
CRETACEOUS AND TERTIARY—MILK RIVER. 121 
24. Sandstone, a few inches. 

a EON I i ib 5 5 oy Aims n'a, 0d pepe RDS 4 6 
26. Yellowish arenaceous clay (lowest bed in . which remains 
of molluscs were found at this place. Ostrea)......... 5 
27. Nodurlarly hardened sandstone............. ....s00- 1 0 
28. Yellowish arenaceous clay............cccccceesceecess 31 0 
Concealed in slope to river, about..... ..........000- 30 0 
Uso Skeets nied nd. ha Pe ine 284 10 
296. The thickness of beds displayed in the above sections, when 
combined, is about 375 feet, and the bottom of the river valley, is 
probably not very far above the base of the Lignite Tertiary formation. 
It will be observed that the genus Ostrea, is here for the first time 
mentioned as occurring in these beds ; further west it becomes one of the 
most usual forms. The conditions of deposit implied by the beds on the 
Milk River, are those of an estuary, or shallow sea margin, where, 
while oysters and corbulas were living, the remains of fresh-water 
shelis and land vegetation were being carried, and mingled with them. 
The salinity of the water just indicated at Wood End, must have been 
here, however, well pronounced; a fact according with the observations 
of Dr. Hayden and other observers to the south. 
297. The superposition of these beds on the Cretaceous clays of 
group 4, is not clear in this locality, as no junction of the two forma- 
tions was observed. Their lithological character, might almost seem 
to render it probable, that they represent the same series as that sup- 
posed to come up from below the Cretaceous clays between the East 
and West Forks of Milk River. The beds here examined, however, can 
be traced almost continuously westward to the country round the Three 
Buttes, and are there found clearly resting on Upper Cretaceous rocks, 
recognizable not only by their mineralogical similarity to those already 
described, but by their fossils. 
298. Eight miles south of the Line, where the Commission Trail 
crosses the valley of Milk River, the banks are somewhat lower and less 
rugged, but exhibit the same rocks with very similar appearance and 
arrangement. 
Cretaceous and Tertiary Rocks from Milk River to the West Butte. 
299. In the coulées and gorges which intersect the prairie on the 
west side of the Milk River, exposures of rocks similar to those described 
in the main valley occur, the beds continuing to maintain a nearly 
horizontal attitude. Eighteen miles west of the crossing place of the 
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