Earth Science Conservation Review
| Introduction to the Late Quaternary and Holocene of Northern Ireland | Antrim, Fermanagh, Londonderry, Tyrone |
| Site Type: | Inland exposure, Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Quaternary (Holocene, Late Glacial) |
| Interest | |
| Other interest: | late-glacial sequence, tephra, Glacial feature, Interglacial deposit, Late-glacial deposit, blanket bog, interglacial peat, raised bog, grassland, temperate forest, tundra |
Description:
INTERGLACIALS AND INTERSTADIALS
Northern Ireland has very few deposits of interglacial age. The only published sequence is from Benburb (Boulter and Mitchell 1977; Gennard 1984). There is still controversy about the age of this and all other Irish interglacial deposits. Some maintain that all the Irish interglacial deposits found so far, and named after the type site at Gort, are of penultimate interglacial age (Mitchell 1976) correlated therefore with the Hoxnian of England. Others maintain that it is unlikely that deposits of penultimate interglacial age would have survived while all deposits of the last interglacial would have been removed (Warren 1985) thereby suggesting that the Gortian deposits are last interglacial (Eemian).
Gennard (1984) distinguished four main phases in the vegetation of Benburb. The first phase has large amounts of pine pollen with birch, alder, willow, hazel, juniper and grass. In the second phase pine and yew are the main contributors. The third phase shows the development of some deciduous woodland with small amounts of oak, beech, ash, elm and alder. Spruce, a European tree absent from our present interglacial, appeared at this time. Phase four begins with the entrance of fir (Abies) and the formation of fir and yew woodlands and the appearance of Rhododendron. Towards the end of this phase the fir diminishes, leaving the yew as the dominant tree. Finally grasses increase as would be expected at a time when climate was deteriorating. The presence of holly and ivy, however, which will not stand severe winters, show that temperate conditions persisted. Neither beech nor Rhododendron is native to Ireland in the Holocene and Rhododendron is now considered to be an aggressive and unwelcome alien.
DEPOSITS OF THE LAST GLACIATION
There are a number of deposits of clays and laminated sediments that are thought to belong to the last glaciation. There are, however, three pollen-containing deposits from warm intervals in the last glaciation. These are from Hollymount (McCabe et al. 1978), Derryvree (Colhoun et al. 1972) both in Fermanagh, and Aghnadarragh in Co. Antrim (McCabe et al. 1987). In each case the organic deposits are sandwiched between deposits of the last glaciation. The deposits at Aghnadarragh were discovered in a trial pit dug by BP Coal. Detailed analysis of wood, seeds, pollen and insects gave a consistent picture of a birch-pine-spruce woodland. Insect evidence suggests a temperature not very different from the present day, but the stratigraphic evidence, particularly the lack of weathering, suggests that the warm period was of short duration. At this time spruce was a significant component of the vegetation, but in the post glacial this tree did not regain entry into Ireland.
THE LATE GLACIAL
It is usual, for descriptive purposes, to treat separately the period called the Late Glacial; that is from the retreat of the ice sheets, to the true start of the Holocene. During this time there were rapid climatic changes well documented in the ice core records from Greenland and also from terrestrial and marine records in and around Ireland (e.g. Lorius 1989 and Coope 1977). It is thought that the driving force behind these rapid changes came from the movement of ocean currents; Ireland is in a particularly sensitive position to record the full intensity of the changes. At least one of the sites in the following report contains excellent sediments of Late Glacial age (Sluggan bog). The ice core records suggest a rapid rise in temperature at about 14,000 years before present (bp). The dates before 10,000 years are based on uncalibrated radiocarbon dates, quoted as bp, those after 10,000 are calibrated and are denoted BP. The latter are directly comparable with dates derived from dendrochronology, from historical sources and from independent estimates such as the ice core records. Clearly one would not expect an instant response from the vegetation. In an environment left bare by the retreating ice there were no seeds for re-colonisation and the nearest plants of some species were perhaps 1000 km away. First the tundra plants that had survived near the southern margin of the ice expanded. Grasses and Artemisia were followed by juniper. There was, at this time, a considerable area of the present continental shelf exposed by the low sea levels that allowed the movement of plants from mainland Europe to England and to Ireland. While birch woodland developed at this time in many parts of NW Europe, in the north of Ireland tree-birch (as opposed to the tiny Betula nana) never got a hold although the presence of seeds shows that at least a few plants were present. At about 12,000 bp the temperature fell somewhat, to the detriment of juniper, and remained a few degrees lower than the present day for about 1000 years. During this time the country was dominated by grasslands. This grassland covered the lowlands and all the upland areas now under blanket bog. There were no lowland raised bogs at this time. It was during this grassland phase that the Giant Irish Deer flourished. Then the climate deteriorated quite drastically at about 10,900 bp. The soil became frozen for part of the year, erosion brought sand and clay into the lakes, leaving the deposits known as the Younger Dryas or in Ireland the Younger Salix herbacea layer, from the leaves of dwarf willow found in sediments of this age. This Late Glacial climate fluctuation was described in Ireland by Jessen and Farrington (1938).
