This chapter will follow the tentative chronological and stratigraphical sequence of the cold and warm stages of the Quaternary deposits over the past 500,000 years as proposed by Mitchell & Ryan.2
| Years Ago | Proposed tentative stratigraphical sequence |
| 13,000–10,000 | Late glacial |
| 35,000–13,000 | Midlandian (Drumlin) cold |
| 65,000–35,000 | Aghnadarraghian mild |
| 79,000–65,000 | Midlandian (Main) cold |
| 122,000–100,000 | Fenitian mild |
| 132,000–122,000 | Eemian warm |
| 302,000–132,000 | Munsterian cold |
| 428,000–302,000 | Gortian warm |
| 500,000–480,000 | Ballylinian warm |
Pollen remains from interglacial deposits
The Ballylinian warm (500,000–480,000 years ago) takes its name from an area south of Castlecomer, Co. Kilkenny, where fossil pollen remains in a 25 m thick deposit of lacustrine clay show that the warm climate allowed the development of open forest containing most of the trees present in Ireland today including fir, spruce, hornbeam, oak, alder, wing-nut (found today in Turkey and present in Ireland from an earlier period) and yew. In the open areas there were grasses and heaths, where rhododendron and many herbs grew.3
The most famous interglacial deposit in Ireland is of peat and mud lying underneath glacial deposits cut by the Boleyneendorrish River near Gort, Co. Galway. It was first discovered and described by Kinahan in 1865 and was reexamined in 1949 by Jessen, Anderson & Farrington. They investigated the pollen remains in the muds and peat and named this warm interglacial stage the Gortian.4,5 The Gortian interglacial has been uncertainly dated as occurring some 428,000–302,000 years ago.2 About 12 other similarly aged deposits have been so far investigated in Ireland, some of whose results have been reviewed by Coxon, Mitchell & Ryan and Watts.1,2,6 The first plant species to appear at the onset of the Gortian interglacial phase, as summarised by Coxon, were the pioneering willow, juniper and buckthorn as well as many herbs and birch scrub. As the weather became milder the extent of pine and birch woodland grew while many other species – oak, elm, holly and hazel – are thought to have migrated into Ireland from other European ice-free areas. Unlike other interglacial sites examined in Britain, these Irish woodlands did not develop into a mature mixed oak forest but into heath, as increasing wet conditions fostered the growth of heather together with alder, yew, spruce and fir trees. The end result was a crowberry wet heath – to be replaced by tundra again when the Gortian period came to an abrupt end as temperatures plummeted. The next cold stage, the Munsterian, persisted from 302,000–132,000 years ago.
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