top of page


There is a history of Earth recorded in rock layers. This wonderful history was revealed to humans once scientists understood the origin of fossils and the details of sedimentary rock formation. Relative dating techniques determined a chronology or arrangement of events in time. Radiometric dating places absolute dates, measured in years, to the relative-dated chronology. 

Relative Dating


Determining the Origin, Composition,
Distribution, and Succession of Rock Strata

Stratigraphy is a branch of geology that studies rock strata with an emphasis on distribution, deposition, age and evidence of past life. Nicolas Steno, William Smith, Georges Cuvier, Alexandre Brongniart, and James Hutton developed the basic rules for the science of stratigraphy. Relative dating uses the principles or laws of stratigraphy to order sequences of rock strata. Relative dating not only determines which layers are older or younger, but also gives insight into the paleoenvironments that formed the particular sequence of rock.

A chance encounter between determined fishermen and a great white shark off the Tuscan coast in 1666 sparked a chain of events that would help change humans views of fossils and Earth’s geologic past (Cutler 2003, pp. 5-8). Nicolas Steno (1638-1686) dissected the head of this shark and realized fossil tongue stones believed to be petrified snake or dragon tongues were actually fossil shark teeth (Prothero 1998, p. 3). One problem still existed, how do fossils become embedded in solid rock? Steno recognized that fossils represent organisms that became buried in sediment, which later turned into rock. The realization that sediments turn into rock was counter to the view that all rocks on Earth formed in a single creation event. Once Steno recognized that the fossils he was contemplating (sharks teeth and sea shells) were formed in the sediments of oceans he was able to work out the basic rules of stratigraphy. Steno formalized the laws of superposition, original horizontality, original continuity and inclusions in his publication entitled De solido intra solidum naturaliter contento dissertationis prodromus (Prothero 1998, p. 3).

The law of superposition is the foundation of Steno’s work on stratigraphy. The principle of superposition states that in an undisturbed sequence of strata or lava flows; each layer is older than the one above and younger than the one below. The law of original horizontality states that sedimentary strata and lava flows are deposited in horizontal sheets. If these layers are not horizontal, subsequent movements have occurred. The law of lateral continuity states that strata and lava flows extend laterally in all directions and pinch out at the edge of their deposition. Inclusions are rock fragments or fossils contained in another rock type. The principle of inclusions states that any inclusion is older than the rock that contains it. Steno's idea that fossils are older than the rock in which they are found hints at this principle, but Hutton is most often given credit for this principle. Steno developed these principles in the context water deposited sediment. It is not clear he was aware of igneous rock formed from lava flows.

The principle of faunal succession states that fossil organisms succeed one another in a definite, irreversible, and determinable order. This law was independently discovered by William Smith (1769-1839), a British engineer, while working on excavations for canals in England (Winchester, 2002 p. 131) and by Georges Cuvier (1769-1832), a French anatomist, and Alexandre Brongniart (1770-1847), a French naturalist and geologist, during their work on the deposits of the Paris Basin. Brongniart was the first to use fossils to date rock strata.

James Hutton (1726-1797) is often considered the father of geology. Hutton developed the theory of uniformatarianism, which states that geologic events are caused by natural processes, many of which are operating in our own time. Put another way, the natural laws that we know about in the present have been constant over the geologic past. The concept of geologic time or deep time was a logical consequence of this theory. In 1788 John Playfair came to see Hutton’s Unconformity in Inchbonny. The unconformity consists of many vertical tilted layers of grey shale overlaid by many layers of horizontal red sandstone. Playfair later commented that, "the mind seemed to grow giddy by looking so far into the abyss of time." McPhee (1998) points out that Hutton removed humans from a specious place in time just as Copernicus had removed humans from a specious position in the universe (p. 74).