THE HOLOCENE
The Late Glacial cold spell did not remove the vegetation, so that when the climate warmed again at 11,000 BP many species were able to flourish. The warming was thought to have been very fast, perhaps within a century or two, with the initial warming in as little as 20 years once the warm Atlantic currents were restored around Ireland (Dansgaard et al. 1989). The first plants to increase as the temperature rose were the grasses, Rumex species and Empetrum, quickly followed by juniper. Algal productivity in the lakes increased dramatically and we see for the first time pollen of plants such as Filipendula that will not tolerate cold conditions. We presume that juniper was present throughout the cold period, probably in the very stunted form we now see on west coast mountains. With the rise of temperature and the lack of competition from other trees it would have been able to grow into a substantial shrub and produce abundant pollen cones.
Its heyday was short however, because tree birch was on the way. As mentioned before, the tree birch had already made an appearance in Ireland in the Late Glacial and presumably was able to move into new areas rapidly. Birch has light wind-blown seeds and young birches set seed when as young as 10 years old. The speed with which this pioneer tree can colonise drained bogs today is well known. For a short time around 10,500 BP birch formed extensive woodland in the north of Ireland. Present with the birch was pine, giving a mixed birch/pine woodland reminiscent of the interstadial forests, except that this time there was no spruce. Although pine was present it only forms a small proportion of the total pollen and the woodlands seems to have been dominated by birch.
By about 10,000 BP the first of the true temperate forest trees and shrubs, hazel, arrived. Although the seeds of hazel are large and not wind-dispersed, it spread rapidly. There are no clear differences in the date of arrival of hazel in different parts of Ireland, although its arrival seems to have been somewhat later in the uplands (Smith and Pilcher 1973). In many sites studied, hazel quickly dominated the landscape. Hazel is able to replace birch as its seedlings can survive under birch's shade whereas birch seedlings can survive only in light conditions. The non-tree pollen is reduced to a few percent at this time and we can envisage all but the highest and most exposed mountain tops being covered by woodland, mostly of hazel. As Jessen pointed out in 1949, this dominance of the woodlands by hazel was restricted to the northern half of Ireland. There is no modern parallel for this hazel woodland and it is difficult to envisage what it might have looked like.
This great flourishing of hazel was short lived as the taller forest trees, oak and elm, were soon to replace it and reduce hazel to the status of an under-shrub in deciduous forest (under which conditions it will produce little pollen). Oak and elm seem to have come into the lowlands at about the same time, around 9500 BP, but oak arrived somewhat later than elm in the uplands as seen for example at Slieve Gallion (Key Site 105). In the Sperrins, oak seems to have expanded very close to 9000 BP (Smith and Pilcher 1973). The next 1000 years was the time of most dense forest cover in Ireland. Pollen samples from close to the summit of the Mournes (Holland 1975), Slieve Croob (Holland 1975), the Sperrins (Pilcher 1973) and the Antrim Plateau (Goddard 1971) all show substantial forest cover. It is reasonable however to assume that there was only a thin woodland cover at the summits of most mountains. Under this woodland many typical plants of the Late Glacial survived, for example Salix herbacea and Empetrum which are still found in the Mournes today. It is also at this time that many of the present raised bogs were reaching the reed-swamp or fen stage of their development (e.g. Sluggan, Fallahogy and Garry Bogs Key Sites 102, 104 and 103).
The next stage in the evolution of the forests was marked by the arrival of alder. Alder had been present in small numbers in the lowlands from before 8000 BP. At about 8000 BP it spread rapidly in the lowlands and more gradually into the uplands. Alder had expanded into the Antrim Plateau by about 7000 BP and into the Sperrins by about 6300 BP. There has been speculation about the explanations for the spread of alder at this time. Today we associate alder with fens, river banks and the wet margins of lakes, and it is tempting to associate the spread of alder with increasing wetness of the climate. Recent work (unpublished) on the sediments of Lough Neagh suggests that this may have been a time of low lake levels. If this is the case alder could have been expanding onto the lakeshores around Lough Neagh.
