Slate, Shale & Mudstone
Nature Bulletin No. 669-A March 4, 1978
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
, SHALE & MUDSTON.
There is a little book by Robert Irving about rocks and minerals. In the
first chapter, "Every Rock Tells a Story, " he tells these stories in a
simple way. Perhaps the title of this bulletin should have been reversed,
or merely MUD, because the story about mudstone, shale and slate
begins with mud.
Since more than a thousand million years ago, vast areas of what is now
the United States were repeatedly covered during long periods by
shallow seas. Each time, until a sea disappeared when the land was
lifted, rivers flowed into it. Where the sediment they carried was mostly
clay from weathered and ground-up rocks, it was deposited in layers of
mud on the sea floor. Shells of marine animals, and wind-blown dust
from volcanoes, sank to the bottom and were buried in mud.
Pressed down by its own weight and that of the water, the mud
sometimes solidified into Mudstone, a tough structureless substance
with no planes of cleavage and no strength. Some outcroppings of coal
in the bluffs along the Illinois River Valley are overlaid with this
material, incorrectly called "soapstone. " When unsupported and
exposed to the weather it soon crumbles into grayish powder. But, if the
bed of mud was very thick, and especially if deep layers of limestone or
of sandstone were deposited on top of it by another sea, it was
compressed and converted into a soft rock called Shale.
Shale, in addition to kaolin, small flakes of mica and bits of quartz
sand, may contain other minerals. It may be gray, or brown because of
an oxide of iron, or black and rich in carbon from the remains of plants
that lived in the sea. Shales are abundant throughout the world. They
are so weak, and so apt to split and erode when exposed to the weather,
that they cannot be used as building stone but some are included in the
manufacture of cement.
Oil shales contain petroleum or asphalt, formerly supposed to be
derived from myriads of tiny animals buried in the clay when it was laid
down. In Scotland, paraffin and olefin oils have been extracted from
shale since 1862. Deposits in New South Wales, Australia, yield 100
gallons per ton. In western Colorado and eastern Utah there are
mountains of oil shale. A process for extracting the oil is being
developed and those colossal deposits could supply the U.S. with
petroleum for many years.
Shale, like sandstone and limestone, is a sedimentary rock. Slate is a
metamorphic rock. It was formed from a bed of shale which, deeply
buried, was subjected to heat and enormous pressure when the earth's
crust shrank, folded, and mountains were created. As a result the shale's
minerals were rearranged and straight lines of cleavage developed at an
angle to the original bedding, so that it is easily split into layers.
Slate is fine-grained, heavy, hard, strong and very durable. It has been
quarried in Europe and the British Isles for hundreds of years. In the
United States there are quarries in several eastern states from Georgia
to Maine but principally in Vermont, northeastern New York (sole
source of red slate), and Pennsylvania where, in one quarry at Pen
Argyll, broad thin sheets of extraordinary size are produced. Slate is
widely used for roofing, blackboards, billiard table tops, laundry tubs,
and the bases of aquariums. Although the commonest color is gray,
shading toward black, slate is marketed in various sizes and thicknesses
for roofing and for flagstone paving in colors such as blue-gray, green
and gray, mottled green and purple, Vermont black, and red.
When we old folks went to school we used slates in wooden frames,
bound with red yarn, and slate pencils that would screech and make the
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Update: June 2012