Chicago's Great Ice Sheet

       So you think the winters in Chicago are cold! Well, you should have been here about a million years ago. Even native Alaskans would have complained of the Chicago climate then. But there were no humans around then, and most of the animals had either gone south as the days and years became slowly colder or, like most of the plants, had died out because of the cold.

       I wasn't at the surface of the earth at that particular time because I had again been trapped in the sea sediments and buried well over my head, as had often happened before. In the North a great quantity of ice and snow was being piled up till the heap was so big that it began to spread out and creep slowly southward at the edges. The great ice sheet finally reached as far as what is now southern Illinois, and the freezing winds blowing from its edges brought great blizzards. Soon the sun could hardly make itself felt at all, and the rock around me got colder and colder. I thought I would never be warm again.

       I could hear sounds of grating and crushing above me as the great tongue of ice spread over the land and rock. The giant wall of ice at the front of the tremendous glacier advanced and scraped all the soil away from the rock underneath. It also scraped, gouged, and smoothed the rock. As time passed, the scraping sounds above me got louder and louder until one day the earth around me trembled and heaved, and almost before I realized it I was again moving along. The rock around me was broken up, and even if I did not go very far I was worn down again and scratched. I seemed to go only a short distance and then I bumped into an obstacle of some sort and stopped moving.

       I stayed in this spot for a long time, piled up with many other pebbles and rocks until the ice above me melted away and I could see the rest of the icy wastes around me. Scores of thousands of years passed in this way, and at last the sun's rays began to get warmer and warmer and the ice began to melt away. Great rivers of water flowed from the edge of the retreating glacier and, where the ground was hollowed out, the water collected in lakes and ponds.

       One of the greatest of these ancient glacial lakes has been given the name of Lake Chicago. It existed on the land where Lake Michigan is today, but it was much bigger. It covered parts of what is now the city of Chicago, and I, too, was covered by its waters. At its highest, the water level of ancient Lake Chicago was about fifty-five feet above the level of Lake Michigan. Because the glacier would melt and flood the lowlands and then again regain its strength, the level of this ancient lake did not stay the same. At times it would grow bigger and the moving water at the bottom would carry me from one place to another, and at other times I was caught in the glacier's icy grip. Finally the lake dwindled to its present size and left me and many like me stranded on this beach.

       In the meantime man had appeared on the scene. Coming from the north, bands of brave hunters followed herds of animals to the south. All the changes I have told you about took millions of years, but with the coming of man there were very rapid changes indeed. If this goes on, I'll have many more strange stories to tell before I am completely worn out.

Sometimes animals called mastodons passed over the ice sheet.