THE MID-HOLOCENE AND THE ARRIVAL OF PEOPLE
The first people to come to Ireland arrived shortly after the start of the Holocene. Their habitations were mainly coastal and along the major rivers such as the Lower Bann and they are thought to have had little impact on the vegetation. The same is not true of the next wave of invaders, the Neolithic people, who arrived about 5900 BP. They brought with them much new knowledge and technology from central Europe. They brought cattle and crop plants and started the process of forest clearance in Ireland. If we look at the pollen diagrams at the time of the arrival of the Neolithic people we see sudden changes in the composition of the forest and see an increase in grasses and plants of disturbed ground. This event in the pollen record has been extensively studied in NW Europe and one of the earliest investigations was by Smith (1958) and then by Smith and Willis (1964) at Fallahogy. Since that time evidence has accumulated that at least part of the vegetational changes may have been due to an outbreak of an elm pathogen like the recent Dutch Elm Disease. What part this may have played is still unclear, but there is a clear co-incidence in the dates of the reduction in elm pollen and the earliest Neolithic settlements (see e.g. Pilcher et al. 1971). After the initial elm decline and agricultural phase, the forest regenerated in most places. The composition of the regenerated forest was somewhat different with shrubs such as holly, rowan and the continued presence of weed and grass pollen even when there is no pollen of cultivated plants.
From this time on there are successive phases of degradation of the forest as waves of human populations or technological innovations reach Ireland. After the Neolithic came the Beaker people at about 4400 BP and then the Early Bronze Age with the first metal tools starting in about 3900 BP. These people had even greater impact on forest cover and forest regeneration was less complete.
At about 4500 BP pine pollen is greatly reduced in all Irish pollen diagrams and in most places it disappears soon after this. Just before its disappearance pine was colonising many of the large raised bogs. Pines from Garry Bog, Sluggan and Fallahogy have been studied from this time. They suggest that this was a period of drying out of the bog allowing pine to spread onto the bog surface. A similar phenomenon can be seen today where bogs have been drained. The bog at the Argory has extensive pine colonisation following drainage earlier in this century and there are a few young pines on the uncut, but rather dry surface of Fallahogy. The death of bog pines from water-logging seems to have contributed to the final demise of pine in Ireland. These events occurred at about the time of the Hekla 4 eruption in Iceland. Tephra (volcanic ash) from this eruption is present in many of the Irish bogs and has been used as a time marker for investigation of the pine decline (Hall et al. 1994a and b). Important sites for this work have been Sluggan, Garry and Fallahogy Bogs.
It is also at about this time (4500 BP) that profound changes were initiated in the uplands when blanket peats started to expand. There is no one single date that marks the start of blanket peat and there is no simple climatic explanation that can be used to explain its initiation. Much more constant, however, is the indication of burning and intensive grazing in the basal deposits under the blanket peat (Goddard 1971). Charcoal is almost always found under the blanket peat, with abundant pollen of grasses and weeds such as plantain. Current opinion would attribute the initiation of blanket peat to a largely man-made origin. It seems fairly certain that had the forest cover remained on the uplands, the bog would not have started to form. By 3000 BP we can see much of the present landscape features appearing. The forest had gone from the uplands and been replaced by blanket bog. The raised bogs had lost their capping of pine trees and were largely Sphagnum dominated. Most of the lowland forest was secondary and composed of alder, hazel, birch and ash with relatively few oak and elm.
Another 1000 years sees further reduction of the woodland cover and probably some deterioration in the climate. At this time there was the construction of trackways across bogs in various parts of Ireland.
The next 1000 years up to 1000 AD, sees the arrival of the horizontal mill and the mould-board plough. The latter opened up the heavier soils previously unsuitable for farming. Tree pollen percentages in the pollen diagrams fall below present-day levels. This is partly because there is in the present landscape some contribution of tree pollen from field hedges not extensive in Mediaeval times. There is a problem in reconciling the very low tree pollen values with the historical mediaeval accounts of vast areas of primeval forest full of "hostile natives" that was said to cover Ireland up to the 17th century. Until recently this part of the pollen record has been little studied.
