Local Geology

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.

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

  • limestone
  • shale
  • glacial till
  • glacial outwash
  • glacial erratics
  • fossil wood

Limestone

enlarged photo of fossiliferous limestoneSouthwestern 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

enlarged photo of shale on streambankShale, 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

enlarged photo of unsorted till in cut bankGlacial 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

enlarged photo of glacial outwashGlacial 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 Erratics

enlarged photo of glacial erraticGlacial erratics are pieces of rocks that 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

enlarged photo of Pleistocene wood in tillThousands 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.