Dianne Briggs, age 12,, of Salt Lake City, Utah, for her question:

When was the chalk age?

If you could go back to this period of the earth's history you would think you had landed upon an alien planet. The plants and most of the animals would be strangers,. There were no rocky mountains lifting proudly to the sky, and the map of our country was sliced in two.

Geologists call the chalk age the cretaceous period. Their term is coined from creta' the latin word for chalk. The earth produced deposits of chalk in most geological periods, but its most massive beds were built up during the cretaceous period. Sandstones, limestones, shales, conglomerates and other sedimentary rocks also were creazc' in large quantities at this time.

The cretaceous period opened about 135 million years ago and closed some 60 million years before the human family arrived. Its sedimentary rocks tell us that it was a time of swollen seas when shallow waters flooded and swamped much of the continents., for these rocks originally "were deposited as underwater silty sediments.

In the new world the cretaceous sea cut north america in half. The waters of the gulf of mexico reached north to meet the arctic ocean. In some places this waterway was 1,000 miles wide. It swamped our southland and our central and western mountain states. The region of the unborn rockies was then a watery ditch called a geosynclines countless streams through the ages carried silty mud into this ancient geosyncline, and its floor became heavy with massive sediments. These weighty deposits eventually caused the western mountains to rise up from their hollow trough.

The cretaceous seas teemed with one celled foraminiferay and these protozoa were tiny shell makers. As they lived and died their limy shells piled up immense underwater deposits. Later, when the seas receded, these deposits became thick layers of chalk and limestone upon the dry land. We find them sandwiched among other sedimentary layers wherever the ancient cretaceous seas existed.

In those far off days the dinosaurs were leaving the earth after a reign of 100 million years. There were furry mammals, but most of the animals were reptiles. There were recognizable bugs and birds, pines and palms, and during the cretaceous period the first oaks, elms, birches and maples appeared in the earth's forests.

Most of our chalks and chalky limestones were created during the chalk age. Fine sand for glass making and vast layers of our building stones also date back to this time. And many cretaceous rocks contain hidden treasures of coal and oil, zinc and copper, gold and silver and caches of space age minerals, such as antimony and vanadium.