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The Ice Age

The Pleistocene, or Ice Age, began over 2.5 million years ago and ended a short 10,000 years ago. During this time, numerous ice sheets formed and moved south from Canada into the United States. Periodically the climate warmed and the glaciers receded, only to form and surge south again.

Why was there an Ice Age?
No single event or action caused the Ice Age. Instead, a number of conditions combined, including the following:

Large land masses near the poles created a surface upon which thick ice could form.

The formation of mountain ranges caused major climatic change by shifting global circulation patterns.

Decrease in atmospheric carbon dioxide reduces the insulating effect of the Earth's atmosphere, resulting in heat loss.

Changes in the position of the North Pole and the Earth's tilt and orbit around the sun affect how much solar radiation it received.

The Last Ice Age
The last Ice Age, the Wisconsinan, began about 50,000 years ago. As the environment cooled, a large ice sheet formed in the Hudson's Bay region, and began to spread south. One lobe entered central Iowa and moved as far south as Greene County. Then, as the climate warmed about 30,000 years ago, this lobe "retreated," or melted. Then as temperatures cooled again, another glacier, the Des Moines Lobe, entered Iowa and pushed down through the center of the state to reach Des Moines about 17,000 years ago. It finally retreated 14,000 years ago.

Link to "The Quaternary" »»

Links to the Iowa Department of Natural Resources' Geological Survey:

Geology of Iowa: Iowa's Earth History Shaped by Ice, Wind, Rivers, and Ancient Seas »»

Landscape Features of Iowa »»

Linked Depressions on the Des Moines Lobe »»

Glacier Landmarks Trail: Iowa's Heritage of Ice »»

Glacial Boulders in Iowa »»



They Came to North America
Humans came to North America near the end of the Wisconsinan glaciation, perhaps about 25,000 years ago. Scientists continue to debate when humans arrived. Discoveries of new evidence continue to push the possible dates of immigration back farther in time, perhaps to more than 40,000 years ago.

There was no single event or condition that allowed for human arrival and settlement in North America. Humans probably arrived here in small groups, and families soon spread across all of North America.

How Did They Get Here?
Scientists debate this, too. Here are some of their theories...

During Wisconsinan glaciation, so much water was locked up in ice that the sea level was about 300 feet lower than today. Today, Siberia and Alaska are separated by the waters of the Bering Straits. But during glaciation, when this land was exposed, grasslands a thousand miles wide connected the two continents. Many scientists believe that across these grasslands (called "Beringia") and down the exposed western coast of North America, plants animals, and humans slowly made their way into this new world.

The Ice-Free Corridor:
Many scientists believe that during Wisconsinan glaciation, an ice-free corridor existed from Beringia down the eastern side of the Rocky Mountains into the middle of the continent, making migration far easier than over glaciers.

Sea Coast Migration:
Proponents of this theory believe that some humans in boats followed the emergent seacoast from Asia down the west coast of North America.

The Atlantic Route:
Another theory holds that a maritime population from Europe traveled by boat, following the southern border of the North Atlantic ice to North America. Some scientists believe this theory because of similarities between the flaked-stone tools of early Paleo-Indians in North America and the Solutreans in Europe.

Evidence of Pale-Indian Culture »»

Des Moines 16,000 Years Ago

Des Moines 16,000 years ago was a place of contrasts. At times, the ice front of the Des Moines Lobe surged to the north edge of the Raccoon River Valley and then retreated. The ice stood hundreds of feet high. Sediment choked the Raccoon River, backing up the water and splitting the channel into interlacing braids. At other times the volume of melt waters increased and the current was deep, fast, and abrasive.

This area was a mix of grasslands and forests of spruce, aspen, and oak. Stretching north of Des Moines, in areas where the ice had melted, marshes and muskeg (or bogs) were common. This environment supported a variety of herbivores.

Fossils from the time reveal that the Des Moines region was home to mammoth, mastodon, giant ground sloth, musk ox, a variety of bison, and elk. We do not know for certain if humans walked the Raccoon River Valley at this time, but they may have. We do know that soon they would be in Iowa and surrounding states, hunting the mammoth, mastodon, and other game.

Within a few centuries, the temperatures warmed and the ice melted for the last time. New forests filled the river valleys, prairies stretched out to the west and south, and marshlands continued to the north. The subtle changes brought great change to the inhabitants—extinction for some, and opportunity for others.

More animals of the time »»

How Do You Make a Mammoth?

The life-size cast of the Hebior mammoth was produced by PAST: Prehistoric Animal Structures, Inc., of East Coulee, Alberta, Canada. The molds for the cast were made from the original fossilized skeleton discovered in 1994 on the Hebior farm near Lake Michigan. Because the Hebior mammoth was 90 percent complete, some missing or damaged bones had to be created.

Candid shots show step by step how the skeleton was made and moved, from Canada to our museum.

See the images »»