282 G. R. MAXSFIELD STRUCTURE OF ROCKY MOUXTAIXS 



Two phases of this compressive activity are illustrated in southeastern 

 Idaho. The earlier phase was that in which the folded and overthrust 

 structures, which form so conspicuous a feature of the mountains, were 

 developed. The compression at this time was localized by a deeply loaded 

 geosyncline. The mountains formed are believed to have been comparable 

 in height to those of today or even higher. They were reduced, however, 

 to a peneplain, or were at least greatly worn down. The second phase 

 was inaugurated by a renewal of the periodic contractional disturbances 

 of the earth. This time the effects were not so definitely localized as 

 before, probably because no accumulation of sediments comparable to 

 those of the former geos^^ncline had taken place and the rigid crust was, 

 therefore, not weighted down as before. The result was a broad uplift, 

 with only gentle foldings or warpings. 



Texsioxal Features ix Mouxtaix Structure 



Kober in his book, about which this symposium centers, takes exception 

 to current views regarding normal faults and undertakes to show that 

 these are compressional features. The Rhine Valley graben, which has 

 become a classic example of normal faulting, he regards as a summit 

 break in a great anticline. He finds overthrust faulting in the vicinity 

 of the graben and an actual overriding of the horsts upon the graben. 

 Kober ascribes the horst and graben structure, which dominates in Ger- 

 many, to the irregular elevation of an old peneplained mountain chain, 

 which, by renewed undulation due to contraction of the earth's crust, has 

 been broken into blocks. The height of the blocks is not equal and not 

 very great ; their borders are broken and overthrust ; the higher uprising 

 blocks tend to overspring the deeper lying basin and trough portions. 

 Kober points out that these structures are very young, that the move- 

 ments are still in progress, and that the superficial shoving together of 

 the horsts may in time go so far as to lead to the overriding of one by 

 another. Thus the intervening graben break might become an overthrust. 



Xo extended discussion of this view may be undertaken here, but it is 

 believed that the studies thus far made in southeastern Idaho have 

 brought out definite evidence unfavorable to it. The Meadow Creek 

 graben, which has been described elsewhere,^* is bounded by horsts that 

 contain folded structures, but no tendency of either horst to override the 

 graben has been recognized. Both horsts and the intervening graben have 

 been cut by transverse faults in such manner that in the area between 



3* G. R. Mansfield : Types of Rocky Mountain sti-uctnre in southeastei-n Idalio. Jour. 

 Geol., vol. 29, 1921. pp. 400-465. 



