Steven Varga, age 8, of Kendall Park, New Jersey, for his question:

How many feet deep is the sand in the Sahara?

When we see the great Sahara in movies and TV shows, the golden sand seems to stretch an and on, endlessly in all directions. So we think that all of this huge desert is at least knee deep in sand. Actually, this is not so. It is true that the Sahara has enough sand to create a strip of beach all the way from New Jersey to California. But it also has high tablelands where there is no sand at all. It even has bare mountain ridges, where snow sometimes falls on the peaks.

The Sahara is the largest desert in the whole world and it is hard to imagine how big it really is. Suppose we could move it from its huge home in North Africa. If we set it down on North America, it would cover all the land south of the Canadian border and sprawl out into the Gulf of Mexico. And as we know, our earth likes to create changes in the scenery. This is why there are sandy patches and bare patches in the Sahara. There also are places where the sand is dust a few inches deep and a few places where it is hundreds of feet deep.

Gritty grains of sand are always restless, ready to blow away with the breezes. And the great Sahara is a very breezy place, where fierce dry winds often blow over the desert for several days. So the shifting sands often move from place to place. Below all that restless sand, there is a solid layer of hard rock. In some places it dips down in crevices. In other places it crops up above the usual level of the sand. In many places, the bare bedrock forms ridges of mountains. And in the center of the great desert, it forms a high plateau. The solid bedrock makes s difference to the depth of the sand.

A long windy spell may use a low rocky ridge as a base to pile up a great hill of sand. When the wind dies down, this pile of sand may be 500 feet deep. Somewhere else, the same dry wind may strip away all the sand and leave a patch of bare rocks. The next hot, dry wind may come from another direction. Then the loose sand blows around and builds deep piles in other places. So the depth of the sand changes all the time. In a certain spot it may be six inches deep at one time and more than 300 feet deep only a little while later.

Then there are those bare mountains and tablelands that make up most of the huge desert. There also are oases, where springs of water feed groves of palms and fields of crop plants. Actually, only about one tenth of the great desert is covered with sand. There the depth of the sand varies from a few inches to several hundred feet    changing with every breeze.

There are wide stretches where the sand is usually as deep as ten feet. If it rests on level bedrock, the breezes pile it into gentle sand dunes and create wavy ripples on the surface. This is everybody's favorite picture of the huge, hot desert. Often the sandy dunes stretch for miles and miles. But this is only part of the vast desert.