Jurassic Formations in 8. E. Idaho, etc. 63 



fauna! relations of some of them, notably certain brachio- 

 pods of the Portneuf limestone, have not been fully 

 studied. It is therefore possible, though perhaps not 

 probable, that some of these strata may be of later age 

 than Lower Triassic. The Timothy sandstone in partic- 

 ular may prove to be of later age because of the uncon- 

 formity described below. 



Unconformities. — In Home Canyon, about six miles 

 east of Montpelier, the Timothy sandstone consists of 

 cross-bedded and coarse- textured sandstones with con- 

 glomeratic layers, in which the fragments are small pieces 

 of limestone like that of the Thaynes. This phase has 

 not been observed elsewhere. The conglomeratic layers 

 indicate that at least locally there was erosion of Triassic 

 limestone while the deposition of the sandstone was in 

 progress. The noteworthy southward thinning of the 

 limestone above the red bed member of the Portneuf, 

 previously mentioned, suggests that this unconformity 

 may be widespread. 



The southeastward thinning of the Timothy sandstone 

 from about 800 feet in the Fort Hall Indian Reservation 

 to less than 200 feet in the Montpelier quadrangle and the 

 pronounced lithologic change with the introduction of the 

 Higham grit, which is more or less conglomeratic and 

 locally strongly ferruginous at the base, indicate a wide- 

 spread unconformity between the Timothy and the 

 Higham although the boundary between the two forma- 

 tions is apparently regular. The unconformity doubtless 

 marks an oblique transgression of the Lower Triassic 

 formations by the Higham and later beds. 



Age of doubtfully Triassic formations. — The great 

 thickness of the formations assigned to the Lower Tri- 

 assic, 5,350 feet, the unconformity at the base of the 

 Timothy sandstone, and the marked unconformity at the 

 base of the Higham grit all suggest that the latter forma- 

 tion and the overlying Deadman limestone and Wood 

 shale may be of later age than Triassic. No fossils, 

 however, have yet been found in any of these formations 

 so they are provisionally retained in the Triassic. 



Jurassic System. 



Subdivisions and thickness. — The Jurassic system is 

 well represented in many parts of southeastern Idaho. 

 It includes four formations, namely, the Nugget sand- 



