334 The Geotectonic Factor 



gresses has an extremely complex action (Lobeck, 1939). The 

 erosive power of water varies with the gradient, the volume, and 

 the abrasive materials suspended in it. Ice may be an active flowing 

 agent as in a glacier, abrading the surfaces over which it moves, 

 or it may freeze in cracks and through expansion disintegrate the 

 surface. The action of wind, like that of water, varies with velocity 

 and the scouring materials which it carries. In all cases the ef- 

 ficiency of these agents is aftected by the texture of the surface 

 layers, harder rocks being generally more resistant to erosion than 

 softer formations. 



These levelling factors are dependent on the dynamics of the 

 atmosphere, either as transport for water vapor and the consequent 

 production of rain, or as a mechanical agent for moving erosive 

 particles. Because the atmosphere is also one of the primary factors 

 which permits the existence of life on the earth, life and the level- 

 ing process have a primary factor in common. 



All of these erosive agents become more effective as the land- 

 scape becomes more rugged because the ruggedness is associated 

 with greater elevation which automatically increases the gradients. 

 That in turn increases the erosive power of water and ice. Thus 

 as mountains go higher, erosion goes faster; as the mountains are 

 worn away, erosion goes more slowly. However, erosion would 

 never come to a complete stop while any land remained about the 

 level of the seas. 



The ridges along the ocean floor may be eroded to some extent 

 by the major ocean currents and may change in ways comparable 

 with surface features. Submarine ridges presumably wear away and 

 afi^ect the course of the very currents that eroded them. This would 

 produce a dynamic character in the "climates" of the ocean. 



Geological evidence from many sources indicates that during 

 periods of minimum crustal unrest, these levelling forces flatten or 

 peneplain large areas of the earth. Under these conditions eco- 

 logical conditions change in the direction opposite to those pro- 

 duced by unusual elevation. Climatic extremes of xeric and cold 

 become modified, and climates over the entire area are thought to 

 approach a fairly even tropical or subtropical humid condition. 

 Under these conditions the tropical rain forest would presumably 

 reach its maximum geographic extent, and the xeric and cold biotas 

 would presumably be at a minimum, restricted to local areas having 

 mountainous terrain or exposure to unusual coastal oceanic cur- 

 rents. The oceanic temperature indications diagrammed in Fig. 131 

 seem to indicate that these periods of widespread warm climates 

 have been much more extensive than those of cold climates. 



