L. E. HICKS AN OLD LAKE BOTTOM. 29 



We come, then, by the method of exclusion to the eolian type of construction, and 

 we soon find that, apart from the objections lying against other hypotheses, the sug- 

 gestion that we have here an example of the influence of a preexisting surface shaped 

 by the action of the wind has much to commend it. The materials of wind construc- 

 tion are drift-sand and dust. The latter does not produce topographic forms of much 

 magnitude, and, in this discussion at any rate, may be disregarded. Drift-sand is, 

 however, an element of construction which produces important topographic results 

 upon the Great Plains at the present time, and it has probably been as busy in pre- 

 vious geologic cycles as it is now. 



The fundamental type of a single sand hill is a half cone lying upon the flat side, 

 its base concave, facing the prevailing wind and forming a "blow-out," and its 

 elongated apex stretching off" to leeward. A succession of these overlapping upon 

 each other gives a serrated ridge running parallel with the prevailing wind. Shift- 

 ing winds give cross-ridges which shut in sections of the troughs h'ing between the 

 ridges first formed and produce closed basins. In a region of newly formed sand hills 

 the ridge-and-trough structure parallel with the direction of prevailing winds is dis- 

 tinctly visible, but where the sands have been long tossed about by shifting winds the 

 leading lines are obscured, the ridges are cut through by fresh blow-outs, and these 

 may be found facing in all directions. 



Such a surface mantled over with lake sediments would present the same forms 

 which we see upon the table-lands. The sharpness of the serrations would be mel- 

 lowed down to graceful curves, the closed basins would form the lagoons, and the 

 whole surface would present gentle and irregular undulations, reminding one of chop- 

 ping waves, after the violence of the storm has passed, arrested and fixed in mid- 

 ocean. The well sections show much sand beneath the surface marl of these table- 

 lands, and, upon the whole, there is good reason to believe that this interesting topo- 

 graphic type is the combined result of eolian and sedimentary processes The char- 

 acter of the climate during the last period of emergence preceding the lake period 

 may therefore be inferred to have been similar to that now prevailing in the same 

 region. The hypothesis of pre-existing sand hills is only intended, to apply to regions 

 of constantly recurring and closely packed lagoons, such as we find in the western 

 part of Custer county. The isolated depressions of other regions may be due to some 

 of the numerous accidents which produce lakes and ponds. 



It may be objected to this hypothesis that in the progressing subsidence which pro- 

 duced the Tertiary lake the sand hills would be leveled down by wave-action on the 

 shore. This result would certainly follow the progressive encroachments of a lake 

 which had already attained considerable dimensions, but in the first stages of its for- 

 mation in the center of the depressed area wave-action would be very slight; the 

 waters would quietly rise above the sand hills, leaving them and the closed basins be- 

 tween them undisturbed, except that slight rounding off and softening of their sharper 

 features which, being still further mellowed down by a light covering of lake marl, 

 produces the gentle undulations which characterize the table-lands. Custer county is, 

 if not in the very center of the old Lake Cheyenne at the time of its greatest expan- 

 sion, at least well removed from its shore-line. 



We have also other distinct evidences that the encroaching lake did not level all 

 before it. Old valleys of erosion, obscured, indeed, but not concealed by the newer 

 sediments, stretch for miles across the table-lands where now no stream flows. It is 

 true that these would be more difficult to obliterate than the sand hills, but their 

 preservation is nevertheless significant. Whatever weight they may have as evi- 



