104 Transactions of the Academy of Science of St. Louis 
Ernst Antevs, who used the varve method in North America, 
considers that the ice sheets disappeared from Europe and North 
America about the same time, due probably to a rise in tempera- 
ture. R. A. Daly in his book, “The Changing World of the Ice 
Age” shares this idea and accepts Antev’s figures for the rate of 
recession of the last ice sheet from North America. After a 
discussion with Antevs and Frank Leverett, George F. Kay de- 
cided that 25,000 years is the safest estimate that can be made 
of the interval since the last ice sheet (Late Wisconsin) began 
its retreat from Iowa. The evidence is conclusive that there were 
several stages of glaciation in Iowa, for glacial deposits are sep- 
arated by erosion surfaces, weathered surfaces, peat formed from 
plant material, and loess, a wind deposit (Figure 4). The four 
stages here include, Nebraskan, Kansan, Illinoian and Wisconsin 
which is made up of Iowan, and Early and Late Wisconsin ( Fig- 
ure 4).The most significant evidence is the weathering of the gla- 
cial deposits, for it affords a means of comparison of the length of 
the interglacial ages with the time elapsed since the retreat of 
the ice from Iowa. The deposits of the last glacial stage (Late 
Wisconsin) have been weathered to an average depth of two and 
a half feet since they were deposited, and to this interval has 
been assigned 25,000 years. The nature and the depth of weather- 
ing of the Iowan deposits are considered by Kay to have required 
2.2 times as long, or 55,000 years (Figure 4). On the same basis, 
the weathering of the Illinoian till would take 4.8 times 25,000 
years or 120,000 years, the weathering of the Kansan 12 times 
as long or 300,000 years and the weathering of the Nebraskan 
8 times or 200,000 years. The combined duration of the inter- 
glacial ages would be 675,000 years. To this must be added the 
time required for the ice to advance to and retreat from Iowa 
during each of the stages. Kay assumes this took place at the 
rate of one mile in ten years and would require about 30,000 
years more, giving a minimum duration of about 700,000 years 
for the Pleistocene of Iowa. This estimate falls about midway 
between the 500,000 and 1,000,000 years favored by most scien- 
tists as the duration of the glacial period. The work of Kay is 
