34 MANSFIELD! MESOZOIC OF FORT HALL RESERVATION 



northern Utah, as described by Richardson, 2 although it was. not 

 practicable in the field work to give much attention to the lower 

 formations. The Mesozoic formations are shown in the accom- 

 panying table (Table I) . 



WOODSIDE SHALE 



The Woodside shale, the lowest Mesozoic formation in the dis- 

 trict, takes its name from Woodside Gulch, in the Park City 

 Mining District, Utah. 3 It has been described in a number of 

 reports and only differences from the usual types need be men- 

 tioned here. In Utah and near Paris, Idaho, some of the beds 

 of the Woodside shale are red, whereas in most of the south- 

 eastern Idaho region the formation is characterized by yellowish 

 and olive-drab tints. Near the base in the Fort Hall Indian 

 Reservation the beds have a distinctive reddish brown tint and 

 are relatively sandy. 



The base of the Woodside shale, which in regions previously 

 studied has been rather sharply distinguished from the underlying 

 Phosphoria formation of Permian(?) age by a lithologic as well 

 as a faunal change, is not so clear here. A Paleozoic fauna, 

 consisting chiefly of the brachiopod Ambocoelia in abundance, 

 together with pelecypods suggesting Paleozoic characteristics 

 but not definitely identified, is found in the Rex chert member 

 of the Phosphoria formation above the chert. It occurs in brown- 

 ish yellow, sandy shales and limestones not easily distinguished 

 lithologically from the Woodside, although the faunas of the ad- 

 jacent formations where well developed are very different. Par- 

 ticular interest attaches to this feature because it appears to in- 

 dicate that the change from Paleozoic to Mesozoic conditions, 

 which in many places is marked by a sharp stratigraphic break, 

 was here more gradual. 



2 Richardson, G. B. The Paleozoic section in northern Utah. Amer. Jour. 

 Sci., N. S., 36:406-416. 1913. 



3 Boutwell, J. M. Stratigraphy and structure of the Park City Mining Dis- 

 trict, Utah. Jour. Gcol., 15:434-458. 1907. 



