SURFACE AND DRAINAGE General surface features.—Illinois is a part of the Great Central Plain of North America. The relief of the state is not sufficient to rorm distinct physiographic areas nor to exert marked influence upon the climate. Illinois is but a portion of an extensive fertile plain.
Although flatness is characteristic of Illinois as a whole, local relief is sufficient in many parts of the state to interfere decidedly with the construction of highways and railroads. The general uniformity of surface is strikingly broken and varied by the valley trenches of the master-streams and their principal tributaries; by the extensive terminal moraines; by the Ozark Ridge; and by the long-continued erosive power of running water in the unglaciated areas of the state.
The highest point in the state, 1,241 feet above sea-level, is Charles Mound in Jo Daviess County, less than a mile from the Illinois-Wisconsin boundary line. The lowest point, 26S feet above the sea, is low-water mark at the junction of the Mississippi and Ohio rivers. The total relief of the state is thus 973 feet. The distance between these two places is 400 miles; the gradient, therefore, averages about 22 feet to the mile, or 1 foot to 2,000 feet, a slope so gentle as to be imper ceptible to the eye and difficult of detection by instruments.
If, however, the traveler should motor from Charles Mound in Jo Daviess County to the river front at Cairo, he would find many miles of his route presenting other than the average gradient. Jo Daviess County alone has a maximum relief of 666 feet, and the traveler finds that the direction of the roads in the first section of his journey is controlled by the mature topography of the unglaciated area, where the relief of every square mile usually exceeds 100 feet. The roads are laid out to follow the gentler slopes and to cross the ridges at their lowest notches. Beyond the unglaciated area the roads begin to follow the section lines on the level prairie lands of the ground moraine of the great ice sheet. As the traveler approaches the broad, steep-sided valley of the Illinois River, he may find it necessary to make a detour of 20 miles or more to find a bridge. The bluffs on each side of the valley now determine the location of the highway until the journey has carried the traveler well beyond the immediate edge of the Illinois Valley. The long stretches of level country are some what broken by the shallow valleys of numerous small streams, while the larger valleys are deep enough to add variety to the landscape and to present steep gradients in the highway. In
southern Illinois the traveler sees a long, even-topped ridge rising abruptly above the level plain, stretching to the east and west as far as the eye can see. The Ozark Plateau now controls the direction and the gradient of the highways, and for the remaining 40 or 50 miles of the journey the picturesque scenery, the steep and winding roadways, and the difficult fords at small streams lead the traveler to question whether Illinois is the level state so frequently mentioned in books. The route leads across the Illinois Ozarks, down the bluffs of the Ohio to Cairo, situated on the narrow strip of level land between the Ohio and Mississippi rivers. From the levee between the city and the Ohio, the traveler sees the river flowing 50 feet below the levee crest, if in the season of low water; if at flood time, he sees the widened stream stretching away to the Kentucky hills at a level well above the streets of the city, which are securely protected by huge levees built at great expense of labor and money.
If we examine the surface of the state, county by county, we shall find that each county has a relief exceeding 100 feet. Among the larger counties, Iroquois has the least maximum relief, 130 feet, while Richland, a smaller county, has a maxi mum relief below that of any other county, 105 feet. Pope County has the greatest relief to be found within any county of the state. The descent from Williams Hill, 1,065 feet above sea-level, the highest point in the Illinois Ozarks, to the Ohio River, is 775 feet, with an average fall of 70 feet per mile, or an average of 1 foot to 75 feet. Among the Ozarks and along the highest river bluffs of the state the relief is frequently 200 to 300 feet or even more within the distance of a single mile. Although irregularities of surface are extensive and very effective in local control of highways, railroads, and industries, yet the state as a whole is noted for its gentle slopes, having a surface remarkably well adapted to the develop ment of the world's greatest industry, agriculture, and to the building of highways and railroads for easy and rapid transportation to every part of the state.