AVERAGE REGIONAL SLOPE 37 



top represents the lowest part of the upland within an area of 

 several hundred square miles and within 10 miles of it there are 

 numerous ridges from 1,000 to 2,000 feet higher. The flat surface 

 is a small but integral part, not of a plainlike, but a decidedly hilly, 

 land form. Discordance between topography and structure is, 

 moreover, as well developed on the sloping hillsides of that land 

 form as on the few flat surfaces that are present within it. 



Neither approximately even sky lines, nor flat or nearly flat 

 areas planing across the structure, are therefore in themselves a 

 proof that the land form within which they occur is of more moder- 

 ate relief than the upland of Interior Plateaus with average slopes 

 as high as 6 per cent. The measurement of the slopes on old 

 erosion surfaces must therefore be made before one can venture 

 to judge of its actual relief or use it in quantitative measurements 

 of earth warping. 



PROPOSED SUBDIVISION 



The following subdivision is concerned only with the stage of 

 old age in the normal cycle of erosion as outlined by Davis. 1 



An old erosion surface is for the purposes of this discussion 

 defined as a geographic unit worn down by subaerial processes 

 alone to a state of moderate relief. By geographic unit is meant 

 a portion of the earth's surface over which topographic conditions 

 and the underlying rock structure were essentially similar at the 

 beginning of the erosion cycle, and over which conditions of erosion 

 remained essentially the same while the cycle was in progress. It 

 is proposed to treat all surfaces in this stage as varying from two 

 types, those of plainlike forms of peneplains and forms corre- 

 sponding in general features to the uplands of the Interior Plateaus 

 of British Columbia which may be referred to as " beveled hills." 

 Following Smith 2 and Davis 3 peneplains are defined as geographic 

 units worn down by subaerial processes alone to a condition of very 

 moderate relief. The theory of the formation of such plainlike 

 land forms does not necessarily imply that all parts of them lay 



1 W. M. Davis, "The Geographic Cycle," Geog. Jour., XIV (1899), 481. 

 2 W. S. Tangier Smith, "Some Aspects of Erosion in Relation to the Theory of 

 the Peneplain," Univ. of California Bull. Dept. of Geol., II (1899), 155-77. 

 3 W. M. Davis, "The Geographic Cycle," Geog. Jour., XIV (1899), 486. 



