Alaska's rivers of ice.

Glaciers are everywhere in Alaska. These huge rivers of ice flow down from the mountains, into the valleys, across the meadows, and down to the sea.

Nothing can stop a glacier from moving. In time, it could even push a whole city out of its way.

Glaciers form in cold places where summers are short and there is a lot of snowfall in winter. Winter after winter, snow piles up in the hollows of the mountains; in the summer, little of it melts. As the snow gets thicker and thicker, the pressure of its own weight will turn it to ice.

First, the snowflakes turn to ice crystals, called firn. In Alaska this may take only about 30 years. In some places it can take more than 1,000 years.

When the ice gets heavy enough, it presses downward so hard that it begins to move. This is partly because gravity is pulling on it and partly because the ice crystals slip against one another. Melting ice along the bottom of the glacier helps it slide more easily. Glaciers move faster in summer and slower in winter. They move faster on steep slopes. Thick glaciers move faster than thin ones.

The center of a glacier moves more quickly than its edges. The glacier slows down when its edges rub against the walls of the valley. A scientist, Louis Agassiz, learned this in Switzerland. He pounded metal stakes in a straight line across the surface of a glacier. When he returned months later, he found that the stakes in the middle of the glacier had moved farther down the valley than those near the edges.

We also know that the ice near the top of a glacier moves faster than the ice at the bottom. Friction between the ice at the bottom and the rock it moves across slows it down.

The top layer of ice on a glacier is brittle and breaks more easily than the softer ice below it. So when one part of the glacier moves faster than another part, the top ice cracks, making loud noises that sound like firecrackers.

These cracks are called crevasses. Some of them are very deep and wide and can be extremely dangerous. Snow often fills up the crevasses. If a person steps in one, he could be killed or injured by the fall.

Glaciers are not pure white. When the snow and ice are clean, the glacier looks blue. This is because ice crystals reflect blue light and absorb other colors. As air forms bubbles between the crystals, the ice begins to look white. After a while, dust and dirt settle on the surface of the snow, making it gray and dingy.

When a glacier is growing in size, it is said to be advancing. When the front of the glacier melts faster than the glacier is moving, it is retreating. But glaciers never move backward. A retreating glacier is still moving forward.

There are three main kinds of glaciers. The first is the valley glacier, which moves downhill between mountains. The Mendenhall Glacier, near Juneau, is a good example of a valley glacier. It is 300 years old and is twelve miles long and two miles wide. The face, or snout, of the glacier stands as tall as a ten-story building.

The U.S. Forest Service has built a visitor center with many glass windows overlooking Mendenhall Glacier. Here you can listen to a naturalist tell about the glacier, watch a movie about glaciers, or go for a hike. Helicopters even take people out to walk on the glacier.

Beautiful Mendenhall Lake lies in front of the glacier, and on clear days it reflects it like a mirror. A large waterfall cascades down one side of the glacier.

A piedmont glacier is one which has come down into a low area and spread out like a fan. Piedmont is French for "foot of the mountain." The Matanuska Glacier north of Anchorage is a piedmont glacier. It's about twenty-seven miles long and has existed since the Ice Age.

Tidewater glaciers are the third type. These reach out into the sea. As the glacier flows into the water, an ice shelf is formed. The back of it is still attached to the glacier, while the front floats on the water.

When tongues of ice from glaciers move to the sea, they often gouge deep valleys that go far below sea level. These fill up with water and are called fjords (fee-ords). The sides of a fjord are usually high cliffs. Misty Fjords National Monument in southeast Alaska has many beautiful fjords.

Huge chunks of ice can break away from the face of a tidewater glacier and crash into the water. This is known as calving. Eskimos called the sound "white thunder."

Glacier Bay National Park has many glaciers. Tour boats take people close to some of them, where they can see and hear the glaciers calving. They can also see seals and sea lions lying on chunks of ice floating in the water.

Glaciers are beautiful, and they have also been useful to us. They deposited much of the fertile farm soil in the U.S. The moving ice also dug out many U.S. lakes and valleys.

Scientists think glaciers may be useful in the future, too. About three-fourths of the fresh water on earth is frozen into glaciers and ice sheets. Some of this may be used to irrigate dry lands. Fresh water for drinking is already being pumped from a number of glaciers to areas nearby.

Plans are being made to tow huge icebergs to faraway places needing fresh water. Scientists think they could cover an iceberg with layers of plastic to keep it from melting too much on the way. It would be towed by a ship to its destination. Once there, the ice would be melted and the water piped to areas that need it.

As you can see, glaciers are interesting and lovely to look at. They are also important to both the past and the future of the world.