8 IOWA STUDIES IN NATURAL HISTORY 
plain’’ now very largely under a high state of cultivation. This 
plain hes between the Missouri and Mississippi rivers above which 
it rises very little, the lowest point in the state, in southeastern 
Iowa, having an elevation of about 480 feet and the highest 
point, in northwestern Iowa, having an elevation of about 1700 
feet. A somewhat ill-defined watershed extends from Dickinson 
county in the northwest to Wayne county in the south central 
part of the state. Approximately two-thirds of the total area 
lies east of this sinuous ridge and less than one-third west of it. 
The so-called Driftless area which has not been covered by an 
ice sheet occupies only a small area of northeastern Iowa. Here, 
deep valleys, high rocky cliffs, steep hills and thin soils are the 
rule. The Drft-covered area which has been covered by well 
defined ice sheets occupies by far the larger portion of the state. 
The Kansan drift which covers most of southern, south central, 
and western Iowa presents steep ravines and rounded hills and 
the entire area is well drained. In southeastern Iowa the [lli- 
noian drift presents a more level and rolling surface. In north- 
eastern Iowa, just to the west of the Driftless area the Iowa drift 
sheet is present and is characterized by the small amount of 
erosion, small streams and, in early days, by many undrained 
sloughs. The Wisconsin drift sheet extends in a more or less tri- 
angular lobe over most of northwestern Iowa and south as far as 
eentral Iowa. Little erosion has taken place and drainage is not 
good so that great numbers of lakes, marshes and sloughs are 
formed. Along the edges of this drift sheet are numerous small 
gravelly hills and mounds. 
According to the divisions made by Dr. C. H. Merriam?, the 
greater part of Iowa lies in the Upper Austral Zone (Carolinian) 
with the Transition Zone (Alleghanian) represented only by a 
short lobe in the northern part of the state. This lobe extends 
from the western third of the state line east nearly to the Mis- 
sissippi river and is not more than two or three counties deep 
at its widest part. 
Cold winters and hot summers, the latter probably attracting 
some of the southern species of insects, are the rule in Iowa. In- 

2Merriam, C. H., Life Zones and Crop Zones of the United States, Bull. No. 10, 
U. S. Dept. A'gr. Bur. Biol. Surv., 1898. 