Hutton gives us three more laws to consider when seeking relative dates for rock layers, one of which, the law of inclusions was described earlier. The law of cross-cutting states any feature that cuts across a rock or sediment must be younger than the rock or sediment through which it cuts. Examples include fractures, faults, and igneous intrusions. Igneous intrusions are sometimes referred to as a seperate principle, the principle of intrusive relationships. Unconformities represent gaps in geologic time when layers were not deposited or when erosion removed layers. This principle includes three types of unconformities. A disconformity is an unconformity between parallel layers. An angular unconformity exists when younger more parallel strata overlie tilted strata. A nonconformity is formed when sedimentary layers are deposited on igneous or metamorphic rock. Hutton’s theory of uniformatarianism and the principles of stratigraphy would be fully developed and made popular by another Scottish geologist Charles Lyell (1797-1875) with his classic three volume work, first published from 1830 to 1833, entitled Principles of Geology: being an attempt to explain the former changes of the Earth’s surface by reference to causes now in operation (Levin, 1999, p. 9).

The science of stratigraphy changed humans’ view of the world from one, which was static to one that was dynamic and changing. Not only did the rock layers indicate changing environments they also revealed that different life forms have existed in different times. 

Absolute Time Scales: Early Strategies

Relative dating provided a history of life on Earth, a history that clearly showed changes in geology, climate, and life. Today radiometric dating places absolute dates on the relative time scale. There were many early attempts at establishing an absolute time scale before the use of radiometric dating. Some of these efforts were biblically based while others represented non-religious estimates. We will survey the work of specific individuals to explore a variety of strategies.

Biblical Chronology
James Ussher (1581-1656) Archbishop of Armagh (Ireland) developed a chronology, entitled Annalium pars postierior in 1654, based upon the life spans of people in the Bible and other ancient documents. Ussher calculated the date of creation to have been nightfall preceding October 23, 4004 BC. The 9:00 a.m. time of creation often attributed to Ussher was actually from previous work by John Lightfoot (1602-1675) a distinguished biblical and Greek scholar (Dalrymple, 1991, p. 21).

Decline in Sea Level
Benoit de Maillet (1656-1738) was a French anthropologist and diplomat. de Maillet estimated the Earth was 2 billion years old. de Maillet's estimate was based upon declining sea levels. Fossils of ocean organisms on land and in the mountains seemed to support that the Earth was once covered by one large ocean. As this ocean evaporated, water levels declined. de Maillet used measured sea level changes to estimate when the Earth was completely covered with water. This theory was based on Descartes theory on the birth and death of the sun and planets. de Maillet's estimate was presented in a fictional conversation between a French missionary and an Indian philosopher Telliamed (de Maillet spelled backwards). de Maillet never published his work for fear of repercussions. Telliamed was published by Abbé J.B. le Mascrier in 1748. le Mascrier did not publish under his real name and changed the estimate from millions to billions. Telliamed was not published in its original format until 1968. Although de Maillet’s formulations were made without knowledge of geological uplift they were important because they represented logical extrapolations from measurements taken from nature. Furthermore, de Maillet’s method excluded human life spans as a measurement (Dalrymple 1991, p.27). The dynamic nature of Earth’s surface was unknown to Maillet, but even during his time it was known that sea levels had dropped in some areas and were rising in others. Sea level change was quickly abandoned as an Earth dating method.

Cooling of Earth
Georges-Louis Lecterc, Comte de Buffon (1707-1788) was a French naturalist. Buffon calculated the world to be much older than Ussher. In his 1778 publication Epochs of Nature Buffon calculated the age of the Earth to be 75,000 years. Buffon arrived at this age by heating small iron spheres and scaling their cooling rates to an earth-sized mass. Sir Isaac Newton was the first to suggest using this strategy to estimate the cooling rate of Earth. The Catholic Church in France condemned Buffon for his calculation and burned his books. Lord Kelvin would later greatly increase the precision of Buffon’s crude methods (Burchfield 1990, p. 34). Kelvin’s methods were mathematically elegant and deductively sound; however, without the knowledge of energy created from radioactive decay his premises and thus his conclusions were wrong. Cooling of the Earth has been abandoned as a method to determine Earth’s age.

