Before the advent of absolute dating in the 20th century, archaeologists and geologists were largely limited to the use of Relative Dating techniques. Estimates of the order of prehistoric and geological events were determined by using basic stratigraphic rules, and by observing where fossil organisms lay in the geological record, stratified bands of rocks present throughout the world. Absolute dating is the process of determining a specific archaeological date. ... (19th century - 20th century - 21st century - more centuries) Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s As a means of recording the passage of time, the 20th century was that century which lasted from 1901–2000 in the sense of the Gregorian calendar (1900–1999... Archaeology or sometimes in American English archeology (from the Greek words αρχαίος = ancient and λόγος = word/speech) is the study of human cultures through the recovery, documentation and analysis of material remains, including architecture, artefacts, biofacts, human remains, and landscapes. ... The Geologist by Carl Spitzweg A geologist is a contributor to the science of geology, studying the physical structure and processes of the Earth and planets of the solar system (see planetary geology). ... Prehistory (Greek words προ = before and ιστορία = history) is the period of human history prior to the advent of writing (which marks the beginning of recorded history). ... World geologic provinces Oceanic crust  0-20 Ma  20-65 Ma  >65 Ma Geologic provinces  Shield  Platform  Orogen  Basin  Large igneous province  Extended crust Geology (from Greek γη- (ge-, the earth) and λογος (logos, word, reason))[1] is the science and study of the solid matter of the earth, its composition, structure... Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ... An ammonite fossil Eocene fossil fish of the genus Knightia Petrified wood fossil formed through permineralization. ... The rocky side of a mountain creek near Orosí, Costa Rica. ... Antarctica Oceania Africa Asia Europe North America South America Middle East Caribbean Central Asia East Asia North Asia South Asia Southeast Asia SW. Asia Australasia Melanesia Micronesia Polynesia Central America Latin America Northern America Americas C. Africa E. Africa N. Africa Southern Africa W. Africa C. Europe E. Europe N...

Though relative dating can determine the order in which a series of events occurred, not when they occurred, it is in no way inferior to radiometric dating; in fact, relative dating by biostratigraphy is the preferred method, and is in some respects more accurate.(Stanley, 167-9) Radiometric dating is a technique used to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances. ... Biostratigraphy is the science of dating rocks by using the fossils contained within them. ...

The Principles

The principles of Relative dating use a combination of fossil study and structural interpretation to learn about the geological history of an area. The rules of relative dating for continuous stratigraphic sequences were worked out long ago, by scientists such as Nicolas Steno (1638-86): An ammonite fossil Eocene fossil fish of the genus Knightia Petrified wood fossil formed through permineralization. ... Nicolaus Steno. ...

• The Law of superposition states that in any undisturbed geologic sequence, older beds lie below younger beds.
• The principle of original horizontality states that whether rock beds are deformed or not, they were originally deposited on a horizontal surface.
• The principle of lateral continuity states that a given rock bed will extend horizontally for some distance, until it thins out or reaches the edge of a depositional basin.
• The principle of cross-cutting relationships states that any geologic feature that off-sets or intersects a series of beds (such as a fault, dike, or sill) must be younger than the beds it cuts across.
• The principle of inclusion states that if a body of rock contains inclusions of another type of rock, it must be younger than the source rock.(Monroe and Wicander, 413-21)

These principles are highly useful in interpreting geologic history. For instance, an angular unconformity with horizontal beds above it indicates a sequence in which: The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. ... Proposed by Nicholas Steno. ... The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. ... Fault in metamorphosed strata near Adelaide, Australia Geologic faults or simply faults are planar rock fractures which show evidence of relative movement. ... Dyke (normal International spelling) or Dike (normal American spelling) can mean several things: A dyke / dike is a long wall built to keep out the sea or enclose land. ... In geology, a sill is a tabular mass of igneous rock that has been intruded laterally between layers of older rock. ... An unconformity is a gap in time in rock layers, where erosion occurs while deposition slows or stops. ...

1. The original strata were deposited horizontally.
2. The strata were tilted and deformed.
3. Much of the strata were eroded away.
4. Deposition later began anew above the unconformity.

Biostratigraphy

Biostratigraphic methods are usually used in tandem with structural ones. For instance, the principle of faunal succession was probably the most important factor behind the elaboration of the geologic time scale, which was more or less complete long before an absolute time scale was available. Beds with a particular fauna can be correlated with others that share it (often globally), and also distinguished from upper and lower beds without them. Biostratigraphy is the science of dating rocks by using the fossils contained within them. ... This article or section does not cite its references or sources. ... // For other uses, see time scale. ...

Rock units that contain a distinct assemblage of fossils are biostratigraphic units, and are based on the "range", or vertical interval in which a taxon is found. A zone, or biozone is the most basic biostratigraphic unit, one bound on its upper and lower boundaries by the ranges of given species; these can be zones where certain species coexist, or which are defined by the earliest appearance or latest disappearance of taxa in neighboring zones. A taxon (plural taxa), or taxonomic unit, is a grouping of organisms (named or unnamed). ...

Index fossils (also guide fossils) are invaluable for biostratigraphy. The best index fossils are: Index fossils (or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). ...

• Abundant.
• Distinct from other flora/fauna.
• Geographically widespread.
• Found in many kinds of rocks.
• Narrow in stratigraphic range.

Unfortunately, few taxa fit all these criteria.(Stanley, 157-8)

Planetological Use

Relative dating is used to determine the order of events on objects other than Earth; for decades, planetary scientists have used it to decipher the evolution of bodies in the Solar System, particularly in the vast majority of cases in which we have no surface samples. Many of the same principles are used; for instance, if a valley on Mars cuts across a crater, the valley must be younger than the crater. Major features of the Solar System (not to scale, from left to right): Pluto, Neptune, Uranus, Saturn, a comet, Jupiter, Ceres which lies in the asteroid belt, the Sun, Mercury, Venus, Earth & Moon, and Mars. ... Note: This article contains special characters. ... Look up crater in Wiktionary, the free dictionary. ...

Craters themselves are highly useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known.(Hartmann, 258)

See also

• Key bed
• Law of superposition
• Stratigraphy
• Structural geology
• Seriation

In geology, a key bed (syn marker bed) is a widespread sedimentary layer that formed at a single time, such that it is useful for geologic correlations and dating over a large area. ... The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. ... Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ... Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces, and their internal fabrics. ... // Seriaton in Archaeology Seriation is a method in relative dating in which artifacts of numerous sites, in the same culture, are placed in chronological order. ...

References:

• Hartmann, William K. Moons & Planets, 4th ed. Belmont: Wadsworth Publishing Company, 1999. ISBN 0-534-54630-7
• Monroe, James S., and Reed Wicander. The Changing Earth: Exploring Geology and Evolution, 2nd ed. Belmont: West Publishing Company, 1997. ISBN 0-314-09577-2
• Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6