THE LOMBARD OVERTHRUST AND RELATED FEATURES 287 



In this region the Ellis formation, consisting of sandy shales 

 and limestones with numerous layers rilled with pelecypod shells, 

 lies on the Quadrant formation with no observed discordance of 

 dip. In the region to the south there is a well-marked reddish 

 sandstone formation of probable Triassic age intervening between 

 the fossiliferous Ellis and the Quadrant. Since the Ellis fossils are 

 considered to be Jurassic in age, it seems clear that there is a dis- 

 conformity in this part of the sections of this region. 



Above the Ellis formation is a series of sandstones and con- 

 glomerates which have been called the Dakota sandstone by Dr. 

 Peale, but they have recently been shown to be more probably the 

 equivalent of the Kootenai formation of the region to the north. 1 

 These sandstones are therefore of probable Lower Cretaceous age. 

 Strata of Montana and Colorado age were identified by Dr. Peale 

 in the hills north of Logan, but there is now some doubt as to 

 whether they can be referred to a horizon as high as that. 



TERTIARY 



All of the Mesozoic and Paleozoic strata were involved in the 

 extensive orogenic movements which began at the close of the 

 Cretaceous in this region. 



The type of folding and the associated overthrust faulting has 

 already been described in this paper. Extensive erosion reduced 

 the region to comparatively low relief in Tertiary times. The 

 great lowland areas were filled in by sedimentary deposits of sand- 

 stone, limestone, and volcanic ash to a great depth. The major 

 features of the present drainage were established on this late Ter- 

 tiary surface and gradually, through uplift and erosion, they were 

 brought into discordance with the underlying structure, as is well 

 shown by the double horseshoe canyon of the Missouri River west 

 of Lombard. 



This whole series of Tertiary valley sediments has been grouped 

 under the heading of the Bozeman formation for convenience in 

 mapping. Dr. Peale's name " Bozeman Lake Beds" seems no 



1 W. R. Calvert, Bull. U.S. Geol. Survey. No. 471-E, 191 2, p. 53. 



