456 University of California Publications in Geology [Vol. 10 



STEUCTUBE 



The dominant structural feature of the area is a rather long anti- 

 cline trending somewhat west of north by east of south, roughly 

 paralleling the front of the Shoshone Mountains. The long strip of 

 Mesaverde shown along the eastern side of the map represents the 

 eastern limb of this fold. The small anticlines mapped are minor 

 folds along the axis of this larger anticline. This larger fold is one 

 of several along the southwestern border of the Bighorn Basin. These 

 folds become broader and natter and progressively younger rocks are 

 exposed on their crests as the center of the basin is approached. 



As shown by the sketch map (pi. 38), three folds whose axes are 

 arranged more or less en echelon form the crest of the larger anti- 

 cline. The most northerly and largest anticline will be referred to 

 as the Sunshine anticline and the most southerly as the Gooseberry 

 Creek anticline. 



Sunshine Anticline. — This fold is about 4 ] /o or 5 miles in length 

 and extends from Redtop Mountain to north of Wood River. In the 

 southern part of the fold the axis trends a few degrees east of north. 

 Just north of the center the axis makes a rather sharp turn to the 

 northwest and then trends N15°W. The southern part is sym- 

 metrical, the dips being 31E on the east and 33°W on the west 

 limb. Toward the northern part the dip of the east limb decreases 

 to 20°, while that on the west increases until it reaches a maximum 

 of 51°. The anticline plunges at both ends, the dip to the south being 

 6°. On the north the angle at which the anticline plunges could not 

 be measured owing to the capping of terrace gravels. The fold dies 

 out. to the south, no trace of it being found south of the Tertiary 

 mantle forming Redtop Mountain. 



The Mowry shale is the lowest formation exposed in the Sunshine 

 anticline. This formation with its hard sandstone members forms 

 the ridge which follows the axis of the fold in the southern and central 

 parts (pi. 34a). Flanking this central ridge are minor ridges separated 

 by small depressions. Outcropping along these flanking ridges are 

 the sandstone members of the Frontier formation, the depressions 

 being occupied by the Frontier shales. Beyond these low ridges along 

 the eastern side of the fold is a broad depression cut from the Cody 

 shale. North and west of Wood River the anticline finds no expression 

 in the topography on account of the flat terraces cut in earlier stages 

 in the history of Wood River. The Frontier formation closes around 



