MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 41 



(2) the Deadman limestone member, (3) the Wood shale member, 

 and (4) the main sandstone or typical Nugget. 



Higham grit member. The basal member of the formation is 

 named from Higham's Peak in sec. 23, T. 3 S., R. 37 E., the high- 

 est summit in the northeastern part of the reservation, which is 

 composed of this rock. The grit is a coarse, white to pinkish, 

 gritty or conglomeratic sandstone, the component particles of 

 which are coarse and subangular. Locally the rock is almost a 

 quartzite. The Higham grit is distinct lithologically from other 

 rocks of the region and is prominent topographically. It forms 

 important strike ridges that are marked by rough, craggy ledges 

 in many places. The pebbles are all of quartzite so far as ob- 

 served, without material derived from immediately underlying 

 formations. The grit appears to lie conformably on the Ankareh 

 sandstone. The rocks are much fractured and slickensided 

 as a result of severe deformations, a condition which causes 

 them to weather in pinnacled and castellated forms. The 

 thickness is about 500 feet. 



Deadman limestone member. Above the Higham grit member 

 is a dense purplish-gray limestone of almost lithographic quality, 

 with subordinate amounts of gray and greenish chert. This 

 member is named the Deadman limestone after a creek in the 

 northeastern part of T. 4 S., R. 38 E., Boise M., in the north- 

 eastern part of the reservation, near the headwaters of which 

 the rock is exposed. The limestone is resistant and in favor- 

 able places forms topographically prominent ledges. Ordinarily, 

 however, it is rendered inconspicuous by the proximity of the 

 more resistant member below. No fossils have been observed 

 in this limestone. The latter is about 150 feet thick. 



Wood shale member. Above the Deadman limestone member 

 is a bright red shale that weathers to a red soil. This member is 

 called the Wood shale from Wood Creek in T. 3 S., R. 38 E., 

 Boise M., in the northeastern part of the reservation, which cuts 

 across the entire Nugget formation. It is less resistant to 

 erosion than the adjacent members on either hand and occupies 

 depressions or gullies. Outcrops are few but the shale may be 

 traced by patches of bright red soil. It is apparently 200 to 250 

 feet thick. 