Cooling of Sun
Hermann Ludwig Ferdinand von Helmholtz (1821-1894) was a German physician and physicist. Helmholtz used the cooling of the sun to estimate Earth's age. In 1856 Helmoltz calculated that it would take the Sun 22 million years to condense down to its current diameter and temperature from a nebula of dust and gas. He assumed the heat of the sun was generated from gravitational contraction. Lord Kelvin’s version of the Helmholtz model included heat generated by meteoric impacts. The gravitational potential energy of the meteor would be converted to kinetic energy and upon collision into heat energy. Although meteoric impact was not supported by the observations made by astronomers Kelvin could see no alternative to gravitational potential energy (Bachall, 2000). Kelvin’s calculations indicated that even the most vigorous chemical reactions could not account for the known age of human civilization, thus chemical reactions could not be a heat source for the sun. With the discovery of nuclear reactions the model developed by Helmholtz and Kelvin became obsolete.

Weathering & Erosion
Charles Darwin speculated on the weathering and erosion of the Weald Chalk formations in his book The Origin of Species by Means of Natural Selection. Chalk formations in North and South Downs at one time extended over the sandstone and clay layers of the High and Low Weald forming a dome. This chalk layer was eroded and now only exits at the edges. Darwin sets up his scientific intuition from his personal geologic field observations. Darwin asks the reader to imagine the immense time needed to weather, erode, and deposit rock. Darwin turns attention to the chalk formations of the Weald and, with what he argues are reasonable rates of weathering and erosion, concludes the denudation of the Weald required 300,000,000 million years (Darwin 1859/2004, pp. 230-232). Although not an estimate for the age of Earth, Darwin’s reasoning represents an interesting attempt at using denudation (exposing of rock strata through erosion) to date a rock formation. Kelvin was critical of Darwin’s estimate when writing on the age of the Sun though cooling. Kelvin believed that the Sun could not have existed for the time required by Darwin’s theory; he was working without knowledge of nuclear energy. Darwin was so shaken by Kelvin’s criticism that he removed his estimate in later editions of the Origin of Species (Bachall 2000). Kelvin’s impressive mathematical models based on physics also influenced the science of geology in both positive and negative ways. The limits Kelvin’s quantitative methods placed on Earth’s age had the unfortunate effect of making uniformatarianism less tenable. Due to Kelvin’s work the science of geology became more quantitative and geologists and physicists interacted with each other (Burchfield 1990, pp. 10-12). It was clear to both sides that scientific hypotheses and theories must be internally coherent and externally consistent with findings in other fields of science.

Sediment Accumulation
John Phillips (1800-1874) was an English geologist. Phillips came under the charge of his uncle William Smith (the father of modern geology) when his parents died. In 1860 Phillips utilized sediment accumulation to estimate the Earth's age at 38 to 96 million years. As a method of determining the age of the Earth, sediment accumulation involved comparing measured rates of continental erosion with the aggregate thickness of sedimentary rock layers from successive time divisions. Charles D. Walcott (1850-1927) an American geologist and paleontologist is probably best known for his discovery and subsequent work on fossils of the Burgess Shale formation of British Columbia, Canada. Walcott made the most detailed model of sediment accumulation (Dalrymple 1991, pp. 59-66). Walcott’s estimate was between 35 and 80 million years. Many uncertainties plagued this method of age determination. These uncertainties included the relationship between areas of erosion and areas of deposition, the rates of sedimentation, and the missing time represented by unconformities, etc. (Burchfield 1990, pp. 16-18). Furthermore, this method could only attempt to reach back to the Cambrium period. These rock layers did not reveal the majority of Earth’s history (Dalrymple 1991, pp 68-69). Sediment accumulation was eventually abandoned as a method to determine the age of the Earth.

