CONTINENTAL DEPOSITS OF ROCKY MOUNTAIN REGION 599 



and clays with prominent belts of conglomerate, the whole increasing in coarse- 

 ness of sediment as it approaches the Uintah on the south and the Wahsatch on the 

 west. Here is an area about sixty miles from north to south by fifty miles from 

 east to west which is essentially a plateau of Vermilion Creek beds." * 



Near the border of this early Eocene basin "conglomerates become more im- 

 portant, until directly north of the upper canyon of Weber river the mountain 

 wall is composed of excessively coarse conglomerate between 3,000 and 4,000 feet 

 thick. It is almost structureless, and lines of stratification can rarely be perceived. 

 The blocks of which the conglomerate is chiefly formed range from the size of a 

 pea to masses with a weight of several tons. . . . The rapidity with which 

 these conglomerates grow finer in advancing from the shore along the Uintah is very 

 conspicuous." f 



The Arapahoe and Denver formations (closely associated with the Tertiary, if 

 not actually belonging in this division of geological time), extending eastward 

 from the base of the Front range in Colorado, consist chiefly of conglomerates and 

 sandstones near the foothills and of finer sediments on the plains. In the foot- 

 hills " the sandy parts of the bed develop in places to wedge-shaped masses, ex. 

 hibiting in their relations to each other and to the conglomerates a very marked 

 cross-bedding." $ On the plains a characteristic feature is " the irregular uncon- 

 formable contact so frequently seen to exist between a conglomerate or grit layer 

 above and a clay or shale layer below. . . . Often the unconformability is very 

 marked. . . . The changes in conditions of sedimentation which gave rise to 

 such stratigraphical relations of consecutive beds were, however, common in both 

 Denver and Arapahoe epochs. Fine sediments were often disturbed and locally 

 removed at the beginning of periods of rapid deposition of coarser materials." § 

 Mention is made of "tree stumps in erect position with roots in mud layers and 

 broken trunks in sand or gravel ; " || and it is recognized that the theory to account 

 for these deposits must be conditioned " by the frequent cross-bedding observable 

 both in sandstone and conglomerate, and by the plant remains and standing tree 

 stumps that abound at certain horizons." 1[ 



Fossils contained in a geological formation are always regarded as highly signifi- 

 cant of the conditions of deposition. The fossils of the western Tertiaries are 

 chiefly land mammals and plants in great variety, to which are added the remains 

 of birds of the air, reptiles of land or fresh water, fishes of fresh water, and mol- 

 luscs of rivers, marshes, and lakes. 



Theories of Lacustrine and Fluviatile Deposits 



If facts such as those above stated were today placed before a geologist for ex- 

 planation he would undoubtedly begin his theorizing by excluding the sea as the 

 place of deposition on account of the absence of marine fossils. He might then 

 provisionally consider the possibility of lacustrine deposition, and here he could 

 begin by reviewing what has been learned concerning the deposits now forming on 



* Fortieth Parallel Survey, vol. i, p. 372. 



•j-Ibid., p. 369. 



X U. S. Geol. Survey Monograph, vol. xxvii, p. 103. 



§ Ibid., pp. 180, 181. 



|| Ibid., p. 168. 



If Ibid., p. 33. 



LXXXIV— Bull. Geol. Soo. Am., Vol. 11, 1899 



