TEXSIOXAL FEATURES IN MOUXTAIX-BUILDING 283 



these faults the distance between the horsts has been increased. Had 

 these faults and the boundary faults of the horsts been overthrusts, this 

 distance should have been diminished rather than increased. 



In general, there seems no good reason for abandoning the idea that 

 tension faults do exist and that they may play an important part in 

 mountain structure. The current view that tensional faulting serves as 

 a method of relief for overstrained or overcompressed structures is be- 

 lieved to be well founded. 



Summary 



On the basis of detailed studies in southeastern Idaho and more cur- 

 sory observations in Montana and elsewhere, some of the broader struc- 

 tural features of the northern Eocky Mountains are discussed. 



A review of the great thrust-faults as described in the literature shows 

 that they vary in length from a few miles to nearly 300 miles, and in 

 amount of dislocation from 2 or 3 miles to nearly 40 miles. They vary 

 also in age, the later ones lying nearer the eastern front of the mountains. 

 Thom regards these faults as parts of a continuous major fracture, along 

 which the great overthrust sheet impinged upon older east-west uplifts 

 and was disrupted by them. Kober considers the eastern ranges of the 

 Eocky Mountains as border chains of a geosyncline thrust forward upon 

 a foreland. These views are briefly discussed as working hypotheses. 



The question of underthrusting as opposed to overthrusting is out- 

 lined and the conclusion reached that the available geologic evidence 

 favors overthrusting, though it is recognized that the direction of move- 

 ment may be relative and that both upper and lower blocks may be 

 actually involved in the movement. 



On the basis of an attempted restoration of structure in southeastern 

 Idaho the following conclusions were reached : 



Crustal shortening by folding alone amomited to 21 per cent, or, con- 

 sidering both folding and overthrusting, to 52 23er cent of the original 

 length of the area studied. 



The depth affected by folding alone was about 22 miles, but if over- 

 thrusting and folding are considered together, this depth is reduced to 9 

 miles. The lower portions of the folded structures should therefore be 

 metamorphosed, but further erosion of at least 0.8 mile will be necessary 

 to expose the metamorphosed" portions. 



Since the overthrusting was a relatively late feature of the deforma- 

 tion, the earth's crust was more deeply affected during the early stages 

 and the more intensive effects were shallower. 



