Late Quaternary river terrace development in the Macgillycuddy's Reeks, southwest Ireland

Abstract

Geomorphological, sedimentological, pedostratigraphical and palaeoecological analyses of fluvial and glacigenic deposits infilling the Gaddagh valley, Macgillycuddy's Reeks, southwest Ireland, are used to elucidate the patterns and controls of Late Quaternary valley floor development in a 3 km reach of the catchment lying immediately downvalley of two mid-basin lakes, Lough Gouragh and Lough Callee. A total of four Holocene alluvial terraces (Terrace Level, TL 2–5) and a modern floodplain (TL 6) have been identified and lie inset within a strath terrace comprising Late Quaternary (Midlandian) till (TL 1). The combination of mid-basin lakes and flanking strath terraces acted to decouple the Gaddagh channel from upper catchment and valley-side debris sources shortly after regional deglaciation. Postglacial fluvial terrace development in the study reach has therefore occurred primarily in response to hydrological changes and reworking of local valley floor deposits rather than catchment-wide debris supply changes. Pollen analyses of valley floor peat deposits interbedded with minerogenic alluvium indicate that anthropogenic disturbance of catchment tree cover had commenced by c.a 1690–1450 cal BC and that the catchment was largely cleared of woodland by later prehistoric times. Major phases of incision and alluviation represented by terraces TL 3–5, however, have been radiocarbon dated to the period after 250–530 cal AD and broadly correspond with Holocene fluvial records elsewhere in the region, upland Britain and northwest Europe. Although fluvial system responses to hydrological changes will have been promoted by anthropogenic disturbances of catchment vegetation cover, the timing of late Holocene incision and alluviation episodes are interpreted as reflecting enhanced flood frequencies and (or) magnitudes associated with late Holocene climatic shifts. In the Gaddagh Valley, however, we hypothesise that the headwater lakes Lough Gouragh and Lough Callee may also have exerted a critical control on the postglacial evolution of the valley floor. In particular, high-magnitude flooding is likely to have been instrumental in valley floor entrenchment and formation of TL 3, TL 4 and TL 5 terraces.