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Figure 2. Maps of the area around the city of Oxford showing the evolution of William Smith’s geological  1770s, Guettard and Lavoisier published a      GSA TODAY | www.geosociety.org/gsatoday
mapping and a comparison with the present day geological interpretation simplified from work by the       series of mineralogical maps of north-
British Geological Survey. (Extracts of William Smith county maps and the William Smith A map are         eastern France showing point locations of
published by kind permission of the Geological Society of London and the P map by kind permission of the  rocks and minerals, but no attempt was
Manuscripts and Special Collections, The University of Nottingham.)                                       made to map strata. Later, in 1809, the
                                                                                                          Scottish-American William Maclure
                                                                                                          (1763–1840) published a geological map
                                                                                                          of the United States that pre-dated
                                                                                                          Smith’s map by six years. Maclure was
                                                                                                          strongly influenced by the work of
                                                                                                          Abraham Gottlob Werner (1749–1817),
                                                                                                          who had developed a theory of universal
                                                                                                          rock genesis based on precipitation and
                                                                                                          erosion within a receding primordial
                                                                                                          ocean (later known as Neptunism). On
                                                                                                          his 1809 map, Maclure used Werner’s rock
                                                                                                          classification, which ultimately proved to
                                                                                                          be a geological cul-de-sac. Maclure did
                                                                                                          meet Smith in 1815 and purchased a copy
                                                                                                          of his map, yet he completely failed to
                                                                                                          understand the importance of Smith’s
                                                                                                          work (Torrens, 2001), for on his 1817
                                                                                                          version of the USA map, the classification
                                                                                                          is essentially the same as the one he had
                                                                                                          used in 1809.

                                                                                                            It is not known whether Smith was
                                                                                                          aware of Werner’s work, but if so, he was
                                                                                                          most certainly not influenced by it. Smith
                                                                                                          realized that an understanding of the
                                                                                                          “ordering of strata” was essential in
                                                                                                          geological mapping, and it was the applica-
                                                                                                          tion of his stratigraphic method that was
                                                                                                          so geologically significant. Smith first
                                                                                                          became interested in this ordering when
                                                                                                          employed as a surveyor on the Somerset
                                                                                                          Coal Canal in 1795. Through detailed
                                                                                                          study of canal sections, he managed to
                                                                                                          separate several repetitious clay formations
                                                                                                          and also to separate the Upper and Lower
                                                                                                          Oolite (Torrens, 2003, p. 161). By August
                                                                                                          1797, Smith had made his first attempt at a
                                                                                                          more general order of strata, starting with
                                                                                                          Number 1 “Chalk Strata” and descending
                                                                                                          to Number 28 “Limestone” below the Coal
                                                                                                          Measures. In June 1799, at the home of the
                                                                                                          Rev. Joseph Townsend, Smith dictated his
                                                                                                          famous “Order of the Strata in the Bath
                                                                                                          area” to the Rev. Benjamin Richardson
                                                                                                          (Phillips, 1844, p. 29) and during the
                                                                                                          course of several iterations it evolved into
                                                                                                          the geological table, part of which is shown
                                                                                                          in Figure 3.

                                                                                                            Like others before, Smith could recog-
                                                                                                          nize strata based on their lithology, some
                                                                                                          rocks (e.g., oolites) being very distinctive.
                                                                                                          However the problem with a purely litho-
                                                                                                          logical approach to stratigraphy can be the
                                                                                                          incorrect correlation of strata of differing
                                                                                                          age but with similar lithology. Smith,

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