GRADATIONS FROM CONTINENTAL TO MARINE CONDITIONS IN CENTRAL MONTANA. 



19 



texture and assortment certainly points to 

 similar physiographic conditions of accumu- 

 lation. This striking resemblance is still more 

 remarkable in view of the fact that the for- 

 mations are separated from one another by 

 several hundred feet of marine shale. The 

 conclusion is inevitable that the oscillations 

 which give rise to the alternate deposits of 

 shale and sandstone during the Upper Creta- 

 ceous were of a gentle, rhythmic type. 



This conclusion at once raises a question as 

 to the age of the Rocky Mountain orogenic 

 disturbance. The petrographic evidence may 

 ultimately have more weight in deciding this 

 question than has yet been assigned to it. At 

 present this evidence seems to warrant the 

 belief that there was no great physical break 

 between the deposition of the formations un- 

 derlying the Lance and that of the Lance and 

 to corroborate the evidence afforded by the 

 attitude of the Lance and Fort Union forma- 

 tions, namely, that the present Kocky Moun- 

 tains are of post-Fort Union rather than pre- 

 Lance age. 



As to the probable source of the material, it 

 is hardly profitable in our present state of 

 Ivnowledge to speculate further than to say 

 that it was evidently derived from the land 

 mass to the west that is supposed to have been 

 in existence during the Cretaceous period. It 

 would yield interestmg data, however, if 

 observations of this character were extended 

 over the entire region involved, and it might 

 then be possible to form more definite con- 

 ceptions, not only as to the source of the mate- 

 rial, but also as to the conditions of its accumu- 

 lation. 



UPPER CRETACEOUS HISTORY. 



Variations in the character of sediments in- 

 volving changes from sandstone to shale and 

 from marine to fresh-water deposits imply cor- 

 responding changes in physiographic conditions 

 during the period of their accumulation and 

 therefore furnish a basis for the interpretation 

 of the past history of the region in which they 

 occur. 



Such changes occurred in north-central Mon- 

 tana dm"ing the Upper Cretaceous epoch. The 

 area herein described happens to be favorably 

 situated in that it furnished more exact evi- 

 dence than has heretofore been available as to 

 the magnitude of some of these oscillations of 



the sea and as to the extent and character of 

 the formations involved. 



An understanding of these conditions will be 

 facilitated by a study of the diagrammatic 

 section on Plate IV which extends from Black 

 Butte, at the north end of the Judith Moun- 

 tains, about 48 miles west of this area, south- 

 eastward to the longitude of Glendive, a total 

 distance of about 210 mdes. Section A is 

 compiled from measurements made by W. R. 

 Calvert in the Lewiston field and those made 

 by the writer in 1912; sections B, F, and G are 

 compiled from data collected by the writer 

 while examining the area herein described ; and 

 information regarding conditions at Glendive 

 is based on the record of a well put down near 

 that place by the Northern Pacific Railway Co. 

 It is not possible to plot this record because 

 the exact horizon of the mouth of the well is 

 not known. The well was begun, however, 

 somewhere near the top of the Pierre shale and 

 continued to a depth of 2,710 feet, which would 

 bring it near the base of the Colorado shale. 



Upper Cretaceous time seems to have been 

 ushered in by a widespread transgression of the 

 sea extending westward beyond the eastern 

 front of the present Rocky Mountains. During 

 the Colorado epoch great quantities of mud 

 and silt were spread out over the floor of this 

 epicontinental sea, and its waters teemed with 

 life, the remains of which were in part entombed 

 in the accumulating muds that now constitute 

 the Colorado shale. The fine texture and 

 homogeneous character of this shale and its 

 freedom from notable amounts of sandy ma- 

 terial, except perhaps near its base, indicate 

 that it was laid down xmder very uniform con- 

 ditions. This implies that the relative eleva- 

 tions of land and sea must have remained 

 nearly constant throughout the Colorado epoch. 

 So the sea was filled to a depth of more than 

 2,300 feet by the accumulated mud and as the 

 land surface was necessarily lowered by the 

 removal of this material, it seems necessary to 

 infer that the relations between land and sea 

 level could not have remained constant unless 

 the sea floor was being gradually depressed at 

 about the same rate as that at which the 

 sediments accumulated. 



This gradual submergence was arrested and 

 the Colorado epoch was brought to a close in 

 the western part of the epicontinental sea by 

 an uplift of the western land mass. As a result 



