Investigating Earth Science in Centerville, Ohio

The Geologic Story of the Stone Used in Centerville's Historic Stone Houses

by Dr. Michael R. Sandy, Department of Geology, University of Dayton, Ohio

(From a brochure produced by the City Of Centerville's Historical Commission, September, 1996)

The limestone buildings of Centerville, Ohio are in fact older than they look! These familiar landmarks in this southernmost part of Montgomery County are constructed of building stone quarried from rock which formed millions of years ago.  Many of the early residences in this area are constructed of the Dayton limestone.  Such rocks record the evidence of the earth's long and turbulent geologic history, literally "written in stone." Geologists study the rocks and fossils left behind as a record of past events and look for clues that may help them better interpret the earth and its changing environment - past, present, and future.

Bedrock

The world's continental areas are those areas above sea-level that we spend most of our lives on.  Wherever there is soil on the continents, if you dig down you will always strike solid rock.  This is bedrock and it forms the geological foundation of all regions.

The bedrock of Montgomery County is composed of sedimentary rock which is composed of fragments of pre-existing rocks, and/or the fossil remains of animals, e.g., limestone or shale (a compacted clay).  Today these rocks form in lakes, river deltas, and the oceans - wherever there is a low spot so that sedimentary particles can gather together (such as sand grains, mud particles, and shells of clams, for example).

Centerville's Geological Story

Ordovician Seas

Centerville's bedrock is made up of limestone and shale formed during the Ordovician Period of geologic time (approximately 440-500 million years ago) and limestones of Silurian age (formed approximately 400-440 million years ago).  During the Ordovician Period a large marine (salt-water) sea extended across the interior part of the North American continent, covering much of Ohio, Indiana, Kentucky, Iowa, and other states.  Dark gray limestones and shales formed during this time.  This was a time of abundant sea-life - the evidence for this is the many beautiful fossils found in these rocks.  These rocks can often be seen where streams have cut down to bedrock, such as in Hole's Creek, Grant Park, and the Cox Arboretum.

Fossils include Horn Corals, Brachiopods (2-shelled fossils like clams), Trilobites (most common as fragments), Bryozoans (colonial animals whose remains look like twigs), and Sea-lilies (despite their common name they are animals, related to star-fish and sand-dollars).  Deposition of these sediments ended approximately 440,000,000 years ago.  At the end of the Ordovician Period the marine sea retreated and a break between the Ordovician and Silurian rocks resulted in this area.

Silurian Seas

The Silurian seawaters then advanced and resubmerged much of the continental heartland including southwestern Ohio.  Sedimentary rocks began to accumulate again, mostly limestones, but some shales, and later during the period, dolomites (a limestone with a high proportion of the element magnesium).  These sediments are again like those of the Ordovician in that they contain the remains of animals (fossils) that indicate this was a saltwater (marine) sea.  Common fossils that can be seen in these rocks are Horn Corals, Sea-Lily Stem Pieces (like tiny washers or "Life Savers," about 1/4 inch or 5 mm in diameter), and Stromatoporoids (calcified sponges that have a layered tecture) often broken into white smooth fragments up to several inches (cms) across.

Most of Centerville's stone layers were formed when this area was under the sea.

For our story it is the Silurian age Dayton Formation that is the most significant geological unit ) this is also known as the Dayton limestone, and Dayton "Marble" - but it is not a true marble).  The significance is that this rock was quarried extensively in Centerville and surrounding communities (notably Dayton, Kettering, and the Xenia area) during the nineteenth century for building stone - the Dayton Limestone.  This is typically a tan, pale cream, to white-colored rock  named the Dayton Limestone because it is well developed in the Dayton area.  The rock, like all limestones, is composed mainly of calcium carbonate.  In addition some dolomite (magnesium carbonate) is present, giving the tan color.

Close-up, Fossils can sometimes be seen, especially Corals and Stromatoporoids.  Some blocks of the limestone show a tan mottling and irregular wavy lines - these are Trace Fossils.  They formed as a result of animals burrowing through sediment on the seafloor before it became solid rock (lithification is the process of turning loose sediment into solid rock).  Some rusty patches and streaks can also be seen.  These formed by the weathering of the mineral Iron Pyrite, also known as "Fool's Gold" because it can be easily mistaken for gold when it is not weathered.

It is the Dayton limestone that gives the stone houses of the Centerville Architectural Preservation District their unique character.  Most of these buildings were constructed between ca. 1806-1840.  In these buildings the Dayton limestone is used for virtually all of the construction apart from the wooden floors, ceilings, roofs, and interior partition walls.  However, the stone is also used in very many other buildings for footings and in basements, window sills, door steps, and there are also examples of limestone horse troughs, gate posts, and mile posts.

Take a closer look at the material that some of these features are made of - are they natural stone or artificial cement?  Cement tends to be much more uniform in appearance, whereas the Dayton limestone will show traces of the original layering indicating how the sediment originally accumulated layer-by-layer on the sea floor over 400 millions years ago!

The next part of the geological story is missing!

There is a very large gap in the rock-record in southwestern Ohio (representing approximately 400 million years of time) between the Silurian rocks and the very recent (geologically speaking) glacial deposits of the Pleistocene Epoch.  The whole Mesozoic Era - the Age of the Dinosaurs - is missing here.  Go West to Colorado or Utah to find Dinosaurs!

Pleistocene Ice

During the Pleistocene enormous ice sheets (glaciers) covered large parts of northern North America.  During the last major glacial phase, the Wisconsinan, the glacier extended over the Dayton area and on to the Cincinnati area.  As the ice melted, approximately 20,000 to 17,000 years ago, it left debris, deposited as sedimentary accumulations by the melting ice or associated meltwaters.  This is how much of the glacial drift that covers the bedrock in the Centerville area originated.

Our Local Landscape

As you travel away from the intersection of Main Street and Franklin Street notice how the ground drops to a lower level.  This is because we have moved off the Silurian limestones (Brassfield and Dayton limestones) on to the Ordovician sediments below.  This is geologically similar to Niagara Falls, where Silurian limestones overlie Ordovician sediments.  The lowest Silurian limestones are referred to collectively as "Niagaran" to reflect this.  So we have our own Niagaran escarpment (slope) in Centerville - but did you notice? - no Falls!

The resistant Brassfield and Dayton limestones in the area form higher ground (geologically known as Outliers).  In these areas the bedrock is often covered by only a few feet of glacial drift making it relatively easy to dig down to the limestones to quarry them. The Dayton limestone was the preferred building material - only small amounts of the orange-colored Brassfield limestone can be seen in local buildings.  It was easy to crush to use for road construction.

The Centerville Formation is a thin green clay-rich sediment only just over one foot thick - it is not suitable as a building stone.  The unit was named after Centerville by the pre-eminent geologist August F. Foerste (who taught in a one-room school in Centerville, 1880-1883).

Geologic Time

The Earth's history is divided into geological periods, each one could be considered like the months in a year - except each month in the Earth's "year" would be many tens of millions of years long.

Geologists use the term "Formation" for a geological rock unit that is distinct enough to be mapped across a region.

There are two major breaks (known to geologists as unconformities) in the geologic succession exposed in the Centerville area, that between the Ordovician and Silurian Periods and that between the Silurian and the Pleistocene deposits.