Margie Erisman, age 12, of Lancaster, Penna., for her question:

Why are some old rocks on top of younger rocks?

The earth's crust is a sandwich of rocky layers. It is from 20 to 40 miles thick and the sandwich has hundreds of layers of assorted minerals. It is not a flat sandwich, for it envelops the round globe. And it is restless, for mighty stresses and strains within the earth and above it constantly work to push and pull it out of shape.
A lifetime is too short to notice much change on the rocky face of the earth. But in a million years, the face of a continent is as restless as a bubbling pot of boiling soup. Mountains and valleys rise and fall, seas and rivers change their boundaries, shallow basins fill with lake water and dry up like puddles. The lithosphere, the earth's crust, is always changing,
The first layers of the lithosphere formed perhaps 4,000 million years ago and newer layers of rocks formed care above another throughout the ages. They are still forming. If the lithosphere were a sturdy, unmovable shell, all the rocky layers would be in order with the oldest at the bottom and the newest on top. But the thick sandwich of countless rocky layers is restless.
We hear of earthquakes and volcanic eruptions which pour rivers of lava on the surface. But such fast, dramatic changes are rare. The important changes in the lithosphere go on steadily without rest, eon by eon. The life span of a mountain range is hundreds of millions of years, As it progresses, deep rocky layers are lifted to the surface. Volcanos with their lava floras and shuddering earthquakes make changes in the rocky, lithosphere sandwich.
Most of the shifting which lifts deep old layers over the newer surface rocks Is done by mountain making.
This job begins with a long, narrow ditch where timeless streams dump silty sediments which form layers of clay and sandstone, slate and limestone. Forces within the earth begin to push up these layers of sedimentary rocks, The multiple sandwich bends in a wrinkle and the wrinkle gets higher. Its sides become steeper and bend in the middle. It becomes strained and finally cracks, perhaps a mile deep into the lithosphere.
The crack is called a fault and it may trigger off earthquakes and volcanos which add to the upset. Stresses continue to push at the fault and one side is shoved over the crack onto the opposite side. It may be a mile thick. Ancient rocks from a mile down are now piled on top of the newest surface rocks. Most of this rock shifting is done by mountains.
Lava flowing over a wide area also covers young surface rocks with rocky material from deep in the earth. On a smaller scale, the rocky layers of the lithosphere are also moved by glaciers, The giant glaciers of the Ice Ages gouged deep layers of rock from the far north toted them far to the south and dumped them on soil and newer layers of surface rock.