Jones: Flora of Illinois 3 



Iowa and Missouri, and on the south by Kentucky. The maximum length is 

 380 miles, and the width more than 200 miles. Its area is approximately 

 57,926 square miles. Physiographically, most of this state except the southern 

 portion lies in the Till Plains Section cf the Central Lowland Province. Bio- 

 geographically, almost all of Illinois lies in the Austral Zone. The great 

 majority of native species of plants are therefore of southern affinities, and the 

 boreal element is extremely small. The Austroriparian Province enters the 

 state only at its extreme southern end. The average elevation above sea level is 

 about 600 feet. The highest point is 1257 feet altitude at Charles Mound in 

 Jo Daviess county along the Wisconsin- Illinois boundary. Although most of 

 the area has a low elevation and comparatively level surface there is a good 

 drainage system with more than 275 streams, which may be grouped in two 

 river systems, one having the Mississippi River, and the other the Wabash 

 and Ohio rivers as its outlet. The soils of Illinois are remarkable for their 

 fertility, and agriculture is one of the important occupations. The better agricul- 

 tural districts are characterized by a black loam, and the alluvial soil of the 

 river valleys is especially fertile. On many of the river bluffs the soil is loess. 

 Nearly all the rocks of Illinois are sedimentary and belong to the Paleozoic 

 era. Igneous rocks are found only in a few places, and metamorphic rocks 

 are almost unknown. 



During the Glacial period there were four advances of the ice-sheet into 

 Illinois. The ice of the third, or Illinoian, stage covered approximately nine- 

 tenths of the state, and extended southward to the Ozark Ridge, the most 

 southerly latitude reached by the North American ice-sheet. Hence, there are 

 only three districts in Illinois that have remained untouched by the Pleistocene 

 glaciation. These are 1) the seven southernmost counties of the state, 2) an 

 area between the Mississippi and Illinois rivers in Calhoun County, and 3) 

 Jo Daviess county and a small portion of Carroll County. The second and 

 third districts are part of a much larger non-glaciated region known as the 

 Driftless Area, which occupies adjacent portions of Wisconsin, Minnesota, 

 and Iowa. 



The flora and fauna of Illinois are similar to those of adjacent states. 

 Extensive forests and grasslands formerly covered the entire region. In the 

 northern part there were large prairies with tongues of forest extending along 

 the principal watercourses. At the present time^ although the vegetation has 

 been greatly disturbed, the flora is still rich and varied, with a large number 

 of species of grasses, as well as other herbs, and ligneous plants. The more 

 extensive forested areas are chiefly in the southern counties, especially on the 

 flood plains of the principal rivers, and in the Ozark Hills. These forests are 

 composed almost entirely of hardwoods. Oak, hickory, maple, and ash are 

 among the more common kinds of trees. 



Formerly one of the most remarkable features of the state of Illinois was 

 its great stretches of prairies covered with rich growth of tall grasses and 

 several hundred species of other herbaceous flowering plants. The most exten- 

 sive of these prairies occurred in northern and central Illinois, and were inter- 

 spersed with numerous swamps and shallow ponds which have long since 



