248 Dalie — Episodes in Rocky Mountain Orogeny. 



the sandstones pass under the Madison rather than lap 

 against the f anlt scarp. 



It is definitely established, then, that there were, in this 

 region, at least two well-marked episodes of deformation, 

 one being pre-Fort Union ( f ), as the term is here nsed, and 

 the other post-Fort Union ( f). Plate VI B of the report 

 of Hewett and Lupton^^ shows Fort Union beds, them- 

 selves dipping at notable angles, resting with plainly vis- 

 ible discordance on more steeply dipping Lance (Ho) 

 beds, and constitutes a decidedly graphic representation 

 of the facts. 



The dating of the above episodes depends upon the cor- 

 rect correlation of the beds involved. The latest beds dis- 

 turbed by the first deformation, in the area studied, are 

 Cody shales, the upper members of which are probably 

 Pierre. Farther east, Mesa Verde, Meteetsee, and Ilo 

 (Lance) are probably also involved. This makes the first 

 disturbance post-Cody, probably post-Lance. And it was 

 closed, or at least in quiescence, before the pebbly beds 

 were laid down. The utmost importance, then, attaches 

 to the age of these pebbly beds. 



The writer has already given his evidence^^ for believ- 

 ing that they are the Fort Union of Hewett 's Shoshone 

 Eiver Section. This, together with some additional evi- 

 dence, is here summed up briefly. The formation de- 

 scribed consists of buff to bright yellow sandstone, in 

 beds two to twenty feet thick, alternating with shaly 

 members which are dominantly gray, but which contain 

 occasional red layers. The sandstones are prominently 

 and intricately crossbedded, and carry many large sand- 

 stone concretions of a darker color (brown to gray) very 

 resistant to weathering, which protrude from the surface 

 of outcrops and at many places lie strewn abundantly 

 over the surface of the ground. At many points, and in 

 several beds, the sandstones are distinctly conglomeratic, 

 the pebbles consisting of red granite and pegmatite, 

 basalt, brown quartzite, sandstone, black and brown chert, 

 and shale. They are well-rounded, and average from one- 

 fourth to one-half inch, but in rare cases exceed two 

 inches. Thin seams of lignite were noted at various hori- 

 zons, and one workable seam of coal was noted, from 



"Hewett, D. F., and Lupton, T. C; loc. cit. 

 " Dake, C. L. ; loc. eit. 



