HO UNIVERSITY OF COLORADO STUDIES 



now prevails. At or soon after the close of Mesa Verde time the brackish- 

 water lagoons ceased to exist and the elevation of the region probably 

 began, though it is doubtful whether this occurred until after the period 

 represented in other localities by the Lewis shales and the Laramie 

 formation, for it seems that the absence of the two latter in the Grand 

 Hogback is due to erosion rather than to non-deposition. 1 The post- 

 Mesa Verde elevation of the land likely progressed through Tertiary 

 time and may even continue to the present day. The entire region has 

 been elevated about a mile, or, to speak more precisely, its position has 

 been changed that amount with reference to sea-level, for whether the 

 difference is due entirely to actual elevation with reference to the center 

 of the earth, or partly at least to the tilting of portions of the earth's 

 crust and consequent withdrawal of the ocean into deepening basins, 

 may never be definitely known. The widespread change in the vertical 

 configuration of the continent at and after the close of the Cretaceous 

 was not uniform, but was accompanied by tilting on a large scale, 

 leaving the portion of the continent bordering the Rocky Mountains 

 high above sea-level, while portions of the continent to the east, west 

 and south were but little elevated. The differential movement was 

 particularly pronounced at places along the present borders of the sedi- 

 mentaries, leaving the strata resting at high angles upon the mountain 

 slopes. The Tertiary beds overlying the Mesa Verde were probably 

 deposited under varying conditions and may be partly subaerial, partly 

 fluviatile and partly lacustrine. During and since the deposition of the 

 Tertiary beds rapid erosion has prevailed and numerous canyons have 

 been cut to depths varying from a few hundred to several thousand 

 feet. During Pleistocene time the higher region in the eastern part 

 of the area explored by us was subjected to the action of numerous 

 glaciers, originating at an altitude of from n,ooo to 12,000 feet, uniting 

 and moving down White River Valley to an altitude of 7,500 feet or 

 less, modifying the valley and upon final retreat leaving behind the usual 

 hummocky topography, moraines and lakes. Glacial sinkholes 2 

 gave the name "Pothole Valley" to that portion just above Buford. 



1 Richardson, U. S. Geol. Surv., Bull. No. 371, p. 18. 



' Hayden's Atlas 0} Colorado, etc., sheet v. 



