PHYSIOGRAPHIC GEOGRAPHY 



provinces. Furthermore, it is significant that he 

 sees first of all the Arid Province placed by nature 

 as a definite boundary and barrier marking off this 

 western region with its distinctive individuality. 

 The peculiarities of the west coast, its problems 

 and its needs, would have been better understood in 

 the eastern portion of the United States had com- 

 munication been more free, and the Panama Canal 

 in a vital, if not in a literal sense, cuts away part 

 of the desert barrier between the two great sections 

 of our country. 



The Arid Province, by reason of its very aridity, 

 has great areas of soil exceptionally rich in plant 

 food and wonderfully productive in the localities 

 where water may be obtained for irrigation. To 

 many visitors the arid region is "a desert" unin- 

 teresting and disagreeable, to be at least partially 

 escaped by travel at night. But viewed either as a 

 lost paradise to be regained by irrigation, or viewed 

 as a physiographic barrier to perfectly free inter- 

 course, which the non-irrigable areas must always 

 remain to a large degree, or viewed as a region 

 fortunately either bare or thinly covered with vege- 

 tation that man may study the forces and processes 

 by which nature evolves scenery from the raw ma- 

 terials, it is full of interest and of charm. Even the 

 discomfort of the heat and dust found in passing 

 through the extreme types of this province in the 

 hottest or windiest day is a valuable experience. 

 The characteristic distinction in the physiographic 

 processes of the type arid region compared with 

 those of the humid climate of the Mississippi Valley 

 and the East is that owing to scanty rainfall the 

 rivers are unable to maintain their courses to the 

 sea. The streams drop the greater part of their load 

 of sediment at the foot of the mountains, forming 

 great alluvial fans, and only the finer material and 

 the matter in solution are carried to the bottom of 

 the various basins where more or less temporary 

 salt lakes are formed. The general result is that 

 while the mountains are being lowered the low- 

 lands around them are being built up, covering their 

 basal slopes, and the amount of relief becomes less 

 and less. The tendency is finally to approximate 

 a plain from which rise the resistant tops of the 

 yet unburied mountains. In contrast with the humid 

 regions where erosion at first deepens the valleys, 

 thus increasing the relief but constantly lowering 

 the average elevation of the entire region and tend- 

 ing ultimately to form a peneplain close to sea level, 

 the typical arid basin levels up as well as down and 

 tends to form a peneplain at the average level of 

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