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 seaboard – as shown by pollen remains from Achill Island, Co. Mayo, and Waterville and Killarney, Co. Kerry. There was permafrost on the lowlands with a scattered plant cover, much of it made up of dwarf willow and other arctic species. The initial vegetation cover of the Irish lowland landscape was a succession of different plant communities consisting of grasses, mugwort and low scrub with juniper and crowberry. Open grasslands later developed, characterised by docks and sedges. The mugwort, Artemisia sp. was also common, as evidenced by the pollen remains, thus giving rise to the description of this period as the Artemisia phase. These grasslands endured for some 1,000 years as large herbivores – giant deer and reindeer – stalked and munched their way through the lush pastures. The Nahanagan cold snap snuffed out much of the start of postglacial life in Ireland and the process had to commence all over again – from a generally bare soil to grasses to shrubs to dwarf trees and eventually to mature woodlands.
During the next 4,900 years, from 10,000–5,100 years ago, the Irish landscape evolved from open tundra to a country almost totally swaddled by woodlands. Only the mountains, poking above the green canopy, and the rivers, lakes and bogs in the lowlands differed from their surroundings. Temperatures continued to rise, more than doubling the July mean temperature from about 7°C to 15°C, approximately the same as today. Because of these new climatic conditions, Ireland became available for colonisation by the flora and fauna that had survived on the ice-free and warmer European mainland and also possibly in parts of Ireland.
How Ireland acquired its flora and fauna is a continuing and unresolved saga. There are three principal scenarios. Firstly, many plants and animals may have entered the country before the Ice Age or during interglacial periods and survived in ice-free areas. The flora and fauna then colonised the landscape at the end of the Ice Age. Forbes first championed this preglacial survival hypothesis in 1846.9 It received support from Praeger in 1932 and Beirne in 1952.10,11 Secondly, there may have been no Ice Age or interglacial survivors, and Ireland’s flora and fauna mostly arrived during the postglacial period, migrating from Britain and southern Europe when sea levels were some 130 m lower than present. This hypothesis was supported by Charlesworth in 1930, Godwin in 1975 and most recently by Mitchell & Ryan in 1997.2,12,13 Finally, postglacial arrival may have been by aerial dispersion, chance methods, and introduction, deliberate or accidental, by early man. This hypothesis was postulated by Reid in 1899 and more recently by Corbet in 1961.14,15
The most probable explanation is likely to be a combination of the three possibilities. Thus it would seem quite plausible for many species to have survived the Midlandian glaciation in the southern ice-free zones, and possibly earlier episodes of extreme coldness, while many other species may have arrived in Ireland during the postglacial period. The postglacial land bridge migration of flora and fauna has been strongly argued by Mitchell & Ryan. They postulated that land bridges between Ireland and Britain existed when the sea level fell to about 130 m below today’s levels exposing dry ridges of land, thus making it possible to cross dry-shod from the Lancashire-Cumbria area to Dublin, from north Wales to Co. Wicklow and from Cornwall to southeast Ireland. Ireland also had an Atlantic coastal ‘pathway’ linking it with southwest England, France and northern Spain. The sea fell to its lowest level some 15,000 years ago. By 10,000 years ago it was back to present day levels. Despite the appeal of the land bridge routes, the country could well have been repopulated from a reservoir of flora and fauna that had survived in the southern ice-free areas of Limerick, Cork and Waterford.
Late glacial Ireland. Column A gives the timescale for the last 13,000 radiocarbon years. Column B gives the names of the Irish type sites. Column C shows temperature trends, largely based on information from fossil animals and plants rather than instrumental measurements. Column D shows geomorphological and soil developments. Column E outlines vegetational developments. Column F lists mammalian records. From Mitchell & Ryan2.
In support of their land bridge concept, Mitchell & Ryan explain the process whereby the retreat northwards of the large wedge of ice that filled the area now occupied by the Irish Sea created a land bridge which was ‘pulled’ northwards with the withdrawal of the glacier. It is argued that the great weight of the ice depressed the land underneath which was squeezed out laterally and at the front of the glacier. Pushing down a fist into a ball of dough would produce a similar effect with the dough squeezed out laterally and rising up around the edges. The squeezed-out land moved out sideways and in front of the ice as a sort of bow of land as the ice pushed southwards. On the retreat of the ice northwards up the Irish Sea area the forebulge of land also retreated. Mitchell & Ryan, drawing upon a detailed study by Wingfield of the British Geological Survey,16 postulated that the fore-bulge moved or was pushed into the south end of the Irish Sea area about 11,000 years ago, when it provided a land bridge link between Devon and Carnsore Point, Co. Wexford. As the glacier melted and retreated northwards up the Irish Sea so the fore-bulge followed, providing a sort of moving land bridge link, of a continually diminishing height, across which plants and animals were able to migrate into Ireland from west Wales. About 9,500 years ago the bridge was enveloped and submerged by the rising waters of the Irish Sea.
Outline curve to indicate possible course of sea