Glossary of Geological Terms

Use the alphabetic list below to find a definitions for common geological terms.



Allochems typically are recognizable fossil shells and shell fragments within a rock. Other types of allochems include intraclasts, pellets, and ooids.


A mineral with the same chemical composition as calcite (i.e., calcium carbonate) but with a different crystalline structure. Aragonite is metastable with respect to calcite. In other words, aragonite tends to convert to calcite over time. This conversion tends to destroy fossils that formed originally from aragonite.


The phylum Arthropoda includes invertebrate animals such as insects, crustaceans, and spiders. The name "arthropod" means 'jointed leg', and refers to the appearance of these animals, which are also caharacterized by their segmented bodies.


A region of the earth that starts at the bottom of the lithosphere and extends to depths of approximately 300 km (180 miles). Conditions of high temperature and intermediate pressure within the asthenosphere lead to this zone being partially molten and relatively soft compared to rocks above and below it.


Bedding Contact

The surface or thin zone that separates individual layers of sediment or sedimentary rock from each other.

Benthic Communities

Sea floor communities containing different types and numbers of various organisms, depending on the location within the community.

Brachiopoda ("lamp shells")

Brachiopoda (or "lamp-shells") are marine invertebrates that live exclusively in shallow, marine-to-brackish (mixed salt and fresh) water. Very abundant during the Paleozoic Era, they do still exist today, but in much smaller numbers. The interior soft parts of brachiopods are not preserved, only the hard valves are preserved as fossils.

Bryozoa ("moss animals")

Bryozoa (or "moss animals") are the most common fossils in Cincinnati area rocks. They inhabited fresh and marine waters and have the appearance of twigs, branches, flattened unsymmetrical masses, or crusts on shells. Bryozoa are actually a zoarium (or "colony") of calcareous (made of calcium) living tubes called zooecia. Bryozoa can range in diameter from that of a pin up to many centimeters.


Calcareous Algae

A group of algae that contain a supportive framework made of calcium carbonate precipitated from sea water.


A mineral that is commonly secreted by marine invertebrate animals to form shells or other types of exoskeletons. The chemical formula of calcite is CaCO3. Aragonite is another mineral with the same chemical formula, but a different crystal structure. Both calcite and aragonite are polymorphs of CaCO3.

Carbonate Ooze

Fine-grained carbonate sediment, carbonate mud.


The outmoded belief that the earth was formed from a series of spontaneous catastrophic events such as floods, volcanoes, and earthquakes.

Cincinnatian Series

The layer of exposed bedrock in southwestern Ohio composed of rock from the Upper Ordovician period.

Clastic Sedimentary Rock

Clastic sedimentary rocks are formed from the products of the mechanical breakup of other rocks; these products include silt, sand, or gravel.

Coelenterata ("corals")

The coelenterata (commonly known as corals) are abundant in the fossils in greater Cincinnati area rocks. Nearly all corals are colonial organisms, with many individual animals (polyps) living within a single large calcium carbonate skeleton. In contrast, the "horn corals" or "cup corals" that are commonly found in ancient rocks such as those in southwestern Ohio were solitary corals with a thick calcium carbonate skeleton.


The circulating masses of material driven by temperature-induced density differences; material heated at depth rises, spreads laterally, then cools and sinks to depths where it is heated again.

Cut Bank

A cut bank is a steep stream bank that forms on the outside of a stream meander due to erosion of the bank by the stronger current in this part of the stream.


Diastemic Bedding Contacts

A diastemic contact is a depositional break of minor extent with little erosion. Also known as sharp contacts, they often indicate rapid changes in sediment character.


A calcium-magnesium carbonate mineral, (CaMg(CO3)2)



The phylum of invertebrates that includes crinoids, sea stars ("starfish"), and sea anemones.

Echinodermata, Crinoidea ("sea lillies")

Although commonly known as "sea lilies," crinoids are actually animals. Crinoids typically live attached to the bottom of the sea by a long, flexible column and root-like base. The main part of the crinoid body has arms that channel food into the mouth. The hard structure of the animal is composed of plates of crystalline calcium carbonate. Crinoids first appeared in Early Ordovician rocks. Although crinoids still exist today, the species found in Cincinnatian rocks disappeared at the end of the Permian Period. Fragments, especially the columns, are abundant in the Cincinnati area.


Economic rocks and minerals are those used as ores or directly in industrial processes.



