EVOLUTION OF MODERN SURFACE FEATURES 15 



mobility of tlie crust reached its climax. Blending was greatest in the 

 eugeosynclinal area, which possessed considerable mobility from the 

 beginning; the orogenic climax was reached earliest here. During 

 this climax the eugeosynclinal sediments and volcanics were strongly 

 compressed and deformed, were more or less metamorphosed, and 

 were invaded by small to large masses of plutonic rocks, ending with 

 masses of granitic composition. 



Detritus eroded from the newly deformed belt was in part washed 

 off its oceanward side, where much of its record has been lost, and 

 was in part spread inland as great sheets of clastic rock that tapered 

 across the miogeosynclinal area, which was as yet undeformed. 

 These clastic sedimentary deposits comprise as much as half of many 

 miogeosynclinal sequences. 



As crustal compression progressed, the miogeosyncline itself was 

 deformed, sometimes in an orogenic period that appears to be dis- 

 tinct from that in the eugeosynclinal area. Its bedded sedimentary 

 rocks were thrown into folds and broken by thrust faults along which 

 rocks above were moved greater or less distances toward the con- 

 tinental interior. Deformation usually progressed as far as the inland 

 edge of the geosyncline, where the sediments thin out, and the 

 interior region was left undeformed. In places, however, deformation 

 was carried beyond the edge of the geosyncline, as in the southern 

 Rocky Mountains, within the region we are considering. 



Effect on Surface Features. The orogenic phase so consolidated 

 and strengthened the rocks of the eugeosynclinal area that they 

 became permanent additions to the continental mass. Also, it 

 ordinarily expelled the seas from the whole geosynclinal area for a 

 long period — or permanently. Nevertheless, there is much question 

 as to how greatly the orogenic phase increased the surface relief. 

 Restoration of folds and fault blocks that are now eroded gives the 

 impression that orogeny might have produced ranges higher than 

 the Himalayas (Fig. 6), but orogeny may have proceeded so slowly 

 that various leveling processes nearly kept pace with it — erosion 

 wearing off the upraised areas and sedimentation filling the de- 

 pressed areas. If so, creation of truly mountainous relief is largely a 

 post-orogenic event, resulting from processes different from those in 

 operation during the orogenic phase. 



Orogenic Land Bridges. Orogenic belts of this sort are thousands 

 of miles long. Some terminate laterally by fading out of effects of 



