IOWA ACADEMY OP SCIENCE 
121 
In seeking to account for the presence of loess on the paha, Norton thus 
accepts McGee’s glacio-fluviatile and fluvio-lacustrine hypothesis, and attempts* 
to meet the arguments advanced by the writer in recent years in support of the 
aeolian hypothesis, without, however, specifically giving their source. 
He offers a number of objections to the aeolian hypothesis in its special appli- 
cation to the loess of the paha, which should not go unchallenged. 
He contends that it does not explain the presence of isolated masses of loess 
on the paha and its absence from intermediate areas. To those who are in the 
habit of considering physical forces only in relation to the subject this may 
appear a serious objection, but in the light of the study of surface condition as 
related to fioral development and distribution, this objection disappears. Indeed 
the application of plant ecology to the problem constitutes one of the strongest 
supports of the aeolian hypothesis. 
The elevations were the first to be drained and to present conditions suitable 
to the development of continuous carpets of vegetation which could receive and 
hold the fine dust descending upon them. They also formed an obstacle 
which operated much as does a snowdrift or sand dune, either of which may be 
gradually built up by fine material driven upward along the surface. Winds, 
strong or gentle, would shift much dust in the same manner, carrying it up- 
ward to greater elevations, where it would lodge in the well-developed vege- 
tation. 
The fiat intermediate areas, being poorly drained, were alternately fiooded 
and exposed, the latter condition providing a source of much of the dust re- 
quired for the upbuilding of loess. Such undrained areas may be seen in many 
sections of the state on the Wisconsin, Iowan and even Kansan drifts. They 
are now uniformly without loess, and usually without much accumulation of soil, 
the increment of fine materials due to decay of plants, dust, etc., evidently be- 
ing fully equalled by the loss sustained during dry periods, when these areas 
are in part exposed. 
The suggestion that the accumulation of loess on the uplands below Waverly, 
adjacent to the narrowest part of the Cedar valley, and the absence of loess 
from the borders of the broad part near Horton, militates against the probability 
of the river-bars being the source of loess materials, and the further statement 
that if elevations accumulate loess dust all should be covered with loess, are 
merely efforts to employ single causes in the explanation of phenomena which 
are dependent upon several causes. 
The fact is that under the aeolian hypothesis several coincident conditions 
are called for, and it is the sum total of these which determines the final result: 
1. — It is necessary that there be a source of material. This may have been 
in river-bars, or in the Waverly territory in the surrounding Iowan plain, or 
even in the adjacent sandy Kansan areas. 
2. — There must further be the agency, wind, the action of which is ma- 
terially modified by local topographic and fioral conditions. In some situa- 
tions the exposure would result in wind erosion, while in others under pro- 
tection an accumulation of dust would result, and this difference could exist 
I 
in closely contiguous areas. 
3. — There must then be an anchorage, furnished in part by local topo- 
graphic features, but chiefiy by plants. The surface must be sufficiently rough 
*Iowa Geol. Sur., vol. XVI, pp. 383-386. 
