"290 Vr. T. LEE BUILDIXG OF SOrXHERX EOCKY MOI'XTAIXS 



but .such illustrations should not be interpreted as proof of lateral com- 

 j)ression or foreshortening of the crust. The upturning of the foothill 

 formations is as readily explained by drag during vertical uplift as by 

 wrinkling under lateral compression, and the isolated remnants high in 

 the mountains are more readily explained by vertical uplift than 

 otherwise. 



In studvinof the foothill region it becomes strikin^lv obvious that, 

 "^ith few exceptions, the sedimentary formations lie nearly horizontal or 

 are only gently warped : but at the foot of the mountains they are either 

 sharply upturned or broken off at nearly vertical fault planes. There 

 is conspicuous absence of minor lateral folds parallel to the main axis. 

 It is difficult to understand, if the moimtains are due to lateral com- 

 pression, why secondary folds are not more numerous. On the other 

 hand, the abrupt upturning of the sedimentary rocks in the foothills 

 and the breaking along vertical planes seem normal in case the upturn- 

 ing is due to drag on the flanks of the rising mass. 



Again allow me to guard these statements with the assurance that I 

 ^m not including areas beyond the limits of the Southern Eocky Moun- 

 tain Province, for in areas beyond this province lateral compression, 

 to some extent at least, is evidenced by overthrust faults and by folds 

 p)arallel to the axis of uplift. Xor am I thinking of the mountain mass 

 ^s a rigid unit. In my opinion, the anticlines and domes in the rela- 

 tively low-lying oil region of southern Wyoming immediately north of 

 the Southern Eockies gives a picture of the more highly lifted moun- 

 tainous .area as it would have appeared had erosion not removed the 

 sediments from the top. Here the strata are thrown into a series of 

 •domes, anticlines and synclines, of different sizes and shapes, which have 

 about as much s^Tiimetrv as the similar heiofhts and hollows would have 



c 



on a mass of yeast-rising dough. I am inclined to think that if this 

 nonmountainous area immediately north of the high mountains had been 

 pushed up vertically several thousand feet higher than it is, it would 

 now. after erosion, present about the same aspect that the mountains 

 liave. 



Perhaps one more consideration in this connection may be excusable. 

 It is a matter of observation that some of the few minor folds which lie 

 parallel to the foothill ridges are asymmetric, with the steeper side 

 toward the mountains. On the supposition that the mountain mass was 

 squeezed up by lateral pressure, these folds may be regarded as incipient 

 overth rusts tending to ride up onto the flanks of the range. On the 

 •other hand, they may be explained as readily by local sag or settling of 

 parts of the moimtain mass following an uplift. Such settling would 



