286 GEOLOGY. 



mollusks are found. This indeed is not conclusive, as this region is 

 at this time remarkable for the presence of southern forms of in- 

 sects and fresh- water mollusks* -Yet it appears to me that an un- 

 usual number of southern forms at this horizon of the Loess must 

 indicate some modification of climate at that period. It may have 

 been only on the eastern shore of this great lake, and caused by the 

 -even temperature which so large a body of fresh water produces on 

 the side toward which the prevailing winds from the lake blow. 

 We have such a phenomenon at the ] resent day on the east 

 shore of Lake Michigan. The Mississippi Valley is by its con- 

 tour eminently favorable to the emigration northward of southern 

 species. 



These Loess deposits, which have done so much to enrich Ne- 

 braska, have received profound attention and study from some of 

 the ablest geologists. But in many of the counties of the State 

 they have not yet been investigated. Much to be discovered must 

 yet remain in them. Though myself long engaged in their inves- 

 tigation, I rarely examine a new section in a well, ravine or railroad 

 cut without finding something new. 



Close of the Loess Period. It was a continuation of the up- 

 ward movement that had again begun during the second de- 

 pression epoch of the Quaternary that brought the Loess 

 period to a final close. As the land rose most towards the west 

 and north, the area of this Loess lake was gradually lessened from 

 these directions, and its remnants were last active on its southeast- 

 ern border. This explains the fact already mentioned in other con- 

 nections, that the long gentle slopes of the bluffs bordering the 

 flood-plains running in an easterly and westerly direction are al- 

 most universally on the north side of the valleys. The closing of 

 the Loess period first clearly outlined the present rivers of Ne- 

 braska, when they covered the whole of the bottoms, from bluff 

 to bluff, and when the mud-flats of the former Loess lake them- 

 selves constituted the flood-plain. So far as known, no convulsive 

 movements to a certainty accompanied the close of this period. 

 Many movements of this kind occurred in the regions of the 

 mountains during the Quaternary, but they have not yet been 

 synchronised with geological events on the plains. 



*Haydeu's Report for 1870, page 467. 



