Geology

Everything about the Headwaters region, from its soil chemistry to its terrain, owes to the massive reordering of the land by ice and water. The Wisconsinan ice sheet that dominated the Headwaters until about 12,000 years ago crushed an earlier landscape under a mile-thick sheet of ice. It left behind glacial debris known as drift - crushed bedrock consisting of dolomite, limestone, and sandstone of varying antiquity. This debris ranges in size from dust to boulders and filled in the ancient valleys, leaving a surface of impressive, if tedious, flatness.

Much of that surface was itself subsequently buried by loess. know to geologists as Peoria Silt, this loess is particles of quartz, feldspar, mica and other minerals mixed with clays that were ground into dust, from bedrock, by glacial ice. On this foundation was built the economy of modern central Illinois. Rich in chemicals, moderate pH, this talcum-powder-like material originally was deposited in great sheets along the floodplains of the Illinois River valley to the west; from there it was carried many miles downwind. Most of Champaign County is blanketed with 24-60 inches of loess; a few miles to the northeast, in Ford County, it is less than 20 inches thick on average. The difference partly explains why grain yields on Ford County farms tends to be lower than Champaign County's.

The richness of the Headwater's best soils also owes partly to their youth. Surface soils there have been acted upon by weather for only about 12,000 years - not long enough for leaching and exposure to seriously drain their mineral fertility. In far southern Illinois, surface soils have lain exposed for 60,000 years, during which time they have lost much of their original wealth of chemical nutrients.

"Glacial erratics" here and there protrude from the surface of the glacial rubble that buries them. Some are chunks of igneous bedrock-like granite that were ripped up by ice and shoved hundreds of miles from their origins north of the Great Lakes. Some blocks of dolomite (mainly in Champaign, Ford and Vermilion counties) are large enough to be mistaken as eruptions of bedrock. Because they were ice-borne only tens of miles rather than tens of hundreds of miles, these behemoths were not crushed and burned rock from these portable quarries in local kilns to make cement, or cut it into building blocks.

A half-dozen state-recognized natural areas in the Headwaters offer glimpses of its glacial past, usually where streams have sliced open the covering mantle of drift. Most of what is visible are the deposits laid by Illinoian - and Wisconsinan-era ice, but one can also see shales and (in the Ocoya Geological Area in Livingston County) fossil-bearing limestone laid down in a warm ancient sea that predated the ice.

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