feed hee we eGR OIN*: BU bak Tl N 17 
westernmost Europe. This means that different forest trees which may 
have originated in various areas of the northern continents had an oppor- 
tunity to mix and disperse between the continents and thus create a tem- 
perate deciduous forest of much greater complexity than had existed 
before. 
Some time after this, chiefly in the Miocene period 20 million years 
ago, the crust of the northern hemisphere underwent a number of striking 
upheavals. In Europe many of the newer mountains arose or were elevated 
to great heights; in Asia the interior was elevated to form the highest part 
of the Himalayas; and in North America the entire plains area and many 
western areas were elevated and some mountain ranges were formed. These 
upheavals produced arid conditions and rain shadows which broke the 
continuous temperate deciduous forest into isolated areas. It must be re- 
membered that this forest can exist only with a fairly high minimum 
rainfall. As the rain supply decreased in various areas, the hardwood trees 
disappeared and other kinds of vegetation took their place. Because of - 
changes in rainfall which accompanied the crustal unrest of the later 
Cenozoic age, the deciduous forest now occurs chiefly in three large areas 
— one in western Europe, one in China, and one in eastern North America, 
plus isolated remnants of just a few speciec of trees in local areas. The 
biotic elements restricted to the temperate deciduous forest have never 
been rejoined since their disjunction of some 15 to 20 million years ago. 
The eastern American segment of this forest may have been isolated 
for millions of years, but it certainly has not been static. Changes of two 
kinds have occurred continuously. First, many new kinds of plants and 
animals evolved, so that today the taxonomic structure of the forest is far 
different from that of the continental deciduous forest of the middle Ceno- 
zoic period. Many species of squirrels, trilliums, and violets evolved in 
this eastern forest. The best-known examples of this species multiplication 
are found in the insects. In genus after genus, insects isolated long ago 
in the eastern deciduous forest have evolved into 15 to 20 species, and in 
some instances into 300 to 400 species. Thus what may have been a re- 
latively sparse biota 20 million years ago has evolved since that time into 
a large and varied fauna and flora. 
The second kind of change has been the geographic shift of climates. 
There is mounting evidence that at times the climate of this area has be- 
come drier, so that the deciduous forest was restricted to some of the higher 
areas east and north of Illinois. The western edge of our deciduous forest 
probably has been a sort of pulsating boundary. But the most spectacular 
changes in climatic conditions were in a north - south direction. 
Another part of the Dunesland history may be found in the pebbles 
on the shore of Illinois Beach State Park. You will notice that very few 
of the pebbles are alike and that they have been gouged and scoured 
and rounded. Do you ever stop to wonder how they got there? There are 
no outcrops of bed rock in Illinois. Most of these bits of weathered rock 
come from ridges and outcrops which occur no closer than central or 
northern Wisconsin and farther to the north and east. The stones were 
brought from the north and northeast by the various glaciers which over- 
rode this part of the country. When the glaciers melted away, they left 
these pebbles in their wake. 
