Illinois and Iowa Under Ice

Most people take for granted the fascinating legacy of the last two glacial events that covered parts of Illinois and Iowa until 12,000 years ago, when the most recent one finally receded.

Addressing the crucial role of ubiquitous glacial deposits left by the last glacier, Ignatius Donnelly wrote in 1883, “It is our earth. It makes the basis of our soils; our railroads cut their way through it; our carriages drive over it; our cities are built upon it; our crops are derived from it; the water we drink percolates through it; on it we live, marry, raise children, think, dream and die; and in the bosom of it we will be buried.”

The contribution of the last two glaciations to the present landscape is readily visible throughout both states.

The mounds and valleys and flat farmland are all remnants of the last great ice sheet as are the five Great Lakes, but the area around Blue Mound and Mount Pulaski, along with the counties south and west of there are glacial drifts and valleys left by a previous ice sheet that covered the area until 125,000 years ago. Known as the Illinois Glacier, it covered most of the state, while the most recent one, called the Wisconsin Glacier, traveled only as far south as Pana and Shelbyville, Illinois

While researchers generally agree on the extent of the ice sheets that covered the area, the cause of the 30 or more glaciations over the last 2 million years is still the subject of some debate, though it is generally accepted as fact that the glaciers were caused by the lack of solar radiation reaching the earth around the 60th parallel, near Hudson Bay, which allowed ice to accumulate for several thousand years.

“In warmer periods, the higher latitudes of the earth tilt toward the sun in summer, allowing some of the snow and ice there to melt, instead of piling up as ice,” said Dr. Eric Grimm, curator and chair of the botany department of the Illinois State Museum.

“But long periods of cold came to the region, it’s now thought, because the orbit of the earth around the sun varies from near circular to elliptical in 100,000 year cycles, and because the tilt of the earth on its axis varies about 5 degrees in 40,000 year cycles and because of some variation in the equinoxes in 20,000 year cycles. The combination of these fluctuations, in time, interrupted the snow-melt cycle allowing the ice to accumulate.”

Grimm said that during the last glacial event, the most recent ice sheet reached a height of about 16,000 feet at Hudson Bay, while at Chicago, it was about as high as the Sears Tower except for the antennas which would probably have been visible above the ice. By the time the ice reached Central Illinois, its depth has been estimated at only a few hundred feet.

Dr. Leon Follmer, of the University of Illinois at Champaign, said when ice accumulates to that extent, something must give.

“It’s like trying to pile up wet cement. Follmer said. “What we see is that the pile is top heavy and as it sinks the cement at the bottom radiates outward until the pressure is stabilized. The same physics apply to a mountain of ice.”

Scientists say that all of the water to make the various ice sheets came from the oceans.

“Sea level dropped 400 feet,” Grimm said. “Water evaporated from the sea and fell as snow over Canada, but because of where the earth was in relation to the various cycles mentioned earlier, the snow never melted.”

Credible evidence has been discovered which suggests that many glaciers have visited the area over the past 2 million years.

“It was thought for a long time there had been only four glaciations,” Grimm said, “but we now know that this phenomenon happens roughly every 100,000 years.”

Grimm said an ocean drilling project that began in 1970 has proven correct the orbit-axis-equinox cycle theory, popularized by the Serbian Mathematician/astronomer Malutin Milankovitch, who, after his death, was awarded a Nobel Prize for his work.

“We have been able to measure the ratio of the heavier oxygen isotope left in the sea water while the lighter isotope evaporated and fell as snow over the land. The correlation of the ratio of the heavier water found in certain plankton, to the lighter water was the same in all the cores from all the seas, which means as the percentage of the heavier isotope increases, it’s a predictor of how much of the lighter isotope evaporated and thus, how much ice formed,” Grimm explained.

The ocean cores also provided clues to the tundra-type weather that Central Illinois and Iowa experienced during the various glaciations.

Dr. Jeff Saunders, curator and chair of the geology section of the Illinois State Museum said the weather was much different in Central Illinois during the ice age.

“The last glacial episode peaked about 20,000 years ago,” Saunders said. “As the temperature increased and the ice receded, it took several thousand years to melt all that ice. Had we been able to live in sight of the glacier then, the most noticeable thing would have been the much cooler summer temperatures,” he said, “The winters though would not have been much colder than the worst winters of our own life time.”

He added that many species of animals that roamed the earth then are no longer with us.

“Mastodons and giant ground sloths, saber toothed tigers and wholly mammoths, all of which are now extinct, were plentiful then, Saunders said. “We would have also recognized many animals that are still with us, including squirrels, rats, mice and beavers.”

The climate, which supported the plants and animals of the Ice Age, has been warming over the last 16,000 years, and while the earth is in a warm period now, we should not become overly complacent.

“In time, the ice will be back,” Grimm said. “Meanwhile, the current warming cycle poses a serious threat of its own. The climate is going to get warmer, due largely to the rising level of greenhouse gasses in the atmosphere,” he said. “As a result, assuming nothing is done to significantly reduce the present level of emissions, sea levels could rise as much at 18 feet, due to melting of the ice cap, which will cause wide-spread flooding of coastal regains, so severe Katrina will look like a non-event.”

Until then, we can simply enjoy working and living on this earth, thankful as we are for the magnificent gifts the glaciers left behind.