AVERAGE REGIONAL SLOPE 35 



level; much more so in the actual view than would be inferred from 

 the crowded contours of the Platte Canyon map sheet." 



Value of certain criteria of peneplanation. — The reasons for mis- 

 takes of the kind referred to are, first, that an uplifted old erosion 

 surface of moderate relief is often seen in juxtaposition to younger 

 topographic forms upon which the slopes are much steeper, so that 

 by contrast the relief on the older surface appears much less than 

 it really is. 



A second and less obvious reason is that certain of the more 

 important characteristics of plainlike erosion surfaces with average 

 slopes of less than 10 feet to the mile are found also on old erosion 

 surfaces with slopes as high as 300 feet to the mile. The criteria 

 referred to are a general flatness of sky line and the planing of a 

 rather flat topographic surface across rocks of different hardness 

 and texture without any apparent change in the character of the 

 topography. 



Flat sky lines : In a rolling hill country ridge lines sloping from 

 100 to 300 feet to the mile may appear quite flat and the complete- 

 ness of the illusion will depend partly on the position of the observer 

 and partly on the distance of the ridge line or lines from him. Flat 

 sky lines often are caused by the blending of more than one ridge 

 line of entirely different elevation in the observer's line of sight, the 

 irregularities of one being neutralized by the other (Figs. 3 and 4). 

 The writer knows of at least one locality in the Beaverdell map area 

 of British Columbia where an observer, climbing up one side of the 

 West Fork River valley and looking across to the opposite side, 

 would see first a flat sky line on a ridge with an elevation of 4,000 

 feet, and as he climbed higher another flat ridge line would come 

 into view with an elevation approximately 700 feet higher and 

 lying 3 miles farther away. The two ridges are shown in cross- 

 section in Fig. 2, profile 2, the flat top of the higher, the St. John 

 ridge, in profile 3. Between the two positions there is doubtlessly 

 one where both ridge lines would blend and appear as one. The 

 flat sky line in this instance evidently does not mean that the 

 ridge tops represented in the sky line are remnants of one nearly 

 flat plain, for the lower one is next to a large river, the higher 3 miles 

 from it, and the slope between them over 200 feet to the mile. 



