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IOWA ACADEMY OP SCIENCE 
terrestrial molluscan fauna being similarly developed chiefly in the grosser 
vegetation of such situations. 
(b) . It explains the difference in thickness of loess on the east and west 
sides of the valleys of our larger streams like the Mississippi and Missouri. The 
prevailing southwesterly and westerly summer winds produce xerophytic con- 
ditions with less abundant vegetation on the east side, where the dust heaps 
up like snow in a drift, while the west side is mostly forest-covered, and re- 
ceives a thinner blanket of finer dust which settles more uniformly in the 
more abundant vegetation. (See Plate III, figs. 1 and 2; Plate IV, figs. 1 and 2.) 
(c) . It throws light on the inequality and irregularity of loess distribution, 
the latter being determined largely by the irregularity of plant distribution, 
and it suggests that the more irregular and unequal loess was deposited on 
drier surfaces, while those which had a dense plant-covering, forest or other- 
wise, received a thinner and more nearly uniform deposit. 
A suggestion of the cause of local irregularities in thickness is furnished 
by the morainic Wisconsin-drift hills in western Lyon county, Iowa. The south- 
western windward slopes and the tops are covered with a tufted scattered vege- 
tation, while that of the leeward slopes is much denser, being partly scrub for- 
est. The bare portions of the former lose their finer materials and coarse drift 
appears at the surface, while the latter retains the finer materials and thus re- 
ceives additional increments, and has already formed a distinct thin loess and 
soil.* (See Plate V, fig. 1.) If the vegetation on the windward slopes 
was gradually reinforced, as during more favorable seasons, and the deposition 
of these finer materials continued until both slopes were covered, the deposit of 
loess so formed would be unequal in thickness on the opposite slopes. 
(d) . It explains the presence of loess in high places, and its almost gen- 
eral absence from the intervening lowlands, for the elevations were earliest 
drained, and first presented conditions suitable for dust-deposition because of 
their greater elevation and their early plant-covering, while the lowlands were 
alternately flooded and exposed. 
II. Structure and composition of loess. 
(a) . The greater coarseness of loess materials on the east side of the large 
rivers has already been noted, together with its probable cause. 
(b) . The presence or absence of lime nodules and iron tubules is of no value 
in determining the age or identity of loess, or loess-like deposits, for they may 
have been formed very recently in cavities formed by or around roots, by 
checking or slipping, or by water. The writer has found both lime nodules and 
iron tubules around fresh living roots. Moreover both sometimes occur in drift. 
(c) . The occasional alternation of sand and loess may be explained on the 
basis of changes in the plant-covering of the area during the formation of the 
deposit. 
Such alternation and interlamination of sand and loess are not uncommon, 
and it is a significant fact that they may be observed, so far as the writer has 
seen, only in the vicinity of modern or manifest old sand-dunes. Pine illustra- 
tions occur in such locations in Iowa, Nebraska, Missouri, Illinois and Indiana. 
In Iowa the finest examples may be found in close proximity to the 
Iowan drift border. 
* See the writer’s paper on “Additional Observations on Surface Deposits in Iowa,’’ 
Proc. la. Acad. Sci., vol. IV, pp. 68-72, 1878. 
