Late 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
Introduction
The late Midlandian period started about 25 000BP and continued up to 10 000BP (P947953). It is the final, predominantly glacial, episode of the Midlandian Stage. The ice mass was retreating significantly before 16 000BP [1] so in Northern Ireland deglaciation features like eskers, glacial lakes, subglacial channels and outwash deposits were forming and maybe continued through to the end of the drumlin re-advance episode (Killard Point event) at about 13 000BP.
The Glenavy Stadial (Last Glacial Maximum) reached a maximum around 24 000–20 000BP and the ‘upper tills’ in Northern Ireland derive from this episode. Ice cover was virtually complete and extended beyond the present coastline in most areas. Strong ice flows moved radially from the Omagh and Lough Neagh ice dispersal centres (P947881) depositing thick glacial diamicts (tills). At Aghnadarragh (P947952), the Glenavy Stadial is represented by a thick till (Unit 10) that has been termed the Glenavy Formation [2]. The till is termed the Gelvin Till in the north and the Maguiresbridge Till in the west. [2] These lodgement tills largely reflect the local bedrock in erratic content and can be around 10 m thick. Extensive northeast-southwest trending rogen moraine formed between the Erne Basin and Co. Armagh [3].
It was at this time that crustal depression caused by the weight of ice reached a maximum so at about 18 000BP, when deglaciation probably started, sea levels were much higher than at the present time compared to the land. There is a raised beach platform at 14.5 m above sea level [C 601 223] near Ballykelly, Co. Londonderry [4]. Hence the extensive glacial outwash deposits pouring into the sea from the Roe and Foyle/Faughan valleys were planed to an elevation of just over 14 m above present sea level. These deltaic deposits (Fruitfield Formation) [5] form an extensive terrace that rises in height southwards up the valleys, reaching 24m south of Limavady. Deglaciation was probably a complex process with ice retreats, standstills and even minor readvances in different areas at different times. Evidence of retreat moraines can be seen along the valleys of the River Foyle [6] and River Roe [7]. Just south of Carlingford Lough, at Cooley Point, there is evidence that the ice margin had withdrawn by around 15 800BP [8] confirming that major deglaciation of the Irish Sea basin had started before 16 000BP.[1]
Meltwater deposits such as the Malone Sands in Belfast probably formed in ice marginal lakes at this time. It is likely that the diamict which covers these sands in the Lagan Valley between Belfast and Lisburn, is the product of debris flow and was not a till deposited directly from an ice mass. Overflow meltwater from ‘Lake Lagan’ initially drained northeastwards along the course of the present River Lagan. When that route became blocked, maybe by ice during a later readvance, the water instead flowed west to Lough Neagh, through the Soldierstown Gap at Moira. It was probably at this time, when the ice support withdrew, that the major block landslides occurred along steep margins of the Antrim Plateau, from Cave Hill and northwards along the Antrim coast to Binevenagh in the west, near Limavady. Particularly catastrophic was the huge slip at Sallagh Braes, north of Larne.
Another feature that developed at the time of the decay of the ice sheets, with the release of vast quantities of meltwater below the ice, are the deeply incised glacial channels. Fine examples occur at Bernisk Glen [H 605 675] 3.5 km east of Sixmilecross, Butterlope Glen [H 492 950] 4 km north of Plumbridge, south of Loughaveema [D 207 354] near Ballycastle and Gortin Gap [H 490 850] in Co. Tyrone.
Killard Point Stadial
Between 14 500–14 000BP ice re-advance and a major drumlin-forming episode in the north of Ireland is correlated with the episode of iceberg discharge in the North Atlantic known as Heinrich event 1 [1]. Initially, ice flow was largely erosive and was responsible for the streamlining, remoulding or removal of bedforms in the area of rogen moraine in the Erne Basin and central Co. Armagh (which were created during the Glenavy Stadial). Linear moraines which extend from Kells in Co. Meath to Killard Point in Co. Down [9] define the southeastern margin of the ice mass (P948096). Calving of icebergs into the sea (at the higher level mentioned above) instigated the propagation of faster ice flow streams northwards from Dundalk Bay through Co. Armagh to the south side of Lough Neagh and involved the headward (north to northwestward) erosion of pre-existing transverse ridges.
The stadial is especially associated with the formation of drumlins and ice-moulded bedforms in Counties Down and Armagh (P947953). At Killard Point [J 613 435], there was ice-marginal deposition of gravel deposits that prograded south from the ice front into the sea.[9] The deposits consist of reworked glacigenic diamict interbedded with the gravel and beds of red marine mud containing in situ marine microfaunas. The melting of icebergs into the sea produced the dropstones that occur in the muds.
Regional evidence now indicates that ice flowed west and north from the Omagh area and, with much greater effect than hitherto, from the Lough Neagh ice axis (P948096). Here, as elsewhere, drumlins occur on low ground mainly below 200 m, but other streamlined rock-cored ridges occur at higher altitudes. Towards the end of the period, linear ridges of moulded till formed in the Clogher Valley beneath ice moving to the southwest from Lough Neagh and by southeasterly ice flow in the area south of Lisnaskea, perpendicular to the orientation of rogen moraine in that area (P948044). In Co. Antrim, ice from Lough Neagh advanced northwards, moulding drumlins across the Antrim Plateau, including the valleys of the Rivers Bann and Main. In north Co. Antrim a slightly later southward advance of ice from Scotland reached the line of the Armoy Moraine. It rode over the pre-existing drumlins without destroying their form, advancing into outwash deposits that were thrust into a series of sub-parallel ridges. The Armoy Moraine, up to 50 m thick, is composed of gravel, diamict, clay, silt, sand and is notable for its glaciotectonic deformation features [10]. Sandwiched between the margins of the two ice masses, extensive sand and gravel was deposited at the edge of the Scottish ice in a lake system fringing the breached, southern margin of the Armoy Moraine [11]. Concurrently, as the ice wasted southwards, north-south trending eskers developed in meltwater channels under the ice sheet. They extend for 15km from Cullybackey [D 055 058] in the south, to the west of Glarryford [D 055 130] and end at The Isles [D 023 204] near Dunloy.
