108 ON THE GEOLOGY AND NATURAL HISTORY 
thence to the mountains, I shall speak of them in a similar manner, acknowledging my in- 
debtedness to his report for several important suggestions. 
Prof. Swallow makes four subdivisions of the superficial deposits of the State of Mis- 
souri, and as seen on the Upper Missouri I will add one more, namely, Erratic Block De- 
posit. We have therefore Ist, Drift; 2d, Yellow Marl Formation; 3d, Erratic Block 
Deposit; 4th, Bottom Prairie; 5th, Alluvium. 
lst. Drift.* 
This deposit consists mostly of much waterworn rocks, gravel, and sand, and underlies, 
to a very great extent, the broad upland prairies of the Northwest. It is usually revealed 
in the channels of streams and varies in thickness from one to thirty feet. It is found to 
a greater or less extent throughout the entire country drained by the Missouri river and 
its tributaries, resting upon rocks of all ages, from the granite to the Tertiary inclusive, 
but is most largely developed in the vicinity of the mountain ranges, as the Laramie 
mountains, the Black hills, and the sources of the Missouri and Yellowstone. So abun- 
dant is the drift toward the sources of these rivers, that it changes their entire character. 
The waters of the Yellowstone at its mouth are turbid, flowing over a bed of marly clay, 
like those of the Missouri from Fort Union to its confluence with the Mississippi; but 
about fifty miles above the mouth of the Yellowstone smooth waterworn pebbles begin to 
appear, small in size and few in number at first, but becoming larger and increasing in 
quantity, and when we reach a point two hundred miles above its confluence with the 
Missouri, they completely pave the bed of the river, and form the greater portion of the 
materials which compose the river bottoms in very large areas to the depth of thirty feet 
and more. From this fact the waters of the Yellowstone gradually lose their turbid cha- 
racter, and near the mountains are clear as crystal. The waters of the Missouri also be- 
come less opaque after passing the mouth of Mussel-shell river, and near the Judith roll 
over a pebbly bed as clear as the mountain streams. The drift seems to underlie all the 
vast table-land to the northward, continually increasing in extent and thickness as we ap- 
proach the base of the mountains, and oftentimes concealing the older rocks over very 
large areas, | 
Much might be said in regard to this deposit and the agencies which have operated in 
- ‘ - well aware that the term “ Drift” as used in this connection has not a fixed or definite meaning. I have 
applied it to certain sand, gravel, pebbly clay and boulder accumulations, which are always found at the base of 
the Quaternary deposits of the West, filling up the inequalities of the surface of the lower Ske and may or may 
not have been accumulated by a force operating over the whole continent. 
