Gregg Hall, age 11, of Peterborough, Ont., for his question:

What is the Pre‑Cambrian Shield?

Geological history is dated in age‑long eras and periods. The Cambrian Period began about 520 million years ago and lasted some 80 million years. Anything which is pre‑Cambrian, then, dates from before Cambrian times and must be very ancient indeed. The Pre‑Cambrian Shield is an area of the earth's surface around Hudson's Bay. Part of it is in Minnesota but, since most of it is in Canada, it is often called the Canadian Shield.

A geological period is but a paragraph in a long chapter of earth's history which is called an era. The Cambrian Period opened the Paleozoic, the third era of earth's history. It followed the Proterozoic Era which reaches back a thousand. million years or more. The rocks of the Pre­Cambrian Shield date back to this remote chapter of earth's history. There, in outcroppings and in stony subsoil, can be found some of the oldest rocks on or near the surface of North America.

There was life of a kind on earth in these distant days. But it was no more than small algae and simple creatures and all life was confined to the ancient seas. The dry land was barren of both plants and animals, The crust of the earth was unsettled then as it always has been. There was violent volcanic activity and there was mountain making. The climate too was variable, for there is evidence of en Ice Age during the Proterozoic Era.

A lofty hump of mountains lifted up in the Hudson's Bay area, more or less in the shape of a shield. We do not know how tall they grew for millions of years of weathering has long since worn them down, Their tops are; gone and also their sloping sides. The rocks of this plain are the roots or cores of those mountains which I. if t ed their proud head in remote Pre‑Cambrian times. Geological time moves

Geological time moves and brings new layers of rocks to cover the old. Muddy creeks and rivers lay down sediments which harden into shales, slates and sandstones. Tiny shellfish add layers of limestone. Volcanoes pour forth lava. The older rocks below become crushed, heated and compressed, Dense, dark rocks of this type are found beneath the overlaying soil in the region of the Pre‑Cambrian Shield.

Such rocks, formed during earth's early history are sometimes called the basement complex. They occur at various depths all over the world. Aside from the Shield in North America, they are revealed in a few bald areas of the Appalachians. The Colorado River has dug down so deep that pre‑cambrian rocks are also laid bare at the bottom of Grand Canyon.