After the ice readvance, deglaciation would have continued in earnest. As the ice retreated southward across northwestern Co. Londonderry and Co. Tyrone to the ice centres in Lough Neagh and the Omagh Basin it became confined to low ground south of the deglaciated Sperrin Mountains. Glaciofluvial deposits on their flanks, such as in the Lough Fea and Pomeroy-Draperstown areas, record positions of the ice margin during retreat [12]. On the north side of the Sperrin Mountains deep lakes formed in long narrow valleys that were dammed by ice flowing from the Omagh and Lough Neagh centres (Figure 18.3C). Gilbert-type deltas and glaciolacustrine sediments were deposited in these ice-marginal lakes which, in the Murnee Hills, created a high level shoreline up to 450m above present sea level (P948045), (P948046).
Woodgrange Interstadial
In the marine environment evidence for ameliorating climatic conditions some 13 000–11 000BP is found at Rough Island, Co. Down. This tiny island in Strangford Lough has, resting on a glacial till, a composite deposit of late-glacial marine laminated mud and sand that has been eroded by tidal currents into a ridge overlain by post-glacial gravel. The mud contains foraminifera dated to 12 700BP.[1] Sea levels during the period must have dropped to below the present day levels. However, the best record of conditions during the time between the disappearance of the ice and the present, is preserved in the vegetation.
A complete record of the changes mentioned above is found in the complex of raised lowland peat at Sluggan Bog [J 099 921] in Co. Antrim (P947879), 3 km northeast of Randalstown [13]. As well as cut sections in the peat, boreholes encountered 5.2 m of raised bog peat, fen peat and lacustrine mud in a shallow, concealed basin. The section is divided into ten pollen zones (P947954) and has a radiocarbon date of 12 500BP for organic detritus at the base of the section, which lies on till.
In the wake of the disappearing ice sheets, initial colonisation was by vegetation of Arctic tundra affinity. The oldest pollen assemblages from Sluggan Bog (Zone S:I) are dominated by dwarf willow (Salix herbaceae) and sorrel (Rumex) indicating a cool climate. In Zone S:II the increasing domination of poplar (Populus), juniper (Juniperus) and birch is indicative of warmer conditions at the start of the Woodgrange Interstadial. It was at this time that Ireland witnessed the arrival of the Giant Irish Deer (Megaceros giganteus) whose antlers have been found in deposits in Larne Lough, Co. Antrim.
Nahanagan Stadial
Climatic deterioration then led to the onset of the Nahanagan Stadial, the final glacial episode in Ireland. Ice accumulation was confined to ice fields and local corrie glaciers in the Mourne and Sperrin mountains. Major ice sheets did not advance across Ireland and evidence for the cold episode is limited to protalus ramparts, fossil rock glaciers and soliflucted material [14]. Once again juniper, mosses and sedges typical of the open tundra dominated the treeless landscape and indicated a temperature depression of up to 8°C.
References
- ↑ 1.0 1.1 1.2 1.3 McCabe, A M, and Clarke, P U. 1998. Ice sheet variability around the North Atlantic Ocean during the last deglaciation. Nature, London, 392, 373–77.
- ↑ 2.0 2.1 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.
- ↑ 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
- ↑ 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.
- ↑ Colhoun, E A. 1972. The deglaciation of the Sperrin Mountains and adjacent areas in counties Tyrone, Londonderry and Donegal, Northern Ireland. Proceedings of the Royal Irish Academy, 72B, 8, 91–147.
- ↑ 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
- ↑ McCabe, A M, and Haynes, J R. 1996. A late Pleistocene intertidal boulder pavement from an isostatically emergent coast, Dundalk Bay, Eastern Ireland. Earth Surface Processes and Landforms, 21, 555–72.
- ↑ 9.0 9.1 McCabe, A M, Dardis, G F, and Hanney, P. 1984. Sedimentology of a Late Pleistocene submarine moraine complex, County Down, Northern Ireland. Journal of Sedimentary Petrology, 56, 716–30.
- ↑ Shaw, J, and Carter, R W G. 1980. Late Midlandian sedimentation and glaciotectonics of the North Antrim End Moraine. Irish Naturalists’ Journal, 20, 67–69.
- ↑ Geological Survey of Northern Ireland 1999. Coleraine, Northern Ireland Sheet 13. Solid and Drift Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
- ↑ Geological Survey of Northern Ireland 1995. Draperstown, Northern Ireland Sheet 26. Solid and Drift Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
- ↑ Smith, A G, and Goddard, I C. 1991. A 12 500 year record of vegetational change at Sluggan Bog, Co. Antrim, N. Ireland (incorporating a pollen zone scheme for the non-specialist). New Phytologist. 118, 167–87.
- ↑ Coxon, P. 1997. Pleistocene climate change: the evidence from Irish sequences. In: Sweeney, J (ed.). Global change and the Irish environment. Royal Irish Academy, Dublin, 17–35.