126 . GEOLOGICAL SKETCH OF THE ESTUARY AND FRESH WATER DEPOSIT 
the coming season. The want of proper facilities for exploration, the wild and desolate 
character of the country, the numerous bands of roving Indians which were constantly 
wandering over this region on their predatory excursions, rendered it impossible for me 
to make any thing more than a mere superficial examination of this locality. 
So intimately do the Estuary beds at the mouth of the Judith seem to be connected with 
Cretaceous Formation No. 1, that it will be important to present such facts as are known 
in regard to it; and, in order to show their true relations to other geological formations 
of the Upper Missouri, I will briefly review the boundaries of these formations as they are 
revealed along the Missouri River. At the mouth of the Platte River we have the lime- 
stones of the Upper Coal Measures with their characteristic fossils. Thirty miles west on 
the Platte, these limestones are succeeded by a coarse, friable, ferruginous sandstone of 
Cretaceous age. About twenty-five miles north of the mouth of the Platte, on the Mis- 
souri, these same limestones are succeeded by the same sandstone just mentioned, which 
sandstone extends up the river to a point about ten miles above the mouth of Big 
Sioux. The Cretaceous rocks of the Upper Missouri have been separated into five divi- 
sions upon lithological and paleontological grounds, and the sandstone formation at the 
mouth of Big Sioux and below, forms the type of No.1. Nos. 2 and 3 are seen reposing 
upon No. 1 at the mouth of Big Sioux, and near the mouth of the Niobrara River, No. 
4 appears upon the summits of the bluffs, surmounting No.3. At the foot of the “ Big Bend,” 
No. 3 passes beneath the water level of the river, and is succeeded by No. 4, which occupies 
the country to Grand River, where No. 5 makes its appearance on the summits of the hills. 
Near the mouth of the Cannon Ball River, the Lignite Tertiary beds begin to overlap 
the Cretaceous strata, but do not entirely conceal them along the banks of the river until 
we reach “Square Buttes,” about thirty miles below Fort Clarke. From this point to 
Milk River in Lat. 48°, Lon. 106°, only the Miocene beds of the Great Lignite basin are 
exposed. The country in the vicinity of the mouth of the Yellow Stone River is covered 
by the Tertiary beds of the Lignite basin alone, containing their peculiar Fauna and Flora. 
The Tertiary beds continue uninterrupted until we reach the mouth of Milk River, 
where, by a reversed dip of the strata, the Cretaceous Formation rises to the surface from 
beneath the Tertiary. “Ihe Tertiary beds continue to overlap the Cretaceous, gradually 
thinning out upon the summits of the hills, until we reach the mouth of the Muscle Shell 
River, where the Cretaceous bed, No. 4, occupies the whole country. We thus see that 
in ascending the Missouri, the dip of the strata is north-west as far as Fort Union or some 
point in that vicinity, and on reaching Milk River we can very distinctly observe the dip 
south or south-east, by which the underlying Cretaceous beds are exposed. We can also 
note the basin-like form in which both Tertiary and Cretaceous rocks were deposited. 
Passing the mouth of the Muscle Shell we soon observe a somewhat remarkable bed rising 