Tidal Effects
William Thompson, Lord Kelvin (1824-1907) was an Irish mathematician, physicist and engineer. Between the years of 1862 and 1897 Kelvin used multiple methods to calculate the age of the Earth. These methods included cooling of the Earth, Sun, and Earth tidal effects. Immanuael Kant (1724-1804) a German philosopher was the first to recognize the breaking effect of the tides on Earth’s rotation. Kelvin was the first to Estimating Earth's age using tidal effects (Dalrymple, 1991, p. 48). As the tides rise and fall gravitational friction causes a reduction in the Earth’s rotation, Moons orbital velocity and an increase in the Moon’s distance from Earth. Kelvin was the first to show that a transfer of angular momentum from the Earth to the Moon caused the Moon to recede from the Earth. Kelvin's estimates using these different methods ranged from 10 million to less than 1 billion years. The 1 billion year figure was arrived at using tidal effects. George H. Darwin (1845-1912) a mathematical astronomer was the son of Charles Darwin. G.H. Darwin refined the method of determining Earth’s age using tidal effects to a high degree (Dalrymple 1991, pp 47-52). Darwin’s minimum age for the Earth using this method was 56 million years. Darwin believed the Earth was most likely much older. Using the Moon’s recession rate from the Earth due to tidal effects did not become an accurate method for determining Earth’s age until the movement of the continents described by Plate Tectonic theory became part of the mathematical model during the 1960’s. The current model is supported by paleontological evidence in the form of tidal rhythmites or tidally laminated sediments (Thompson, 1999). The current age of the Earth using this model is in agreement with radiometric dating.

Salt Accumulation in Oceans
Edmund Halley (1656-1742) was an English Astronomer. In 1715 Halley hypothesized that the age of the Earth could be calculated by determining the rate at which salt is added to the ocean through erosion. Halley recommended that salt concentrations be measured on a regular basis to establish the rate at which they were added so that future scientists could use his method. Thomas Mellard Reade (1832-1909) an English geologist was the first to apply Halley’s suggestion of using the oceans as a sort of “salt clock” in 1876. Instead of using changes in salt concentrations over time as suggested by Halley, Reade used estimates for the amount of salts added by erosion. Reade knew the estimated amounts of chloride and sulfate salts added to the oceans from the major rivers of the world. He used this data to determine how long it would take the ocean to reach its present salt concentrations. His estimate was 25 million years for sulfates and 200 million years for chlorides (Dalyrumple 1991, p. 53). Reade called his salt clock model “chemical denundation”. John Joly (1857-1933) an English geologist would refine Reade’s model. Jolly used sodium concentrations as his salt clock because he believed that sodium is added, but not withdrawn from the oceans. Jolly’s estimate for Earth’s age was 90 to 100 million years. Today we know that the salts are not only added but also removed from the ocean. Salt accumulation in the oceans was abandoned as a way to determine Earth’s age.

Eccentricity of Mercury’s Orbit
In 1918 Harold Jeffreys (1891-1989), British Astronomer and Geophysicist, used the changing eccentricity of Mercury's orbit over time to estimate the age of our solar system to be 3 billion years (Dalrymple, 1991, p. 17). All of the above methods would become obsolete with the discovery of radioactivity.

Radiometric Dating

Early attempts at establishing an absolute time scale utilized the following concepts: declining sea levels, cooling of the Earth, cooling of the Sun, Earth tidal effects, sediment accumulation, and changes in ocean salinity. Using changes in Earth's temperature or the Sun's use of energy failed because energy from nuclear reactions was unknown. Once this energy was discovered a new, successful strategy called radiometric dating would be developed. Today radiometric dating places absolute dates on the relative time scale.

Radioactive Decay Discovered

Wilhelm Konrad Rontgen (1845-1923) published a report in 1895, which described his discovery of a mysterious source of energy emitted by a cathode ray tube. This source of energy caused barium platinocyanide coated on paper to luminance. Furthermore, this energy could penetrate cardboard and even walls to fluoresce barium platinocyanide. Rontgen used the mathematical symbol for unknown to name this energy X-rays. The interest in Rontgen’s discovery would launch the second scientific revolution; the first being launched by the work of Galileo (Asimov, 1984, pp 514-515). In 1896 Rontgen demonstrated the use of X-rays to view skeletal structure by taking a picture of the German biologist Rudolf Albert von Kolliker’s (1817-1905) hand.

