ISOSTATIC FEATURES 



279 



plains may have a bearing on the problem. On the other hand, the 

 Columbia River basalts lie in an area in which there is a gravity defect. 

 Extensive sedimentary areas occur in Colorado, Utah, Wyoming, Mon- 

 tana, and farther eastward. These have plus anomalies which seem to 

 indicate that much of the mountain-built area is not fully compensated, 

 and that the batholiths of central Idaho and Montana may not have a 

 close isostatic relationship to the folded rocks of the northern Rockies. 



In the symposium on isostasy at Amherst last year Reid^- called atten- 

 tion to the fact that isostatic sinking tends to offset the effects of folding, 

 thereby militating against the elevation of mountains to great heights by 

 compression. He raised the question whether folding can actually pro- 

 duce high mountains. The present study affords a means of testing this 

 question. 



The Snowdrift peneplain, which is the highest recognized erosion sur- 

 face in southeastern Idaho, represents the surface to which the mountains 

 were there reduced by erosion after the Laramide folding and after the 

 sinking of the folded mass under isostatic conditions. This peneplain, 

 which now stands 9,000 feet, or 1.7 miles, above sealevel, probably affords 

 a maximum measure of the amount of subsequent uplift which the folded 

 area has experienced in being elevated to its present position. If this is 

 subtracted from the theoretical elevation, 6.1 miles, suggested by the 

 restoration of the upper beds of the Cretaceous, figure 2, there still re- 

 mains a difference amounting to 4.4: miles. How much of this difference 

 to allow for the erosion that prevented the mountains from attaining the 

 full height to which the folding might otherwise have raised them it is 

 impossible to say from present data. If it be assumed that this erosion 

 consumed half or two-thirds of the folded mass, the mountains might 

 have reached elevations ranging from 1.5 to 2,2 miles. It appears, there- 

 fore, that folding of the strata, with due allowance for isostatic sinking, 

 may produce mountains comparable in height to those of today. This is 

 without regard for any allowance that might be made for changes in 

 density of material within the mountain mass induced by unloading 

 through erosion, which would tend to further increase the height of the 

 mountains. The later uplifts, which most of the higher ranges have 

 experienced, represent separate mountain-building activities of perhaps 

 another sort, but it is not necessary to infer from their presence that high 

 mountains may not be formed by folding alone. 



3"-* H. F. Reid : Isostasy and earth movements. Bull. Geol. Soc. America, vol. 33. 1922, 

 pp. 323-324. 



XIX — Bull. Geol. Soc. Am., Vol. 34. 1922 



