32 LEOPOLD REIN EC KE 



carry a suggestion of the actual land form to the mind of the reader. 

 For the measurement of such slopes either topographic maps or a 

 number of traverses across the region to be examined are essential. 

 An appreciation of the significance of the slopes can, however, be 

 attained only by traveling over them. 



The following is an outline of the methods followed in obtaining 

 data from topographic maps of portions of the southern section 

 of the Interior Plateaus of British Columbia. These methods 

 were applied, in part, to the measuring of slopes upon topographic 

 maps of certain sections of the United States, with satisfactory 

 results. 



The first step to be taken is the drawing of a number of profiles, 

 some in the direction of the main drainage and others at right 

 angles to it. The profiles should include as many of the pertinent 

 topographic features of the region as possible. If they are plotted 

 with a vertical scale somewhat larger than the horizontal, they will 

 assist both in determining whether the land form under considera- 

 tion is the result of one or more cycles of erosion, in discovering 

 whether processes other than subaerial erosion have been respon- 

 sible for existent forms, and in detecting tilting or warping of the 

 crust subsequent to the formation of the surface. The usefulness 

 of profiles is discussed more fully farther on. If forms due to more 

 than one cycle are present, slopes should of course be measured on 

 each of those forms separately. In the Interior Plateaus two cycles 

 are represented, an older upland and younger valleys intrenched 

 in it. There is a distinct break or topographic unconformity 

 between the upland and the valley forms (Fig. 2, profiles 1 to 7). 

 In this instance the measurements of slopes on the older land form, 

 the upland, were made by taking a large number of horizontal 

 measurements on the topographic map from a dominant ridge line 

 to the bottom of a large upland valley. If a deep valley of the 

 younger cycle occupied the site of the bottom of the old valley, the 

 measurements were made to the point where the break in slope 

 occurred between the old and the young forms (profiles 1 and 2). 

 Horizontal measurements were made as long as possible, and never 

 less than one mile. The vertical difference was read directly from 

 the topographic map. Measurements were taken in both directions 



