78 ON THE GEOLOGY AND NATURAL HISTORY 
twelve broad, Ammonites eighteen inches to two feet in diameter, also the vertebre of a 
huge Cetacean. Near Little Rocky Mountain creek, No. 4 begins to rise toward the 
summit of the hills, and about fifty miles below the mouth of the Judith it caps the bluffs, 
still containing its characteristic fossils. It continues to be seen in thin outliers to the 
vicinity of Fort Benton, perhaps even farther, but its limits in that direction have not 
yet been ascertained. 
On the Yellowstone river I observed this formation in but one locality, about eighty 
miles above the mouth of that river. The Cretaceous strata here have an extent of only 
about eight miles, and are exposed only along the banks cut by the river, yet in that space 
they reveal the remains of marine mollusca in a profusion which I have seen in no other 
locality. In ascending the river its first appearance is a lightish blue clay, containing a few 
concretions. At its best exposure above the water’s edge, we have the following section: 
a. Dark ash-colored clay, upper part of a bluish cast, slightly indurated, filled with concretions fully charged with 
shells. The fossils are so abundant in the concretions that they form large masses of shell conglomerate, 
cemented with a fine blue calcareous clay, exceedingly hard and breaking with an irregular fracture. This is 
probably but an extension in a northwest direction of the same shell zone seen at Moreau and Grand rivers, 
forks of Shyenne, Sage creek, &c. 20 feet. 
b. A very dark indurated clay, presents similar characters to its equivalent at Great Bend, and contains fewer fossils 
than the bed above. 
The fossils of bed a indicate a blending of formations 4 and 5. The whole thickness of 
Cretaceous rocks exposed on the Yellowstone at this locality is not more than twenty-five 
feet, and the distant hills on either side are composed of Tertiary beds. 
We will now return to White or Smoking Earth river below Fort Pierre, and trace 
this formation into the interior of that interesting region. Passing up the valley of White 
river, we find it occupying the country bordering upon that stream for about fifty miles 
above its mouth. Near this point outliers of the White river Tertiary basin begin to 
cover the highland, and No. 4 is seen along the river for about twenty miles farther, when 
it is concealed by Tertiary strata. ‘The intervening country east and northeast of the Bad 
Lands to the Shyenne river, a distance of one hundred and fifty miles, is for the most part 
underlaid by this bed, except an extension of Fox ridge, to the sources of the Teton river, 
which is composed of formation No. 5 of the vertical section. The extensive area drained 
by the Shyenne river is composed of No. 4, excepting the sources of a few of its tributaries. 
Sage and Bear creeks take their rise in the White river Tertiary basin, but flow mostly 
through this formation, revealing large quantities of Cretaceous fossils. A few small 
tributaries have their origin in the Fox ridge, and Cherry river has its source in the Lig- 
nite Tertiary basin, near the head waters of the Little Missouri. 
