TBT: Where it began

Photo from the Abraham Lincoln Presidential Library & Museum
Illinois exports $8.2 billion in agricultural commodities overseas, roughly six percent of all the agricultural exports of the United States. We are #2 in the nation in exporting soybeans and feed grains, and our state has ranked first in the nation in soybean production in the recent past, while standing in second place in corn production. We rank among the top ten states in the nation in dozens of agricultural categories, from sweet corn production to the total value of our state’s farm real estate.

All that production from our more than 72,000 farms has also opened the door to an enormous agribusiness economy. Illinois agricultural commodities generate nearly $20 billion every year, with just over half of that total coming from corn alone. Add to that the economic impact of the manufacturing of farm equipment and the jobs at the more than 2600 food manufacturing companies in our state.

In short, Illinois is an agricultural powerhouse. But how did it get to be this way?

According to the Illinois Department of Agriculture, “about 89 percent of the state’s cropland is considered prime farmland, ranking the state third nationally in total prime farmland acreage.” All that good farmland is the result of the fertile soil which was dropped here tens of thousands of years ago by the advance and retreat of glaciers during the great ice ages of the past.

Illinois, along with most of North America, was at some time in its past covered by ice. Today we are in the midst of what is known to geologists as the Quaternary Period, consisting of the most recent 2.5 million years of the Earth’s history. During this time, the Earth’s temperature has fluctuated, causing those glaciers to move back and forth over the land that is now known as the state of Illinois. At one point, 90% of the land area of our state was glaciated, as far south as a line roughly parallel to state highway 13 in southernmost Illinois. The glaciers brought Illinois its rich farmland, and also the basis for the life that it sustains.

Map from the Illinois State Geological Survey
Researchers, including the Illinois State Geological Survey, estimate that there were at least four glaciations in North America over the past two million years, possibly as many as eight, with others possibly occurring in the geological periods pre-dating that time. The glaciers stretched southward during a time lasting thousands of years in which temperatures were considerably lower on average than now. The result was a snow pack which never completely melted, and instead just grew more densely packed and expanded. Present-day Illinois, located in the lowlands between the Appalachian and Rocky Mountains, would have made an easy target for these southward-flowing glaciers.

As they moved, the glaciers scraped across the land, flattening rises and pushing the sediment into valleys, thus levelling off the landscape, creating prairies. But not all of present-day Illinois experienced this phenomenon. Somehow, these glaciers evaded the northwest corner of Illinois, leaving that area with its rolling hills, including the highest point in Illinois in Jo Daviess County. South of the line marking the glaciers’ farthest advance in Illinois lie the Shawnee Hills and the densely-wooded Shawnee National Forest.

Oftentimes, when a glacier was halted by some obstruction, it would begin to melt. Its meltwater would erode away the obstruction over a number of years, and the glacier would begin to move again. Rocks were pushed or carried ahead of the glaciers, only to be dropped where the glaciers finally halted and began to melt for good. As they melted, the meltwater carved new drainage paths in the landscape, or scoured out existing routes, in the process creating the great rivers of North America; including the ones so much of our agricultural commerce uses to get to markets today.

When the glaciers retreated, the sediments they carried were deposited. The glaciers left behind a prairie in the middle part of North America with a depth of between 400 and 500 feet of rich soil which today feeds our nation and the world. The last step in this process occurred about 15,000 years ago; after the glaciers began their final retreat; when a period of high winds carried to Illinois a layer of topsoil called loess, which proved to be very fertile for plant growth.

The ice age’s effect on present-day Illinois was spread across several different glacial episodes. The approximate dates of these episodes were determined using sediment records and studies of buried soils and the deposits from each glacier.

The glaciation which reached the farthest to the south in Illinois is believed to be the third of the Quaternary Period, and it is known, conveniently, as the Illinois glacial episode. It occurred between 190,000 and 130,000 years ago. At its peak, the glaciers covering Illinois could have had a thickness of as much as one mile. As the glaciers retreated and the weather warmed, it was replaced by the Sangamon interglacial episode or Sangamonium Stage, which lasted around 50,000 years.

The glaciers began advancing again around 75,000 years ago, a period known as the Wisconsinan glaciation or Wisconsin glacial episode. This glaciation lasted until about 11,000 years ago, but did not affect as much of present-day Illinois as the previous episodes. The Wisconsin glacial episode covered the northeastern quarter of Illinois, reaching as far south as Paris and Shelbyville and as far west as Peoria and Princeton. It left a series of moraines throughout northeastern Illinois, most notably the Marseilles Moraine which arcs from the Elgin area southwest around Chicagoland and through the area just south of Kankakee, comprising such features as the Fox River, Minooka Ridge and parts of the Illinois River.

The Wisconsin glacial episode ended with the dawning of the Holocene geological epoch, which extends right up to the present day. It was around this time that the first human settlers reached Illinois. But the ice wasn’t quite finished with Illinois just yet. During its final years, the retreat of the Wisconsinan glaciers carved out a large proglacial lake which filled with meltwater. It was the creation of the body of water we now call Lake Michigan, which along with its sister Great Lakes was created over the course of about 10,000 years around the end of the Wisconsinan glaciation.

While the greatness of Illinois history has much to do with historical figures like Abraham Lincoln, the groundwork for Illinois’ present and future was quite literally laid by a series of glaciers which swept through the area tens of thousands of years ago. They created the farmland that feeds the world, the rivers that move our products to markets and the lakeshore which led to the settlement of our largest city. Illinois recently celebrated 200 years of statehood, but the movement of the glaciers marked the time where Illinois as we know it really began.


A much more extensive analysis of ice age glaciation and its effect on Illinois has been conducted by the Illinois State Geological Survey.