The uppermost parts of lowland raised bogs contain the history of the climate and the development of the recent landscape of Northern Ireland. In many places the records of climate and landscape change are contained in the top metre of peat. These surface peats are most vulnerable to desiccation by drainage and removal by mechanised cutting. Problems with establishing a secure timeframe for the study of the recent past using radiocarbon dating have been overcome with the development and geographical expansion of tephrochronological studies in Northern Ireland. Volcanic ash from historic volcanic eruptions in Iceland has been detected at lowland sites. Tephra in peats from Sluggan and Garry Bogs is known to have originated from eruptions of Hekla in AD 1104 and AD 1510 with further finds of ash from the eruption of Oraefajokull in AD 1362. Dating by tephrochronology has revolutionised historic palaeoenvironmental research in Northern Ireland, but this work is only possible where the bog surfaces remain intact. One pass by a tractor-drawn peat extractor and the story is lost forever.
For site specific information on the Late Quaternary and Holocene of Northern Ireland see the following sites:
Key Site 102 - Sluggan Bog Key Site 103 - Garry Bog Key Site 104 - Fallahogy Bog Key Site 105 - Slieve Gallion Key Site 106 - Garvaghullion Bog Key Site 107 - Claraghmore Bog Key Site 108 - Glen West Bog
Northern Ireland has very few deposits of interglacial age. The only published sequence is from Benburb (Boulter and Mitchell 1977; Gennard 1984). There is still controversy about the age of this and all other Irish interglacial deposits. Some maintain that all the Irish interglacial deposits found so far, and named after the type site at Gort, are of penultimate interglacial age (Mitchell 1976) correlated therefore with the Hoxnian of England. Others maintain that it is unlikely that deposits of penultimate interglacial age would have survived while all deposits of the last interglacial would have been removed (Warren 1985) thereby suggesting that the Gortian deposits are last interglacial (Eemian).
Gennard (1984) distinguished four main phases in the vegetation of Benburb. The first phase has large amounts of pine pollen with birch, alder, willow, hazel, juniper and grass. In the second phase pine and yew are the main contributors. The third phase shows the development of some deciduous woodland with small amounts of oak, beech, ash, elm and alder. Spruce, a European tree absent from our present interglacial, appeared at this time. Phase four begins with the entrance of fir (Abies) and the formation of fir and yew woodlands and the appearance of Rhododendron. Towards the end of this phase the fir diminishes, leaving the yew as the dominant tree. Finally grasses increase as would be expected at a time when climate was deteriorating. The presence of holly and ivy, however, which will not stand severe winters, show that temperate conditions persisted. Neither beech nor Rhododendron is native to Ireland in the Holocene and Rhododendron is now considered to be an aggressive and unwelcome alien.
DEPOSITS OF THE LAST GLACIATION
There are a number of deposits of clays and laminated sediments that are thought to belong to the last glaciation. There are, however, three pollen-containing deposits from warm intervals in the last glaciation. These are from Hollymount (McCabe et al. 1978), Derryvree (Colhoun et al. 1972) both in Fermanagh, and Aghnadarragh in Co. Antrim (McCabe et al. 1987). In each case the organic deposits are sandwiched between deposits of the last glaciation. The deposits at Aghnadarragh were discovered in a trial pit dug by BP Coal. Detailed analysis of wood, seeds, pollen and insects gave a consistent picture of a birch-pine-spruce woodland. Insect evidence suggests a temperature not very different from the present day, but the stratigraphic evidence, particularly the lack of weathering, suggests that the warm period was of short duration. At this time spruce was a significant component of the vegetation, but in the post glacial this tree did not regain entry into Ireland.
