Illinois was prairie.
Modern scientists recognize six main subclasses of prairie in Illinois.
Flat terrain and deep loess soils made most Illinois prairies ideal for agriculture. The breaking of the Illinois prairies began in earnest with the invention in 1837 of a self-scouring steel plow both strong enough to slice through the dense mat of prairie plant roots and slick enough to slip through sticky loam soils. Vast stretches of prairie were destroyed between about 1840 and 1900. Four out of five of the state's prairie remnants are smaller than ten acres and one in three is smaller than one acre. Of the 253 prairie sites identified by the inventory, four out of five are not protected as dedicated nature preserves.