Alfred Libutti, age 10, of rochester, N.Y., for his question:

Was there really a bridge betweena Alaska and Siberia?

We know that the earth is very old and that its surface is changing constantly, though the changes as a rule are very slow. Shorelines change as the seas steal and return parcels of land. Islands appear and disappear. Tracing these past events calls for very tricky, scientific detective work.

The Indians, of course, were in America long before Columbus arrived and even before Leif Ericson and his vikings came from Greenland. The experts tell us that they resemble the Asiatic people. Most of their kinfolk were left behind in Asia, and our Indians did not arrive here all together. They immigrated to the new world in groups through several thousand years. Large numbers came about 11,000 years ago.

Alaska in North America and Siberia in Asia are now separated by the narrow Bering Strait. The islands and shorelines of this region are still restless, and it is logical to suppose that the bering strait might once have been land, or a bridge of land between the old world and the new. Most experts think that such a strip of dry land might occur during an ice age.

The mighty glaciers of the ice ages did much to remodel the earth's crust. The weight of a glacier 1,000 feet thick presses down with 30 tons on every square foot of ground, and some of the ice age glaciers were a mile thick. The crust of the earth was crushed and distorted. Countless tons of water were frozen in the ice and the sea leve1 sank. These and other factors may have caused the Alaskan land bridge to Siberia.

There are plenty of reasons as well as evidence to prove that large migrations of Indians used this link of land during the last ice age. Most experts took it for granted that it was a frozen strip of a glacier, but some new evidence may change these ideas. The arctic may have been much warmer than the ice covered regions farther south. Fossils and records have been found throughout a wide area within the arctic circle.

Many tribes enjoyed the warm climate north of the glaciers, and the land bridge was part of their homeland. When the ice melted, the rising seas cut off siberia from alaska. The land lifted from the weighty ice, and a high plateau formed along the western mountains. As the warm arctic became cold, the Indians took the high road southward and scattered throughout the two Americas.

Earth scientists figure how land and water areas changed climates in the past. Samples from the ocean beds tell them the history of ancient seas. The shell of a fossil oyster will reveal the temperature of the water in which it lived. It can tell the year and even the season when the oyster was born, maybe 300,000 years ago. Someday this detective work krill reveal the detailed history of the old land bridge between Asia and America.