452 



N. H. DARTON PALEOZOIC AND MESOZOIC OF WYOMING 



characteristic of the jSTiobrara formation. It is conspiciious west of Lar- 

 amie, on the east side of the moiintains east of Laramie, about Eock Eiver 

 and ]\Iedicine Bow, in Little ]\Iedicine yalley, about the Preezeout and 

 Shirley hills, and on Mudd}' creek southeast of Casper. It also appears 

 south and southwest of Douglas and Casper, but west of the ISTorth Platte 

 it rapidly loses its calcareous components, and the characteristic colonies 

 of Ostrea congcda cease to occur. 



^Yind River-Old Creel: region. — In the "Wind Eiver basin the Colorado 

 formation varies from 750 to 1,650 feet thick, the greater amount being 

 along the south side of the Owl Creek uplift. The stratigraphic succes- 

 sion is nearly uniform throughout. The lowest beds are several hundred 

 feet of dark shale containing near the base a sandstone from 2 to 20 feet 

 thick. 'Next come the Mowry beds, which average 100 feet thick and 

 give rise to prominent round-topped ridges, which are especially conspic- 

 uous along the north side of the valley of Mud creek, in the flexures east 

 and west of Thermopolis, and about the head of Dry creek and along the 

 outcrops northeast and south of Lander. In places the impressions of 

 fisli scales are so abundant in the Mowry beds as to show from 5 to 10 

 on a surface 6 inches square. Near Port "Washakie and southwest the 

 Mowr}!- l)eds are about 500 feet above the base of the formation, but 

 farther north there is less shale below them. Next above are several 

 hundred feet of gray shales, the amoimt diminishing to the south, con- 

 taining near the top one to three beds of gray to biiff sandstone, usually 

 constituting ridges, as shown in plate 30. Near Lander there are three 

 of these beds, the top and lowest ones thin and partly shaly, the middle 

 one liard and thick, locally attaining a thickness of 40 feet. A thin bed 

 of coal occurs in one area which near Dallas has a local thiclcness of 18 

 inches. A fourth sandstone is included near Port Washakie. Near the 

 top of the formation are from 100 to 150 feet of dark shales which con- 

 tain lens-shaped concretions, mostly from 2 to 3 feet in size, containing 

 Prionocychis icooJgari — a form highly characteristic of the uppermost 

 portion of the Benton (Carlile formation) in most regions. It often 

 attains a diameter of from 14 to 20 inches. The following section, meas- 

 ured east of Black mountain, illustrates the stratigraphy of the- formation 

 in the western portion of the Owl Creek uplift: 



Section of Colorado Formation on Red Creek, 3 Miles East of Blade Mountain 



Feet 



Gray, massive sandstones, mostly hard; many fossils 35 



Dark shale with sandstone beds 200 



Light gray, sandy shale, weathering light buff 40 



Coarse, buff sandstone. , , 10 



