Collins New Naturalist Library. David Cabot

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      The Pliestocene geology of Ireland showing the areas considered never glaciated and the extent of the older Munsterian glaciation (302,000–132,000 years ago) and more recent two cold phases of the Midlandian: Main: 79,000–65,000 years ago and Midlandian Drumlin: 35,000–13,000 years ago. From J.B. Whittow (1974). Geology and Scenery of Ireland. Penguin Books, London.

      The Gortian floral assemblage contains several species whose history in Ireland is a matter of much conjecture. The occurrence of pollen from Mackay’s heath, Dorset heath and St Dabeoc’s heath opens up the possibility of their survival through the subsequent cold phases rather than a more recent postglacial arrival on land bridges from their southern headquarters in Portugal, Spain and France. Rhododendron, another Gortian species, possibly moved into Ireland to escape declining temperatures elsewhere in Europe at the time but is generally considered to have become extinct in Ireland at the end of the Gortian phase. Its reintroduction came during the eighteenth century and it has since spread into many habitats, especially deciduous woodland and peatlands. Two further species, considered north American in their current distribution – the slender naiad and pipewort – were also present in Gortian deposits. They, like the heaths and heathers above, could possibly have continued their tenure in the country through the subsequent Munsterian cold stage in areas not subjected to intense coldness, having arrived before the glacial period by migration through Greenland and Iceland when the water barriers were not so great. This would make the need for other explanations unnecessary – such as their arrival on the feet of migratory waders and geese from western Greenland and northern Canada and perhaps America from the end of the late glacial period onwards.

      Palaeobotanists have found it difficult to correlate the Gortian interglacial deposits with other such deposits in Britain and Europe but Mitchell & Ryan believe that the closest fit is with the Hoxonian period in Britain and the Holsteinian period in Germany. Whatever the correlation, the Gortian interglacial is considered by some scientists to have been the last warm interglacial before the onset of the very cold Munsterian stage.⁷ The Gortian period provided the opportunity for the development of some 100 taxa of higher plants of which some 20 are not native of Ireland today.

      Before the Munsterian ice was fully in place, the low ground turned into a polar desert. Only the toughest species of the Gortian vegetation could have survived these conditions while others migrated southwards to avoid the falling temperatures. Jessen was of the opinion that many of the species that migrated southwards before the advancing cold in Europe ended up in the Black Sea area. During the Munsterian glacial period large masses of ice flowed into Ireland from the Scottish Highlands and probably covered much of Ireland during its maximum extent. Limited areas of high land in the west and south probably remained ice-free. Low-lying areas, even along the Atlantic coastline, were characterised by a cold polar desert climate. Only the hardiest forms of flora and flora could have survived in Ireland when the Munsterian cold stage was at its maximum extent.

      Mitchell & Ryan have put forward some evidence for the occurrence of two warm or mild phases (the Eemian and Fenitian) which followed the Munsterian cold stage and lasted from approximately 132,000–100,000 years ago, but more research is needed to establish the full nature of these interludes before the onset of the next cold phase, the Midlandian (Main) cold stage. Around 79,000 years ago it became severely cold with arctic and dry conditions until ice sheets formed and spread out from their two main centres located in an area from Donegal to Belfast and in the Midlands. A tongue of Scottish ice also passed down the Irish Sea. There were also ice caps in the Wicklow Mountains and the Cork and Kerry mountains. There were, however, substantial areas south of a line approximately between Askeaton, Co. Limerick, and the Wicklow Mountains that remained ice-free, and it was in this very cold region that many plants and animals would have had the opportunity to survive to then recolonise Ireland with the onset of warmer conditions commencing some 13,000 years ago.

      During the Aghnadarraghian period, approximately 65,000–35,000 years ago, mild conditions set in. Remains of fossil beetles indicate summer and winter temperatures similar to those of today. The relatively warm conditions encouraged the development of temperate cool woodlands with hazel and yew. The earliest mammalian remains in Ireland, molar teeth, tusks and broken bones of woolly mammoth and musk ox bones (but see below), were found in gravel deposited on top of a band of lignite (brown coal) and date from over 50,000 years ago. The warm Aghnadarraghian mild phase was brought to an end with the onset of dry cold conditions which persisted for some 8,000 years before the development of more ice marking the Drumlin phase of the Midlandian cold stage, but conditions were sufficiently mild to allow the development of open grasslands with scattered birch and willow woodlands. It was in this environment that many mammals flourished, evidence of their occupation provided by bone remains in caves. The renewed ice possibly peaked around 25,000 years ago and then lasted until about 15,000 years ago when it started to melt, a process that took about 2,000 years.

      Grass covered eskers, sinuous ridges composed of glacial outwash gravel.

      The late glacial period and the development of woodlands

      By the end of the late glacial phase some 13,000 years ago the ice sheets had melted and the final ordering of the rocky skeleton and cosmetic adjustments to the skin of the Irish landscape were complete. Mountains, hills and rocks had been scraped, scoured and polished by the flowing ice. Soil and boulders had been lifted up, moved over huge distances and dumped as rude morainic material and glacial till: sinuous ridges of outwash gravels or eskers, some extending over several kilometres and reaching 20 m in height, had established their presence in the Midlands. Miniature hilly landscapes made of drumlins or small hilly lumps of glacial drift, possibly formed underneath the melting ice sheets or dropped as dollops of material, had appeared. Lake basins were scooped out, valleys were formed.

      As it emerged from the cold, Ireland entered what is known as the Woodgrange interstadial phase; a sort of mini-interglacial period without the full development of woodlands. The name comes from the shallow lake basin lying between drumlins at Woodgrange, Co. Down, where pollen was blown, settled, and remained preserved in the organic muds. Originally described by Singh,⁸ the Woodgrange pollen signatures were later recounted by Mitchell & Ryan. They chronicle the succession of plants that settled and spread in this area over a period of 3,000 years.

      The first plants to emerge and fix themselves in the bare soil were sorrels, grasses and the dwarf willow. This initial growth is known as the grass/sorrel phase. Five hundred years later juniper and birch flourished while other pollen deposits showed crowberry growing close to the Atlantic coastline near Roundstone, Co. Galway. In those days the Irish landscape must have approximated that of arctic tundra with a smattering of birch woodland and juniper scattered over the ground. However, this initial growth was brought to an abrupt end as a renewed drop in temperature killed off the pioneering species.

      Pollen diagram from Woodgrange, Co. Down. From Singh⁸.

      A cold snap, triggered by a southerly movement of arctic waters down the Atlantic coast of Europe around 10,600 years ago, suppressed the birch and juniper development and opened up bare patches of soil only suitable for the more resistant grasses. This period, lasting some 600 years, is named the Nahanagan stadial by Mitchell & Ryan, after Lough Nahanagan in the Wicklow Mountains where glacier ice reformed as it did in other mountain corries under the renewed influence of freezing temperatures. Such extreme conditions only allowed the emergence of a sporadic plant cover, mainly of arctic-alpine species, growing at low altitudes and also at sea level along the western СКАЧАТЬ