The Geotectonic Factor 333 



Although crustal movements take place slowly in terms of our 

 own sense of time, sufficient time has elapsed since the origin of 

 life on the earth for myriads of these crustal movements to occur. 

 In some areas, such as the Paleozoic Arbuckle Mountains of Okla- 

 homa, orogenic movements have been few, and no new major up- 

 thrusts have occurred since the Paleozoic. In other areas crustal 

 changes have been frequent, forming orogenic belts in which moun- 

 tain building has been fairly continuous since pre-Paleozoic times 

 (Fig. 146) (Umbgrove, 1947; Love, 1939). 



There is good evidence in the geologic record (Schuchert, 1924) 

 that whereas mountain building has been almost continuous, it has 

 not always been of the same intensity. This activity has reached 

 high peaks during some periods, with long intervening periods of 

 lesser activity. The Laramide orogeny during the late Cretaceous 

 and early Cenozoic, seems to be one such period of increased oro- 

 genic activity, as have been late Pliocene and Pleistocene times. 

 Umbgrove (1947) believed that these successive active times are 

 cyclic phenomena which he described as "the pulse of the earth." 

 King (1958) suggested that the Mesozoic and Cenozoic orogenic 

 phenomena of western North America form a single long cycle. 

 Attempts to ascertain past oceanic temperatures give indications 

 of a climatic cycle of about 75 million years (Fig. 131), consider- 

 ably shorter than that proposed by King and more nearly like that 

 proposed by Umbgrove. However, even between the orogenic 

 "revolutions" mountains did rise locally and crustal elevations did 

 occur. 



The net effect of crustal movements is to increase topographic 

 contrasts on the earth's crust and as a direct result of that to in- 

 crease ecological contrasts. At the height of the elevating process, 

 through a combination of high mountains, intense rain shadows, 

 and other effects, the greatest extremes of hot and cold, humid 

 and xeric conditions would also occur. These extremes would be 

 expressed as ( 1 ) the greatest variety of different climates which 

 would exist together at one time, and (2) the greatest propor- 

 tionate expanse of cold climates and xeric climates and the small- 

 est expanse of hot, humid climates. In such times, areas of tropical 

 rain forest would be at a minimiun whereas areas of warm xeric 

 climates and of temperate or arctic climates would be at a max- 

 imum. 



THE LEVELLING PROCESS 



As soon as crustal elevations occur, the action of wind, water, and 

 ice immediately start wearing them down. The erosion which pro- 



