A revised lithostratigraphic subdivision of the Dinantian rocks at the southern end of the Isle of Man is presented. Resting on the Cambro-Ordovician Manx Slate Group is the Langness Conglomerate, representing a locally-derived alluvial fan deposit. The succeeding Derbyhaven and Castletown Formations are both thinly-bedded, fine-grained dark limestones with variable amounts of shale partings; the Derbyhaven Formation has yielded fossils suggesting an early Arundian to Holkerian age, whilst the Castletown Formation appears to be mostly of early Asbian age. The latter is diachronously overlain by the thick Balladoole Formation, composed of lime-mud mounds, of Asbian age. The Poyllvaaish Formation is of similar mudmounds and beds, with original depositional dips of up to 20° and yielding late Asbian to early Brigantian faunas. The penecontemporary Close-ny-Chollagh Formation consists of black limestones and shales enclosing an unusual complex of slipped and foundered blocks from the upper Poyllvaaish limestones, with considerable distortion of the black beds around the blocks. The Scarlett Volcanic Formation consists of subaqueous volcanic breccias, vesicular and pillow basaltic lavas and fine-grained tuffs, with local intercalations of black limestones of early to mid-Brigantian age. Disruption surfaces and contortions, previously thought to be due to thrust-faulting, are now regarded as contemporary mass-flow and eruptive disturbances.The Carboniferous Limestone and associated volcanics near Castletown on the south coast of the Isle of Man are magnificently exposed along some 10 km of foreshore and low cliffs. Inland there are few outcrops owing to widespread glacial deposits on low-lying ground which barely reaches 60 m above sea level.
Historically, four workers, Cumming (1846), Lamplugh (1903), Hind (1907) and Lewis (1930), developed the stratigraphic framework of the Castletown area. Lamplugh (1903, p. 189) followed the divisions recognised by Cumming (1846):-(1) Basement Conglomerate, (2) Castletown or Dark Limestones, (3) Poolvash or Pale Limestones, (4) Posidonomya Beds and (5) Volcanic Series of Scarlett.He presented a description of the coastal outcrops, summarized earlier work and included a detailed bibliography. The three-fold division of the limestone sequence used by Lamplugh (after Cumming) was modified by Lewis (1930) who recognised nine divisions (Fig. 1). Lewis's major contributions were the presentation of the first detailed map of the area (Lewis 1930, plate 25) together with palaeontological work and a suggested correlation of the limestone beds with those of mainland Britain. Lewis proposed a dual lithological and palaeontological subdivision which correlates well in the upper part of the sequence, though lack of correspondence between the two leads to confusion in the lower part. Lewis's lithological sequences (1930, figs. 1 and 2) were presented in a somewhat generalized manner without designation of type sections. Our remapping (largely based on the first author's Ph.D. thesis, Dickson 196...
