Lee Morrison, age 15, of Greenville, Miss., for his question:

WHAT EXACTLY IS LOESS?

Loess is a kind of silt that forms a fertile topsoil in some parts of the world. Loess consists of tiny mineral particles brought by wind to the places where they now lie. These mineral particles are finer than sand but coarser than dust or clay.

Topsoils made up of loess are found in the central and northwestern parts of the United States, in central and eastern Europe and in eastern China.

Two great sources have provided most of the world's loess. One source is the area once covered by the great ice sheets of the Ice Age. When the ice melted, it left vast plains of bare mud. When these plains dried, winds blew the mineral grains to the grasslands farther south.

The other source of loess is the great deserts of central Asia. These deserts furnish the loess now in eastern China.