Examine glacial scratches on rocks from Switzerland to New York City for evidence of Earth's icy past

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NARRATOR: A glacier really is like a river of ice, with swiftly moving sections and lazy eddies and backwaters.

And like a river of liquid water, glaciers scour and scrape and shape the Earth's surface.

Since Agassiz, most scientists have become convinced that there were times in the past when the Earth was nearly covered by glaciers.

In fact, the evidence indicates that several times during the last one million years, drops in global temperature have caused glaciers to turn much of the land on our planet into glistening fields of ice.

These frozen periods are called ice ages.

The evidence for ice ages can be found in many parts of the world.

Glacier-scratched [music out] rock is found not only in Switzerland, but also in the western United States, around the Great Lakes, in New England, and even in New York's Central Park.

And wherever these rocks appear in North America, the scratches are always parallel, and they tend to run from north to south.