Limestone is exceedingly common, is exposed over a wide area, and is abundantly fossiliferous. It has the further virtue of preserving its fossils
with little crushing and in fine detail. Limestone is so widely quarried to produce crushed rock, agricultural lime, and building stone that exposures of both fresh and weathered material are easy to find.
Limestone differs from other sediments in the way it is formed. Sandstone, conglomerate, and shale are made up of fragments, granules, or silt washed in or otherwise deposited in the place where they were consolidated into stone. These the geologist calls clastic sediments. Most limestone, however, is basically a precipitate of calcium carbonate from calcium salts dissolved in water, usually salt water, although there are freshwater limestones. The precipitate, in the form of microscopic crystals of calcite, settles on the bottom. In time it is capable of forming layers miles thick.
In this limy ooze, shells of brachiopods, clams, oysters, and snails accumulate. They, too, construct their homes of calcium carbonate stolen from the water. In the same way, cephalopods, crinoids, calcareous sponges, some algae, protozoa, blastoids, echinoids, bryozoa, and even starfish add their calcareous hard parts to the ooze. Coral reefs, made up in a like manner of the accumulated remains of the tiny coral animal, form islands miles in diameter. Such fossil coral reefs from the Silurian period are often quarried in the Midwest where they outcrop at the surface as prehistoric islands.
The empty shells and hard parts become incorporated in the limy mud on the bottom. Animals and plants lacking calcareous structures, such as trilobites, graptolites, and plant leaves, are buried in it, too. Calcium carbonate recrystallizes easily, so that the layer of soft lime with its embedded organisms gradually turns into limestone.
Some limestones are massive; they do not part easily into thin slabs. These are limestones that have formed over a long period of time and under constant conditions. But conditions in other prehistoric seas must have been more changeable, because in them formed thin layers of limestone interbedded with thin shales or limestones of different composition. These thin layers are often rich in fossils or almost entirely composed of them. Thick layers of Burlington limestone, quarried in Iowa and Missouri, are made up of clearly visible crinoid stem segments and plates of the crinoid cups. Scarcely any cementing material is visible in the limestone.
Shells of clams make up most of the substance of a distinctive limestone found in Florida that is used as a building stone. A limestone whose major
constituent is one type of fossil is known by the name of this fossil; examples are crinoidal limestone, blastoidal limestone, and coquinal limestone. What is commonly called coquina rock is a rock formed not necessarily of coquina clams, but of broken pieces of any shell, or of coral fragments with little visible cement and little filling of cavities between shells.
Limestone is a good final resting place for organisms. While mollusks and trilobites are often flattened and disorted in shales, they remain plump in limestone. Shale is still the place to find fossils of the soft-bodied and rare animals, but limestone is not a bad second in this respect. The Bertie limestone, through which canals were built a century ago in New York State, yielded beautifully carbonized fossils of eurypterids, complete in detail even to their legs.
The quarries of Solnhofen, Germany, have produced a fine-grained limestone for more than 400 years. It is still used for lithographic stones. On these slabs of Jurassic sediments are occasionally found extraordinary fossils, such as the feathers of birds. Fish with all scales in place are
fairly common, as well as insects and shrimp with antennae intact, horseshoe crabs, and even jellyfish.
The famous crinoid locations at Le Grand, Iowa, discussed in Chapter 1, and at Gilmore City, Iowa, occur in layered limestone. Thousands of coral specimens have been collected from Devonian limestone beds in northern Iowa and at the Falls of the Ohio River near Louisville, Kentucky. Much of the state of Texas exposes a fossil-rich layer of Cretaceous chalk, which is a soft limestone. The word Cretaceous, in fact, comes from the Latin word for chalk. Another kind of limestone formed of a soft, very limy mud is called marl. It is often filled with fossils.
Many of the same types of fossils can be collected from the limestone as from shale, with a few exceptions. Graptolites are commonly found in dark shales, always compressed. They were not found in limestone until 1890, when complete, uncrushed specimens were etched from the rock. These provide good material for scientific study, but they are rare. Leaf fossils are mostly confined to shales, probably because most of them were deposited in freshwater or ocean-shore environments where limestone would not be likely to form.
Large corals are generally found in the limestone formations which they helped build. Pyritized fossils are rare in limestones but are not uncommon in shales. On the other hand, silicified fossils are rare in shales, while beautiful specimens have been collected from limestones, such as the exquisitely preserved brachiopods from the Glass Mountains of Permian age in Texas, or silicified Ordovician trilobites from West Virginia. Miocene limestone nodules found in one area of California often contain silicified insects, preserved as perfectly as the insects in Baltic amber, with every antenna and eye facet in place.
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