288 AV. T. LEE BUILDIXG OF SOUTHERX EUCKY MOUXTAIXS 



opening of the Upper Cretaceous eix)eli they were worn down to baseleveh 

 In the meantime a broad highland of continental proportion seems to 

 have arisen in the region of the present Great Basin, cutting off con- 

 nection with the Pacific, and detrital matter from this new highland 

 spread eastward over the space between this newly formed continent 

 and the last survivals of the vanishing Ancestral Eockies. 



As the submergence of the baseleveled Ancestral Eoc-kies is an im- 

 portant consideration in this discussion, I pause here to note that some 

 still cling to the belief of the earlier workers in this region, that certain 

 ^'islands'^ persisted in the Southern Eoeky Mountain Province throughout 

 Cretaceous time. But the announcement years ago that no such 

 "islands'* could be found, and that there are good reasons for believing 

 that the Upper Cretaceous formations once extended uninterruptedly 

 over the site of the Southern Eockies, has not been challenged. Until 

 this concept is shown to be untenable, I may reasonably assume that the 

 Eock}' Mountain geos}Ticline which formed in western Xorth America in 

 Cretaceous time included the site of the baseleveled Ancestral Eockies. 



The sea entered the geos}Ticline and for a long time received and 

 distributed the detrital matter from the newly formed uplands to the 

 west. It would seem from some writings that the trough is supposed to 

 have formed rapidly, and that the interior sea was deep and open 

 throughout Upper Cretaceous time. Opposed to this is the radically 

 different conception that the geosyncline formed slowly ; that the waters 

 throughout the epoch were so shallow that fine material was distributed 

 somewhat uniformly over the bottom; that the filling practically kept 

 pace with the sinking of the bottom, and that several times during the 

 Upper Cretaceous epoch sand and silt entering from the west built low 

 marginal plains over large areas from which the sea water was tempo- 

 rarily excluded. The slow subsidence of the trough, coupled, perhaps, 

 with a rise of sealevel, and the conflict between sea and sediments con- 

 tinued until several thousands of feet of material, mostly sand and silt, 

 had accumulated. 



Then, at the close of the Cretaceous period, the crustal movement was 

 reversed. The deeply buried base of the Ancestral Eockies was raised 

 and highlands reappeared where the sea had been. Here again a com- 

 mon conception seems to be that a radical change occurred and a rapid 

 rise of land took place to mountains of such proportion as now exist. 

 Opposed to this is the conception that the first uplift — that is, the one 

 which closed the Cretaceous period — was relatively mild. The signifi- 

 cant fact, however, is the reversal in the direction of movement. The 

 rate and extent of uplift are of minor consequence. 



