GEOLOGIC TIME All history is subdivided into segments based on some significant event. The most fundamental subdivision of human history, for example, is into pre-historic time and historic time with the divide being the advent of the written word. Geologic history is divided in much the same way except it is subdivided into eons, eras, and periods. Eons Eras Periods Longer interval of geologic time Shorter interval of geologic time When you see this symbol, click on it and it will return you to this page. {

GEOLOGIC TIME: EONS Earth’s 4.5 billion-year old history is subdivided into two eons of time, the Cryptozoic Eon and the Phanerozoic Eon. In both cases, the stem -zoic is from the Greek word meaning “life”. Cryptozoic Eon Phanerozoic Eon However, recent work on fossil jellyfish found in the Edicara Hills of Australia suggest that the animals lived 700 million years ago. The fossils of these soft-bodied animals have lead some geologists to recommend moving the beginning of the Phanerozoic Eon back to 700 million years ago. As is the nature of science, other geologists accept the fossils as truly those of animals who lived during the Cryptozoic Eon. Today 550,000,000 years ago 4.500,000,000 years ago Historically, the dividing line between the Cryptozoic and Phanerozoic eons has been placed at the first appearance of abundant fossil remains. Currently the most accepted date for this division is million years ago.

Cryptozoic Eon Some geologists subdivide the Cryptozoic Eon into the Archean and Proterozoic Eras. This arrangement is based primarily on the distinguishing characteristic shown in the box below.. The prefix crypto- comes from the Greek word for “hidden”, referring to the fact that very few fossil remains are found in Cryptozoic rocks. For reasons that will become apparent in the following discussion, the entire Cryptozoic Eon is commonly referred to as the pre-Cambrian. Proterozoic Era 2.5 billion to 550 million years ago Distinguishing characteristic: shallow-water marine life forms Archean Era 4.5 to 2.5 billion years ago Distinguishing characteristic: no large continents exist

Phanerozoic Eon The prefix phanero- comes from the Greek word meaning “apparent” and refers to the abundant fossil remains found in these rocks. Historically, the dividing line between the Cryptozoic and Phanerozoic eons has been placed at the first appearance of abundant fossil remains about 550M years ago. As a result, the Phanerozoic Eon represents the last 550M years of Earth history. For reasons that will become apparent in the following discussion, the entire Cryptozoic Eon is commonly referred to as the pre-Cambrian. Avoid term confusion! Geologists Technical Term = Cryptozoic Eon Commonly Used Term = pre-Cambrian Cryptozoic Eon = pre-Cambrian

GEOLOGIC TIME: ERAS The Cenozoic Era extends from 60 million years ago to the present. The prefix ceno- comes from the Greek word kainos meaning “recent.” The Mesozoic Era extends from 250 million years to 60 million years ago. The prefix meso- comes from the Greek word mesos meaning “middle” referring to the position held by the reptiles between the more “ancient” invertebrates of the Paleozoic and the modern mammal-dominated Cenozoic. The Paleozoic Era lasted from 550 million years ago to 250 million years ago. The prefix paleo- from the Greek word palaios meaning “ancient” or “old”, referring to the predominance of invertebrates in the fossil record. Cenozoic Era Today to 60,000,000 years ago Mesozoic Era 60 to 250 million years ago Paleozoic Era 250 to 550 million years ago PHANEROZOICEONPHANEROZOICEON Eons of time are subdivided into eras of time. For example, the Phanerozoic Eon is subdivided (older to younger) into the Paleozoic Era, the Mesozoic Era and the Cenozoic Era.

GEOLOGIC TIME: PERIODS Eras are subdivided into periods of time. Rocks of Mesozoic and Cenozoic age are not common in West Virginia. The true story of West Virginia’s geologic evolution takes place in the Paleozoic Era. For this reason, grasping the sequence and age of the periods of the Paleozoic Era is considered “need to know” material. The Paleozoic Era is subdivided into seven periods of time. From oldest to youngest these are: 1) Cambrian, 2) Ordovician, 3) Silurian, 4) Devonian, 5) Mississippian, 6) Pennsylvanian, and 7) Permian. The best way to remember these names and their sequence is to develop a pneumonic device. For example, Can Ollie See Dave Missing Paula’s Pet? Period names are usually derived from the geographical locality where the rocks were first described. CLICK HERE FOR MORE INFORMATION ON CLICK HERE FOR MORE INFORMATION ON EACH PALEOZOIC TIME PERIOD EACH PALEOZOIC TIME PERIOD

GEOLOGIC TIME: PALEOZOIC PERIODS mya = millions years ago Named for Perm Province of Russia where these rocks were first studied. Named after extensive coal deposits of Pennsylvania. Predominate age of West Virginia rocks. Named for excellent exposures of the limestones (also called carbonates) along the Mississippi River Valley. NOTE: Until the 1950’s the Mississippian and Pennsylvanian were combined into one period called the Carboniferous. Named after Devonshire, England. Named after the Silures, another ancient tribe that once lived in Wales. Named after the Ordovices, an ancient tribe once living in what is now Wales. Named after Cambria which was the Roman name for Wales, the site where the rocks were first described. PALEOZOICERAPALEOZOICERA PERMIAN mya PENNSYLVANIAN mya MISSISSIPPIAN mya DEVONIAN mya SILURIAN mya ORDOCIVIAN mya CAMBRIAN mya