NO. lO NEBRASKA ARCHEOLOGY STRONG 237 



affected by the sharp rise of the highland. This has stopped the mi- 

 gration of the dunes, for they are now sodded over and in places 

 exhibit scant tree growth. The nearest sources for these dunes are 

 not less than lO miles distant from where they are now, and the more 

 likely sources (according to the direction of the prevailing winds) 

 are some 14^ to 18 miles away. Estimating the time required for dune 

 movement, at the highest speed the most reliable figures permit, it 

 would have taken these dunes some 2,110 years to reach their present 

 location. If they come from the north or the northwest, which is 

 more logical, it would have taken them at least 3,100 years. More 

 conservative estimates, based on slower rates of dune migration, would 

 raise this period to about 5,200 years. Even then, the dunes would 

 have to have been remarkably persistent in their movement. If more 

 humid conditions intervened, as seems probable from the soil layers 

 on the butte, the speed of the dunes would be materially reduced, 

 and a temporary cessation of movement would be expectable. It is 

 not likely that all the loess on Signal Butte and other high places in 

 the vicinity came from such sand dunes, but it is believed that the 

 origin of the dunes and of this loess came about at one and the 

 same time. As the dunes began migrating and sifting their materials, 

 the deposits on the highlands beyond, including Signal Butte, should 

 likewise have been begun. Hence the estimated time of dune move- 

 ment in this case should roughly correspond to the minimum age of 

 the aeolian deposit on top of Signal Butte. 



The two old soil horizons in the earth cap on Signal Butte are 

 important factors in this discussion. These^ correspond with human 

 occupation level I (a thick, black, soil line) and human occupation 

 level II (a very thin and faint soil line). Level I lies directly on what 

 we believe to be water-carried deposits of Pleistocene age and marks 

 the beginning of the period of aeolian deposition. Above this comes 

 a sterile layer of wind-borne material apparently representing a dry 

 period. Following this, a thin soil horizon appears with evidences 

 of human occupation (level II), which is covered by another sterile 

 aeolian layer, again deposited under dry conditions. This is capped 

 by a third period of occupation (level III) which immediately un- 

 derlies the present sod line. This horizon is only slightly darker, 

 especially at the top, than the sterile aeolian layer which underlies 

 it. It is therefore obvious that the dififerent strata on Signal Butte pre- 

 sent evidence of climatic fluctuations during the Recent period. 



Unfortunately, Sears could find no pollen in soil samples from any 

 of these levels,*" hence a botanical correlation with his suggested 



*° Letter from Paul B. Sears to Maurice E. Kirby, February 25, 1933. 

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