70 Upper Cretaceous Seacoast in Montana [268 



From this and from other evidence at hand it seems rea- 

 sonably certain that the strand lines of the late Mesozoic lay 

 nearly parallel to the present Eocky Mountain front, and 

 that it migrated to and fro in response to the recurrent 

 depressive movements of the period, which were gradually 

 counterbalanced by intervening intervals of sedimentation. 



DISTRIBUTION AND CHARACTER OF THE JUDITH RIVER SEDI- 

 MENTS 



The sediments of the Judith River formation are now well 

 exposed north of Billings by reason of the extensive denuda- 

 tion which the region has undergone; hence it is possible to 

 form an accurate estimate of the variations of the section, 

 both vertically and horizontally. 



Near Gibson, 50 miles northwest of Billings, the formation 

 is of freshwater origin and consists of sandy .shale and nu- 

 merous beds of hard, rather muddy, quartizitic sandstone 

 containing reed stems and fragments of coniferous material. 

 Both shale and sandstone are characteristically of a yellowish 

 or tawny hue, though some of the beds, particularly among 

 the softer units, indicate by their texture and peculiar green- 

 ish brown color the presence of the tuffaceous material which 

 was thrown out so copiously by the volcanoes of the Crazy 

 Mountains during later Upper Cretaceous and early Eocene 

 time. 



Farther east the volcanic material disappears and the sec- 

 tion gradually assumes the aspect typical of the formation as 

 it is developed along the Missouri and in the vicinity of 

 Havre on Milk River; an alternation of light-gray to white 

 clay shales with lignitic shale and carbonaceous sandstone 

 giving the exposures a peculiar striped appearance. With 

 the advent of the carbonaceous zones in the section the 

 brackish water shell Ostrea subtrigonalis also makes its ap- 

 pearance at different horizons in the area immediately north 

 of Broadview, though it is conspicuously present in but a 

 single very thick shell bed of extraordinary uniformity and 



