Forest Preserve District of Cook County, Illinois
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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


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 teacher mad.

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