Concretions

Concretions are found in shale, occasionally in sandstone, and even in coal. If coal balls are described as concretions, coal may also be included. Concretions are most commonly composed of calcium carbonate or iron carbonate, although they may occur in sediments that are lacking in either carbonate. Their surface is usually curved and even spherical. They may be compared in form to a french-fried shrimp or onion ring. Such an object, dipped repeatedly in batter, builds around itself layer after layer until gradually it loses all except the rudimentary outline of its shape. Some concretions are plainly layered, while others may have grown more subtly — perhaps, like the fat lady in the circus, adding to their bulk from within. What causes concretions to form around fossils still challenges paleontologists.

Not all shales contain concretions, but those that do often contain pro-

Sandstone Septarian Nodules
Massive gray shale with concretion in place. The iron carbonate concretion quickly weathers out of the soft shale and darkens to a reddish color. Pennsylvanian; Terre Haute, Indiana.
Plant Fossil Concretions

Concretions may reach mammoth size, such as this one weathering free along a Pennsylvania road.

Neuropteris Gigantea

Illinois and Indiana are famous for the superb plant fossils found in concretions such as this. When struck on edge, they break along the fossil surface, exposing two halves. Neuropteris gigantea; Pennsylvanian; Braidwood, Illinois.

Leaf Fossils Ellsworth Kansas
Split concretion showing the rough outside and the seed-fern leaf Neuropteris scheuch-zeri inside. The layering of the shale in which the concretion formed is evident in this specimen. Pennsylvanian; Mazon Creek, near Morris, Illinois.

digious numbers of them. Concretions are not common in early Paleozoic rocks, but they suddenly proliferated in the Pennsylvanian period. Many shales exposed in midwestern coal mining contain them. Rocks of later periods, particularly the Permian, Triassic, and Cretaceous, are good sources of large concretions containing fossils of leaves and animals.

Concretions are easy to see and to collect from shale. They weather free until they litter the shale banks, or they wash into nearby streams. Any gray or reddish rounded rock found in shales or near shale exposures, therefore, is worth breaking open.

Strip coal mining in the Midwest has exposed thousands of acres of shale, from which millions of beautiful Pennsylvanian plant fossils have been taken. The Mazon creek area in Illinois is the best known, but similar plant fossils in almost identical concretions have been found several hundred miles south in Illinois, in western Illinois, at Terre Haute,

Plant Fossil Concretions
Not uncommonly, sandstone concretions containing large leaves are found weathering out of Cretaceous formations in Kansas. Populites; Ellsworth, Kansas.

Indiana, and as far away as Mineral Wells, Texas. Some 500 species of plants have been described from these concretions. "Fern fossils" in concretions have also been unearthed at Dudley, England. In the same Pennsylvanian concretions that contain the plants are occasionally found animal fossils, such as horseshoe crabs, insects, worms, fish, and, rarely, amphibians.

Other coal mines of the Midwest yield extremely large concretions which, when cracked open, often reveal a center filled with well-preserved brachiopods, snails, clams, and goniatites (small coiled cephalopods). Some of these are pyritized, particularly in western Illinois coal mines.

Among the most spectacular concretions are the septarian nodules of Knoxville, Iowa, which are related to those described above because they undoubtedly formed around a fossil. Shrinkage of the concretion caused cracks to form, radiating from the center. Solutions dissolved the fossil and deposited bright crystals of calcite along the walls of the cracks. It is from these walls, called septaria from the Latin word for partition, that the nodules get their name. Smaller nodules of this type, when sawed across to show the starlike pattern, are prized by mineral collectors.

The Cretaceous period abounded in shales and their progeny, concretions. Giant concretions weather out of shales in South Dakota, sometimes with a beautifully preserved ammonite at their center. Others contain large clams, oysters, or belemnites. The concretion may also display a septarian pattern, or it may contain golden barite crystals in the cavities.

