PENEPLANATION OF THE PLATEAU 307 



walls disappear and the rivers flow out on great alluvial fans and make 

 their way over them into the lake. 



Peneplanation of the Plateau 

 character of surface and its elevation 



The most notable feature of the plateau is its peneplained condition; 

 the surface is beveled in a marked way and without reference to foliation 

 or other rock structures. The plateau surface has a strong gradient de- 

 veloped by the peneplaining activities; this gradient slopes toward the 

 ocean as if the sea had in some way controlled its development. The 

 gradient, while averaging 18 to 20 feet to the mile, is by no means uni- 

 form, but varies in such a way as to suggest that there have been a series 

 of tectonic basins or platforms connected with each other by slopes or 

 scarps. It may be that the plateau was broken up into a number of great 

 fault-blocks at one stage of its history. If so, the scarps and other irregu- 

 larities have been smoothed down or removed by erosion and the whole 

 surface graded to the fairly uniform slope tliat now exists. 



Much of the plateau lies at an elevation of several thousand feet above 

 sealevel. Under normal conditions, at such altitudes there would be 

 marked relief developed, as, for example, in the case of stream valleys. 

 The plateau, however, lacks these elements of relief; indeed, over much 

 of its more arid parts deep valleys are conspicuous by their absence. 



It seems probable that the plateau is being peneplained at high alti- 

 tudes, without reference to sealevel or Avithout the aid of stream erosion, 

 at least in any large measure. As this method of peneplanation differs 

 from the normal method, it is necessary to consider the factors entering 

 into it. Of the various factors, the principal one is the climatic one. 

 Other minor ones are the depth of the water table, texture and structure 

 of the rocks, absence of high mountain ranges to furnish constant sup- 

 plies of water for through trunk streams. 



CLIMATIC FACTOR 



The climate apparently has a more important bearing on the question 

 of peneplanation than any other factor. From the climatic standpoint 

 it is possible to divide the country into zones parallel to the coast. The 

 rainfall at the coast and for 30 or 40 miles back is about 32 inches annu- 

 ally, though this amount fluctuates greatly. Tlien follows a belt about 

 275 miles wide in which rain is scanty. In tliis belt, however, the higher 

 elevations, such as the summits of the monadnocks and the more elevated 



