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Rock or Mineral?

Minerals are naturally occurring, solid, and inorganic substances that have a definite chemical composition (or range of compositions) and a regular internal atomic arrangement.

A rock is a solid, cohesive aggregate of one or more minerals or mineral-like materials. There are 3 main rock types:

  • Igneous–formed by the cooling and solidification of molten rock
  • Sedimentary–forms either from the deposition and consolidation of fragments of preexisting rocks, or through the precipitation (biological or chemical) of minerals from a solution
  • Metamorphic–result of the transformation of preexisting rocks 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.

Geology of Oxford

The geology of the Oxford, Ohio area includes (but is not limited to) the following rocks and features:


Southwestern Ohio's limestone (a sedimentary rock) is saturated with life — life from about 450 million years ago, that is. Our local limestone layers reflect times of relative prosperity for the wide variety of marine invertebrates living in a warm, shallow sea that covered much of North America. However, this prosperity was interrupted time and time again by large storms. Records of these conditions have been left behind in the layers of limestone and mudstone in local bedrock. The fossils that these layers contain are world-famous for the details that they record about life on Earth during the Late Ordovician Period.

Besides preserving pieces of Earth's history, limestone has many modern uses, such as road surfacing material, building stone, and concrete. Pure limestones are white or almost white, but impurities, such as clay, sand, organic remains, iron oxide, and other materials, cause them to exhibit different colors, especially on weathered surfaces.



Shale, also called mudstone, is a sedimentary rock formed from fine particles such as mud, silt, and clay. Shale is formed by finely bedded material that splits into thin layers. The pieces commonly found in Southwestern Ohio were formed about 450 million years ago during the Late Ordovician Period, when a warm shallow sea covered much of North America.

Shale typically formed in the deeper parts of this sea, where fine-grained sediments were deposited in quiet waters. These sediments originally came from an active volcanic mountain chain near the present-day Appalachians, and were brought here by hurricane-sized storms.

Although local shales typically contain fewer fossils than the fossil-rich limestone with which they are interlayered, they nevertheless preserve valuable information about where the sediments came from as well as clues to sea level changes over time. Mudrocks, mudstone, and shale comprise about 65% of all sedimentary rocks.



Glacial Till

Glacial till is unsorted sediment deposited directly by glacial ice. Glacial till is a soft rock identified by large angular rock fragments on the surface and within the soil. Because of their huge mass, ice sheets flow outward as if they were huge piles of peanut butter. As the ice moves, it pushes along soil, bedrock chunks, and other sediments and surface materials. Indeed, glaciers are extremely effective at eroding and transporting these materials and everything from clay- to boulder-sized particles are moved as one large mass. As a result, ground-up bedrock, plant fragments, and even animal remains can be found in glacial till.

Glacial till can make excellent farmland. In fact, the great agricultural potential of the soils of the till-rich plains of western Ohio drew settlers to the area with hopes of becoming highly successful farmers. The glacial till located in the Bluffs area of Collins Run Creek was deposited approximately 24,000 years ago.



Glacial Outwash

Glacial outwash is sand and gravel deposited by running water from the melting ice of a glacier. When rivers of meltwater wash away at glacial till deposits, they carry the finer clay-sized materials away and leave behind thick outwash deposits of sand and gravel that concentrate in valleys. The glacial outwash then forms an outwash plain, known as a sandur, formed from the meltwater of glaciers.

In southwestern Ohio, the melting of Ice Age glaciers filled many such valleys with hundreds of feet of sand and gravel. These deposits make excellent aquifers, or buried sources of groundwater. In Ohio, the largest of these aquifers is in the Great Miami-Mad River Valleys, which stretch between Bellefontaine and Cincinnati.



Glacial Erractics

Glacial erratics arepieces of rocksthat are carried by glacial ice and are erratic because they do not match the bedrock in the area where they are found. They are transported southward from as far away as Michigan and Canada by Pleistocene (Ice Age) glaciers and were left in our area when the glaciers melted.

Because the glacial erratics that we find in southwestern Ohio come from such a large area, they consist of a wide variety of rock types, including igneous, sedimentary, and metamorphic. These erratics can range in size from pebbles to massive pieces that can be found miles away from their original location. Although they match the bedrock of the area from which they came, they look quite different than our local bedrock. A common use of glacial erratics is for landscaping purposes.



