Mike Hackney, age 12, of Nashville, Tenn question:

How is chalk formed?

Chalk is a form of the mineral calcite or calcium carbonate. This mineral is a chemical compound formed from the elements calcium, carbon and oxygen. When it forms in the earth's crust, the carbon and oxygen are taken from carbon dioxide in the air or waters Limestone, marble, onyx and alabaster are also forms of the mineral calcite. The chemistry of chalk is interesting in itself, but far more fascinating is the story of how it is formed in the earth’s crust.

This story goes back perhaps 130 million years and  is still going on, The leading characters are tiny little sea dwellers called foriminifera, a name which means the window makers. The dramatic events include the long struggle between land and sea and movements of the earth's crust. Most of our chalk deposits were started during the Cetaceous Period, meaning chalk making period, which began some 130 million years ago and lasted for perhaps 70 million years. But a certain amount of chalk is still being made in a few warm shallow seas.

The Cretaceous Period was the heyday of the dinosaurs. All shapes and sizes of reptiles stalked the land and a few spread leathery wings and took to the air. The sea teemed with life of all kinds, for this was a period when shallow waters slopped over much of the lands of Europe and North America. The little chalk makers, foriminifera, thrived in countless numbers.

These tiny mites used chemicals in the water to make their dainty miniature shells of limy calcite. Under the microscope we can see that chalk is made from these little shells, They show up as dainty whorls, buttons, buns, braids of bread and countless other shapes. Every shell has a hole or window through which the living tenant kept in contact with his watery world.

Through the ages, untold generations of these little window makers lived  and died arid their shells sifted down to the floor of the sea.

Great piles of the limy deposits collected on the floors of the ancient seas. Then the geography of the earth changed and shallow seas receded from the land. The beds of limy calcite were left high and dry. This happened in what is now Kansas and in certain areas of central North America. Here we have beds of chalk or chalk mixed with muddy silt. In the Kansas chalk deposits, we find not only the shells of foriminifera, but fossil remains of little sea serpents, water birds and other creatures that lived during the Cretaceous Period.

Some of the best chalk beds were formed when the seas receded from Northern Europe. Here the foriminifera shells were mixed with little or no mud and the beds are almost pure chalky calcite. It is these beds of chalk which fcrm the famous White Cliffs of Dover along the shores of southern England.