Morrison 



426 N. H. DAK.TOX STRATIGRAPHY OF THE BLACK HILLS, ETC. 



The basal, reddish sandy member of the Morrison at this place is an 

 unusual feature in Colorado. East of Lyons there are excellent ex- 

 posures of the Morrison exhibiting the very unusual thickness of about 

 420 feet, consisting of the following beds : 



Feet 



" Dakota " Sandstone, hard, massive, bnff , 



j Olive green massive shale, with some sandstone layers. . 150 



Light grayish green massive shale 30 



Soft to hard gray sandstone, fine grained 15 



Red, maroon, and green massive shale 150=fc 



Massive buff sandstone, moderately fine 10 



Grayish green to maroon massive shale, with thin layers 



of fine grained sandstone 40 



[_ Covered 25 



Upper Wyoming. . Soft buff sandstone 30 



The typical Morrison formation in the region west of Denver has been 

 denned by Eldridge* as consisting of fresh water marls having an 

 average thickness of 200 feet, the amount varying somewhat. Its upper 

 limit is sharply defined by the " Dakota " sandstone, while the brown 

 and pink sandstone, regarded as the top member of the u upper Wyo- 

 ming," clearly marks its lower limit. 



Mr Eldridge's description of the formation is as follows : 



" The marls are green, drab, or gray and carry in the lower two-thirds numerous 

 lenticular bodies of limestone of a characteristic drab color and a texture compact 

 and even throughout. A small but persistent band of sandstone and limestone in 

 thin alternating layers occurs about 20 feet above the base. In some places the 

 arenaceous elements largely predominate, and near Mount Vernon, 3 miles north 

 of Morrison, and in the vicinity of Van Bibber creek there are at about this hori- 

 zon from 10 to 15 feet of dull gray or yellowish sandstones." 



" The upper one-third of the formation is generally a succession of sandstones 

 and marls, of which the former predominate." "The most important sandstone 

 occurs just above the lower clays, is very persistent, and from contained saurian 

 remains has been called the saurian sandstone. It varies in thickness between 5 

 and 35 feet, and in its distance below the Dakota from 10 to 125 feet, although 

 more generally it lies 50 to 80 feet below." "The sandstone is locally divided 

 into several layers by thin intercalations of drab clay. In the vicinity of Turkey 

 creek these clays reach the unusual thickness of 20 to 30 feet, the sandstones ag- 

 gregating about 20 feet." "The shales overlying this sandstone are similar to 

 those comprising the bulk of the formation, but carry through them a number of 

 minor sandstones and occasionally one or two strata of limestone." 



"The cause of the variation in the thickness of the upper half of the formation 

 could not be determined from conditions existing in the Denver field, but an oft- 

 suggested unconformity at the base of the Dakota may be the explanation." 



* .Monograph U. S. Ueol. Survey, vol. 27, 18%, pp. 61-G2. 



