284 G. R. MANSFIELD STRUCTURE OP ROCKY MOUNTAINS 



The larger folds actually seen are parts of still larger (major) folds- 

 15 miles or more wide. 



Consideration of isostatic conditions in the northern Eockies shows a. 

 less favorable relationship between the distribution of gravity anomalies 

 and mountain structures there than in the Alps. The northern Eockies 

 are apparently not fully compensated. The relationship of the Snowdrift 

 peneplain to the restored structures seems to indicate that isostatic sink- 

 ing does not fully offset the effects of folding and that high mountains 

 may be formed by folding alone. 



Several hypotheses of mountain-building are discussed in their bearing 

 on the northern Eockies. It is concluded that compressive stresses, origi- 

 nating in gravitative readjustments within the earth and influenced to a 

 greater or less degree by magmatic movements, were localized by the 

 heavily loaded geosyncline, which became folded and overthrust, produc- 

 ing high mountains that were subsequently peneplained. Later com- 

 pressive stresses, originating in a similar manner but not localized by 

 sedimentary accumulations, caused broad upwarpings and gentle folds,, 

 which gave the mountains their present elevations. 



Kober's view, that horsts and graben are due to compressive agencies,. 

 is not supported by geologic evidence in southeastern Idaho. Such struc- 

 tures are still regarded as of tensional origin and as affording means of 

 relief from the overcompression incident to mountain-building. 



