The Ordovician Period began about 488 million years ago and ended about 444 million years ago. These are the oldest rocks in the state that are exposed at the surface, in southwestern Ohio along the axis of a positive structural feature known as the Cincinnati Arch or Platform, which formed in the Ordovician. The exposed rocks are from the upper part of the Ordovician System, except for limited exposures of Middle Ordovician rocks along the Ohio River. Older Ordovician rocks are present in the subsurface throughout Ohio.
The oldest Ordovician unit, the Knox Dolomite, began to be deposited in Late Cambrian time in a tidal-flat environment. During the early part of Ordovician time, deposition of the Knox ceased as the sea level fell as the land rose. It is thought that this gentle elevation of the land surface was a bulge (peripheral or forebulge) created by the beginnings of continental collision along the east coast of North America. This collision became a major mountain-building event later in the period and is termed the Taconic Orogeny. During this episode of emergence, deep erosion took place on the surface of the Knox Dolomite, forming a prominent unconformity. It was originally thought that his unconformity marked the boundary between the Cambrian and Ordovician Systems; however, later studies placed the boundary lower in the Knox sequence.
Although collision of island arcs and continental masses during the Taconic Orogeny took place far to the east of Ohio, this episodic mountain-building event had significant influence on Ohio. Shallow seas once again flooded the state and thick limestones were deposited in the quiet seas. Late in the period, streams eroding the Taconic Mountains carried sediment westward, building a major delta known as the Queenston Delta. The fine-grained clay and silt from the delta were carried out into the Ohio sea, forming thin beds of shale that alternated with thin limestone beds.
Large volcanoes associated with the island arcs had major eruptions as recorded by beds of volcanic ash preserved over a wide area, including Ohio. Some of these ash beds (known to geologists as bentonites) can be traced from the Mississippi River eastward to Europe and Russia. It has been estimated that these eruptions generated about 5,000 times the volume of volcanic ash as was produced by the eruption of Mt. St. Helens in 1980.
The sequence of alternating thin beds of limestone and shale exposed in southwestern Ohio was deposited in a warm, shallow sea and is known to geologists as the Cincinnatian Series. These beds are highly fossiliferous with exquisitely preserved marine invertebrate fossils. This rock sequence is so complete and well known that it is the official reference section for Upper Ordovician rocks in North America.
Many of the limestone and shale beds in Ordovician rocks of southwestern Ohio consist mostly of shells and other hard parts of marine invertebrates. Brachipods, clams, snails, cephalopods, bryozoans, echinoderms, and trilobites, among others, can be found at almost any outcrop in the area. Most sought after by collectors are the fossils of extinct marine arthropods known as trilobites. The most common one is Flexicalymene, which is often found in a rolled-up position. The most sought-after trilobite is perhaps Isotelus, a large trilobite that reached up to two feet in length. Isotelus was named the official state fossil of Ohio in 1985.
Although some of the limestone beds in the Cincinnati area were quarried for building stone and other uses, this industry is no longer active. Economically, Ordovician rocks in Ohio are most famous for oil and gas deposits. In 1884, oil and gas were discovered in the Middle Ordovician Trenton Limestone in northwestern Ohio. This marked the discovery of the first giant oil and gas field in North America, which eventually stretched from Toledo to Indianapolis. It is estimated that 100,000 wells were drilled, 76,000 of them in Ohio. The Trenton produced at least 500 million barrels of oil and 1 trillion cubic feet of gas before the field was depleted by 1910. Several American oil companies had their beginnings in this field.
The hilly terrain in the greater Cincinnati area is prone to landslides where an Upper Ordovician shale unit, the Kope Formation, is present. Highly weathered shale of the 220-foot thick Kope is prone to slope failure, particularly when wet.