The biggest collecting area at a strip mine is the badlands created by dumping of overburden. All rock layers seen in the working face can be found somewhere in these dumps.
Gray shales are the commonest rocks here. They may contain plant fossils. Magnificent fronds of ferns preserved as black carbon films on the gray shale occur in the layers lying right above the coal. Such fossils must be collected by splitting blocks of the freshly mined material, because these gray shales collapse into sticky clay after a few rains. Shales in Pennsylvania and other eastern mines are a bit more durable, but the plant fossils are not as sharply preserved as the midwestern ones.
If one block containing fossils is found, there are more around; these plant-bearing gray shales are more persistent in extent than the black shales.
After a few months or years of weathering, the hills become smooth piles of clay. The weathering, however, has released any hard fossils preserved in the shales and left them on the slopes while the clay washed away. Typical Pennsylvanian marine fossils can be found weathered free in some areas, particularly western Illinois, southern Indiana, and a few places in southern Illinois. These fossils are usually calcified, but are often badly crushed and distorted. The tiny ditches and ravines cut in the older hillsides are the best place to look, even in mining areas overgrown with grass and trees. There are always a few open ditches, even in the oldest stripped areas.
Nearly 500 species of plants have been found in the fossil-bearing concretions of the strip mining area around Braidwood, Illinois. The concretions bear the fossil in the center surrounded by shale hardened by iron carbonates in concentric layers around the fossil nucleus. Their origin, like that of the coal balls, is mysterious and the cause of much debate. Iron carbonate is highly resistant to weathering, and as the soft gray shale washes away, the hard concretions remain behind. When "ripe," the concretions conveniently turn a warm, red-brown color to contrast nicely with the remaining gray mud.
Near Braidwood, Illinois, is the mine whose concretions have provided science with more than a hundred new species of soft-bodied animals and insects, in addition to plants. Large concretions are found weathered loose on the waste piles of strip mines in western Illinois. Each may contain hundreds of well-preserved brachiopods and cephalopods, often pyritized. There is no surface evidence of the fossils inside. Marine fossils found in these concretions are generally not crushed like their neighbors in the shale enveloping the concretions.
Not all large concretions contain fossils. Those uncovered during mining in southern Iowa are full of calcite crystals but no fossils. Eastern coal fields rarely disclose concretions of interest to a fossil collector. But all concretions must form around a nucleus, usually a fossil, so all are worth breaking open. Any reddish or grayish rounded rock found on the stripmine waste hills is likely to be a concretion.
Small yellow or gray limestone slabs may show up sporadically in strip-mine dumps. These are usually highly productive of marine fossils. Limestone boulders brought down by the glaciers may also appear in the topmost overburden of northern strip mines. These are usually weathered to a yellowish color. Traces of fossils may appear on the outside, but the boulders must be broken apart to release undamaged fossils inside. There
This benevolent little face is not part of a trilobite, but is merely a concretion that happens to resemble a trilobite. Many such pseudofossils are thought to be heads, feet, wings, and bones by overimaginative collectors.
may be some difficulty in identifying fossils and even in determining their age in the stray limestone lumps.
Sandstone layers several feet thick may occur as a blanket deposit in some coal mines. These evenly bedded sandstones may show plant remains, especially where a sudden flood washed in sand that buried standing plants. Such fossils usually lack the fine detail preserved when the plants were entombed in fine clay and mud. However, thick stems, great slabs of bark, and compressed trees can be found in these sandstones as casts.
Masses of crushed Lepidodendron and Sigillaria are common in a two-foot-thick sandstone layer exposed south of Pella, Iowa. Quantities of leaves are buried at odd angles throughout a thick sandstone near Ottawa, Kansas. Giant stumps and logs of Coal Age trees stand upright in the thick Millstone Grit formation of Indiana and created quite a nuisance during the last century for producers of millstones. A similar occurrence is seen in a roadcut near Omaha, where Calamites stems stand upright in the layers of sandstone. Energetic collectors have dug down as much as four feet to remove a long section of stem.
Plant leaves in sandstone are often preserved as a carbon film, and a
spray of lacquer or liquid plastic may be necessary to fix them to the matrix for safe transport home.
Shaft coal mines offer only one place to collect —the dump, which is a mixture of rock from the mine's many levels, most of it from digging the shaft and widening the underground workings. Collectors are not allowed underground, and most active mines will not even allow them on the dump. On an old dump, go around and up and down, breaking concretions and blocks of rock, as well as keeping alert for loose fossils.
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