Article: The graptolite assemblages and zones of the Brikhill Shales (Lower Silurian) at Dobb's Linn
The graptolite assemblages and lithologies of the Birkhill Shales (Lower Silurian) at Dobb's Linn are described. The condensed 141 ft. (43 m.) euxinic sequence is divided into eight graptolite zones in descending order: Rastrites maximus, Monograptus sedgwickii, M. convolutus, M. gregarius, M. cyphus, Cystograptus vesiculosus, Akidograptus acuminatus, and Glyptograptus persculptus. These zones are compared with the original work of Lapworth (1878), and with equivalent zones elsewhere in Great Britain. The base of the Middle Llandovery at the type locality for the graptolitic Llandovery (Valentian) is taken at the base of the gregarius Zone (triangulatus Zone in Wales), and not at the base of the magnus Zone, as is the current practice in Wales.