428 



HISTORICAL GEOLOGY 



epoch of crustal deformation, which resulted at the close of 

 the period in the formation of many important mountain 

 ranges, especially in the western hemisphere. Besides wide- 

 spread warping and changes of level in western United States, 

 the rocks were folded along a belt from Mexico to Alaska, 

 and also apparently the entire length of South America. 

 This marks the beginning of the present Rocky Mountains 



FIG. 450. Gentle folds characteristic of the Rocky Mountains in Wyoming. 



and the Andes, although the present height of those moun- 

 tains is due chiefly to later movements. The folding in the 

 Rockies at this time was by no means so intense as it was 

 in the Appalachians at the close of the Permian. The folds 

 are chiefly broad arches with troughs between (Fig. 450). 

 Near the Canadian boundary the lateral compression was 

 relieved not only by folding, but by a profound dislocation, 



FIG. 451. The great overthrust in the Rocky Mountains of Montana. 

 The Proterozoic rocks on the left have been pushed up over the Mesozoic 

 rocks on the right. 



the older rocks having been pushed up over the Mesozoic 

 strata along a great thrust plane. At one point the Algon- 

 kian rocks have been thrust out over the Cretaceous beds 

 to a distance of at least seven miles (Fig. 451). 



While the folding and warping were in progress, volcanoes 

 came into existence in many parts of western America. 

 Volcanic mountains comparable to the modern cones of 

 Vesuvius and Fujiyama were built up in Colorado, Montana, 



