CRETACEOUS UPLIFT OF OYSTER RIDGE. 251 



which the eastern side of the fold has disappeared. In their parallehsm, 

 their generally steep angles, and the persistency of direction of their strike, 

 these folds resemble those of the Appalachian system more than those gen- 

 erally observed in this region. 



At the northern limit of the map, Ham's Hill is composed of white 

 shaly sand-rocks and massive sandstones, having a strike a little east of 

 north and dipping 20° to the westward, which, from its general lithological 

 character, is considered to belong to the Fox Hill group. A probable con- 

 tinuation of this fold to the northward has been observed by Professor Cope 

 on Fontanelle Creek, where he notes a well-defined anticlinal fold, in which 

 are exposed by erosion conformable beds as low as the Upper Carbonif- 

 erous limestone. To the west of Ham's Hill, the eastern foot-hills of Aspen 

 Plateau, as seen from a distance, are composed of almost horizontal beds of 

 the pinkish gravels of the Vermillion Creek series. This region not having 

 been thoroughly explored by our parties, it is not impossible that, in some 

 of the deeper ravines, there may not be exposures of underlying Cretaceous 

 rocks, which have not been indicated on the map. 



At the point where the Little Muddy cuts through Oyster Ridge, similar 

 sandstones to those found on Ham's Hill form the western edges of the ridge, 

 with a dip of 20° to the westward The strike at this point is north 15° 

 east, but assumes a northern direction a few miles farther north. In the 

 little circular valley just west of the ridge, at this point, are exposed some 

 clayey beds, too much' disintegrated to show a definite structure; but as they 

 are succeeded on the east by the same sand-rocks, dipping 25° to the east- 

 ward, with a strike of north 15° east, they have been colored as belonging 

 to the Colorado series. At no other point was the eastern member of this 

 fold observed. The overlying striped red and white clays of the Vermillion 

 series form a semicircular wall to this valley on the east, dipping eastward 

 at an angle of 3° to 4°. 



In Oyster Ridge, south of the gap, the two strikes above mentioned are 

 observed in conjunction. From the gap a few miles southward, the strike 

 continues nearly north and south, and then bends to the westward, assuming a 

 position of 30° east of north, while the dip at the curve steepens to 45° west. 

 In the sandstone beds, which form the crest of this ridge, are found immense 

 quantities of remains of Ostrea soleniscus, a long narrow shell from 8 to 12 



