Editing Cambrian (section)

== Etymology and history ==
The term ''Cambrian'' is derived from the Latin version of ''[[Cymru]]'', the Welsh name for Wales, where rocks of this age were first studied. It was named by [[Adam Sedgwick]] in 1835, who divided it into three groups; the Lower, Middle, and Upper.<ref name="Peng-2020" /> He defined the boundary between the Cambrian and the overlying Silurian, together with [[Roderick Murchison]], in their joint paper "''On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales''". This early agreement did not last.<ref name="Davidson-2021">{{Cite book |last=Davidson |first=Nick |title=The Greywacke |publisher=Profile Books Ltd |year=2021 |isbn=9781788163781 |edition=1st |location=London}}</ref>

Due to the scarcity of fossils, Sedgwick used rock types to identify Cambrian strata. He was also slow in publishing further work. The clear fossil record of the Silurian, however, allowed Murchison to correlate rocks of a similar age across Europe and Russia, and on these he published extensively. As increasing numbers of fossils were identified in older rocks, he extended the base of the Silurian downwards into the Sedgwick's "Upper Cambrian", claiming all fossilised strata for "his" Silurian series. Matters were complicated further when, in 1852, fieldwork carried out by Sedgwick and others revealed an unconformity within the Silurian, with a clear difference in fauna between the two.<ref name="Sedgwick1852">{{cite journal |last=Sedgwick |first=A. |year=1852 |title=On the classification and nomenclature of the Lower Paleozoic rocks of England and Wales |url=https://zenodo.org/record/2432137 |journal=Q. J. Geol. Soc. Lond. |volume=8 |issue=1–2 |pages=136–138 |doi=10.1144/GSL.JGS.1852.008.01-02.20 |bibcode=1852QJGS....8..136S |s2cid=130896939}}</ref><ref name="Davidson-2021" /> This allowed Sedgwick to now claim a large section of the Silurian for "his" Cambrian and gave the Cambrian an identifiable fossil record. The dispute between the two geologists and their supporters, over the boundary between the Cambrian and Silurian, would extend beyond the life times of both Sedgwick and Murchison. It was not resolved until 1879, when [[Charles Lapworth]] proposed the disputed strata belong to its own system, which he named the Ordovician.<ref name="Davidson-2021" />

The term ''Cambrian'' for the oldest period of the Paleozoic was officially agreed in 1960, at the 21st [[International Union of Geological Sciences|International Geological Congress]]. It only includes Sedgwick's "Lower Cambrian series", but its base has been extended into much older rocks.<ref name="Peng-2020" />