AVERAGE REGIONAL SLOPE 45 



that rocks must be cracked if results are to be obtained, and the 

 physiographer who depends largely upon distant views will miss 

 a great deal of the detail which helps to prove or disprove field 

 hypotheses. 



SUMMARY 



In the study of the physiographic development of the Interior 

 Plateaus of British Columbia, certain characteristics commonly 

 accepted as criteria for peneplanation were found well developed 

 upon an erosion surface with regional slopes of 3 to 6 per cent. 



Stress is laid on the value of the study and measurement of the 

 regional slopes of old erosion surfaces, and a quantitative subdivision 

 of old erosion surfaces on the basis of their average regional slopes 

 is suggested. Regional slope is defined as the general slope of the 

 land toward main drainage lines, and an outline is given of the 

 methods of measuring regional slopes, as followed in the work on 

 the Interior Plateaus of British Columbia. 



The determination of the regional slopes of a land form is of 

 value in furnishing clews to the agencies which have affected its 

 development, and in separating the products of the different cycles 

 of erosion through which it may have passed. The measurement 

 of regional slopes is essential if a land surface is used as a datum 

 for the measurement of movements of the earth's crust. For if 

 such measurements are not made, an uplifted old erosion surface 

 is very likely to be considered of much lower relief than is actually 

 the case. Such an assumption leads to serious errors in estimates 

 of the manner and extent of movements of the crust. It is caused 

 partly by optical illusions, and partly from the fact that the char- 

 acteristic flat horizon lines and the discordance of topography and 

 structure which prevail over old erosion surfaces of plainlike form 

 are also found well developed upon surfaces of much greater relief. 



It is proposed that old erosion surfaces be divided into two 

 central types "peneplains" and "beveled hills." Peneplains are 

 to be characterized by average regional slopes up to 2 per cent by 

 discordance between topography and structure, by the absence of 

 local irregularities of slope, such as cliffs and flat areas, and by 

 deep soil covering. 



