THE LOMBARD OVERTHRUST AND RELATED FEATURES 273 



but it is approximately two miles, and strata which are strati- 

 graphically about 6,800 feet apart are here in contact. 



The age of the Lombard overthrust cannot be definitely deter- 

 mined, but it is certainly younger than the Cretaceous strata ex- 

 posed near Lombard, and probably older than the Lower Oligocene 

 deposits which occur near the southern end of the ridge and are 

 apparently undisturbed. It may therefore be assigned with some 

 uncertainty to very late Cretaceous or early Tertiary time. 



A normal fault. — The only normal fault observed in this region 

 appears in the highest part of the ridge north of Three Forks and 

 west of the Missouri River. This fault cuts across the western limb 

 of the elongate dome already noted, and has caused a repetition 

 of the upper part of the Gallatin formation and the base of the 

 Jefferson limestone (see Fig. 2, section C-C). This fault has a 

 length of about two miles and a diminishing throw to the south. 

 It could not be traced to its intersection with the overthrust fault, 

 but the displacement apparently dies out in that direction also. 



The age of the normal faulting is considered to be the same as 

 that seen farther south in the Three Forks quadrangle, which is 

 dated as probably Pliocene. 



STRATIGRAPHIC GEOLOGY 

 PRE-CAMBRIAN 



The oldest rocks exposed in this region are a series of somewhat 

 altered sediments which occur below the base of the Cambrian, 

 and are considered to be part of the Belt Series, which are typically 

 exposed in the Little Belt Mountain region to the north and north- 

 east. The exposures of the Belt formation occur along the Gallatin 

 River east and northeast of Logan, and also north of Three Forks, 

 in a widening strip which trends northeastward and crosses the 

 Missouri River at the double horseshoe bend west of Lombard 

 (Fig. 1). 



The exposures north of the Gallatin River are of rather coarse 

 micaceous sandstones and shales with thinly bedded siliceous lime- 

 stones. They are not divisible on the basis of lithological char- 

 acters into the various formations which characterize the Belt 

 Series at the type localities. 



