Hilary Kuzuw, age 11, of Somerset, N.J., for his question:     

WHAT CAUSED THE WHITE CLIFFS OF DOVER?

On clear days the French people of Calais can look across the English Channel and see the famous White Cliffs of Dover, glistening in the sunshine. There are other white chalk cliffs along the French and English shores on opposite sides of the narrow channel. They have stood there since the Ice Age, but their story began millions of years ago‑‑under the sea.

The geography of the earth is constantly changing, inch by patient inch. The shores of the great oceans are spreading, causing the continents to drift like lazy rafts around the globe. Mountains rise and decline, new layers are added to the earth's crust, often with the help of living creatures.

For example, the chalky White Cliffs of Dover were built by foraminifera, tiny sea creatures that deposited layers of their discarded shells on the ocean floor. Their mini shells were made of chalky calcium carbonate, extracted from salty sea water. This was during the Cretaceous Period of geological history, which began about 130 million years ago and lasted for roughly 70 million years.

During this patient chalk‑building period the world's climate was mild and the global ocean was brimful of water. Shallow seas slopped over on much of the land. They covered southern England and parts of northern Europe. These warm, shallow waters suited the teeming foraminifera just fine. They thrived and multiplied. Through the ages, thick beds of. chalk were deposited all the way from Scandinavia to southern Russia.

The Cretaceous Period was followed by climate changes, and new mountains began to arise. The mighty dinosaurs departed, and the age of the mammals arrived. In Europe and Asia, the land was lifted by the growing Alps and Himalayas. This changed the shorelines, and some of the great chalk deposits rose above the water.

Then came the Ice Age, when much of the world's water was frozen and trapped in enormous glaciers. There was less water in the ocean, and the sea level sank. More of the old chalk beds became dry land. In one region the rising Alps and the declining sea level hoisted aloft the white cliffs along the shores of the English Channel.

The chalky material in these snowy white cliffs is very clean, embedded with hard gobs of flint. This suggests that the Cretaceous seas were surrounded by dry, sandy deserts. For little or no muddy clay was mixed with the chalky shells. The flint nodules could have been formed from sand, blown in from the arid deserts.