The Kimmswick Limestone is an Ordovician geologic formation in Arkansas, Illinois and Missouri. Fossils occurring in the Kimmswick include corals, bryozoans, brachiopods, conodonts, trilobites, crinoids and mollusks.
The Maquoketa Formation is a geologic formation in Illinois, Indiana. Iowa, Kansas, Minnesota, Missouri, and Wisconsin. It preserves mollusk, coral, brachiopod and graptolite fossils dating back to the Darriwilian to Hirnantian stages of the Ordovician period.
The Roubidoux Formation is a geologic formation in the Ozarks of Missouri. It preserves fossils dating back to the Ordovician period.
The Powell Formation or Powell Dolomite is a geologic formation in northern Arkansas, southeast Missouri and Virginia. It contains gastropod, cephalopod, and trilobite fossils dating back to the Ordovician Period.
The Cotter Formation is a geologic formation in Arkansas, Missouri, Oklahoma and in Virginia. It preserves fossils dating back to the Ordovician period.
The Fernvale Limestone is a geologic formation in Alabama, Arkansas, Illinois, Missouri, Oklahoma, and Tennessee. It preserves fossils dating back to the Ordovician period.
The Theodosia Formation is a geologic formation in Missouri. It preserves fossils dating back to the Ordovician period.
The Bonneterre Formation is an Upper Cambrian geologic formation which outcrops in the St. Francois Mountains of the Missouri Ozarks. The Bonneterre is a major host rock for the lead ores of the Missouri Lead Belt.
The Eminence Formation or Eminence Dolomite is a geologic formation in Missouri. It preserves fossils dating back to the Cambrian period.
The Potosi Formation is a geologic formation in Missouri, Illinois and Indiana. It preserves fossils dating back to the Cambrian period.
The Burlington Limestone is a geologic formation in Missouri, Iowa and the Midwest region. It preserves fossils dating back to the Mississippian subperiod.
The Compton Limestone is a geologic formation in southwest Missouri. It preserves brachiopod and echinoderm fossils of the Mississippian subperiod. The Compton rests unconformably on the Cotter Dolomite of Ordovician age. The Compton was named for the community of Compton, Missouri, as the type sections were described for outcrops along the James River and its tributary the Compton Branch.
The Jefferson City Formation or Jefferson City Dolomite is a geologic formation in the Ozarks of Missouri and Arkansas. The Jefferson City is in part not differentiated from the Cotter Formation of northern Arkansas. It preserves fossils dating back to the Ordovician period.
The Leemon Formation is a geologic formation in Missouri. It preserves fossils dating back to the Ordovician period.
The Rich Fountain Formation is a geologic formation in Missouri. It preserves fossils dating back to the Ordovician period.
The Cape Limestone is a geologic formation in Missouri. It preserves fossils dating back to the Ordovician period.
The Dutchtown Formation is a geologic formation in Missouri. It preserves fossils dating back to the Ordovician period.
The Joachim Dolomite is a Middle Ordovician geologic formation in Arkansas, Illinois, and Missouri. The name was first introduced in 1894 by Arthur Winslow in his study of the geology of Missouri. Winslow designated a stratotype along Plattin Creek, which was misidentified as Joachim Creek, in Jefferson County. The name was introduced into Arkansas in 1911, replacing part of the, now abandoned, Izard Limestone.
The Plattin Limestone is a Middle Ordovician geologic formation in Arkansas, Illinois, and Missouri. The name was first introduced in 1904 by Edward Oscar Ulrich in his study of the geology of Missouri. A type locality was designated at the mouth of the Plattin Creek in Jefferson County, Missouri, however a stratotype was not assigned. As of 2017, a reference section has not been designated. The name was introduced into Arkansas in 1927, replacing part of the, now abandoned, Izard Limestone.
The Cason Shale is a Late Ordovician to Middle Silurian geologic formation in the Ozark Plateaus of Arkansas. The name was introduced in 1894 by Henry Shaler Williams in his study of Arkansas. Williams designated a type locality at what was known as the Cason tract and mine, near Batesville, Independence County, Arkansas, however, he did not assign a stratotype. As of 2017, a reference section has not been designated for this unit.
United States 
