The State of Illinois forms the lower part of that slope which embraces the greater part of Indiana, and of which Lake Michigan, with its shores, forms the upper part. At the lowest part of this slope, and of the State, is the city of Cairo, situated about 350 feet above the level of the Gulf of Mexico, at the confluence of the Ohio and the Mississippi; hence, the highest place in Illinois being only 800 feet above the level of the sea, it will appear that the whole State, though containing several hilly sections, is a pretty level plain, being, with the exception of Delaware and Louisiana, the flattest country in the Union.
The State contains about twenty-five considerable streams, and brooks and rivulets innumerable. There are no large lakes within its borders, though it has some sixty miles of Lake Michigan for its boundary on the east. Small clear lakes and ponds abound, particularly in the northern portion of the State.
As to the quality of the soil, Illinois is divided as follows:—
First, the alluvial land on the margins of the rivers, and extending back from half a mile to six or eight miles. This soil is of extraordinary fertility, and, wherever it is elevated, makes the best farming land in the State. Where it is low, and exposed to inundations, it is very unsafe to attempt its cultivation. The most extensive tract of this kind is the so-called American Bottom, which received this name when it was the western boundary of the United States. It extends from the junction of the Kaskaskia and Mississippi, along the latter, to the mouth of the Missouri, containing about 288,000 acres.
Secondly, the table-land, fifty to a hundred feet higher than the alluvial; it consists principally of prairies, which, according to their respectively higher or lower situations, are either dry or marshy.
Thirdly, the hilly sections of the State, which, consisting alternately of wood and prairie, are not, on the whole, as fertile as either the alluvial or the table-land.
There are no mountains in Illinois; but in the southern as well as the northern part, there are a few hills. Near the banks of the principal rivers the ground is elevated into bluffs, on which may be still found the traces left by water, which was evidently once much higher than it now is; whence it is inferred, that, where the fertile plains of Illinois now extend, there must once have been a vast sheet of water, the mud deposited by which formed the soil, thus accounting for the great fertility of the prairies.
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As we have said, the entire area of Illinois seems at one period to have been an ocean-bed, which has not since been disturbed by any considerable upheaval. The present irregularities of the surface are clearly traceable to the washing out and carrying away of the earth. The Illinois River has washed out a valley about two hundred and fifty feet deep, and from one and a half to six miles wide. The perfect regularity of the beds of mountain limestone, sandstone, and coal, as they are found protruding from the bluffs on each side of this valley, on the same levels, is pretty conclusive evidence that the valley itself owes its existence to the action of water. That the channels of the rivers have been gradually sunken, we may distinctly see by the shores of the Upper Mississippi, where are walls of rock, rising perpendicularly, which extend from Lake Pepin to below the mouth of the Wisconsin, as if they were walls built of equal height by the hand of man. Wherever the river describes a curve, walls may be found on the convex side of it.
The upper coal formation occupies three-fifths of the State, commencing at 41° 12' North latitude, where, as also along the Mississippi, whose banks it touches between the places of its junction with the Illinois and Missouri rivers, it is enclosed by a narrow layer of calcareous coal. The shores of Lake Michigan, and that narrow strip of land, which, commencing near them, runs along the northern bank of the Illinois towards its southwestern bend, until it meets Rock River at its mouth, belong to the Devonian system. The residue of the northern part of the State consists of Silurian strata, which, containing the rich lead mines of Galena in the northwest corner of the State, rise at intervals into conical hills, giving the landscape a character different from that of the middle or southern portion. Scattered along the banks of rivers, and in the middle of prairies, are frequently found large masses of granite and other primitive rocks. Since the nearest beds of primitive rocks first appear in Minnesota and the northern part of Wisconsin, their presence here can be accounted for only by assuming that at the time this region was covered with water they were floated down from the North, enclosed and supported in masses of ice, which, melting, allowed the rocks to sink to the bottom. A still further proof of the presence of the ocean here in former times is to be found in the sea-shells which occur upon many of the higher knolls and bluffs west of the Mississippi in Iowa.