional 

 history of the range has been more accurately determined. It is 

 evident, however, that this surface was produced in the erosion 

 cycle which was begun by the deformation that elevated the earlier 

 peneplain, and was closed by the uplift that resulted in the excava- 

 tion of the present canyon-hke valleys to depths of a few thousand 

 feet. If the summit peneplain is as old as the middle Miocene it 

 appears not improbable that the sub-summit peneplain is as young 

 as the PHocene, but if the former is of Pliocene age it is probable 

 that the latter is of late PHocene or early Quaternary age. More- 

 over, although the younger peneplain must considerably antedate 

 the existing deep valleys, the fact that fiattish remnants as large as 

 10 to 20 square miles in vulnerable positions have been but sHghtly 

 dissected by tributary valleys strongly suggests that it may have 

 been completed and elevated as recently as the Quarternary. The 

 boldness of the mountain front, its abrupt rise for thousands of 

 feet above the plains, and the shght amount of dissection over large 

 areas support this view. 



Quarjternary events. — This second epoch of peneplanation was 

 termina:ted by a vertical uplift of several thousand feet, which 



