RESTORATION OF STRUCTURES IN IDAHO 277 



Willis/^ on the basis of experiments by Adams and Bancroft, thinks that 

 this depth may be as great as 40 miles; bnt the smaller figure seems to 

 accord better with geologic evidence. The average altitude of the present 

 surface in the western half of the structure section is 1.3 miles, the aver- 

 age for the eastern half is 1.35 miles, and for the whole section 1.33 miles. 

 If these figures are subtracted from the restored heights given above, the 

 results are 5.4 miles for the western half, 4.35 miles for the eastern half, 

 and 4.8 miles for the whole section. These figures, which represent the 

 depth of the present surface below the restored surface, fall well above 

 the depth of the zone of flowage as given by Van Hise and indicate that 

 the visible structures were developed considerably above that zone. This 

 result is in accord with the general absence of regional metamorphism 

 "throughout the district represented by the section. 



If the average altitude of the present surface be added to the figures 

 ■obtained above for the depth of folding and compression below sealevel, 

 the average result will be 23.53 miles if only folding is considered, or 

 10.43 miles if both folding and overthrusting are taken into account. In 

 -either case the lower part of the compressed mass should fall within the 

 zone of flowage and the deeper structures should be metamorphosed. On 

 the basis of the figures given above, further erosion, amounting to 0.8 

 mile or more, will be necessary before the average depth of the meta- 

 morphosed portions may be reached. 



The principal folding in southeastern Idaho occurred before the over- 

 thrust faulting. According to the computations above, the depth affected, 

 when only folding is considered, is greater than the corresponding depth 

 when both folding and overthrusting are taken into account. These data 

 ^suggest that in the earlier stages of compression the earth's crust was more 

 deeply affected than at later stages. In other words, the zone of com- 

 pression as it became more intense migrated outward toward the surface. 



It has been stated that some of the folds mapped in southeastern Idaho 

 ■exceed 50 miles in length and 3 miles in width. Seen in the field, each 

 -of these structures appears to be relatively large and independent, though 

 many smaller folds are present. The restoration, figure 2, shows that 

 the large folds in turn are members of still larger structures, and that 

 the region traversed by the structure section contains the greater part of 

 two such folds, an anticline and a syncline, each of which is approxi- 

 mately 15 miles wide. The size of these larger structures, which may be 

 termed major folds, affords a means of comparison of the intensity of 



2s Bailey Willis : Discoidal structure in the lithosphere. Bull. Geol. Soc. America, vol. 

 .31, 1920, pp. 247-302. 



