328 The Geotectonic Factor 



to affect the configuration of the earth's surface: the elevating and 

 levelhng processes. Each has its distinctive action on the surface 

 and through this its effect on the biotic environment. 



THE ELEVATING PROCESS 



The most conspicuous results of the elevating process are the moun- 

 tains, but these are only one of many related phenomena. The 

 process should more properly be called crustal deformation, for it 

 includes not only great upthrusts which produce mountains, but 

 also warping and faulting of the surface and upwellings of molten 

 materials from the interior of the earth. 



Mountains produce high elevations, hence cool temperatures on 

 the land surfaces which precipitate large amounts of moisture from 

 clouds moving across them. This results in decreasing precipita- 

 tion on the lee side of the mountains and produces in the moun- 

 tains themselves cooler and more moist habitats than exist in the 

 surrounding country. This dual effect is especially marked in the 

 western part of North America where much of the moisture comes 

 from the Pacific Ocean. Successive mountain chains running roughly 

 north and south provide cool temperate to near arctic conditions 

 southward into areas which are subtropical at sea level, but they 

 also produce rain shadows which result in extremely xeric areas 

 on the eastern side of each range. Deflection of winds produces 

 similar changes, resulting in greater or lesser rainfall depending 

 on the moisture content of the winds or in temperature changes. 



Mountain-like ridges arising from the ocean floors undoubtedly 

 influence the direction of ocean currents and in this way affect the 

 temperatures prevailing in different areas and at various depths 

 in the ocean. These effects are not, however, limited to the ocean 

 itself. Upwellings of cold water at the surface may cause precipita- 

 tion from air moving above it. The Humbolt current is such a cold 

 upwelling, and it reduces rainfall for thousands of miles along the 

 western coast of the tropical and subtropical Americas. The warm 

 surface Japanese current in the North Pacific produces the opposite 

 effect, contributing to the unusually warm and humid condition of 

 the western coast of North America northward from San Francisco. 



At times great areas of the continents have been raised to con- 

 siderable heights. The Great Plains region and the Colorado plateau 

 in North America were elevated possibly 4,000 or 5,000 feet in 

 middle Cenozoic time. Large areas of Africa were similarly ele- 

 vated. It is likely that when these areas were raised, entirely new 