The French physicist Antoine Henri Becquerel (1852-1908) wondered if X rays might be among the energy given off by fluorescent materials. In 1896 Becquerel discovered that potassium uranyl sulfate (K2UO2 (SO4)2 did create energy that penetrated black paper to expose photographic film. To his surprise the compound could do this even when it was not fluorescing. This energy did not only have penetrating power it also ionized the air as shown by an electroscope.

In 1898 the Polish-French physicist team of Marie Sklodowska Curie (1867-1934) and her husband Pierre (1859-1906) discovered that thorium also gave off radiation. The phenomenon of emitting penetrating, ionizing radiation was given the name radioactivity by the Curies. The Curies found that the radiation was proportional to the Uranium content of the compound. This indicated that radioactivity was a property of atoms, not molecules. They also discovered that compounds containing U and Th emitted more radiation than either element by itself. The Curies deduced that other radioactive elements must be present, which led to their discovery of the elements polonium and radium (Dalrymple, 1991, p 69).

In 1902 the New Zealander-English physicist Ernest Rutherford (1871-1937) and the English chemist Frederick Soddy (1877-1956) published results from their experiments with radioactivity. Rutherford and Soddy made four important observations. First, the decay of what we now call the parent isotope is exponential. Furthermore, the formation of the daughter isotope is exponential and inverse to the parent decay. Second, atoms of radioactive elements are unstable and spontaneously decay to other elements by the emission of alpha and beta particles. Thus the transmutation medieval alchemist desired was in fact being done by nature continuously. Third, radioactive decay is proportional to the number of atoms present. Finally, helium might be a product of radioactive decay (Dalrymple, 1991, pp 70-71). In 1905 Rutherford suggested that radioactive decay could be used as a method to calculate absolute time.

Radioactive Decay & Chemical Ages

Early attempts at using radiometric dating are referred to as “chemical ages”. They were done without knowledge of isotopes or the decay rates and intermediate products of Uranium. Furthermore, the decay of Thorium to lead was unknown. Even so, early work done by Bertram Borden Boltwood (1870-1927), an American chemist and physicist demonstrated several important consistencies. First, the same Pb/U ratios are obtained in samples from the same geologic age. Second, Pb/U ratios are a function of geologic age. Third, altered samples have lower Pb/U ratios. Finally, as Arthur Holmes (1890-1965), a British geologist pointed out, ages obtained using these new methods were often in agreement with what many geologists had speculated (Dalrymple, 1991, pp. 72-74). The fact that this new method of dating utilized empirical evidence gathered in the field and analyzed in the lab with methods based upon experimentally determined physical principles made it promising and powerful. It soon became clear that the Earth was much older than many of the earlier methods indicated. With time, radiometric dating became more sophisticated and accurate, by 1931 radiometric dating had proven to be the credible method for determining the age of igneous rocks. For a good discussion on modern radiometric methods see The Age of the Earth by Dalrymple.

Radiometric Vocabulary Terms

1. Radiometric Dating-Measuring the passage of time by the regular rate of decay of radioactive isotopes.

2. Isotopes-Same element, but different number of neutrons. There are 350 different isotopes. Some isotopes are stable and others are radioactive.

3. Parent Isotope-Radioactive isotope incorporated during crystal formation.

4. Daughter Isotope-Stable decay product of parent isotope.

5. Radioactive isotopes decay or change into a stable element at an exponential rate that does not change. The decay rate is not affected by heat, temperature, pressure, or chemical reactions.

6. Half-life-The time it takes for half of the parent sample to decay to the stable daughter isotope.

7. Examples:

a. U-238 to Pb-206 (4.5 billion years)
b. U-235 to Pb-207 (704 million years)
c. K-40 to Ar-40 (1.3 billion years)
d. C-14 to N-14 (5,730 years)

Correlation of Strata

Correlation of Rock Strata
Matching Rock Strata Formed from
Different Locations that were Formed at the Same Time

1. Index Fossils-Used to correlate rock sequences from different locations. Using index fossils it is possible to determine that two different rock types were formed at the same time. Index fossils have the following parameters:

a. Must have lived for a short geologic time span (geologically this could be millions of years).
b. Must have had a wide geographic distribution.
c. Must be easily identified.

