1 64 ROLLIN T. CHAM BERLIN 



at least two distinct stages in this process. Since the weaker 

 strata of the plains reached a stage of advanced old age, two 

 episodes of erosion have left their impress there, according to the 

 studies of Davis.^ 



Quaternary glaciation has also greatly modified the details 

 of the mountainous belt. By the cirquing and stoping process, 

 serrate peaks and sharp ridges have been carved and sculptured 

 out of the rounded monadnocks and remnants of the summit 

 peneplain (Fig. 3). Rounded U-shaped valleys with capacious 

 cirques testify to the potency of glacial erosion in the adjoining 

 valleys. 



'W. M. Davis, op. cit., p. 31. See also N. M. Fenneman, "Geology of the 

 Boulder District, Colorado," U.S. GeoL Survey, BulL 26 j (1905), Fig. i and PI. 

 IIIA. i 



[To be continued] 



