Shale Quarries

Shale is sometimes quarried for the manufacture of bricks and ceramics. Some is mixed with ground limestone in making cement. For some reason, shale quarries are usually called clay pits or brick pits. They are so designated on geologic maps.

Such pits can be located by inquiring of the Geological Surveys or by asking questions locally. They are rarely as deep as limestone quarries and are short-lived, designed to remove a thin layer of profitable clay near the surface before being abandoned. Booklets from the state geological survey list clay and shale producers of the state and may list the age and formation of the shale being removed.

Some fossils are best collected from fresh shale, and others are at their prime after long weathering. Puryear, Tennessee, is a classic area for collecting Eocene leaf fossils, but only by splitting extremely fresh shale. A few days of weathering and rain will destroy a leaf that has been in the rock for millions of years. Fragile carbon imprints of ferns are not uncommon in midwestern and eastern strip coal mines but disappear after the slightest weathering. Tougher creatures such as brachiopods, corals, and crinoid stems, on the other hand, will be cleaned free of matrix by weathering.

The recomended method is to find a fresh block of shale near the working face of the shale pit. Split it along bedding planes and look for soft-bodied fossils, then search the weathered piles along old pit walls or scattered around the area.

Iowa Limestone Quarries
Shale pit (right center) of Rockford Brick & Tile Company, Rockford, Iowa, a notable collecting site. (Photo courtesy Rockford Brick & Tile Company)

As in limestone quarrying, unsalable shale will be hauled aside and left in heaps for fossil hunters. Thin limestone layers may occur in the shale. Frequently these are very fossiliferous while the accompanying limestone layers are not. Usually inquiry at the quarry office while registering there will disclose where piles of the shale may be found.

Many clay deposits of Pleistocene age, particularly those in the northern United States, formed at the bottom of small frigid lakes near the terminus of a glacier. They consist of finely ground rock dust produced by the glacier. Few living things braved these arctic waters, either for living or swimming, and such clays are only sparsely fossiliferous. In contrast, clay quarries in Cenozoic, Mesozoic, or Paleozoic rocks are remarkably fossiliferous. Ammonites are found in Cretaceous shale pits of Dallas, Texas, brachiopods of Devonian and Mississippian age in Indiana and Iowa.

It is profitable to check the working face of the shale pit carefully, layer by layer, At the Medusa quarry at Sylvania, Ohio, for example, a thin, two-inch stratum of Devonian shale produces most of the rolled-up trilobites in that area. Another thin layer a few feet above the trilobite horizon yields most of the complete crinoids found in the quarry, while a third layer is the final resting place of rare phyllocarids. Although the product of the Medusa quarry is basically limestone, a thick layer of shale lying above the limestone is removed separately. Trilobites and crinoids can be collected from the shale dumps, but they are mixed with barren material. If the horizon of the fossils can be located in the wall of the quarry, it can be followed, and usually the shale is soft enough to allow some digging into this layer.

Many shale quarries are worked down layer by layer. If the collector is on hand when the proper layer is exposed it can be a bonanza. A shale pit near Annapolis, Indiana, was worked in this way, and when one layer was almost completely removed, a paleontologist from the Field Museum in Chicago found that that particular layer was crowded with rare Penn-sylvanian-period fish fossils. Thousands had been destroyed. While digging a ship canal on the south side of Chicago, workmen dumped a thin seam of shale along a quarter-mile of bank. Many years later a collector found that this shale contained 400-million-year-oId worms preserved faithfully as a carbon film —the only ones of their kind in the world.

Most commercial shales and clays are light-gray or tan. These generally will contain marine fossils. Black shales found in some clay pits and strip mines are typical of near-shore deposits rich in organic material. They may contain fish or plant fossils. Small concretions that weather free from light-colored shales are worth cracking open; a small fossil, perhaps pyri-tized, may lie at the center. Clay pits operating in Cretaceous gray shales, such as around Dallas, Texas, contain huge amonite-bearing concretions that are neatly piled up in the pit, awaiting the collector.

When collecting from weathered dumps, the collector can get a clue to the layer from which the shale came by the color of the weathered material. A group of collectors examining shale piles in a southern-Indiana quarry found a few perfect blastoids weathered free. One experienced collector noticed that the large, perfect specimens were always lying on piles a shade lighter than the other weathered shales. He searched for these light spots, and found dozens of the blastoids, much to everyone's amazement.

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