AVERAGE REGIONAL SLOPE 33 



at right angles to the trend of the ridge, and also along its crest. 

 Variations in the slopes along ridge crests in the Beaverdell map 

 area of southern British Columbia are illustrated in Fig. 2, profiles 

 3 and 4. The average slope measured on four or five ridges in this 

 area lay between 100 and 300 feet to the mile, and averaged over 

 200 feet except in places where certain Tertiary formations occurred 

 over which the slopes ranged between 500 and 900 feet to the mile, 

 and averaged 600. The Tertiary areas occupied less than one- 

 eighth of the area of the whole upland; the average slope along 

 ridges therefore averaged between 200 and 300 feet to the mile. 

 Slopes across ridges were in this area of very nearly the same 

 magnitude and did not average over 300 feet to the mile. 



Slopes as high as 900 to 1,000 feet to the mile were found in a 

 few places only, and could have been omitted from the general 

 average without changing the result to any great extent. Such 

 local irregularities of slope are more likely to occur in land forms 

 with fairly high slopes than in those which are of a plainlike char- 

 acter. Of twelve measurements on the Caldwell, Kansas, map 

 sheet for instance, six lay between 14 and 21 feet to the mile, four 

 between 32 and 35 feet, one was 47 feet, and one 10 feet to the mile. 



VALUE OP THE MEASUREMENT OF REGIONAL SLOPES 



The study and determination of the regional slopes upon old 

 erosion surfaces is both useful and necessary. It is useful: (a) in 

 helping to determine the agencies which have carved and molded 

 the topography to its present form, and (b) in separating forms due 

 to different erosion cycles. It becomes necessary (c) when an old 

 erosion surface is to be used as a datum for measuring diastrophic 

 movements. 



a) The study of regional slopes often will indicate the agencies 

 which have carved or assisted in carving a land form. This is 

 illustrated by BarrelFs 1 work along the New England coast. He 

 found that certain flat-topped ridges in the interior sloped gently 

 toward the coast, and the plainlike surfaces, of which the ridge 

 tops were residuals, occurred in terraces of successively higher 



1 Joseph Barrell, " Piedmont Terraces of the Northern Appalachians," Bull. 

 Geol. Soc. Am., XXIV, No. 4 (December, 1913), 688-90. 



