Shale is composed of clay, silt, or mud — materials smaller in size than the sand and pebbles of sandstone and conglomerate. Clay is made up of microscopic particles of aluminum silicates, such as mica, and of clay minerals such as kaolin, with fine particles of feldspar, quartz, and iron oxides. Shale cemented by calcium carbonate is appropriately called limy shale, but if there is more calcium carbonate than clay, it is shaly limestone. Both will fizz when touched by a drop of hydrochloric acid, but pure shales will not.
Black shale was formed where organic muds accumulated on the bottom of quiet waters like the Everglades, the Okefenokee swamp in Georgia, and some northern lakes. The bulk of the shale is composed of coallike rotted particles of plants and animals with a small amount of clay as a binder, now hardened into rock.
The muddy bottoms of early seas were home to brachiopods, clams, crinoids, and trilobites; almost every type of fossil can be found in marine shales. The crinoids of Crawfordsville, Indiana, lived and died and were buried in such deposits. So were the Cambrian trilobites of southern Utah and California: Elrathia kingi from a Utah location is common in collections. The handsome black Phacops rana of Sylvania, Ohio, is collected from a shale seam. The fine grain of clays preserves the structure of fleshy parts of soft animals as no other rock can. The unique Cambrian fossils of soft animals found in 1910 by C. D. Walcott in Canada were carbon films in the slaty black Burgess shale. A fine-grained shale is the matrix of the concretions that yield the Tully monster and other members of the Essex fauna of the Illinois strip mines.
Plant fossils are commonly found in shales. Carpeting the coal seams of the Illinois basin, the Iowa fields, and the eastern United States is a layer of shale formed, so geologists believe, where mud was washed in on top of thick layers of fallen vegetation. This must have occurred as the area sank slowly, drowning and killing the plants and turning the forest into a shallow, brackish lagoon or bay. Leaves, seeds, branches, cones, and trunks of the drowning trees and plants were buried and later carbonized. They are so well preserved that even the hairs that grew on
the leaves of Neuropteris scheuchzeri, a seed-bearing fern, can be measured and counted.
At Puryear, Tennessee, clay shales are quarried that contain quantities of carbon-coated impressions of leaves that look like those from modern trees. This was probably a lake deposit, since a leaf will not drift very far in a stream before decaying or disintegrating.
The extensive Green River formation in Wyoming, outcropping in hills over a large area, was once the site of an Eocene lake where fish swam.
Killed by unknown causes, many thousands periodically sank to the bottom and were buried in a mud formed of volcanic dust. The water of this lake must have become poisonous, because the fish show no sign that they were killed by predators that certainly were present in the lake.
Shales, then, are an important source of fossils. Freshly quarried shales laid down in quiet waters may yield specimens in which all the delicate detail of the living animal is preserved. In weathered shale, however, nature leaves only the hard parts for the collector.
Limestone weathers slowly, shale rapidly, so that the limestone in a road cut juts out over the shale layers, which are often rich in fossils. Massive limestones such as these are usually poor in fossils.
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