GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 



21 



and thrusting, which produced the principal 

 tectonic features of the Rocky Mountain sys- 

 tem. Since that time erosion has reduced the 

 land surface to its present form, though it may 

 be that between the epochs of deformation and 

 erosion the Oligocene and Miocene deposits now 

 found farther east were deposited over parts of 

 this region as well. 



By way of summary it may be said that 

 eastern Montana remained beneath the sea and 

 was not affected by any oscillations of land and 

 sea level throughout the Colorado and Montana 

 epochs, whereas in the central part of the State, 

 at least as far west as the front of the present 

 Rocky Mountains, there were oscillations of 

 land and sea level produced either by rhythmic 

 gentle depressions of the sea floor or by uplifts 

 of the land mass to the west. The first of these 

 uplifts, which resulted in the deposition of the 

 Eagle sandstone, was of the shortest duration 



and was effective in the area here described for 

 only a few miles east of its western boundary. 

 The uplift during the Judith River epoch was 

 of longer duration, to judge by the thickness of 

 material deposited, and was considerably more 

 widespread. As a result of this uplift the sea 

 withdrew to about the longitude of 107° 30' 

 and the site of sedimentation was shifted to 

 some point east of the longitude of Forsyth, 

 perhaps nearly as far east as Glendive. Be- 

 cause of this change the Montana group is in 

 the central part of the State divisible into the 

 Eagle, Claggett, Judith River, and Bearpaw 

 formations but farther east consists of a single 

 lithologic unit — the Pierre shale — to which a 

 thin representative of the Fox HiUs sandstone 

 is added near the eastern boundary. In this 

 eastern region also there is little if any lithologic 

 distinction between the shales of the Colorado 

 and Montana epochs. 



