54 The Great Lakes-Mid

The Great Lakes FOCUS Question How did the Great Lakes form?

The Largest Lakes In the middle of North America lie five lakes, all connected by rivers. These are no ordinary lakes. They're the Great Lakes. Together they hold over 20 percent of Earth's fresh water. If the Great Lakes were spread out evenly, their waters would flood the continental United States 3 meters (9 ft.) deep! Like small seas, the Great Lakes have waves, powerful currents, and even tiny tides. Where did these enormous lakes come from? The formation of the Great Lakes goes back to the most recent ice age. The lakes, and the land around and under them, reveal their incredible history.

The Great Glaciers Across Canada and the northern United States, saber- toothed cats and mammoths once roamed snow-covered lands. This ice age lasted from about 110,000 to 12,000 years ago. It was just one of a series of "cold snaps" that have hit Earth over the past 2.5 million years. Scientists call these cold snaps glacial periods. During the last glacial period, snow accumulated and never melted. Over thousands of years, the snow became tightly packed under its own weight. It formed enormous sheets of ice. These ice sheets spread and grew up to 3 kilometers (1.8 mi.) thick.

Sculpted by Ice The heavy ice sheets acted like bulldozers as they grew and shifted. They scraped away at the rock and pushed away soil. The land beneath the ice sheets sank down under the heavy weight of the ice. During each glacial period, ice sheets scooped and pushed away more land. By 12,000 years ago, they had carved enormous basins in the middle of North America As Earth slowly warmed up again, the ice sheets melted and water flooded the low-lying basins. This process formed the Great Lakes.

Today's Great Lakes There are five main Great Lakes that flow from one to the next in a single system of fresh water. Lake Superior "Superior" is more than just a name. Lake Superior has the largest surface area of any lake on Earth and contains Earth's third-largest volume of fresh water. That's more water than all the other Great Lakes combined.

Lake Huron and Lake Michigan Water flows out of Lake Superior and down the St. Mary's River into Lake Huron. Lake Huron is connected to Lake Michigan by the Straits of Mackinac (MA-kuh-naw), which are 8 kilometers (5 mi.) wide. Since water flows back and forth through the straits, some scientists consider Lake Michigan-Huron to be one huge lake. Lake Huron contains the largest island in a body of fresh water on Earth. Manitoulin Island has its own lakes. Some of these lakes even have their own islands! Lake Michigan is only connected to the other lakes through Lake Huron. It is also the only Great Lake entirely within the United States.