SEDIMENTARY ROCKS 475 



Eiver), on figure 13, show that the typical Thermopolis black shale was 

 deposited in a trough between Yellowstone Park and the Black Hills, 

 with a shallow "reef" extending from the Big Horn northward to the 

 Porcupine. On the east side of the trough the Thermopolis becomes 

 very thin ; on the west side it becomes sandy, due to the proximity of the 

 Eocky Mountain shoreline. 



This "trough" of bituminous black shale can be followed northward 

 into Montana by referring to sections 4 (Musselshell Eiver), on figure 

 12, 3 (Great Falls-Cat Creek), on figure 11, and 2 (Milk Eiver), on 

 figure 10. Each one shows the thinning out to the east and the increas- 

 ing sandiness to the west, and on section 2, figure 10, the position of 

 the Sweet Grass Hills relative to the Thermopolis trough can be seen. 

 The final result of the increasing sandiness westward is shown on section 

 1, figure 9, the sanely Thermopolis grading into the Blackleaf phase, 

 and this into the thick, partially continental Blairmore formation of 

 the Canadian geologists. 



To sum up, the early Colorado Sea entered Montana from the south, 

 and first crept northward in an embayment now marked by the thick 

 Thermopolis black shales. On the west, high lands supplied coarse 

 detritus and sand, which were deposited along the western border, while 

 the black shale was laid down in the center. On the east were no high 

 lands, but probably a very shoal sea with no deposition. 



These conditions changed under the impetus of a quick oscillation 

 which first shoaled the sea, permitting fine-grained sand (Mowry) to be 

 spread broadcast over the entire region, and then abruptly submerged 

 the whole plains region to a depth which permitted the deposition of 

 only fine calcareous shales over central Montana, with interbedded lime- 

 stones east of Billings. The western shoreline retreated far west of 

 the present Eocky Mountains. The Sweet Grass Hills sections reflect 

 these changes. Above the sandstone that marks the top of the Mowry 

 are brown and blue calcareous shales, with a very little black shale and 

 almost no sandstone. The thickness of this meniber is 1,000 feet, and 

 it represents the upper Benton and Niobrara stages of Cretaceous time. 



Like the Thermopolis Sea, this upper Colorado Sea advanced from 

 the southeast. The Mowry shoaling of the sea, which was most pro- 

 nounced in the Yellowstone Park region, did not reach the Black Hills, 

 and the deposits of the Upper Colorado Sea are far thicker in eastern 

 and central Wyoming than in central and northern Montana. Being 

 3,300 feet thick at Basin, Wyoming, this formation decreases, as we 

 have seen, to 1,000 feet in the Sweet Grass Hills. Its chief submembers 

 in the southeast are : 



