Darla Valure, age 11, of Duluth, Minnesota, for her question:

How do we know about past ice ages?

This is one of those summery days when a few thoughts of past ice ages might be of some help. Sometimes we feel a bit cooler when we think of something frosty. And during the ice ages, the region around Duluth was very frosty indeed. Maybe you can find some proof of those chilly glaciers if you explore certain rocks out in the wide open spaces.

We now know that the earth's crust has been modeled and remodeled through more than four billion years. Modern geologists can explain how and when most of its rocks were formed or reformed. But around 1800, many experts thought that the earth was no more than a million years old. They observed many displaced boulders and other rocky oddities. But they assumed that these were caused by Noah's flood.

Then a young Swiss mountaineer named Louis Agassiz took another look. Among his lofty Alps, he saw stones and boulders frozen into the glacial ice. He also noted how mountain glaciers slowly slide downhill, like frozen rivers. As they moved, they carried their stony debris with them, often from far up the slopes. In time, the moving glaciers reached valleys and the ice melted. There the stony debris was dumped on the ground.

This, he decided, is how mysterious boulders from one region now squat far from home. They are transported by glacial ice sheets. Louis Agassiz found similar misplaced stones way down in the valleys, far from the glaciers and their melting streams. Surely this meant that in times past the glaciers were bigger. In fact, more of this evidence helped to prove that ice age glaciers once covered most of Europe.

Some experts thought that this evidence was nonsense, mainly because if true, the world was much older then they thought. Others explored further and discovered traces of ancient glaciers in Europe, North America and many other parts of the globe. Modern geologists take this evidence for granted and look for traces of old glaciers all over the map.

Some ice age glaciers were more than a mile thick and their mighty weight left depressions on the land. Stones and boulders, gravel and pebbles were frozen into the ice    and as the glaciers inched forward, they were toted far from home. The moving ice sheets also scraped down hills and gouged deep scratches on solid bedrock.

In North America, evidence of past ice ages still exists on scraped and scratched rocks, in misplaced boulders and in gouged hollows now filled with lake water.

In some regions we can trace where the glaciers reached their limit and started to melt. Here we find piles and banks of rubble, once dumped by glacial streams. In fact, wherever the massive ice fields encroached, they completely remodeled the face of the earth.