Origin of Geologic Periods

The Origin of the Definition of the Geologic Periods
Despite the influence of the German school of the late 1700's to early 1800s, sedimentary and bio-stratigraphy made rapid advances in the British Isles. Here industrialization and the associated mining of coal and iron ores, and the building of canals spurred on a closer examination and mapping of the sedimentary and section. At least three men became catalysts for the great advances made in geology at this time. These men and their advances were

1) John Strachey, who in 1719 constructed a geological cross-section of the coal bearing strata he mined beneath his property in Somerset (Fenton and Fenton, 1952, Mintz 1972, Dott and Batton 1976, and Fuller, 1995);

2)  William Smith who surveyed that same property in 1791 to 92, and began to map the sedimentary strata of the Cotswold Hills in order to build a canal to exploit these coal reserves (Fuller 1995); and

3)  James Hutton, who in 1785 pioneered the principles of the uniformitarianism, (processes we see operating on the earth today also operated in the past), and the concept that the earth is in dynamic and constant change (Fenton and Fenton, 1952, Dott and Batton 1976).

William Smith, through his activities as a surveyor went on to draw a geological map of the whole of England. He mapped the sedimentary strata of this country on the basis of their lithology, their fossil content, and "erosional" breaks. However like many of his contemporaries his ideas on the origins of the sediments and their fauna involved the great flood of Noah, and a confused and misled lack of understanding of their relationship to time. Nevertheless there is no doubt that the work of Strachey and Smith were the major inspiration for the seminal ideas of Lyell (1 & 2), Sedgwick, Murchison and Lapworth who all formalized and synthesized models to explain accumulations of sedimentary rocks and used fossils determine their relative ages. Smith's use of lithology and fossils for correlation; Hutton's recognition unconformities; and Lyell, Sedgwick, Murchison and Lapworth's use of fossils to establish the relative age of sedimentary sequences; all lead to the establishment of a relative geologic time scale (see the table below compiled from Mintz (1972)).

  • Sedgwick proposed the Cambrian in 1835
  • Lapworth proposed the Ordovician in 1879
  • Murchison proposed the Silurian in 1835
  • Murchison and Sedgwich proposed the Devonian in 1839
  • Coneybeare and Phillips proposed the Carboniferous in 1822
  • Murchison proposed the Permian in 1841
  • Von Alberti proposed the Triassic in 1834
  • Von Humboldt proposed the Jurassic in 1799
  • D'Halloy proposed the Cretaceous in 1841
  • Aduino proposed the Tertiary in 1760
  • Denoyers proposed the Quaternary in 1829.

This pioneer work in geology influenced Charles Darwin, who used Lyell's descriptions of fossil successions in The Origin of Species (1859). Interestingly this followed the naming of the Geologic Periods. Further Charles Darwin and Alfred Wallace were encouraged by Lyell to read their papers on their parallel ideas on evolution at a meeting of the Linnaean Society of London (Dott and Batton 1976). In 1869, inspired by Darwin, Waagen and Karpinsky traced the evolution in a group of ammonoids (Dott and Batton 1976) and laid the foundation for many of the concepts that are currently used today by geologists to date the stratigraphic section with bio-stratigraphy, and understand the cyclic and punctuated character of the sedimentary record. From this paleontologists, like Van Hinte (Van Hinte 1976a &b, 1978 and 1979), have built time scales tied to radiometric dates and magnetic reversals. Further, building from this, De Graciansky et al (1998), and a large group of contributors, have written a massive book containing papers and charts on the Mesozoic and Cenozoic sequence stratigraphy of European basins. Finally the ultimate tool is that of Shaw's technique of graphic correlation, which is used to track the first occurrence and last occurrence of fauna.


Sunday, May 15, 2016
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