IOWA ACADEMY OF SCIENCE 
15 
Apart from the sheets of glacial drift, which constitute the most important 
of the Pleistocene deposits, whether viewed from the economic or the purely 
scientific side, the Survey has demonstrated the stratigraphic position and 
areal extent of interglacial and postglacial sands and gravels that may not he 
neglected. These are important not only as furnishing sands for use in build- 
ing, and gravels for the improvement of roads, hut with the multitudinous uses 
of cement in modern construction they attain an economic value not easily 
calculated. There are at least three well defined sand and gravel horizons in 
the Pleistocene; two are interglacial, one postglacial. Between the pre-Kansan 
and the Kansan drifts are extensive beds known as Aftonian, which, as recent 
work has demonstrated, are very extensively developed throughout southern, 
southwestern and western Iowa. Studies hy Shimek during the last field season 
have broadened our knowledge of the extent and distribution of these Aftonian 
beds, and when we remember that they occur in regions where there are few 
stone quarries and the modern streams fiow through mud fiats, without sand 
bars, the economic importance of the Aftonian sands and gravels may be 
appreciated. 
Great, however, as is their economic value, the purely scientific significance 
of the Aftonian beds is even greater. The Aftonian gravels, especially up and 
down the western slope, have been yielding remains of animals that flourished 
during the interglacial interval, while peat and soil beds of the same age are 
telling us of the forests and humbler plants that furnished shelter and food 
for the Aftonian fauna. The results of these Aftonian studies have not yet 
been published, but we are prepared to say that during Aftonian time Iowa 
supported three great elephants, ElepJias imperator, Elephas columdi and E. 
primigenius, one mastodon, Mammut americanum, at least one large camel, a 
large stag intermediate between the moose and the elk, and at least two species 
of horses. In addition to the bones and teeth of these great mammals the Af- 
tonian sands have furnished shells of river mollusks, identical, according to Shi- 
mek, with the species living in our modern streams. The Aftonian climate was 
not very different from that experienced by modern Iowa. The important and 
significant result of these discoveries in western Iowa lies in the fact that for 
the first time, the exact horizons of the Equus and ElepJias imperator beds has 
been definitely fixed. These same great mammals have been found in Pleistocene 
deposits in western Nebraska, in Texas, in Oklahoma, and in other regions 
which were not covered v/ith glacial drift; but the precise position of the beds, 
stratigraphically, could not be determined. The work in Iowa gives us an un- 
doubted, definite horizon. The studies in Iowa further demonstrate that the 
Aftonian was a real interglacial interval, that its deposits do not represent a 
mere “fluctuation and temporary local withdrawal of the pre-Kansan ice” as 
some with meager and partial, knowledge of the subject are disposed to insist. 
The Aftonian, as w^e now see it, was a time when extensive grassy plains 
alternated with luxuriant forests, a time when soils were developed, when?, 
great peat beds accumulated, when heavy rains fell and swollen rivers fre- 
quently overflowed their flood plains, when this latitude had a climate whichi 
permitted the modern types of mollusks to occupy the streams and when great 
herbivores found an abundance of food on the open plains, in the warmer river 
valleys, or over the thinly wooded slopes. 
There is another interglacial interval between the Kansan and the Illinoian 
stages of ice invasion, the records of which have been determined by Leverett 
