Kevin Cashman, age 11, of San Francisco, California, for his question:
Why are the cliffs of Dover white?
They look like gleaming white walls, facing across the English Channel toward the shores of France. This stretch of water is about 20 miles wide and usually its grey waves are rough and stormy. But white gulls wheel overhead to match the white cliffs and the tide strokes the shore with foamy white fingers. The handsome cliffs are made from chalk and the patient earth took at least a million years to make them.
Chalk and chalky white limestone are fossil minerals, originally made by living creatures of the sea. Most of our deposits of soft crumbly chalk were formed during the Cretaceous Period, around 100 million years ago. At that time the world climate was mild and the seas were much higher than they are now. About half of North America was under water. So was much of Europe, including the part where the White Cliffs of Dover now stand high and dry above the sea.
The seas, as always, teemed with shell builders, including multitudes of micro¬scopic creatures called the foraminifera. These midgets extracted minerals from the water to build their shells of calcium carbonate. Foraminifera means the window makers and these midgets made mini windows in their shells. As they lived and died, thick masses of their discarded shells accumulated on the floors of the seas. In Europe, these soggy deposits were mostly clean fragments made by foraminifera, certain sponges and other manufacturers of calcium carbonate. In other parts of the world, the chalky deposits were mixed with mud and often embedded with the fossils of sizeable prehis¬toric animals.
In Europe, the shallow Cretaceous seas came and went. For a time, parts of England were submerged under lakes that later became dry land and flooded again. In southern England, one of these lakes stayed long enough to accumulate massive deposits of fine white chalk. When the Cretaceous Period ended, the map of Europe was remod¬eled.. The Alps began to rise. With this mighty upheaval, volcanoes erupted in France, in the Scandinavian countries, Iceland and parts of England.
The submerged shorelines along England and France were lifted up above the water, though the sea still claims a channel between England and northern Europe. Most of the Cretaceous chalk beds that covered this wide area have since been covered by various other deposits and scarred by glaciers of the ice ages. But chalky white layers were left exposed as cliffs along parts of the English and French coasts.
The showiest ones stand along the shores of Dover. They are white because very little mud was mixed with the soggy deposits under the ancient shallow sea. Here and there, modules of jet black flint are embedded in the soft, white chalk. In other parts of the world, chalky deposits were crushed below other rocks and changed into limestones. In a few mountainous regions, limestones were remade into marble. All these minerals started as chalky little seashells.