The oldest fossiliferous rocks in Britain are found in North Wales. Cambria is the medieval Latin name for Wales, so these rocks were named Cambrian. When rocks older than the Cambrian were discovered, they were labelled Precambrian.
| Superion | Eon | mya | Life forms | ||
| Precambrian | Hadean | 4600 | Earth molten | ||
| Archaean | 4000 | stromatolites | |||
| Proterozoic | 2500 | eukaryotes | |||
| Eon | Era | Period | mya | Life forms | |
| Phanerozoic | Palaeozoic | Cambrian | 541 | invertebrates | |
| Ordovician | 485 | land plants | |||
| Silurian | 444 | jawed fish | |||
| Devonian | 419 | amphibians | |||
| Carboniferous | 359 | reptiles | |||
| Permian | 299 | seed plants | |||
| Mesozoic | Triassic | 252 | mammals | ||
| Jurassic | 201 | dinosaurs | |||
| Cretaceous | 145 | birds | |||
| Era | Period | Epoch | mya | Life forms | |
| Cenozoic | Neogene | Palaeocene | 66.0 | mammals | |
| Eocene | 56.0 | modern mammals | |||
| Oligocene | 33.9 | new mammals | |||
| Miocene | 23.0 | more mammals | |||
| Pliocene | 5.33 | hominids | |||
| Quaternary | Pleistocene | 2.58 | humans | ||
| Holocene | 0.012 | civilisation | |||
Fossils
Fossils are the remains (bones, shells) or traces (footprints, bite marks, droppings) of organisms preserved in rocks. A fossil is formed when a dead plant or creature is accidentally buried in sediment. When a burial does not occur, the normal process of decay or scavenging returns a dead organism’s bodily chemicals to the environment in which it had lived.
Sediment is the natural product of rock weathering, and it is carried by rivers to the bottoms of lakes and seas. Organisms living on the lake or sea floor, or washed there after death, are constantly showered with fine sediment that will eventually bury anything that does not move. Because land animals have little chance of being buried, the fossil record is strongly biased towards sea creatures.
Relative Dating
Important in archaeology is the ordering of things into sequences. Stratification is based on the principle that the top layer of a bed or stratum is usually the most recent and the succeeding layers are progressively older. It follows that:
(a) A feature that cuts across or into a bed or stratum must be younger than that bed or stratum;
(b) Material from an older bed may be incorporated in a younger bed but not vice versa; and
(c) Strata may be correlated based on the sequence and the uniqueness of their floral and faunal content.
When we observe the things around us we can mentally arrange them into some form of chronological sequence: some things look older than others. An artefact such as a pot can be described by its material, shape and decoration. Pots with the same attributes constitute a pot-type and typology groups artefacts into such types. As the change in the style of artefacts can be gradual it is reasonable to put similar artefacts together to obtain a chronological sequence. The process of determining this sequence is called seriation.
Absolute Dating
All absolute dating methods are based on a permanent time-dependent reference, for example, Earth’s yearly orbit of the Sun. In 1878 Baron Gerard Jacob De Geer (1858-1943) noticed that the appearance of certain layers of clay was similar to that of tree rings. He called these annual sedimentary layers varves and presented an outline of his discovery in 1882.
Tree-ring dating, or dendrochronology, was developed by Andrew Ellicott Douglass (1867-1962) in the early decades of the twentieth century, although it had been known long before then. Each year, trees add a ring of growth, and the age of living trees is determined by counting their growth rings (it is not necessary to fell the tree, a sample is extracted by boring a hole). The amount a tree grows each year is affected by fluctuations in climate, for example, in arid regions rainfall above average will produce a thicker annual ring. By finding older and younger trees that overlap in age, a relative chronology can be constructed to which the tree-ring pattern of timber can be compared. The longest sequence to date is based on the ‘bristlecone pine’ and extends back some 5000 years.
The radiocarbon technique can provide reliable ages back to 50,000 years ago (50 kya), and under extreme conditions back to about 75 kya. Carbon-14 (14C), a radioactive isotope produced in the atmosphere, is absorbed by plants during photosynthesis and passed on to animals that feed on those plants. In living organisms any 14C lost is replaced and the amount remains constant but at death no more 14C is absorbed and the amount declines. Because the rate of loss is known it is possible to measure how long ago an organism died by determining how much 14C is left.
Practically all natural materials contain small amounts of radioactive isotopes. These decay at a known rate and the particles are so energetic that when they collide with an atom they can remove an electron. In some crystalline materials such as ceramics the lattice contains defects that can trap and hold these electrons and over time more and more energy is trapped in the form of these electrons. If heated rapidly to above 500°C the electrons are freed and lose their energy as a flash of light known as thermoluminescence. The amount of light emitted is proportional to the number of electrons released, which in turn is proportional to the time that has elapsed since the last time the material was heated (dosimetry). In principle this method can date inorganic material before 50 kya but it is less precise than radiocarbon.
Potassium-argon dating is based on the decay of the radioactive isotope potassium-40 to the inert gas argon-40 in volcanic rock. The radioactive clock is set by the formation of the rock through volcanic activity, which drives off any argon formally present. This method is applied to volcanic rock at least 100,000 years old. It is used to date early human sites in Africa, which can be up to five million years old.
Uranium series dating is based on the radioactive decay of the isotopes uranium-238 and the less abundant uranium-235 into daughter elements. Parent isotopes are soluble in water, whereas the daughter isotopes are not. When the waters precipitate on cave walls and floors, daughter elements are produced and the radioactive clock is set going. Dates using this method stretch from thousands of years to 350 kya.
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