ARENIG GROUP

arenig group, in geology, the name now applied by british geologists to the lowest stage of the ordovician system in britain. the term was first used by adam sedgwick in 1847 with reference to the "arenig ashes and porphyries" in the neighbourhood of arenig fawr, in merioneth, north wales. the rock-succession in the arenig district has been recognized by w.g. fearnsides ("on the geology of arenig fawr and moel llanfnant," _q.j.g.s._ vol. lxi., 1905, pp. 608-640, with maps) as follows:-- ordovician caradoc / _dicranograptus_--shales. \ defrel or _orthis_--limestone. / rhyolitic ashes = upper \ upper ashes | massive ashes = middle > of llandeilo < acid andesitic ashes = lower / arenig. group | daerfawr shales. zone of _didymograptus murchisoni_. | platy ashes \ lower ashes of arenig \ great agglomerate / (hypersthene andesites). / olchfa or _bifidus--shales_ (_didymograptus bifidus_). | filltirgerig or _hirundo_ beds \ _didymograptus arenig < erewnt or _ogygia_--limestone / hirundo._ group | henllan or _calymene_--ashes \ _didymograptus | llyfnant or _extensus_ flags / extensus._ \ basal grit \~~~~~~~~~~~~~~~~~~~~~~~/ (unconformity) the above succession is divisible into: (1) a lower series of gritty and calcareous sediments, the "arenig series," as it is now understood; (2) a middle series, mainly volcanic, with shales, the "llandeilo series"; and (3) the shales and limestones of the bala or caradoc stage. it was to the middle series (2) that sedgwick first applied the term "arenig." in the typical region and in north wales generally the arenig series appears to be unconformable upon the cambrian rocks; this is not the case in south wales. the arenig series is represented in north wales by the garth grit and ty-obry beds, by the shelve series of the corndon district, the skiddaw slates of the lake district, the ballantrae group of ayrshire, and by the ribband series of slates and shales in wicklow and wexford. it may be mentioned here that the "llanvirn" series of h. hicks was equivalent to the bifidus-shales and the lower llandeilo series. references.-adam sedgwick, _synopsis of the classification of the british palaeozoic rocks_ (1885); sir a. ramsay, "north wales," _geol. survey memoir_, vol. iii.; c. lapworth, _ann. mag. nat, hist._ vol. vi., 1880; g.a.j. cole and c.v. jennings, _q.j.g.s._ vol. xlv., 1889; c.v. jennings and g.j. williams, _ibid._ vol. xlvii., 1891; messrs crosfield and skeat, _ibid_. vol. lii., 1896; g.l. elles, _geol. mag._, 1904; j.e. marr and t. roberts, _q.j.g.s._, 1885; h. hicks, _ibid._ vol. xxxi., 1875. see also ordovician. (j. a. h.)