Geologic Section of West Tennessee Above.

Silurian Period 443 to 417 Million Years

The first true terrestrial ecosystem is believed to have become well established by the Silurian Period. The first vascular plants appear in the fossil record with terrestrial arthropods. Marine Arthropods, trilobites, were beginning to fade while the terrestrial arthropods were starting a new chapter of life on land. Vertebrates particularly fish became common and diversify. The diverse bony fish group appears in the fossil record and is the largest group of vertebrates today. The first ammonites appear in the fossil record.

The extensive Glade system of western Tennessee has some of the best Silurian Period fossils one can collect. This remarkable fossiliferous Glade system along the Tennessee River has outcrops of the Beech River Formation, lower Brownsport Group, middle Silurian. The fossils are wonderfully preserved, diverse, and include crinoids, the cystoid Caryocrinites, a blastoid Troostocrinus, diverse assemblage of sponges, brachiopods, bryozoans, corals, gastropods, and trilobites. The Upper Silurian was removed by Devonian and Mississippian Period erosion, the Devonian was removed by Mississippian and/or Cretaceous erosion, and the Mississippian age sediments were removed by Cretaceous Period erosion. Cretaceous Period sediments can be found atop most of the hills in the Silurian Cedar Glade area, and immediately west along the Mississippi embayment within the Tennessee Valley area.

Devonian Period 417 to 354 Million Years

The Devonian is known as the “Age of Fishes” as it was nicknamed in the 1840s for the famous middle Devonian fossil fish sites in Scotland. Fish dominated the vertebrate community while coral reefs flourish in the oceans. On land forests develop with insects, arachnids and vertebrate tetrapods appearing in the fossils record for the first time.

Very little Devonian sedimentary rocks are present in the Tennessee Valley and North Alabama Region. The only Devonian Period fossils found are located on the northern edge of the Silurian Glade area in western Tennessee. In north Alabama and in Tennessee the black, lightly radioactive, Chattanooga Shale is upper Devonian lowermost Mississippian age, and has very few fossils for one to find. I have devoted very little time collecting Devonian fossils because there is very little to be found in the Tennessee Valley Region.