234 Part II. Chapter 3. 
it prevails over at least three-fourths of Nebraska to a depth ranging from 
5 to 150 feet and furnishes a soil of extraordinary strength and fertility. Th 
altitude of the loess deposits is by no means uniform. In the lower part 
the Mississippi valley, it is found at an elevation of about two hundred and 
fifty feet. In the upper Mississippi valley, it rises to a height of seven hundred 
feet above the bed of the river’). No general depression of the region will 
account for these irregularities and we must have recourse to wind action. 
Loess forms a mantle at the surface over all the Kansas area and covers with 
a thin sheet most of the southern portion of the Iowan drift. Asa rul 
the deposit is a very fine-grained, yellow silt covering alike the summits, 
slopes and valleys., At numerous points, the loess attains a depth of fıftee 
to twenty five feet. In many of these deeper deposits, the shells of pulmonate 
land mollusks belonging to the genera Zonites (Zonitoides, Euconolus), Patı , 
(Oreohelix, Pyramidula), Helicodiscus, Ferussacia (Cochlicopa), Pupa (Bifidaria 
Vertigo (Sphyrodium), Mesodon (Polygyra), Vallonia, Succinea have been fount 
while a few species of Limnaea, found in fresh, water occur. In such bet 
the loess is usually quite calcareous and segrations of lime form small nodul 
("loess-kindchen”). 
rofessor CALVIN has shown that the materials of the loess of central an 
southern Iowa were probably derived from the finer constituents of the lo 
drift”). It would seem that the conditions during the glacial period would I 
exceptionally favorable for strong northerly and westerly winds whose sweep. 
over the ice sheet would be unobstructed. Then too the finer, loose material; 
liberated by the melting of the ice, would be in prime condition to be ig 
thered up and swept along by the winds. During all these physiograpäl | 
changes which began with the oncoming of the glacial period and closed with 
the formation of loess, we must believe that the original grass flora of th 
plains maintained itself, but during the ice age typically in a more sou 
locality for in the north central prairie region, the vegetation and prairie veg 
tation was destroyed and replaced by an arctic flora along the ice front. 5 
boreal trees as Betula papyrifera, Populus balsamifera, Populus tremuloides, 
Acer glabrum were left behind according to CLEMENTS after the disappearance 
of the northern flora. As a closed vegetal formation the grass vegetation 
prevented trees from ever reaching the prairie areas. We have evidence 
of a continuous grassy covering of parts of this central plain from the time 
1) WRIGHT, G. FREDERICK: The Ice Age in North America 1889: 360. 
2) SAVAGE, T.E.: Loess. Iowa Geological Survey XII (1902) 242. 
