160 



HISTORICAL GEOLOGY 



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and Salisbury). The reason 

 for such differences is not far 

 to seek. For instance, Ordo- 

 vician rocks are actually more 

 widespread than the Pennsyl- 

 vanian, but they are largely 

 concealed under later rocks, 

 while the Pennsylvanian rocks 

 were never extensively cov- 

 ered by later deposits (except 

 Glacial drift and to a small 

 extent by Permian strata 

 in the Appalachian district). 

 Shortly after their deposition, 

 toward the close of the Pale- 

 ozoic, the region was elevated 

 and has remained a land area 

 ever since. Post-Paleozoic 

 erosion has been sufficient to 

 remove much of the Permian 

 and some of the Pennsylva- 

 nian, though large areas of 

 the latter rocks still remain as 

 shown on the accompanying 

 map. 



Pennsylvanian rocks are 

 extensively developed in the 

 western United States except 

 in Montana, Idaho, Washing- 

 ton, and Oregon. They prob- 

 ably underlie much of the 

 Great Plains just east of the 

 Rockies from Wyoming to 

 Texas. They are also pretty 

 widespread in Alaska. 



In the Appalachians (Fig. 

 94), Nova Scotia, and New 

 Brunswick, and in the moun- 

 tains of the west, the rocks are 

 highly folded or tilted, but 



