SHALE is a common and important sedimentary rock composed of compacted
clay or mud. It is so fine grained that the minerals forming it
generally cannot be identified without the aid of X-ray.
Shales are composed mainly of clay minerals but, like other sedimentary
rocks, generally include other minerals. Shales containing calcium
carbonate are called calcareous shales. Most shales contain silt or sand
particles; if silt or sand is present in large quantity, the rock is
called silty shale or sandy shale. If mica minerals are present in
quantity in a shale it is called micaceous shale.
The particles of most clay minerals are thin and flat and overlap each
other.
Shales have a wide range of colors but most of those in Illinois are
gray. A gray, black, or blue-gray color is caused by organic matter in
the shale; shades of red, brown, yellow, or green are caused by iron
compounds.
Shale is widely distributed in Illinois, especially in “Coal Measures”
rocks, and is used in manufacturing bricks, drain tile, building tile,
and lightweight aggregate.