OF THE UPPER MISSOURI. 
113 
slope of the Rocky Mountains, and as they all indicate these western slope terraces to be¬ 
long to the same great system, the remarks that will apply to one portion of the country 
will apply to a greater or less extent to all. 
Near the sources of the larger streams as they flow from the steep sides of the moun¬ 
tains, these terraces form a very conspicuous feature of the valleys. There are usually 
from three to four of them, forming quite narrow benches, and gradually increasing in 
width but diminishing in height as we approach the Mississippi. For example, in the 
valleys of the three forks of the Missouri, there are three and four of these terraces on 
each side of the river-bed ; first terrace near the channel of the river, three feet above 
water-level, varying in width from a few feet to ten or twenty yards ; second terrace ten 
to twenty feet above the bed of the river, from ten to fifty yards wide; third terrace thirty 
to fifty feet high, and from twenty to forty yards wide; fourth terrace one hundred to one 
hundred and fifty feet high, and varying from one hundred yards to half a mile in 
width. As we descend the Missouri, the fourth terrace seems to be wanting, and the 
first, second, and third terraces vary greatly in width, often forming broad, level, plains. 
Near the mouth of the Yellowstone, the first terrace is from three to six feet above the 
water, and is on a level with the islands and sandbanks. Most of the timber grows upon 
this terrace; the second terrace forms what is called the bottom, and is from ten to fifteen 
feet in height, and from a few yards to half a mile in width; and the third terrace ascends 
very gradually to the foot of the bluffs, varying from one mile to five miles in width, and 
twenty to forty feet in height. Upon this terrace Fort Union is built, and it has never yet 
been known to be overflowed. It is covered with a thick deposit of marl, underlaid with 
small waterworn pebbles, generally to the depth of from ten to twenty feet, and then we 
come to the Tertiary beds. The first and second terraces are in most cases composed en¬ 
tirely of the more recent superficial deposits. Along the valley of the Big Shyenne and 
near the Black hills, we again see these terraces, forming very broad, level, beautiful plains. 
Near the mouth of Plum creek, the first terrace is a fine grassy bench about four feet 
above the bed of the stream, and the second terrace is fifteen to twenty feet in height, and 
the third forty to fifty feet high. This last terrace is covered with a thick deposit of 
pebble-stones, sand, and gravel, resting upon Cretaceous formation No. 4, which is the 
prevailing rock of this region. Near Fort Randall, the first terrace is composed of fine 
sand, clay, and pebbles, six to ten feet above the river; second terrace, stratified sand and 
clay, from ten to twenty feet high; third terrace is composed mostly of dark stratified in¬ 
durated clay, with numerous seams of pebble-stones. It is evidently derived from the 
eroded materials of Cretaceous formation No. 4, and at one point forms a series of bluffs 
sixty feet in height, worn into ravines like bluffs on a larger scale. As we approach the 
settled portions in descending the Missouri, we find that most of the towns and villages 
VOL. xii.— 15 
