222 E. B. BRANSON ORIGIN OF RED BEDS OF WESTERN WYOMINO 



Almost all the Eed Bed sandstones are ripple-marked on plane surfaces 

 and the beds are uniform in thickness and texture for long distances. 

 Beds that occur 15 miles south of Lander may be traced along the out- 

 crops almost continuously to Bull Lake Creek, a distance of some 60 

 miles, with slight variations in thickness and texture — variations so small 

 that only detailed measurements could detect them if they are present. 

 This is true for all the beds noted by the writer excepting the gypsum, 

 Popo Agie beds, cross-bedded sandstone, and the limestone. Such regu- 

 larity is found only in deposits made in bodies of standing water. Bar- 

 rell says: "It appears, therefore, that typical water-made ripple-marks 

 associated with regularity of bedding in sandstones is especially associated 

 with the subaqueous plain of deltas and the bottoms of shallow seas." ^^ 



The cross-bedded sandstone is succeeded by red sandstone, and gypsum 

 beds occur only a few feet above it. In the Lander region the gypsum 

 rano-es from a few inches to 40 feet, but maintains a thickness of a few 

 feet for long distances along the outcrop. The thick deposits are limited 

 in extent, rarely running for more than a mile, and seem to be fillings 

 of depressions in the main basin floor when the deposition took place. 

 The g}'psum is remarkably pure and deposits 40 feet in thickness contain 

 only a few thin partings. 



It is the writer's opinion that the gypsum beds are strong evidence of 

 the subaqueous origin of the Eed Beds, but Mr. Scliuclieit nia])S the IJecl 

 Beds as subaerial in his paleogeographic maps, and an eminent stratigra- 

 pher wrote me, after reading the first draught of this paper: "Xote the 

 extreme vai-iiil)ility of your gypsum deposits. Translate this into topog- 

 raphy and you get a series of pools. . . . T would rather you test out 

 the idea that these are fresh-water deposits under an arid climaTe such 

 as the Triassic was in this region." 



It seemed worth while to give this liyjiothesis n quantitative test and 

 the results are outlined below. 



CoxDrriONs of the uppki; 1\i:i) I>ki) (ivi'sr:\[ Drposits 



L The gyi'suni outcrops in ])laces oxci- an area of at least "iO,!)!)!! 

 square miles. 



2. Probably half of the 20,000 square miles is underlain by gypsum 

 averaging 1 foot thick and one-tenth oL' ihe ai'ea by beds 10 feet thick 

 or more. 



3. It thickens and thins reinarkal)ly in <lioii dis(;in(a'<. i^angini; fi-oni 

 a few inches to 40 feet. 



liBull. Geol. Soc. Am., vol. 23, p. 420. 