Fauna denotes the entire animal population, living or fossil, of a given area, environment, geologic formation, or time span.
(Dictionary of Geological Terms)

Foote of Philadelphia

Foote was a very prevalent collector and retailer of mineral samples about a hundred years ago. There are mines and collections named after this collector. Foote's mineral specimens are very valuable.


A formation is a set of rock layers that consists dominantly of a certain rock type or combination of types.

Fossiliferous Limestone

Limestone that contains abundant fossils.


Fossils are the remains and traces of plants and animals preserved in the geological record.



A glacier is a mass of ice that persists from year to year and moves or flows over land under its own weight.

Gradational Bedding Contacts

Gradational contacts between adjacent beds indicate continuous deposition of sediments that vary in composition over time.


Granite is coarse-grained, plutonic igneous rock, typically rich in quartz and feldspars.


Graptolites were minute individual tube-shaped animals grouped together into colonies called "rhabdosomes", which frequently look like a single, two-sided jigsaw blade. In life, the rhabadosome was suspended from a "float" by a slender thread called a nema.



The amount of time that it takes half of the atoms of the parent isotope found in a mineral to decay and form stable daughter isotopes.


Igneous Rock

This type of rock is formed by the cooling and solidification of molten rock (magma or lava).


Not living, for example, rocks and minerals


Atoms of the same element that have different numbers of neutrons.


Late Ordovician Period

The Ordovician Period refers to the interval of geologic time from approximately 505 to 438 million years ago and is sometimes called the "age of marine invertebrates". Late Ordovician refers to the approximately the last third of this time interval.


A sedimentary rock consisting mostly of calcium carbonate, usually in the form of limy mud, calcareous sand, and/or fossils.


Solidification of sediment into rock. Lithification typically involves compaction, from the weight of overlying sediments, and cementation, caused by the precipitation of mineral cements.


The outermost brittle layer of the earth is the lithosphere.


Metamorphic Rock

Metamorphic rocks are the result of the transformation of preexisting rocks (igneous, sedimentary, or metamorphic) through the process of metamorphism. Metamorphism refers to mineralogical, textural, and/or chemical changes that occur when a rock is subjected to changes in temperature, pressure, and/or chemical environment. Metamorphic changes occur in the solid state, i.e., in the absence of melting.


A meteorite is a rock that has fallen from space onto a planetary surface.
(Encyclopedia of Rocks and Minerals)


1 micron = 1/1000 mm.


A mineral is a naturally occurring, solid, inorganic substance with a definite chemical composition (or range of compositions) and a regular internal atomic arrangement.

Mollusca, Cephalopoda

Existing cephalopoda include squid, octopus, and the pearly nautilus, all very important in modern seas. Some Ordovician cephalopoda around Cincinnati grew up to nearly 4 meters long, had good eyesight, strong beaks, and could move rapidly. Most Cincinnatian cephalopods had straight, conical shells. Generally, only the internal mold of the shell is found as a fossil.

Mollusca, Gastropoda ("snails")

The number of snail species exceeds that of all members of the animal kingdom except insects. The majority of gastropods are aquatic — some living as deep as 5 kilometers in the ocean — but some are not and may be found as high as 5 kilometers above sea level.

Mollusca, Pelecypoda ("clams, scallops, oysters, mussels")

Animals in the class pelecypoda are sluggish bottom-dwellers in marine or fresh water. Good specimens of pelecypod fossils are uncommon in the Cincinnati area. This is because their shells originate as aragonite, which transforms over time into calcite; this transformation can destroy the original fossil.


Ordovician Period

The Ordovician Period is the name given to the interval of geologic time between approximately 505 and 438 million years ago.



A packstone is a rock where grains touch and form a framework, and contains micrite or recrystallized micrite (microspar or pseudospar) between the grains.

Paleozoic Invertebrates

These animals lacked spinal cords. Examples of Paleozoic invertebrates include brachiopods, trilobites, nautaloid cephalopods, and bryozoans that dwelt in tropical seas of the Paleozoic Era.
(McGraw-Hill Encyclopedia of the Geological Sciences)


Preserved signatures of Earth’s magnetic fields (of the past) expressed by magnetic minerals in igneous rocks and certain types of sediments. Paleomagnetic data from rocks can be used to show:

  • That the earth's magnetic polarity has changed many times throughout the past,
  • That the continents have moved around on the earth's surface over time, and
  • How the bedrock of the ocean floors is generated.

