IOWA ACADEMY OP SCIENCE 
59 
Those who have studied the Iowan drift in this state agree that the ice-sheet 
was comparatively thin. When the ice receded upon the great plain thus bared 
there were projecting points and ledges of rock as well as irregular drift eleva- 
tions left by the ice. These elevations formed nuclei for the collection of 
minute dust particles from the broad plain in the same manner in which an 
obstacle on the surface of the earth may form the nucleus of a sand-dune or 
snow-drift. Such a dune or drift will itself form an obstacle which will in- 
tercept other material brought to it by winds, and will thus assist in its own 
upbuilding. In the same manner, the paha and the rocky ridge formed nuclei 
about which gathered the dust whose accumulations form the loess caps on 
these elevations. The comparative thinness of the ice-sheet also resulted in the 
early exposure of the surfaces of these elevations while the great intermediate 
plains still remained ice-covered. No doubt much fine material was washed out 
upon the surface of the ice and this probably furnished in some places the first 
material which was deposited on the exposed elevations. However, by far the 
greater part of the loess material was evidently deposited after the ice had re- 
ceded and the climate had become sufficiently moderated to permit the develop- 
ment of an extensive vegetation, which supported a more or less abundant 
molluscan fauna. The fact has repeatedly been noted that along the border of 
the Iowan drift and on the paha within its borders there are great accumula- 
tions of sand lying immediately beneath the loess. This sand in some places 
shows no lamination and was probably deposited by the Iowan ice, though it 
may be of Kansan origin. Very much of it, however, is distinctly laminated in a 
manner which suggests the structure of an ordinary sand-dune. These lamina- 
tions follow the vertical contours and in some places alternate with narrow 
belts and bands of fossiliferous loess, the sand being entirely free from fossils in 
all cases mentioned. The structure of these underlying sands suggests that 
soon after the recession of the Iowan ice there were extensive sand-dunes 
formed along the borders of the Iowan drift, and that there were isolated dune 
areas within the Iowan border. These shifting sands were probably bare for a 
time and the winds easily removed the finer particles of dust in great quantities. 
The successive changes were probably such as take place in recent times 
in any sand-dune, region. At first a scant vegetation consisting, perhaps, of 
tufted sedges and grasses and leguminose plants, is developed. Each plant 
forms a nucleus about which a small amount of finer soil material is gathered. 
While each plant thus serves as an anchorage for fine material, there are at first 
comparatively broad areas of loose shifting sands between these plant-tufts. 
As the scant vegetation gathers the finer soil, it prepares the way for a denser 
vegetation, which will accumulate more fine soil to the advantage of still other 
plants, until the whole area is covered with a carpet of plants which will check 
if it does not wholly prevent wind erosion. (See Plate V, fig. 2.) In the 
Iowan sand-dune area the fine material thus blown out of the sands was, after 
the recession of the Iowan ice, carried to the above noted elevations, and there 
lodged. It should be born in mind that these elevations presented the first 
suitable surfaces for terrestrial plants, for, no doubt, for a long period the 
fiatter areas were at least swampy, resembling probably the corresponding areas 
upon the Wisconsin drift plain. Other portions were, evidently, for a time 
shifting sands upon which vegetation gained a foothold with much greater diffi- 
culty. Upon the elevations alone was there sufficient drainage to permit the 
development of an early terrestrial vegetation. 