Other marine fossils, notably ammonites, are found in large concretions in the Eagle Ford shale of the Cretaceous period exposed and quarried around Dallas, Texas. Coastal areas of California and Oregon have produced marine fossil-bearing concretions of similar age.

Fossil leaves similar to modern ones are found in concretions of Cretaceous age that weather out of sandstones and shales in a belt extending from the Dakotas down into Kansas. These are often large and unwieldy. Fossil fish, wonderfully preserved, are found in Brazilian concretions. English shales of Cretaceous age abound in ammonite-bearing concretions.

Jurassic and Triassic concretions are uncommon in the United States but elsewhere in the world carry fossils similar to the Cretaceous ones.

More recent rocks contain concretions, too. The fossil crabs of Washington State are found in cannonball-shaped concretions that lie in soft sandstone of Oligocene age. These crabs are also found as well-preserved

Fossils Found Ontario
Crab fossils are found in concretions from sandstones of Oligocene age in the Pacific Northwest. Zanthopsis vulgaris; Washington.

fossils in the sandstone with no trace of concretion around them. Other marine fossils are common in the sandstone, but the concretions rarely contain anything but crabs. Even the recent Pleistocene has concretions: in Ontario, small fossil fish are found in concretions of that period.

COAL BALLS

Coal balls can be considered concretions, as they are rounded masses of a mineral different from the surrounding rock and deposited before consolidation of the host rock, which is coal. The compost of Coal Age forests settled in the swamps, and calcium carbonate infiltrated masses of matted vegetation, forming the coal balls. As the plant debris was compressed, these rounded masses were already petrified and remained as swellings in the coal seam.

These coal balls are rounded or lenticular, from fist size up to giants weighing a ton, but they seem to average basketball size. They are unmistakable when found embedded in the coal seam: nothing else so large and solid and round exists in the coal. The smaller ones are scooped up with the coal, separated in the washing plant, and discarded on the dump. Very large ones are left in the mine.

Coal balls occur sporadically. One Kansas coal mine ran into so many

Dipped Geode Ring

This chalcedony-lined geode was once a coral. Covered with clay and soft limestone, the coral dissolved, leaving only a mold of the outer surface. Later, silica-rich waters deposited layers of agate and chalcedony in the cavity. This specimen has been cut in half, showing exterior and interior. Miocene; Tampa, Florida.

This chalcedony-lined geode was once a coral. Covered with clay and soft limestone, the coral dissolved, leaving only a mold of the outer surface. Later, silica-rich waters deposited layers of agate and chalcedony in the cavity. This specimen has been cut in half, showing exterior and interior. Miocene; Tampa, Florida.

that it became uneconomical to rid the coal of them, and the mine was closed. But in a nearby mine there were none. Mines in Iowa, Kansas, Illinois, Indiana, and Kentucky are particularly rich in these concretions. The Mazon Creek region in Illinois, famous for its fern-fossil concretions, is currently producing a number of coal balls. They are partly pyritized. Many mines produce such pyritized coal balls; they are useless for research as they cannot be properly "peeled" or sectioned.

Coal balls can be appreciated only when they are examined under a microscope. Their value lies in the perfectly preserved cell-for-cell petrifaction of the original woody tissues. The cell walls are still there, as are spores still in the spore sacs of 275 million-year-old fruiting bodies. Preparation of these fossils is treated in Chapter X.

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Responses

  • Elizabeth Frank
    Where to find fern fossils in Indiana?
    8 years ago
  • alberic
    What fossil is indiana famous for?
    8 years ago
  • COSIMO
    Are concretion rocks found in washington state?
    8 years ago
  • Fatimah Medhanie
    Do concretions contain fossils?
    7 years ago
  • amanda
    Do pierre shale concretions contain fossils?
    7 years ago
  • Fesahaye
    How does sandstone containing fossil leaves formed?
    7 years ago
  • cornelia
    Are there fossils in sandstone concretions?
    6 years ago
  • silke
    How to split sandstone concretions?
    6 years ago
  • christopher
    How to open fossil concretions?
    2 years ago

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