Long before radiometric dating was used to determine the absolute ages of rocks, geologists had realized that the sequence of geologic events in a certain area could be determined relative to one another. For example: In a sequence of horizontal sedimentary rocks that apparently have not been overturned, the oldest rock layer (strata) is on the bottom and the youngest is on the top.
Geologists began to group rock layers into time groups and assigned each group a name, creating a relative geologic time scale. Each name in the time scale marks a highlight in the historical development of geological science over the past 200+ years. Nearly every name is associated with a significant event or an important place where a rock was identified or important scientific discovery was made. Over the last 100 years, the study of fossils has defined the time scale even further. For example, the Cambrian Period is so named for Cambria, the Latin name for Wales, where these rocks were first studied. The Carboniferous Period is so named because during this time period a large amount of coal-bearing rocks formed, with coal being composed mostly of carbon. The period is further broken down into the Pennsylvanian (named after the state of Pennsylvania) and the Mississippian (named after the upper Mississippi Valley) epochs that contain these coal-bearing formations.
The Geological Time Scale is divided into the following categories from largest to smallest time divisions: Eon - Era - Period - Epoch - Age
An understanding of the meaning of the eon/era names and events that marked each one is necessary in order for a basic understanding of geologic time.
Precambrian– This eon represents the bulk of the time scale, spanning from about 570 million years ago to the evolution of the Earth at 4.54 billion years ago. Rocks from this time are mainly complex metamorphic and igneous rocks that form a large volume of the continental crust. There are very few fossils from this time period, due to the fact that life was primitive and had not evolved hard body parts. However, in some special circumstances, such as in the Ediacara Hills in South Australia, these primitive softbodied life forms can be found well preserved.
Paleozoic– means “ancient life” and the era marks the time (570 million years ago) at which fossils become abundant. In fact, the start of the Paleozoic is referred to as the “Cambrian Explosion” due to the seemingly sudden development of a vast array of life forms that are preserved as fossils. The variety of life that evolved is astounding, including marine organisms, fish, amphibians, land plants, and even huge insects. The Paleozoic era ended about 250 million years with the opposite event, the “Permian Extinction,” during which more than 80% of all of Earth’s life forms disappeared.
Mesozoic– means “middle life”. Fossil reptiles and invertebrates dominate these rocks. Many of the Earth’s major mountain ranges also were formed. The era’s most famous denizen is the dinosaur, and reptiles (as well as flora) thrived in the generally mild climates that dominated most continents. Pangaea, a huge supercontinent that formed during the late Paleozoic, began to break up during the Mesozoic. The era again ended with another mass extinction, whose cause is often attributed to a collision of a comet or meteor. This extinction however caused the destruction of the dinosaurs.
Cenozoic– means “recent life” and is currently still in progress. Most fossils are related to modern forms, including mammals, modern plants, and invertebrates. Mammals are the dominant vertebrate life form. The first fossils of humans and humanlike animals appear about 3 million years ago, with Homo sapiens (modern humans) evolving about 50,000 years ago.
This area was once the bottom of a shallow sea. This allowed the formation of sedimentary rock. Over time the gradual flow of water and heavy rain brought sediments to the bottom of this sea. The sediments became compressed and changed into sedimentary rock.
In the nearby valley of Clinch River the folding and general deformation is severe and the rocks suffer a number of minor contortions that complicate greatly the geology of the region.
That belt of greatly contorted strata is bounded on the northwest by one of the most profound breaks in the quadrangle; it is a broad overthrust fault, the plane of which dips lightly toward the southeast, and as a result the Cambrian limestone has presumably overridden the coal field to a considerable extent.
In view of this, the exposure here represented is of great interest and should be seen by every student of geologic structure who enters this region.
The Earthcache coordinates will bring you to an area formed during the Paleozoic Era and where the highway road cuts through and exposes sandstone and limestone.
Please email me the answers to the following questions: 1. How many different rock layers (strata) are visible. 2. Estimate the height of the oldest visible strata.
You are welcome to post photos of your visit to the site, but this is optional and not part of the logging requirements.
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