The Silurian Period began about 444 million years ago and ended about 416 million years ago. Silurian rocks form the bedrock surface throughout much of the western half of the state, although outcrops are limited in many areas because of a thick cover of sediments deposited by glaciers of the Pleistocene Ice Age. These rocks are present in the subsurface beneath the eastern and northwestern portions of the state.
During the Silurian Period, Ohio was in tropical latitudes near the equator and the shallow sea that covered the state was dominated by the precipitation of limestone, dolomite, gypsum, anhydrite, and halite (salt). The dominance of chemical rocks and the minor amounts of clastic rocks such as shale and sandstone, indicate that the Taconic Mountains of the Ordovician were worn down and contributing only a little sediment to the sea in Ohio. Some of the limestone beds contain a diverse assemblage of marine invertebrate fossils.
The boundary between the Ordovician and Silurian Systems is marked by an unconformity, indicating a time of emergence and erosion. It is probable that this episode of emergence is related to a drop in sea level due to an extensive glaciation in the southern continental mass known as Gondwana.
An extensive series of reefs began forming in Ohio and adjacent areas late in the Silurian Period. As these reefs grew, they formed barriers that restricted the inflow of seawater across eastern Ohio, western Pennsylvania, Michigan, and portions of western New York. As the seawater evaporated in a hot, dry climate, it became increasingly salty and began to precipitate gypsum, anhydrite, and salt. Periodically, seawater refreshed the restricted basins and the precipitation cycle continued. The result was thick, economically important deposits of salt and lesser amounts of gypsum.
At the end of the Ordovician Period there was a major extinction of marine invertebrates that is thought to have been due to the lowering of sea level due to glaciation and the disappearance of shallow marine shelf habitats. As these habitats were reestablished with the rise of sea level in the Silurian, many new species of invertebrates appeared. The Silurian is sometimes referred to as the Age of Corals because these organisms built extensive reefs during this time. Many Silurian rock units in Ohio are dolomites in which recrystallization has destroyed most fossils. However, many of the limestones and shales, particularly in outcrop areas in southern Ohio, have abundant and often well-preserved fossils. Perhaps the best-known Silurian fossils from Ohio include a large pelecypod, Megalomoidea Canadensis, and the brachipods Pentamerus and Trimerella. Although not know from Ohio, the first well-established land plants appeared during the period and fishes began to diversify.
Silurian rocks in Ohio are of great economic importance. The western half of the state is dotted with quarries that extract limestone and dolomite that is used for road construction, commercial building, concrete, and agricultural and chemical lime. Rock salt (halite) was discovered by drilling beneath eastern Ohio in 1886. Two salt mines, about 2,000 feet beneath Lake Erie, mine rock salt for snow and ice control. These vast subsurface salt deposits have also been exploited by a process known as solution mining. Hot water is pumped into the salt bed where it dissolves the salt. The salty water, known as brine, is pumped to the surface where it is evaporated to recover the pure salt. Ohio has long been an important producer of salt. It is estimated that the salt deposits beneath eastern Ohio could supply the entire nation for 32,000 years.
Oil and gas have also been produced from Silurian rocks. Of particular importance is a sandstone beneath eastern Ohio, known to drillers as the “Clinton.” Large amounts of natural gas have been produced from more than 75,000 “Clinton” wells drilled in eastern Ohio.
Many museum-quality specimens of crystals of celestite, fluorite, sphalerite, galena, pyrite, and marcasite, among others, have been collected from pockets or vugs in Silurian limestones and dolomites, particularly in northwestern Ohio. Silurian rocks in Adams and Highland Counties in southern Ohio have produced mineral specimens.
In some areas of the state, thick, erosion-resistant limestones and dolomites form scenic cliffs and waterfalls. Of particular note are John Bryan State Park and Clifton Gorge State Nature Preserve in Greene County, where the Little Miami River and its tributaries have cut through the Cedarville Dolomite. Small solution caves are developed in these rocks in this area and in Silurian rocks on South Bass Island in Lake Erie.