Fossil Wood

Thousands of years ago, during a slightly warmer part of the Pleistocene Epoch, there was a spruce forest where Peffer Park now stands. Mammoths, saber-toothed cats, ground sloths, and other now-extinct beasts roamed the forest. As the climate cooled, an ice sheet advanced and plowed the forest beneath it, burying the spruce trees in the clay-rich glacial till at its base.

Sealed off from oxygen in the dense glacial till, the wood from these trees was preserved well enough to allow geologists to date it using naturally radioactive carbon-14. Radiocarbon dating of wood from the till at the Bluffs has shown that the wood is about 24,000 years old. In some instances, fossil wood may be petrified, where all the organic materials are replaced with minerals. The study of fossil wood is called palaeoxylology.

Glossary of Geological Terms

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

A - Z Glossary


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

Aragonite - 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.

Arthropoda - 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.

Asthenosphere - 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.

Calcite - 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.

Catastrophism - 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.

Convection - 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.

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


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


Fauna - 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.

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


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

Graptoloidea - 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.


Half-life - 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).

Inorganic - Not living, for example, rocks and minerals

Isotope - 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.

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

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

Lithosphere - 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.

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

Micron - 1 micron = 1/1000 mm.

Mineral - 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.


Packstone - 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)

Paleomagnetism - 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.)

Pleistocene - 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 - 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.\

Polymorph - 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.

Rock - 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 - 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.

Shale - 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 - 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 - 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)

Terrigenous - 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.

Trilobite - 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)


Unconformity - 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.

Uniformitarianism - 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.


Wackestone - 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.

Places of Interest

There are several great nearby places to see geology for yourself, whether you're interested in collecting Late Ordovician fossils, exploring glacier-carved landscapes, or observing streams as they shape the modern landscape.

Brookville Lake

Brookville Lake, located in southeastern Indiana, was formed due to the construction of a flood-control dam in the Whitewater River Valley in the 1970s. Additionally, the lake is used as a recreational area and is known for the prehistoric Indian mounds found in several locations in the valley.

Caesar Creek State Park

Caesar Creek State Park is one of the foremost outdoor recreation and nature preserve areas in the state of Ohio. The 7,900-acre park offers a wide variety of recreational, geological, and educational opportunities. You can enjoy the quite solitude of a wooded hiking trail filled with fascinating geology or hunt for fossils in Caesar Creek's spillway to learn about local geology.

Great Parks of Hamilton County

Great Parks of Hamilton County is composed of 21 different parks, in the greater Cincinnati area, which span more than 16,000 acres. Many different types of rocks can be found in Great Parks, from local fossil-rich limestone to igneous and sedimentary rocks brought here from the north by glaciers during the last Ice Age. 

Hueston Woods State Park

Hueston Woods State Park, located in Butler and Preble counties, is nearly 3,000 acres of natural resources for outdoor recreation, such as hiking, fishing, canoeing, and unique to this region — fossil hunting. The park surrounds 625-acre Acton Lake, with campsites, cabins, and a resort lodge.

Peffer Park

Once a pastureland and farm, this 80-acre plot was purchased as two parcels in 1955 and 1966 by Miami University. Later, Fred C. Yager generously donated money to Miami University in order to develop a park on this land south of campus along Highway 27.

Peffer Park Tour

Once a pastureland and farm, this 80-acre plot was purchased as two parcels in 1955 and 1966 by Miami University. Later, Fred C. Yager generously donated money to Miami University in order to develop a park on this land south of campus along Highway 27. Yager stipulated that the park was to be developed in memory of his nephew, G. Maurice Peffer. William Amos and Dorothy R. Amos contributed funds for the park's development as well.

A small portion of the land is a recreational park. The balance is maintained as a natural area to promote outdoor recreation, education, and research in the natural sciences. Common activities include hiking, snow skiing, mountain biking, birding, and a unique program conducted by Miami University's Outdoor Pursuit Center.

Karl E. Limper Geology Museum

Dr. Kendall Hauer, Director
126 Shideler Hall
Oxford, OH 45056