62 
IOWA ACADEMY OP SCIENCE 
interglacial periods, and do not connect the drift with loess. Such are the fol- 
lowing examples which the writer has personally examined: 
A post-Kansan loess between Kansan and Illinoian drifts in cut facing 
Hershey Ave., Muscatine, Iowa;* a similar loess between the Kansan and Iowan 
drifts in Jefferson twp., northwest of Iowa City;** a loess between the Kansan 
and Wisconsin drifts in Des Moines, Iowa;*** two loesses between the Kansan 
and Wisconsin drifts near Carroll, Iowa;t a loess between the Illinoian and 
Wisconsin drifts in the Farm creek exposure near Peoria, Ill.;t and other 
similar exposures. In all the examples mentioned the loesses are fossiliferous, 
and their relation to the drifts is clear. 
These interglacial loesses present no evidence of close genetic relationship 
with the contiguous drift-sheets — on the contrary, their abundant terrestrial 
fossils indicate that the conditions which existed during the formation of these 
loesses were similar to those under which the same species exist today through- 
out the loess-covered area, and these species now exist abundantly on our up- 
land surfaces in a temperate climate! 
These loesses clearly represent interglacial periods of long duration, and 
the foregoing cases cannot be considered in connection with the occasional 
gradual transition from drift to loess which seldom results in the formation 
of a stratum more than a few inches in thickness. But even this gradual 
transition, which as noted may sometimes be observed, does not prove the com- 
mon origin of loess and drift, for it is entirely consistent with the aeolian 
hypothesis and gives substantial support to it. Such gradual transition from 
sand or drift to finer material, such as loess, would occur on surfaces on which 
the covering of vegetation increased gradually, particularly where the sand-dune 
or drift surface was adjacent to a broader territory over whose surface the same 
gradual increase in vegetation had taken place. While vegetation was still 
practically absent, the surface materials were freely shifted about, even coarse 
sand being thus moved. As vegetation gradually increased the coarser material 
was less frequently disturbed, or when shifted became mixed with finer ma- 
terials, and finally only the finest dust, now forming the loess, was transported 
and lodged on the more densely covered surfaces. 
Where the change in vegetation was very gradual there was a corresponding 
change upward from coarse to fine material. Where the changes were abrupt, 
because of changes in drainage, etc., the lines of demarkation became sharp, 
and where these conditions alternated, corresponding hands of loess and sand 
were formed, if the surface was quite sandy. 
III. Conditions existing during deposition of loess. 
The presence of numerous fossil mollusks in the loess proves that there was 
an abundant vegetation on the surface at the time (or successive times) of 
deposition. These mollusks are herbivorous, and they are terrestrial and air- 
breathing, chiefiy upland species, and their distribution is determined by the 
character of the plant covering, which serves both for shelter and food. A 
comparison of the distribution of modern terrestrial mollusks and the plants 
^Reported by the writer in Bull. Lab. Nat. Hist. , St. Univ. of Iowa, vol. V, pp. 362-3. 
**Reported by the writer, ibid., p. 366. 
*** Reported by McGee and Call, Am. Jour. Sci., vol. XXIV, pp. 202, et seq., 1882; and by 
the writer, Bull. Lab. Nat. Hist. St. Univ. of Iowa, vol. V, p. 367. 
t Reported by the writer, ibid. , p. 367. 
tReportedby Leverett— Monographs, U. S. Geol. Sur., vol. XXXVIII, p. 187, 1899. 
