Knobs are ridges of flint limestone, intermingled and covered with earth, and elevated one or two hundred feet above the common surface. This species of land is of little value for cultivation, and usually has a sprinkling of dwarfish, stunted timber, like the barrens.
The steep hills and natural mounds that border the alluvions have obtained the name of bluffs. Some are in long, parallel ridges, others are in the form of cones and pyramids. In some places precipices of limestone rock, from fifty to one or two hundred feet high, form these bluffs.
Ravines are formed amongst the bluffs, and often near the borders of prairies, which lead down to the streams.
Sink-holes are circular depressions in the surface, like a basin. They are of various sizes, from ten to fifty feet deep, and from ten to one or two hundred yards in circumference. Frequently they contain an outlet for the water received by the rains. Their existence shows that the substratum is secondary limestone, abounding with subterraneous cavities.
There are but few tracts of stony ground in the State; that is, where loose stones are scattered over the surface, and imbedded in the soil. Towards the northern part of the State, tracts of stony ground exist. Quarries of stone exist in the bluffs, and in the banks of the streams and ravines throughout the State.
The soil is porous, easy to cultivate, and exceedingly productive. A strong team is required to break up the prairies, on account of the firm, grassy sward which covers them. But when subdued, they become fine, arable lands.
Rivers, &c.—This State is surrounded and intersected by navigable streams. The Mississippi, Ohio and Wabash rivers are on three sides,—the Illinois, Kaskaskia, Sangamon, Muddy, and many smaller streams are entirely within its borders,—and the Kankakee, Fox, Rock, and Vermillion of the Wabash, run part of their course within this State. The Mississippi meanders its western border for 700 miles. Its principal tributaries within Illinois, are Rock, Illinois, Kaskaskia, and Muddy rivers. The Illinois river commences at the junction of the Kankakee, which originates near the South Bend in Indiana, and the Des Plaines, which rises in the Wisconsin Territory. From their junction, the Illinois runs nearly a west course, (receiving Fox river at Ottawa, and Vermillion near the foot of the rapids,) to Hennepin, where it curves to the south and then to the south-west, receiving a number of tributaries, the largest of which are Spoon river from the right and Sangamon from the left, till it reaches Naples. Here it bends gradually to the south, and continues that course till within six miles of the Mississippi, when it curves to the south-east, and finally, to nearly an east course. Its length, (without reckoning the windings of the channel in navigation,) is about 260 miles, and is navigable for steamboats at a moderate stage of water to the foot of the rapids. The large streams on the eastern side of the State are Iroquois, a tributary to the Kankakee, Vermillion of the Wabash, which enters that river in Indiana, Embarras, that has its source near that of the Kaskaskia, runs south-easterly, and enters the Wabash 9 miles below Vincennes, and Little Wabash near its mouth. Along the Ohio, the only streams deserving note are the Saline and Bay creeks, and Cash river, the last of which enters the Ohio six miles above its confluence with the Mississippi.
Productions.—These are naturally classed into mineral, animal and vegetable.
Minerals. The northern portion of Illinois is inexhaustibly rich in mineral productions, while coal, secondary limestone, and sandstone, are found in every part.
Iron ore has been found in the southern parts of the State, and is said to exist in considerable quantities in the northern parts.