Early Midlandian–Middle Midlandian, Pleistocene, Northern Ireland
| Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast. |
R A B Bazley
Early Midlandian
Fermanagh Stadial
The earliest glacigenic deposits in Northern Ireland probably date from the early part of the Midlandian Stage. They include glacial tills deposited by ice from an Irish centre and from a Scottish source. The latter moved onshore along the east and north coastal fringes of Northern Ireland as well as sweeping across Counties Down and Armagh. Evidence of ice flowing to the south and southwest from the Irish Sea and North Channel is found in the distribution of clasts of Cretaceous chalk and flint and in the concentration of distinctive erratics of riebeckite-microgranite derived from Ailsa Craig in the Firth of Clyde [1].
The slopes south of Lough Foyle, Co. Londonderry have three tills demonstrably one above the other. The basal two tills are probably from this early glaciation and the highest is from the late final glaciation. South of Ballykelly [C 612 191], at the base and in contact with bedrock, is the 2 m thick Lackagh Till [2]. Erratics indicate deposition from an Irish ice source to the south. The overlying Bovevagh Till is about 7m thick and is widely exposed between Eglinton and Limavady. This till is present up to a height of 200 m above sea level and erratics show that it was deposited by an ice sheet moving southwards from Scotland. It is a brown, calcareous lodgement till with clasts of basalt, chalk, flint, Triassic and Carboniferous sandstone, and Dalradian lithologies. It also includes in some places grey mud with numerous shells of marine bivalves, gastropods and foraminifera that probably originated as offshore deposits picked-up by the ice sheet and transported inland. At some localities the glaciomarine deposits are distinct clasts within the till. However, at Sistrakeel [C 626 213] near Ballykelly, it has been suggested that the glaciomarine deposit lies beneath the Bovevagh Till and has not been moved. The section consists of 3 m of stratified diamict and cobble gravel and a 2 m thick mud unit. The mud contains a microfauna of foraminifera and ostracods of high boreal to arctic aspect and broken shells of marine molluscs that give amino acid dates around 90 000BP [3]. It is also now suggested that the type section of the Bovevagh Till at Bovevagh Old Church [C 668 140] includes in situ marine muds [3].
The basal lodgement tills at Derryvree, Aghnadarragh, Hollymount and Greenagho (P947879) are ascribed to the Fermanagh Stadial (P947951). The presence of a suite of erratics common to all these localities indicates an origin from one ice sheet that moved radially from central northern Ireland, blocking the on-shore movement of Scottish ice thereby restricting its till to the coastal fringe (P947880). However, further south the Scottish ice pressed inland for tens of kilometres. Near Crossmaglen (P947879), glacial gouges on roches moutonnées were produced by ice flowing to the southwest, and probably date from this time [4].
Aghnadarragh Interstadial
Evidence for the stratigraphical position and age of the Aghnadarragh Interstadial occurs in Unit 6 in the stratotype section at Aghnadarragh [J 125 735] [5]. The lowest part of the succession, represented by Units 2–6 (P947952), is typical of a low-angle, prograded delta infill and records a gradual climatic transition from glacial-deglacial-interstadial at the end of the early Midlandian. Organic material from Unit 6 gives a minimum 14C age of 48 180BP. After climatic amelioration during the Aghnadarragh Interstadial, conditions deteriorated and the upper peat horizon (Unit 8) gave a minimum 14C age of 46,850BP for the ensuing cold phase.
Middle Midlandian
Evidence for cold, non-glacial climatic conditions during the middle Midlandian is found at three sites in Co. Fermanagh. Freshwater organic mud occurs at Hollymount [H 356 403], near Maguiresbridge [6] and at Greenagho, near Belcoo [7]. Pollen assemblages indicating open countryside conditions of northern aspect at Hollymount gave a minimum 14C age of 41 500BP. No pollen was found at Greenagho and the 14C date of 32 460BP is regarded with caution as a minimum date. It is not possible to correlate the peat in Unit 8 at Aghnadarragh with either the Hollymount or Greenagho deposits because of the absence of reliable dates.
At Derryvree in Co. Fermanagh [H 361 390], a lower till, deposited during the early Midlandian Fermanagh Stadial, is separated from a late Midlandian upper till by organic-rich layers with a flora of freshwater mosses and sedges and an insect fauna of cold northern aspect [8]. A 14C age of 30 500±1170–1030BP was obtained for the organic-rich layers.
References
- ↑ Charlesworth, J K. 1939. Some observations on the glaciation of north-east Ireland. Proceedings of the Royal Irish Academy, 45B, 11, 255–95.
- ↑ Bazley, R A B, Brandon, A, and Arthurs, J W. 1997. Geology of the country around Limavady and Londonderry. Geological Survey of Northern Ireland. Technical Report GSNI/97/1
- ↑ 3.0 3.1 McCabe, A M. 1999. Ireland. In: Bowen, D Q (ed.). A revised correlation of Quaternary deposits in the British Isles. Geological Society, London, Special Report, 23, 113–24.
- ↑ McCabe, A M, Knight, J, and McCarron, S G. 1999. Ice-flow stages and glacial bedforms in north central Ireland: a record of rapid environmental change during the last glacial termination. Journal of the Geological Society, London. 156, 63–72.
- ↑ McCabe, A M, Coope, G R, Gennard, D E, and Doughty, P. 1987. Freshwater organic deposits and stratified sediments between Early and Late Midlandian (Devensian) till sheets, at Aghnadarragh, County Antrim, Northern Ireland. Journal of Quaternary Science. 2, 11–33.
- ↑ McCabe, A M, Mitchell, G F, and Shotton, F W. 1978. An inter-till freshwater deposit at Hollymount, Maguiresbridge, County Fermanagh. Proceedings of the Royal Irish Academy, 78, 77–89.
- ↑ Dardis, G F, Mitchell, W I, and Hirons, K R. 1985. Middle Midlandian interstadial deposits at Greenagho, near Belcoo, County Fermanagh, Northern Ireland. Irish Journal of Earth Sciences, 7, 1–6
- ↑ Colhoun, E A, Dickson, J H, McCabe, A M and Shotton, F W. 1972. A Middle Midlandian fresh-water series at Derryvree, Maguiresbridge, County Fermanagh, Northern Ireland. Proceedings of the Royal Society of London, 180B, 273–92.