'Lake Pontiac' covered Fairbury

The land around Fairbury is relatively flat. The glaciers from the last ice age leveled it, and it became some of the most productive farmland in the world.

Five primary ice ages have occurred in the last 2.5 billion years of Earth's history. In chronological order, these ice ages include the Huronian, Cryogenian, Andean-Saharan, Paleozoic, and the Quaternary Ice Age, which ended about 12,000 years ago.

Scientists are still working to determine what exactly caused the past ice ages. Most theories revolve around subtle changes in the Earth's orbit around the sun. These changes can cause major temperature changes on the planet, and they are likely cyclical in nature.

If global temperatures enter a cooling period, snow accumulates yearly and does not melt in northern areas. When enough snow accumulates, it turns into ice, and the ice sheet flows in a southerly direction. The last major glacier to cover most of Illinois occurred during the Wisconsin glacial period. The Wisconsin glacial period started about 75,000 years ago and ended about 12,000 years ago. This ice sheet reached a thickness of about two miles high in Canada and one mile high in Wisconsin. The thickness of the ice sloped gently down to the furthest southern point of the glacier.

The ice sheet from the Wisconsin period covered the eastern half of Illinois and extended south down to about 180 miles south of Chicago. Fairbury is 100 miles south of Chicago.

The glacier acted like a giant bulldozer and flattened any hills on the land. The rocks and soil sheared off became part of the glacial ice, and the rocks in the ice made the glacier like a sheet of sandpaper, smoothing the land.

When global temperatures rose, the glaciers began to melt and retreat back from Central Illinois to Canada. Sometimes, the temperature would cycle from warmer to colder, and the glacier would advance again. When the southern edge of the ice sheet melted, the glacier left a pile of rocks and debris called a moraine. The glacier retreated gradually, and each major step left a different moraine.

The prominent moraines in Livingston County left from the Wisconsin period include the Cropsey Moraine, the Chatsworth Moraine, and the Marseilles Moraine. The Cropsey Moraine is further subdivided into the Inner, Middle, and Outer Cropsey moraines.

Moraines can be thought of as small earthen dams. As the glaciers melted, a tremendous amount of water was released. When this meltwater became trapped between different moraines, temporary lakes were formed. Sometimes, the water would erode the Moraine and open a channel to allow the lake to drain away.

Lake Pontiac was the largest lake in Livingston County left after the Wisconsin-era glacier melted. This lake covered what is now Fairbury and Pontiac. The second largest lake was Lake Watseka in the southeastern part of Livingston County.

It is unknown how long Lake Pontiac existed before it drained. It may have existed in the Fairbury and Pontiac area for hundreds of years.

On May 23, 1936, the Illinois State Geological Survey conducted a geologic field trip for high school science teachers. Participants drove in their cars from the Pontiac High School north to Odell, south down to Forrest, west to Fairbury, northwest to McDowell, and back to Pontiac. They stopped at various locations, and the state geologists pointed out the Marseille moraine and various glacial lakes. A similar geology field trip was done in 1963 in Pontiac, and the program is 33 pages long. Copies of these programs still exist and are in the Livingston County Historical Society archives.

In 1955, Fairbury Township High School sponsored a geological science field trip in the Fairbury area. The general public who attended this field trip drove in their automobiles to various stops, similar to the 1936 Pontiac field trip.

The geologists who conducted this tour noted that the Inner Cropsey Moraine is one in a series of three moraines. The other two are the Outer and Middle Cropsey, which, like the sub-crests of either one, are not equally distinguishable or equal in magnitude at all places. South of Fairbury, the Outer, and Middle Moraines are both narrower than the Inner and the Middle Moraine, on which the town of Cropsey is located, is higher and more prominent than the Outer or Inner Moraines.

The geologists also noted that as a glacier receded, meltwater doubtless accumulated in local ponds or lakelets between the ice front and the last formed Moraine, except where there were channels through the Moraine through which the water could drain. Where such drainage channels are absent, it may be presumed that as the ice front continued to recede, the local ponds and lakelets gradually merged into one large lake that persisted until the glaciers uncovered some passage or until some outlet river eroded a channel through which the lake could be drained.

Geological studies show that such a lake, or a series of lakes, existed behind each of the three Cropsey moraines. Whether the older ones still existed or were drained before the later ones developed has not been determined. However, we know that for a time, the lake's waters that locally existed behind the Inner Cropsey Moraine flowed southward through a gap in the Moraine, which is now part of the valley of Indian Creek.

The 1955 Fairbury area tour program had 17 pages, including the tour routes and various maps of the moraines and glacial lakes. A copy of this program is in the Fairbury Echoes Museum archives. One of the maps was modified to show the moraines and glacial lakes in Livingston County, and this modified map accompanies this article.

Glaciers and ice sheets have been part of Earth's history for billions of years. The next ice age, which will form large ice sheets in Illinois, is expected in about 50,000 years. This next ice age may flatten out our relatively flat farmland even more!

(Dale Maley's weekly history feature on Fairbury News is sponsored by Dr. Charlene Aaron and Antiques & Uniques of Fairbury)