Shale is a sedimentary rock made up of tightly packed clay and mud
particles. It has a smooth appearance because it is so fine grained. In
fact, most of the particles in it are too small to be distinguished with
a magnifying glass. These particles are the weathered remains of earlier
rocks. They were carried by creeks and rivers to other parts of the land
or to the sea, where they formed layers of clay and mud. Later, other
sediments were deposited on top of them. The weight of these new
sediments squeezed the clays and muds together to form firm, compact
shale.
Shale looks very much like some clays. It, like clay, can be almost any
color. If the shale contains animal or plant matter, it is black, gray,
or blue. If it contains iron oxide (many minerals containing iron alter
to this material), it is a shade of red, yellow, or brown. Shale is soft
and can be easily scratched by a knife. It also is brittle and crumbles
easily. This rock has a property that will help you to distinguish it
from clay: the particles that make up the shale were deposited in
layers, and the shale splits into flat, thin flakes along these layers,
which clay will not do.
Shale is fairly abundant in Texas, especially in Mississippian,
Pennsylvanian, and Cretaceous formations. For example, Pennsylvanian
shales are found at the surface in north-central Texas, in the area
around the Llano uplift of central Texas, and in the Marathon and
Solitario uplifts of west Texas.
Many of shale’s uses are the same as those of clay. Some of it can be
used to make brick, tile, and other products, and some is often used
instead of clay in making portland cement. Cement plants at Dallas, El
Paso, Fort Worth, and Waco are located at places where Cretaceous
limestones, which also are used in cement making, and Cretaceous shales
are found near each other at the surface.
Oil shale, from which petroleum can be obtained by heating, has been
found in central Texas. It occurs in Mississippian formations in
Lampasas, McCulloch, and San Saba counties. Because oil is much less
expensive to obtain from wells, it is not produced from these shales.