Rocks and minerals make up most of the outer layer or crust of our
earth—the actual ground beneath our feet. The crust is approximately 18
to 30 miles thick beneath the continents. In general, the outermost part
consists of many layers of stratified rocks, one above another. The
older rocks normally make up the bottom or the deeper layers, and the
younger rocks form the upper layers. Not all the layers are perfectly
flat and parallel—some are lenticular (lens-shaped), some are tilted,
some are partly eroded away, and some are present in one place and
absent in another. Beneath the continents, the layers of rock rest on
ancient metamorphic rocks and on great masses of igneous rock such as
granite. These lower rocks are known as the _basement_.
[Illustration: Earth’s outer crust (thickness not drawn to scale).]
Over much of the land surface of the earth, the outermost layer is
made up of layers of rock
On the continents, the layers of rock rest on metamorphic rocks and on
igneous rocks such as granite