CRETACEOUS AND TERTIARY FORMATIONS 527 



below and another above. A band six to eight inches thick is 

 in places closely packed with the shells, which Dr. T. W. Stanton, 

 who visited the locality in the summer of 1909, refers to Ostrea 

 subtrigonalis and Ostrea glabra. While in the Little Beaver Creek 

 region there was an abrupt change at the close of Fox Hills time 

 from marine to land or fresh-water deposits, as Dr. Stanton points 

 out, this oyster bed is evidence that in this neighboring area 



marine or at least brackish water conditions continued for some time after 



non-marine deposition began Such an oyster bed must have been 



formed in tidal waters connected with the sea, and its presence here argues 

 strongly for the assumption that the underlying portion of the Lance forma- 

 tion was formed near sea level so that a slight downward movement permitted 

 temporary admission of brackish water into the low lying swamps and marshes 

 in which coal was forming. It is, therefore, most probable that the abrupt 

 change from marine to fresh-water and land conditions seen near Marmarth 

 is purely local, and that the eroded surface at the top of the Fox Hills does 

 not represent a time interval of any geologic importance. 1 



The lower portion of the Lance formation contains many 

 dinosaur bones among which those of Triceratops are most abun- 

 dant. Many were found in the badlands at the mouth of Bacon 

 Creek. Mr. Gilmore, who examined them, referred some tenta- 

 tively to the species Triceratops horridus ( ?) and one belonged 

 to the genus Trachodon. 



Yellowstone Valley area. — The Lance beds, as already stated, 

 are found along the valley of the Yellowstone for a distance of 

 nearly 150 miles, or from the vicinity of Forsyth to a point about 

 fifteen miles below Glendive. They are well exposed near the latter 

 town, and in the vicinity of Iron Bluff, about ten miles southwest, 

 the following section occurs: 



Feet 



8. Coal bed, burned, but probably 6 feet thick 



7. Shale with a few thin beds of sandstone; one of these sandstone layers 



20 feet above the base contains many plants 1 50 



6. Sandstone, massive, gray 40 



5. Shale and sandstone; a few fossil plants at base 160 



4. Sandstone, massive, white; most prominent stratum in the region. ... 35 



3. Sandstone, brown, forms summit of Iron Bluff 75 



2. Shale and sandstone 75 



1. Shale, dark, with calcareous concretions carrying abundant marine 



shells (Pierre) ; exposed to river level 100 



Total 635 



1 Am. Jour. Sci., XXX (September, 1910), 183. 



