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The geologic record in
stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...
,
paleontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
and other
natural sciences Natural science is one of the branches of science concerned with the description, understanding and prediction of natural phenomena, based on empirical evidence from observation and experimentation. Mechanisms such as peer review and repeat ...
refers to the entirety of the layers of rock
strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...
. That is, deposits laid down by
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
or by deposition of sediment derived from
weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...
detritus (
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
s,
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
s etc.). This includes all its
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
content and the information it yields about the history of the Earth: its past
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
, geography, geology and the
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
of life on its surface. According to the
law of superposition The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the ...
,
sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
and
volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plat ...
rock layers are deposited on top of each other. They harden over time to become a solidified (
competent Competence may refer to: *Competence (geology), the resistance of a rock against deformation or plastic flow. *Competence (human resources), a standardized requirement for an individual to properly perform a specific job *Competence (law), the me ...
) rock column, that may be intruded by
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
s and disrupted by
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
events.


Correlating the rock record

At a certain locality on the Earth's surface, the rock column provides a cross section of the natural history in the area during the time covered by the age of the rocks. This is sometimes called the ''rock history'' and gives a window into the natural history of the location that spans many geological time units such as ages, epochs, or in some cases even multiple major geologic periods—for the particular
geographic Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, a ...
region or regions. The geologic record is in no one place entirely complete for where geologic forces one age provide a low-lying region accumulating deposits much like a layer cake, in the next may have uplifted the region, and the same area is instead one that is
weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...
and being torn down by chemistry, wind, temperature, and water. This is to say that in a given location, the geologic record can be and is quite often interrupted as the ancient local environment was converted by geological forces into new
landform A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, ...
s and features. Sediment core data at the mouths of large riverine drainage basins, some of which go deep thoroughly support the law of superposition. However using broadly occurring deposited layers trapped within differently located rock columns, geologists have pieced together a system of units covering most of the
geologic time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...
using the law of superposition, for where tectonic forces have uplifted one ridge newly subject to
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
and
weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movement ...
in folding and faulting the strata, they have also created a nearby trough or
structural basin A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat-lying strata. They are geological depressions, the inverse of domes. Elongated structural basins are also known as syncline ...
region that lies at a relative lower elevation that can accumulate additional deposits. By comparing overall formations, geologic structures and local strata, calibrated by those layers which are widespread, a nearly complete geologic record has been constructed since the 17th century.


Discordant strata example

Correcting for discordancies can be done in a number of ways and utilizing a number of technologies or field research results from studies in other disciplines. In this example, the study of layered rocks and the fossils they contain is called
biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of ...
and utilizes amassed geobiology and
paleobiological Paleobiology (or palaeobiology) is an interdisciplinary field that combines the methods and findings found in both the earth sciences and the life sciences. Paleobiology is not to be confused with geobiology, which focuses more on the intera ...
knowledge. Fossils can be used to recognize rock layers of ''the same or different geologic ages'', thereby coordinating locally occurring geologic stages to the overall geologic timeline. The pictures of the fossils of monocellular algae in this
USGS The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
figure were taken with a scanning electron microscope and have been magnified 250 times. In the U.S. state of
South Carolina )''Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no) , anthem = " Carolina";" South Carolina On My Mind" , Former = Province of South Carolina , seat = Columbia , LargestCity = Charleston , LargestMetro = ...
three marker species of fossil algae are found in a core of rock whereas in
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth are ...
only two of the three species are found in the
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...
Series of rock layers spanning three stages and the geologic ages from 37.2–55.8 MA. Comparing the record about the discordance in the record to the full rock column shows the non-occurrence of the missing species and that portion of the local ''rock record'', from the early part of the middle Eocene is missing there. This is one form of discordancy and the means geologists use to compensate for local variations in the rock record. With the two remaining marker species it is possible to
correlate In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...
rock layers of the same age (early Eocene and latter part of the middle Eocene) in both South Carolina and Virginia, and thereby "calibrate" the local rock column into its proper place in the overall geologic record.


Lithology vs paleontology

Consequently, as the picture of the overall rock record emerged, and discontinuities and similarities in one place were cross-correlated to those in others, it became useful to subdivide the overall geologic record into a series of component sub-sections representing different sized groups of layers within known geologic time, from the shortest time span stage to the largest thickest strata eonothem and time spans eon. Concurrent work in other natural science fields required a time continuum be defined, and earth scientists decided to coordinate the system of rock layers and their identification criteria with that of the geologic time scale. This gives the pairing between the physical layers of the left column and the time units of the center column in the table at right.


Gallery

File:Dinosaur Provincial Park 19.jpg, Well-
stratified Stratification may refer to: Mathematics * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Data stratification in statistics Earth sciences * Stable and unstable stratification * Stratification, or st ...
and fully exposed sedimentary formations in Dinosaur Provincial Park,
Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest T ...
,
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
) extend over large areas exposing eons of rock history through numerous wind and water exposed strata layers Image:DeepSiltOffMirabeau.jpg,
New Orleans New Orleans ( , ,New Orleans
after
Hurricane Katrina Hurricane Katrina was a destructive Category 5 Atlantic hurricane that caused over 1,800 fatalities and $125 billion in damage in late August 2005, especially in the city of New Orleans and the surrounding areas. It was at the time the cost ...
: Unlithified sediment layers laid down in historic times. This cut was an attempt to find bedrock near a residential street near the lower breach of the
London Avenue Canal The London Avenue Canal is a drainage canal in New Orleans, Louisiana, used for pumping rain water into Lake Pontchartrain. The canal runs through the 7th Ward of New Orleans from the Gentilly area to the Lakefront. It is one of the three main drai ...
after restoring the levees which has been plowed/excavated clear by the Army Corps of Engineers, showing a nascent stratigraphy in the large deposits of silt deposited by flooding in recent earth history. Image:Tremp formatie.jpg, Three eras of deposition and two discordances are visible in this highway cut in the Netherlands. Note the color and slight angular change between the lower red bed layering and the middle strata. The upper strata are tilted yet again relative to the bottom layerings well demonstrating the cycles this land formation went through as part of the sea floor. Image:Péry-Reuchenette Oxfordian.JPG, Oxfordian (Upper Jurassic) cyclic sediments at Péry-Reuchenette, near Tavannes, Bern canton, Switzerland. Alternating layers are
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
(light, more competent) and
marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part ...
/
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...
; dominant cycle is the 200,000 year-cycle. Image:Time-Honored Rockfall.jpg, An ancient rockfall which protected the rock records beneath its impact site from further large scale erosion. Taken along Burr Trail,
Grand Staircase–Escalante National Monument The Grand Staircase–Escalante National Monument (GSENM) is a United States national monument protecting the Grand Staircase, the Kaiparowits Plateau, and the Canyons of the Escalante ( Escalante River) in southern Utah. It was established in ...
, Utah, USA. Image:PS2492-2 0017-1298 photo.jpg, Sediment core, taken with a gravity corer by the research vessel POLARSTERN in the South Atlantic; light/dark-coloured changes are due to climatic variation of the
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...
; basis age of the core is about 1 million years.


References

{{DEFAULTSORT:Geologic Record Paleontology Zoology Fossils Historical geology Geologic time scales of Earth Geochronology Stratigraphy