Pulverized rock is part of it. Ice age glaciers were awesome creatures, some up to 8000' high.
The glaciers (4 for Illinois) of the Ice Ages formed the soils and terrain of Illinois. The northeastern and central areas of the state are very flat where the last glacier ground up rock and soil in its path. It deposited sand, gravel, dirt, and some rocks from farther north when it melted. Dust and sand also blew onto land surrounding the edges of the glaciers. All these deposits (loess and till primarily) make up the fertile soil of the central area.
Loess covers most of Illinois, in some places reaching a depth of more than 20 feet. It is rich in minerals, contains no rocks, and has a uniform texture that retains moisture - all qualities of good soil for growing crops.
Till - As the ice front receded, leaving an average of a 100-foot thick layer of sediment. As the ice sheet melted and receded, this sediment of clay, sand, gravel, pebbles, and rocks was dropped and packed down over the bedrock. It is called till, which is material directly deposited and covered by series of glacial ice sheets.
Some publications from the Illinois State Geological Survey discuss this well; if I can locate one I'll send you a link.
