PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
35 
soft rock being carved into valleys, and hard rocks standing 
out as ridges. Old age is characterized by the comparative 
absence of cliffs upon the hill sides, by the fact that differences 
in rock structures are no longer brought out by the topography, 
and by relatively low relief. Thus one may, by determining 
which of these characteristics a land form possesses, determine 
to what stage it has progressed in the Geographic Cycle. 
Land forms are, however, very seldom the product of one 
cycle of erosion, and mountainous regions especially are often 
made up of the remnants of land forms produced in several 
distinct cycles; the earlier cycle existing as the smaller rem- 
nants upon the higher points. In frying to determine where a 
composite form is to be placed, in which different parts have 
different characteristics, one is led into more speculation than 
seems warranted; hypotheses upon the origin of composite land 
forms can generally be verified only by detailed studies over large 
areas with the aid of topographic maps. The one thing, however, 
which can be determined for any given region with satisfaction, 
is the relative relief and the average regional slopes. By average 
regional slopes we mean here such as are obtained by taking 
differences of elevation upon the higher interstream areas and 
in the lower valleys that lie between them, thus indicating the 
average fall of the waters which aid in denuding and degrading 
the land. It is fairly well established that there is a definite 
diminution of average slope after a region has passed the stage 
of early maturity, that such diminution is very rapid during 
maturity and becomes very slow during the later stages of 
old age. It would, therefore, seem possible to classify land forms 
according to their average regional slopes into a system which 
would not violate the accepted theories of the origin of such 
forms, and would place definite limits to the various stages. 
The necessity of such definition of land forms can be es- 
pecially illustrated by the use of the term peneplain. This 
term is continually being applied to land forms having a mature 
appearance irrespective of their actual relief. Thus the uplands 
of the Interior plateaus of British Columbia, with regional 
slopes of 160 to 300 feet to the mile (3 to 6 per cent), have been 
