18 DHE (AU. Dsus BION BU ieee 
The ice sheet which carried pebbles from the north down to our 
Dunesland area also over-rode the life of the land. This terrain was once 
covered by a mile of ice. I will not dwell on the details of the ice ages. 
Four main ice sheets descended over this area. During the last ice period, 
called the Wisconsin. the tremendous Lake Michigan glacial globe spread 
to central Illinois. The ice retreated, advanced, retreated, and readvanced, 
until finally it passed to the north. The last ice thrust to enter Illinois 
occurred about eleven thousand years ago and covered only this north- 
eastern part of the state. A combination of its churnings and the fluctuat- 
ing levels of Lake Michigan produced the series of sandy and gravely ridges 
and swales which make up the Dunesland. What we have here, then, is a 
relic of the living things which colonized some of the bare areas left in 
the wake of the glacial action. 
Most of the bare areas have been colonized by plants and animals 
typical of other colonizing communities of the temperate deciduous forest, 
but many species are found seldom if at all in other areas of Illinois. The 
restricted plant species include trailing juniper, low bush juniper, and the 
bearberry, which occurs on the ridges, and fringed gentian and arrow grass, 
which occur in the swales. The result of the colonization is a series of 
distinctive communities. The ridges of the Illinois Dunes are a mixture of 
scrub oak and prairie. The prairie communities characterized by the 
bluestem grasses, juniper, and bearkerry, are the finest example of this 
aggregation in the state. Along with the plants occurs a galaxy of unusual 
insects. The swale communities are equally attractive, with various rushes 
and sedges, aquatic cinquefoil and arrow grass, and with fringed gentians, 
orchids, and bog birches around their edges. These also have distinctive 
insects feeding on them. 
One of the most unique features of this area is a group of northern 
caddisflies in the Dead River. These caddisflies are aquatic insects whose 
larvae make either portable cases or a fixed shelter. The larvae pupate 
either in their cases or in a special cocoon in the water. When mature, 
the pupae swim to the surface, crawl up on a plant stem, and emerge as 
adults. These insects abound in the waters of the Dead River and three 
species of them have been found nowhere else in Illinois. 
All of these distinctive Dunesland insects and plants are northern and 
many of them are widespread. The juniper, bearberry, and fringed gentian 
are found in the high meadows and screes of the Rocky Mountains, as 
well as areas to the north and northeast of Illinois. Many of the Dunesland 
insects are also northern. This includes the three caddisflies which we 
mentioned, the bearberry leafhopper and aphid, and a leafhopper and an 
aphid on meadowsweet. A few of these northern insects are of unusual 
interest in that they represent some of the elements which have evolved 
within the eastern temperate deciduous forest and have never spread to 
other areas. 
Why are some of these northern species found only in this corner of 
Illinois? If we draw lines through points having the same temperature 
(such lines are called isotherms), we find that during fall and winter the 
lines generally slant from southwest to the northeast. In spring and summer, 
however, the isotherms slant the other way. Evidently the ranges of many 
of these northern organisms are restricted by critical conditions during 
