The St. Louis Limestone is a large geologic formation

Geologic formation

A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...

 covering a wide area of the midwest of the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. It is named after an exposure at St. Louis, Missouri

St. Louis, Missouri

St. Louis is an independent city on the eastern border of Missouri, United States. With a population of 319,294, it was the 58th-largest U.S. city at the 2010 U.S. Census. The Greater St...

. It consists of sedimentary limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 with scattered chert

Chert

Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...

 beds, including the heavily chertified Lost River Chert Bed in the Horse Cave Member. It is exposed at the surface through western Kentucky

Kentucky

The Commonwealth of Kentucky is a state located in the East Central United States of America. As classified by the United States Census Bureau, Kentucky is a Southern state, more specifically in the East South Central region. Kentucky is one of four U.S. states constituted as a commonwealth...

 and Middle Tennessee

Middle Tennessee

Middle Tennessee is a distinct portion of the state of Tennessee, delineated according to state law as the 41 counties in the Middle Grand Division of Tennessee....

, including the city of Clarksville, Tennessee

Clarksville, Tennessee

Clarksville is a city in and the county seat of Montgomery County, Tennessee, United States, and the fifth largest city in the state. The population was 132,929 in 2010 United States Census...

. The limestone deposit is Mississippian

Mississippian (geologic period)

The Mississippian is a subperiod in the geologic timescale or a subsystem of the geologic record. It is the earliest/lowermost of two subperiods of the Carboniferous period lasting from roughly 359 to 318 Ma...

 in age, in the Meramecian series, roughly 300 million years old.

Fossils commonly found in the St. Louis include the rugosa

Rugosa

Disambiguation:The Rugosa Rose is also sometimes just called "Rugosa". For the moon in Star Wars: The Clone Wars, see .The Rugosa, also called the Tetracoralla, are an extinct order of coral that were abundant in Middle Ordovician to Late Permian seas.Solitary rugosans are often referred to...

n corals Lithostrotion

Lithostrotion

Lithostrotion is a genus of rugose coral which is commonly found as a fossil within Carboniferous Limestone. Lithostrotion is a member of the Lithostrotionidae family within the order Stauriida, class Rugosa in the Cnidaria phylum...

and Lithostrotionella and the bryozoan Fenestrellina.