Welcome to You Ask Andy

 Becky George, age 11, of Richmond, Va

 How did the Great Lakes form?

This is another story of the ever changing face of the earth. A million years ago, the silvery blue Great Lakes did not exist. Their place was occupied by a region of rivers and river valleys worn deep with time. One ancient river flowed to ,join the Mississippi north of St. Paul. One flowed over what is now Chicago td meet the Illinois. Another merged with the Ohio and still another carried its waters all the way to the Atlantic Ocean.

Then, inch by inch, icy foot by icy foot, the great glaciers of the Joe Age crept down from the polar regions. The region of the old rivers was buried under massive ice sheets two or three miles thick. With their tremendous weight they dug into the old river valleys, gouging out great holes. As Natures bulldozers crept forward relentlessly, untold tons of rock and debris became trapped and frozen in the ice, adding to the weight.

When the ice at last melted, this rocky debris was dumped in huge piles. The inland rivers were blocked in their paths by this glacial drift, no longer able to join the Mississippi or her tributaries. Meantime the ground rose as the weight of the massive glaciers was removed. The vast watershed was tilted downward, toward the northeast. The rain and glacial water tended to flow away from the southwest, away from the great Mississippi.

The ancient river valleys, gouged deep by the glaciers, now filled with water and became the Great Lakes. The river which joined the Mississippi near St. Paul became the mighty Lake Superior, biggest fresh water lake in the world. The river that joined the Illinois became Lake Michigan and the river that joined the Ohio became Lake Erie. Lake Ontario filled

Lake Ontario filled a deep basin in the valley of the old river which flowed to the Atlantic. This river, the beautiful St. Lawrence, gathered in the waters of the Great Lakes and toted them to the sea.

The floods of glacial water from the melting ice has long since gone and you may wonder how the Great Lakes keep themselves filled. No great rivers drain into them and surely that much rain and snow does not fall down upon their silvery surfaces. Together, they form the largest area of fresh water in the whole wprld and yet they gather only a little of this water from rivers and rainfall.

Most of their water comes from ground water, far below the surface, Rain and winter snow fall over a vast watershed around the lakes and much of it seeps down into the ground. There it rests, trapped in porous rocks. The top of this ground water, the water table, is down below the surface of the ground, All the Great Lakes stand with their feet deep in this buried reservoir of ground water. As their surface waters flow out to join the sea, it is replaced by ground water. Some seeps in from ,porous, water logged rocks and some gushes in from underground springs.

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