New Biostratigraphic Evidence of the Post-Glacial Colonization of Ireland and for Mesolithic Forest Disturbance

Abstract

Recent road works have exposed a thin but extensive deposit of calcareous tufa resting on till (Dublin Formation) at Newlands Cross near Tallaght, County Dublin. Close serial sampling and quantita- tive analyses of the contained fossils (non-marine Mollusca, ostracods and vertebrates) have been undertaken. The mollusc data reveal successional patterns that can be directly related to the vegetational history of the area by means of pollen analyses. A chronology for the more important biostratigraphic changes has been provided by radiocarbon dating using accelerator mass spectrometry. Thus the onset of tufa formation is dated at 9720?300 BP (OxA-569) during a Gramineae-Juniperus-Betula pollen assemblage zone. An open-country mollusc assemblage including several arctic-alpine species, e.g. Vertigo genesii, V. geyeri and notably Columella columella (new to the Irish Quaternary) existed at this time. This fauna was gradually replaced by a series of shade-demanding snails during the period of Corylus expansion. A thin buried soil horizon represents a brief period of retarded tufa growth when the land surface became relatively dry. A fresh flint-flake was recovered from this surface, which also marks the main expansion of the shade-demanding snail, Discus rotundatus. A radiocar- bon date of 7600?500 BP (OxA-568) was obtained from the fruits of Eupatorium cannabinum from the tufa immediately overlying this soil. The presence of Mesolithic man in the area is unequivocally demonstrated. Teeth of woodmouse (Apodemus sylvaticus) at this level confirm the native status of this species in Ireland. Other biostratigraphic changes in the upper levels of the tufa are almost certainly attributable to anthropogenic disturbance involving some forest clearance. Woodland conditions are again evident towards the top of the sequence and tufa formation, at this point, probably ceased by -7000 BP. This sequence, the most detailed yet reported from Ireland, is compared with equivalent sites in Britain and elsewhere and the biogeographic questions posed as regards the possible existence and timing of a landbridge connection are discussed.