Phylum Mollusca

The phylum mollusca includes classes cephalopoda, gastropoda, and pelecypoda.

Plate Tectonics

A theory or conceptual model that encompasses continental drift, sea-floor spreading, and the interaction of lithospheric plates and crustal movement.
(Adapted from Christopherson, R.W. Geosystems 4th Edition, Prentice Hall, NJ: 2002.)


The Pleistocene Epoch is the name of the time interval that occurred between about 2 million to 10,000 years ago and was characterized by extensive continental glaciation. This interval is also referred to as the "Ice Age".


Plutonic rocks are igneous rocks that crystallize below the earth's surface. They are characterized by visible crystals arranged in a three-dimensional interlocking mosaic.

Point Bar

A point bar is a body of sediment, often sand and/or gravel, that forms on the inside of a stream meander, where the current slows.\


When two minerals have the same chemical composition, but different crystal structure, they are said to be polymorphs. For example, calcite and aragonite are polymorphs of calcium carbonate.

Porifera ("sponges")

A sponge has a system of holes and canals the animal uses to circulate water through its body. Sponges are multicellular but because its cells are not organized into tissues, they are considered "primitive" animals.


Radioactive Decay

The spontaneous natural process by which radioactive parent isotopes decay to form daughter isotopes. By measuring the radioactive elements, geologists can determine the absolute ages of certain types of rocks and minerals.


A rock is a solid, cohesive aggregate of one or more minerals or mineral-like materials (such as volcanic glass).


Sedimentary Rock

This type of rock forms either from the deposition and consolidation of fragments of preexisting rocks or through the precipitation (either biologically-mediated or strictly chemical) of minerals from solution (in water).


Sediments are loose, unconsolidated accumulations of mineral or rock particles that have been transported by wind, water, or ice, or shifted under the influence of gravity, and redeposited. Sediments can also be materials that precipitated, either chemically or biologically, from chemicals dissolved in water.


A fine-grained clastic sedimentary rock formed from the lithification of clay- and silt-sized particles, i.e., mud. Shales typically display fissility, the ability to be broken along thin subparallel surfaces. Mudstones are similar to shales except that they tend to be more massive, i.e., they do not display fissility.


Slump is a type of mass wasting in which a mass of soil moves as a single unit down slope along a curved failure surface. This phenomenon is commonly seen along soil-dominated stream banks and highway margins.

Subduction Zones

Areas where the oceanic lithosphere is sinking into the earth’s mantle.

Sub Tidal

In the ocean, the range of depths below the influence of tides.



Tektites are objects composed almost entirely of natural glass formed from the melting and rapid cooling of terrestrial materials (rocks and soil) by the energy accompanying the impacts of large extra-terrestrial bodies (such as meteorites).
(McGraw-Hill Encyclopedia of the Geological Sciences)


Derived from the land. Terrigenous sediments are typically clastic sediments eroded from a nearby landmass.

Till (or glacial till)

Till is a poorly-sorted sediment, containing particles ranging from clay- to boulder-size, that is deposited directly by glacial ice.


Trilobites (phylum Arthropoda) are extinct marine invertebrate animals related to crabs, spiders, and insects. Trilobites are found among the oldest well-preserved fossils in the Lower Cambrian period. Cincinnati area rocks are filled with trilobite fragments, but only the hard outer skeletons are commonly found.

Type Fossil

A type fossil is an individual or specimen from which the description of the species or subspecies has been prepared and upon which the specific name has been based.
(The American Heritage Dictionary of Science)



A break or gap in the geologic record presented by rocks or sediments in a given location. Unconformities can be caused by an interruption in sedimentation and/or by erosion of previously existing rocks.


The belief that the landscape of the earth was formed, and is still forming, slowly over time. This slow development is punctuated with the occasional catastrophic event such as meteorite impacts.



A wackestone contains greater than 10% grains occurring in a matrix of micrite, which is lithified lime mud (ooze).


  • Cipriani, Nicola. Encyclopedia of Rocks and Minerals. Barnes & Noble Books, Toledo, Spain. 1996.
  • Parker, Sybil P. (editor). McGraw-Hill Encyclopedia of the Geological Sciences, second edition. McGraw-Hill Book Company Inc., New York. 1988.
  • Barnhart, Robert K. The American Heritage Dictionary of Science. Houghton Mifflin Company, Boston. 1986.
  • Dictionary of Geological Terms, American Geological Institute.
  • Cincinnati Fossils, Cincinnati Museum of Natural History.