28 BULLETIX 16 9, UXITED STATES NATIONAL MUSEUM 



They include nothing really comparable with the Lebo, and the re- 

 semblance to the Melville is too vague to have any real correlative 

 value. Paleontological correlation alone seems to have any sig- 

 nificance between this region and that near the Crazy Mountains. 



STRUCTURE 



The beds of this field are almost nowhere exactly horizontal and in 

 places are steeply tilted. Even in the course of purely paleontological 

 work it is necessary to take strict account of the structure, since 

 relative levels between the numerous isolated exposures can seldom 

 be measured directly but have to be calculated from the structural 

 data. 



Along the north side of Fish Creek, from the mountains eastward 

 to sec. 23, T. 6 N., R. 15 E., there is a strong monoclinal flexure dip- 

 ping south at angles of about 40° to 75°. North of this fold only 

 Cretaceous beds are exposed. The flexure itself involves the upper- 

 most Cretaceous, Bear, Lebo, and basal Melville. The strike is 

 sinuous but is mainly east and west to the end of the Crazy ^fountains, 

 where the Melville beds mostly run into that range, while the lower 

 beds swing around its northern end. In the section noted, this fold 

 ceases to affect the Fort Union beds, w^hich are affected south and 

 southeast of here by a broad anticline with northeast-southwest trend 

 and low dips up to about 12°. Erosion along the axis of this anticline 

 has formed a great embayment, about 6 miles wide at its mouth and 

 of about the same depth, north and south, surrounded by a high rim 

 on the basal Melville. Widdecombe Creek flows along the soft No. 2 

 zone on the eastern limb of the anticline. Directly east or southeast 

 of this anticline is a parallel syncline the axis of which is occupied by 

 remnants of the basal Melville sandstone rising abruptly above the 

 No. 2 slopes and valleys. Puet Creek cuts deeply into this, separating 

 the axial elevation into the long isolated Bear Butte to the northeast 

 and Lion Butte, to the south and connected with the great mass of 

 No. 3 beds extending westward to the mountains. 



The Hell Creek, Bear, and Lebo swing around the north end of 

 Bear Butte, dipping toward it. The dip increases in intensity here 

 away from the actual Butte, until in the upper Hell Creek northeast 

 of the Butte it reaches about 30°. The Lebo flanks the long east scarp 

 of Lion Butte with low dips, usually 4° or 5°, toward the latter, vari- 

 able and afi'ected by slight local disturbances. South of here, toward 

 the Glass Lindsay Lakes, the Lebo is nearly horizontal, with dips up 

 to 1° or 2°, erratic in direction but oftener to the west. 



Over the greater part of the Melville beds area, south of the strong 

 monocline and west of the Widdecombe Creek anticliae, the dips are 

 prevailingly westward and fairly consistent at 4° or 5° over a large 



