Stratigraphy is the study of rock strata or layers and is concerned with aspects of the rock layers such as their succession, age relationships, lithologic composition, geometry, distribution, correlation, fossil content, and environments of deposition. The main aim of stratigraphy is to understand and interpret the rock record in terms of paleoenvironments, mode of origin of the rocks, and the causes of similarities and differences between different stratigraphic units. These units can then be compared across regions, continents, and oceans to obtain an understanding of the conditions on Earth at the time the rocks were deposited.
The most basic unit of stratigraphy is the formation, a distinctive series of strata that originated through the same formative processes. Formations must be distinctive in appearance and easily recognizable. They can be recognized on the basis of lithology, which consists of the composition of the mineral grains, color, texture of grains, thickness and geometry of stratification, character of organic remains (fossils), and outcrop character. According to the code of stratigraphy, there is a hierarchy of naming different layers in rocks. A single layer is called a stratum, and the many layers within a formation are called strata. Groups consist of several related formations, whereas systems of strata consist of several groups of strata.
Early stratigraphers tended to regard Earth history and the stratigraphic record in a simplistic way. Most believed that the stratigraphic divisions being named in Europe were present and of the same age worldwide, developing a model of the Earth that was like a layered cake, with uniform stratigraphic layers around the globe. Gradually it became recognized that there may be some lateral variations between formations. For instance, in 1789 the French polymath, chemist, geologist, farmer, and engineer Antoine Lavoisier (1743-1794) suggested that similarities of fossils in similar sedimentary rocks might reflect environmental factors more than age. Lavoisier showed that shallow, near-shore marine sediments are coarser and contain organisms adapted to rough water, whereas deeper marine, quiet-water sediments are finer and contain delicate bottom-dwelling organisms, as well as floaters and swimmers. Lavoisier showed that different sedimentary products may form in different environments and have different groups of fossils, even though they formed at the same time. Five years later Lavoisier was beheaded during the French revolution, since he was one of 28 tax collectors and branded a traitor.
In the 1830s the British geologists Adam Sedgwick (1785-1873) and Sir Roderick Impey Mur-chison (1792-1871) also found that in Britain, the nonmarine Old Red Sandstone was laterally equivalent, in part, with marine sandstones. These two famous geologists found that the marine and nonmarine sandstones had an interfingering relationship, meaning one sediment type or formation grades laterally into another.
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