2. Volcanic Ash Falls-many volcanoes can erupt explosively leaving a layer of ash over large areas. This rapid geological event can leave a single layer of ash over a large area making time correlation from different areas very accurate.


Nineteenth century geologists determined the relative ages of rock by studying changes in fossil assemblages. Geologist came to realize that rocks reveal an amazing story of life on Earth; different organisms have lived at different times and environments change. Furthermore, this history of life is irreversible. Thus, a relative time scale could be developed from boundaries defined by the appearance and disappearance of life forms as well as changes in rock types. The history of life on Earth was a revelation from the fossil record. This revelation would throw light on extinction and evolution. To learn more about human's evolving views of fossils and the patterns they reveal, visit the fossil section of our website.

Each period and epoch has an introduction that outlines some of the important biological patterns revealed by fossils. In addition to the introduction, galleries containing example fossils representative of the period or epoch may be explored. 
There has been an on-going debate as to whether or not the Quaternary should be included in the timescale. Many marine geologists have argued that the Quaternary is a climato-stratigraphic unit, not a chronostratigraphic unit defined by clear markers in the rock record. Geologists who find this time period useful have come to view the Quaternary and Pleistocene as marked by the start of global cooling and glaciation 2.6 million years ago.

In 2008 the International Commission on Stratigraphy (ICS) published a timescale with the Quaternary as a full period starting at 2.6 million years ago. The Pleistocene epoch was pushed back from 1.8 to 2.6 million years ago to align with the Quaternary. The International Union of Geological Sciences (IUGS) ratified the ICS recommendation in June 2009 (Gramling, 2009, p. 13). For now, the debate as to whether or not the Quaternary should be included in the timescale seems to be settled.


  1. Phanerozonoic, ERA:

    1. Cenozoic, PERIOD:​

      1. Quaternary, EPOCH:​

        1. Holocene​

        2. Pleistocene

      2. Neogene, EPOCH:

        1. Pliocene

        2. Miocene

      3. Paleogene, EPOCH:

        1. Oligocene

        2. Eocene

        3. Paleocene

    2. Mesozoic, PERIOD:

      1. Cretaceous

      2. Jurassic

      3. Triassic

    3. Paleozoic, PERIOD:

      1. Permian

      2. Carboniferous

      3. Devonian

      4. Silurian

      5. Ordovician

      6. Cambrian​

  2. Protezoroic

  3. Archean

  4. Hadean

The following websites can be used to explore geologic time in more detail. Each site will open as a separate window.

The Geological Society of America

ICS Website


Understanding Geologic Time


  • Asimov, I. (1984). The History of Physics. New York: Walker and Company.

  • Bachcall, J.N. (2000). How the Sun Shines. Published on see

  • Burchfield, J.D. (1990). Lord Kelvin and the Age of the Earth. Chicago: The University of Chicago Press.

  • Cutler, A. (2003). The Seashell on the Mountaintop. New York: Dutton.

  • Dalrymple, G. B. (1991). The Age of the Earth. Stanford California: Stanford University Press.

  • Dalrymple, G.B. (1991). The Age of the Earth. Stanford, California: Stanford University Press.

  • Darwin, C. (2004). The Origin of Species. New York: Fine Creative Media. (Originally published in 1859).

  • Geology of the Weald printed on see:

  • Gramling, C. (2009). A Matter of Time: The Quaternary's Back. Earth, Sept.

  • Levin, H.L. (1999). The Earth Through Time [6th Ed.]. New York: Harcourt Brace College Publishers.

  • McPhee, J. (1998). Annals of the Former World. New York: Farrar, Straus, & Giroux

  • Prothero, D. R. (1998). Bringing Fossils to Live: An Introduction to Paleobiology. New York: McGraw-Hill.

  • Thompson, T. (1999). The Recession of the Moon and the Age of the Earth-Moon System. The TalkOrigin Archieves see:

  • Winchester, S. (2002). The Map that Changed the World: William Smith and the Birth of Modern Geology. New York: Perennial.

bottom of page