666 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Creek beds north of Twin Creek on two carefully measured sections was found to be 3,500 and 

 3,800 feet. * * * 



The Beckwith formation, which directly overhes the Twin Creek, has been so named from 

 its occurrence and extensive development on leased State lands now forming part of the Beck- 

 with ranch, situated just east of Beckwith station on the Oregon Short Line. It is here and 

 throughout the west side of this area composed of two rather distinct members, a lower red-bed 

 member, composed of interbedded sandy clays, sandstones, and conglomerates 2,500 feet thick; 

 and an upper member, composed of rather light colored interbedded sandstones and clays. 

 The sandstones are commonly rather light yellow and the clays vary from yellow to light piakish 

 red. * * * The upper member has a thickness of 3,000 feet or more. 



In the eastern belt of older rocks these two phases merge; in the area along the west side 

 of the great fault just west of Hilliard these beds, while having a predominant reddish cast, are 

 all hght in color. The conglomerates, which near Beckwith are deep red, are here white to 

 yellow. These reddish beds are exposed along the whole of the eastern side of the eastern belt 

 just east of the Cretaceous exposures, and for the most part form the crest of the pronounced 

 antichne which skirts this region on the east. These beds here reach a total thickness of 4,000 

 feet. * * * 



This formation is hthologically distinct from the very fossiliferous, dark-colored beds which 

 overhe and underlie it, and compared with them is essentially unfossiliferous. Dr. Stanton has 

 found marine Jurassic fossils at two points in beds which are regarded as in the lower part of 

 the Beckwith formation. At old Bear River City, just west of the great fault hne and east of 

 the principal conglomerate bed at this point. Dr. Stanton found Belemnites densus M. and H., 

 Trigonia quadrangularis H. and W., Myacites (Pleuromya) weberensis Meek ( ?). In the exposures 

 north of this point this horizon appears to be distinctly underlain by the characteristic unfossil- 

 iferous Beckwith beds, and unless it represents a portion of the Twin Creek which has been 

 faulted up — and this is not regarded as probable — it is distinctly in the Beckwith formation. 

 This fossil-bearing layer is here about 1,600 feet below the Bear River formation and 2,400 

 feet below the Aspen shales. South of RoclqDort, on Weber River in Utah, Dr. Stanton found a 

 specimen of Trigonia quadrangularis H. and W. (?) about 2,000 feet above the characteristic 

 fossiliferous blue thin-bedded limestone and shale of the Twin Creek formation and 3,500 

 feet or more below the lowest observed black shales with fish scales, representing the Aspen 

 formation, which here, in the absence of the Bear River, is the base of the known Cretaceous. 



The lower part of the Beckwith formation is thus clearly upper Jurassic, and the remainder 

 probably contains time equivalents of the lower Cretaceous and Dakota beds, if these occur in 

 this area. 



Veatch gives a list of fossils from the Twin Creek formation. 



K 12-13. NORTHWESTEBN COLOBADO. 



In describing the Rangely oil district, Colorado, Gale^^'" gives an account of 

 the Jurassic strata which constitute the White Cliff and Flaming Gorge formations 

 of Powell. He says: 



Immediately below the Dakota sandstone is a mass of variegated badland-forming shale 

 and clay, including some harder beds of limestone and sandstone, together with a pecuUar 

 dark cherty, siliceous conglomerate almost exactly like that commonly found with the Dakota 

 formation above. The shale or marl is prevaihngly of greenish and pinkish shades, with some 

 beds that weather yellow. The lower 100 to 150 feet of this formation is composed of darker- 

 colored beds, fine greenish sandstone and calcareous rock. Limestone layers composed largely 

 of shells are present near the base, although the general character of the beds seems to indicate 

 much limy or marly material throughout. These beds have an approximate thickness of 800 



