[Illustration: Shale bluff at a strip mine near Columbia.]
Shale is a compressed, and layered or laminated clay or mud rock.
Consequently it will return to mud if it is wetted with water and
rubbed. This may serve as a test for shale. It may occur in thick layers
or formations, five, ten to fifty or more feet in thickness, and it
ranges downward to paper-thin partings between beds of limestone. It is
also commonly associated with coal beds. The color of shale varies from
light gray to black, or it may be tan, yellow, red, rust, purplish, or
green. It is platy, and these thin plates or laminae, piled on each
other, make up the shale bed.
[Illustration: Hand specimen of shale shown in preceding picture.
Note the characteristic thin layering or lamination.]
Some shales are hard, tough, and strong enough to serve as temporary
mine roofs. Hard shales are sometimes called “slate” but this name is
technically incorrect. _True slate_ is a metamorphic rock, composed
chiefly of the mineral mica in very fine flakes, and will resist the
action of water (weathering) for a long time. Therefore, it is a good
roofing material for buildings, whereas shale is composed chiefly of
clay minerals, and despite the strength and compactness of the more
“slaty” varieties soon disintegrates in water. Missouri “slaty” shale
would not serve as satisfactory roofing material.
The red “burned” shale found on burned-out coal mine dumps is called
“shale” locally. It is, of course, shale which has been fired more or
less to the condition of building brick by the hot burning waste coal.
The same original shale could be crushed, molded into brick, “burned” in
a kiln, and become a satisfactory building brick. The “burned shale” of
the coal mine dumps is used in many places as a drive-way covering.
“Soapstone” is a name applied by some persons to some soft, slippery to
greasy shales, but this name is incorrect in a technical sense. True
soapstone is a metamorphic rock (shale is sedimentary) which is composed
chiefly of the mineral talc. Soapstone occurs abundantly in certain
parts of the Appalachian Mountains but is exceedingly sparse in
Missouri.
The chief commercial uses of shale are in the manufacture of common
brick, building brick, building tile, drain tile, sewer pipe, Portland
cement, and other ceramic products. Many shale beds and occurrences are
technically suitable for these uses but have no real commercial value
because other necessary factors are lacking. In order to make brick,
tile, or cement there must be sufficient fuel available at low cost,
low-priced bulk transportation of the raw and finished products,
available labor, capital for the erection of a plant, and above all a
large near-by, dependable market for the manufactured product. The value
of a shale deposit, therefore, depends as much upon outside conditions
as upon the properties of the rock (shale) itself.
The shales of Missouri were formed from deposits of mud that settled out
in sea water which in the past covered this state. Fossil remains of
sea-living organisms which are preserved in the shale give evidence of
the marine conditions once existent here. Like the muds that are
accumulating along the Atlantic coast and in the Gulf of Mexico, where
the Mississippi River is discharging its load of silt and clay, so did
mud form layers on the bottom of geologically ancient interior seas. In
some cases sand was later washed in and covered the mud; in other cases
limestone-forming material (like off the coast of Florida today) was
deposited on top of the mud. The weight of the overlying beds and the
slow movement which raised the sea bottom up to land squeezed out the
excess water, compressed and compacted the muds into thin layers, and
brought about the shale rock which is exposed to us today.
[Illustration: Soft, easily eroded bed of shale between two more
resistant beds of limestone near Columbia.]
Black muds, rich in humus and other organic material, formed black
shales; red and yellow clays colored by red and yellow iron oxides (iron
rusts) formed red and yellow shales; and sandy muds were compacted into
gritty, sandy shales. All of them were derived from eroding land and
soils just as today our eroding soils contribute to the formation of
more shale now in the long, slow process of formation.
The chemical composition of an average shale is not simple, as is shown
by the subjoined composite analyses of sedimentary rocks taken from U.
S. Geological Survey Professional Paper No. 127.