GEOLOGICAL SURVEY OF THE TERRITORIES. 175 



A careful study of these modern deposits will undoubtedly show consec- 

 utive links by which it was united to the tertiary period, in the same 

 manner as the cretaceous and tertiary are connected in the case of 



Fig. 17. 



the great tertiary lake now indicated by the deposits on White and 

 Niobrara Elvers, in Nebraska, in which the waters continued to cover a 

 greater or less area through most of the quaternary period, at least, 

 as is shown by the thick deposits of fine sand, with bones of mammals 

 and shells of existing species, on Loup Fork and its tributaries. The 

 same may be said of the bluff deposit, or loess, which is so well displayed 

 along the Missouri from Fort Pierre down below St. Louis, and, probably, 

 to the Gulf of Mexico. At a modern period it is probable that the 

 waters of the ocean swept high up inland, reaching nearly to the foot of 

 the mountains. The great water-courses had already been marked out, 

 consequently we find the yellow marl or loess fifty to one hundred and fifty 

 feet thick in the immediate valley of the Missouri, but thinning out as we 

 recede from it, or the valleys of any of its branches. The existence of 

 so many fresh-water mollusca and the entire absence of any marine 

 forms indicate that the waters of the Mississippi and Missouri were 

 either cut off from the direct access to the sea, or that the influx of such 

 a vast quantity of fresh water as must have flowed down from the 

 mountain districts rendered completely fresh the inland portions. 



We may suppose the temperature just prior to the present period to have 

 been extremely low, and that the elevated portions of the West were 

 covered with vast masses of snow and ice ; that as the temperature be- 

 came warmer this snow and ice melted, producing such an accession to 

 the already existing waters that they covered all the country, except- 

 ing, perhaps, the summits of the highest peaks ; that masses of ice filled 

 with fragments of rocks, worn and unworn, floated off into this great 

 sea, and melting, scattered the contents over the hills and plains below ; 

 that as the waters diminished these masses of ice would accumulate on 

 the summits of the foot-hills of the mountains, or at certain localities in 

 the plains ; and thus account for the great local accumulations of stray 

 rocks at certain places. The materials, also, which must have been re- 

 moved from all portions of the West drained by the Missouri and its 

 tributaries by surface denudation, as is illustrated by the " bad lands," 

 &c, were also swept into this vast inland lake, and then, carried beyond 

 the reach of currents, would settle quietly to the bottom, almost with- 

 out lines of stratification, as we observe in the loess. The last act was 

 the recession of these waters to their present position, and the forma- 

 tion of the terraces. We believe the terraces constitute the last change 



