60 
IOWA ACADEMY OP SCIENCE 
During' the summers many of these areas were exposed year after year by 
the evaporation of the waters and furnished new supplies of dust which accumu- 
lated upon the elevations in the well-developed flora which covered them. There 
are in some localities clear evidences of changes in surface conditions. The 
typical locality in the northern part of Madison township, Johnson county, will 
serve as an illustration. At the point in question there is a range of low hills 
on the south side of the Iowa river and parallel with it. These hills follow 
the river eastward to Curtis and thence to a point east of North Liberty. Ex- 
posures at various points indicate that the structure of this ridge is essentially 
that of the paha. At the intersection of this ridge and the C. R. & I. C. electric 
railway two cuts exhibit the structure here selected as an illustration. (See 
Plate VI, fig. 1.) The entire exposure in these cuts exhibits a succession of sand 
and loess bands following approximately the vertical surface contours. The 
loess bands, even where quite narrow, contain the characteristic fossil shells of 
terrestrial mollusks. 
Upon the adjacent surfaces of the ridge there is now a forest covering and 
among the leaves and leaf-mould of the surface the same species of mollusks 
are found living. 
Westward the ridges become lower and the river-valley expands into a sandy 
plain. During cycles of dry seasons the covering of vegetation becomes more 
scant and quantities of loose sand are shifted about by the winds forming' small 
sand dunes or drifts in the fields and along the road sides and encroaching more 
or less upon the wooded tract. During wetter seasons the covering vegetation is 
more abundant and much less material is shifted about. The position and the 
structure of the ridge both suggest that in the past there were cycles of favor- 
able seasons during which a dense vegetation, probably chiefly forest, was de- 
veloped upon its surface and that this plant-covered area received only the finer 
dust blown from the adjacent surfaces. This dust formed the loess bands and 
entombed the mollusks living upon the surface. During dry seasons the ex- 
posed sandy areas were very much increased in extent and the sand being no 
longer retained by plants was carried by the winds into the forest, thus forming 
the sandy bands. Upon such sandy surfaces no land-snails grow today and no 
shells were imbedded in the sands in the past. 
The structure herein discussed has been heretofore considered only in 
connection with the Iowan border, but may be observed in various localities 
remote from the Iowan border, and it is significant that in all these localities it 
is exhibited on ridges contiguous to manifest sand-dune areas. 
For example, south of New Harmony, Indiana, along the east side of the 
Wabash valley there are sand ridges, evidently old dunes, which are covered 
with loess and exhibit essentially the same structure as that of the Iowan 
border as noted above. South and west of these areas there is a territory now 
covered with more or less shifting sand-dunes. 
At Gladstone, Illinois, the bluffs near the town consist of drift, with super- 
imposed sand which is often laminated, and this is capped with a layer of 
typical fossiliferous loess.* (See Plate VI, fig. 2.) Southward there extends a 
great sand plain which probably furnished the greater part of the loess dust. 
The northward continuation of the same bluffs east of New Boston shows again 
the same structure. There are now extensive sand-dune areas south of this point. 
