Geologists have subdivided the 4.5-billion-year history of the earth into two dominant eons: the Precambrian and the Phanerozoic. The Precambrian represents most of the earth's history up till the last 550 million years; the Phanerozoic covers from 550 million years ago to the present. Most of the fossils found on earth are Phanerozoic. The Phanerozoic is divided into three eras: the Paleozoic (550 to 250 million years ago), the Mesozoic (250 to 65 million years ago), and the Cenozoic (65 million years ago to the present). The Paleozoic has been called the Age of Invertebrates because of the rapid development of invertebrate animals during that time. Invertebrate animals (animals without backbones) lived before the Paleozoic and are still the most abundant animal life on earth. The Mesozoic Era is sometimes called the Age of Dinosaurs because the dinosaurs lived entirely within the Mesozoic. However, the first dinosaurs were not present till several million years after the start of the Mesozoic. The Cenozoic is commonly called the Age of Mammals because of the rapid rise of mammals after the extinction of the dinosaurs. Actually, the first mammal-like animals were present during the later stages of the Paleozoic, and mammals or mammal-like animals existed throughout the Mesozoic Era.
The Paleozoic Era is divided into the following ages or periods: Cambrian (550 to 510 million years ago), Ordovician (510 to 440 million years ago), Silurian (440 to 410 million years ago), Devonian (410 to 360 million years ago, Mississippian (360 to 325 million years ago), Pennsylvanian (325 to 290 million years ago), and the Permian (290 to 250 million years ago).
The Mesozoic Era is divided into the following ages: Triassic (250 to 205 million years ago), Jurassic (205 to 140 million years ago), and the Cretaceous (140 to 65 million years ago).
The Cenozoic Era is divided into the following ages: Paleogene (65 to 23 million years ago, Neogene (23 to 1.6 million years ago), and Quaternary (1.6 million years ago to the present). The Paleogene and Neogene are sometimes lumped together as the Tertiary. The Paleogene is divided into three series: the Paleocene (65 to 53 million years ago), Oligocene (53 to 32 million years ago), and the Eocene (32 to 23 million years ago). The Neogene is divided into two series: the Miocene (23 to 5.3 million years ago) and the Pliocene (5.3 to 1.6 million years ago). The Quaternary is divided into two series: the Pleistocene (1.6 million to 10,000 years ago), and the Holocene or Recent (10,000 years ago to the present).Geologic Ages Represented in Kentucky
Most of the rocks exposed at the surface in Kentucky are flat-lying sedimentary rocks of Paleozoic age. The oldest exposed rocks in Kentucky date back about 450 million years to the Ordovician Period. Most of the older rocks exposed in Kentucky (Ordovician, Silurian, Devonian, and Mississippian age) are limestones, dolostones, and shales, which were formed in ancient seas when the eastern part of the continental United States was submerged under seas. These rocks are stratified (layered), and marine fossil remains can be found in them in many places. The younger rocks exposed at the surface in Kentucky (Pennsylvanian, Cretaceous, and Tertiary) are sandstones, shales, and coal beds, which were deposited in shallow seas and on low coastal plains. Plant remains are found in some of these rocks and sediments and give evidence of ancient forests and swamps. The youngest rocks and sediments are of Tertiary age and are located in western Kentucky. Quaternary sediments (not yet cemented into rocks) are very recent, and are located in Kentucky as valley-bottom deposits.
The sedimentary rocks below the surface in Kentucky are late Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian in age and occur as deep as 12,000 feet or more. Below the sedimentary rocks are igneous and metamorphic rocks of Precambrian age, which occur as shallow as 5,000 feet in some places in Kentucky. Little is known of these Precambrian rocks because few drill holes have penetrated them.