SUMMIT LEVELS AMONG ALPINE MOUNTAINS 125 



two theories regarding the geologic and physiographic history of 

 the range makes the decision of primary importance. The existence 

 of broad, little-dissected plateaus remnant from a greater plateau 

 nearly or quite coextensive with the range having internal structures 

 of alpine complexity, is a positive criterion favorable to the peneplain 

 theory. From such remnant plateaus must be distinguished the 

 elevated shelves due to high-level glacial erosion; to wind erosion; 

 to the local control of internal structure; or to changes in conditions 

 whereby the floors of former, high-lying basins of erosion, through 

 deformation or through the migration of divides, become the sum- 

 mits. Peneplain remnants must further be distinguished from the 

 common, often broad, ridge-summit formed by the meeting of two 

 gentle slopes where the low angles of the slopes are incidental to 

 general gradation of the mountain above tree-line. Analogous forms 

 on a much smaller scale are never absent from the ridges in bad land 

 topography where there is no suggestion of peneplanation. 1 



On the other hand, the remnant plateaus may not appear in the 

 present topography of a range, and the accordance of summit alti- 

 tudes may characterize peaks and serrate ridges only. Such accord- 

 ance may give a comparatively even sky-line in the views from any 

 dominating point, but the full force of the composite explanation of 

 accordance as above outlined is directed against the reference of that 

 even sky-line to the direct or inherited profile of an ancient, uplifted 

 peneplain. 



1 G. K. Gilbert, Geology of the Henry Mountains (Washington, 1877), p. 122. 



