420 blackwelder: new formations in Wyoming 



2. Dolomite; dense, slabby, pale gray. Beds 1 to 4 inches 

 thick. Sprinkled with small crinoid stems, and traces of 

 bryozoans (?) Base not visible 2.0 



Massive member of Bighorn dolomite {Ordovician) 



1. Extremely massive, cream-colored dolomite, mottled with 



gray in alga-like patterns over 100 



The Leigh member differs from the rest of the Bighorn in being 

 characterized by thin, dense and brittle, flaggy strata with smooth 

 milk-white surfaces. It is stated verbally by Kirk and Tomlin- 

 son that, to the southwest in the Bear River Range of Utah and to 

 the northeast in the Bighorn Range of Wyoming, the thin milk- 

 white dolomites of the Leigh member are interbedded with the 

 very massive rough weathering strata typical of the Bighorh. 

 Nevertheless, it seems probable that correlations w4th the Leigh 

 may be made in some districts outside of western Wyoming, 

 where it ite typically developed. 



Although the dolomite generally appeals to be barren of or- 

 ganic remains, a characteristic fautia, consisting largely of ostra- 

 cods with some pelecypods and gastropods, has been found at 

 several localities. This fauna is assigned by Ulrich and Kirk 

 to the Richmond horizon of the ate Ordovician. The member 

 takes its name from Leigh Canyon, on the west slope of the Teton 

 Range, for on the south side of that valley there are excellent 

 exposures of the dolomite in its typical condition. 



DARBY FORMATION 



The introduction of this new term is made necessary because 

 none in present use fits the stratigraphy of w'estern Wyoming. 

 In the Threeforks section Peale gave the name Threeforks shale 

 to the upper division of the Devonian, but included the lower 

 dolomites, which are also Devonian, in his Jefferson limestone. 

 Hague, Weed, and others in the Yellow^stone National Park 

 later misused the term ''Threeforks" to include also the thin- 

 bedded, dark-colored dolomite of the upper part of Peale's Jef- 

 ferson, thus limiting the latter term to the very massive beds of 

 gray dolomite below. The Darby formation is apparently equiv- 

 alent to Peale's Three Forks shale plus the upper part of his 