THE LATE GLACIAL
It is usual, for descriptive purposes, to treat separately the period called the Late Glacial; that is from the retreat of the ice sheets, to the true start of the Holocene. During this time there were rapid climatic changes well documented in the ice core records from Greenland and also from terrestrial and marine records in and around Ireland (e.g. Lorius 1989 and Coope 1977). It is thought that the driving force behind these rapid changes came from the movement of ocean currents; Ireland is in a particularly sensitive position to record the full intensity of the changes. At least one of the sites in the following report contains excellent sediments of Late Glacial age (Sluggan bog). The ice core records suggest a rapid rise in temperature at about 14,000 years before present (bp). The dates before 10,000 years are based on uncalibrated radiocarbon dates, quoted as bp, those after 10,000 are calibrated and are denoted BP. The latter are directly comparable with dates derived from dendrochronology, from historical sources and from independent estimates such as the ice core records. Clearly one would not expect an instant response from the vegetation. In an environment left bare by the retreating ice there were no seeds for re-colonisation and the nearest plants of some species were perhaps 1000 km away. First the tundra plants that had survived near the southern margin of the ice expanded. Grasses and Artemisia were followed by juniper. There was, at this time, a considerable area of the present continental shelf exposed by the low sea levels that allowed the movement of plants from mainland Europe to England and to Ireland. While birch woodland developed at this time in many parts of NW Europe, in the north of Ireland tree-birch (as opposed to the tiny Betula nana) never got a hold although the presence of seeds shows that at least a few plants were present. At about 12,000 bp the temperature fell somewhat, to the detriment of juniper, and remained a few degrees lower than the present day for about 1000 years. During this time the country was dominated by grasslands. This grassland covered the lowlands and all the upland areas now under blanket bog. There were no lowland raised bogs at this time. It was during this grassland phase that the Giant Irish Deer flourished. Then the climate deteriorated quite drastically at about 10,900 bp. The soil became frozen for part of the year, erosion brought sand and clay into the lakes, leaving the deposits known as the Younger Dryas or in Ireland the Younger Salix herbacea layer, from the leaves of dwarf willow found in sediments of this age. This Late Glacial climate fluctuation was described in Ireland by Jessen and Farrington (1938).
THE HOLOCENE
The Late Glacial cold spell did not remove the vegetation, so that when the climate warmed again at 11,000 BP many species were able to flourish. The warming was thought to have been very fast, perhaps within a century or two, with the initial warming in as little as 20 years once the warm Atlantic currents were restored around Ireland (Dansgaard et al. 1989). The first plants to increase as the temperature rose were the grasses, Rumex species and Empetrum, quickly followed by juniper. Algal productivity in the lakes increased dramatically and we see for the first time pollen of plants such as Filipendula that will not tolerate cold conditions. We presume that juniper was present throughout the cold period, probably in the very stunted form we now see on west coast mountains. With the rise of temperature and the lack of competition from other trees it would have been able to grow into a substantial shrub and produce abundant pollen cones.
Its heyday was short however, because tree birch was on the way. As mentioned before, the tree birch had already made an appearance in Ireland in the Late Glacial and presumably was able to move into new areas rapidly. Birch has light wind-blown seeds and young birches set seed when as young as 10 years old. The speed with which this pioneer tree can colonise drained bogs today is well known. For a short time around 10,500 BP birch formed extensive woodland in the north of Ireland. Present with the birch was pine, giving a mixed birch/pine woodland reminiscent of the interstadial forests, except that this time there was no spruce. Although pine was present it only forms a small proportion of the total pollen and the woodlands seems to have been dominated by birch.
By about 10,000 BP the first of the true temperate forest trees and shrubs, hazel, arrived. Although the seeds of hazel are large and not wind-dispersed, it spread rapidly. There are no clear differences in the date of arrival of hazel in different parts of Ireland, although its arrival seems to have been somewhat later in the uplands (Smith and Pilcher 1973). In many sites studied, hazel quickly dominated the landscape. Hazel is able to replace birch as its seedlings can survive under birch's shade whereas birch seedlings can survive only in light conditions. The non-tree pollen is reduced to a few percent at this time and we can envisage all but the highest and most exposed mountain tops being covered by woodland, mostly of hazel. As Jessen pointed out in 1949, this dominance of the woodlands by hazel was restricted to the northern half of Ireland. There is no modern parallel for this hazel woodland and it is difficult to envisage what it might have looked like.
This great flourishing of hazel was short lived as the taller forest trees, oak and elm, were soon to replace it and reduce hazel to the status of an under-shrub in deciduous forest (under which conditions it will produce little pollen). Oak and elm seem to have come into the lowlands at about the same time, around 9500 BP, but oak arrived somewhat later than elm in the uplands as seen for example at Slieve Gallion (Key Site 105). In the Sperrins, oak seems to have expanded very close to 9000 BP (Smith and Pilcher 1973). The next 1000 years was the time of most dense forest cover in Ireland. Pollen samples from close to the summit of the Mournes (Holland 1975), Slieve Croob (Holland 1975), the Sperrins (Pilcher 1973) and the Antrim Plateau (Goddard 1971) all show substantial forest cover. It is reasonable however to assume that there was only a thin woodland cover at the summits of most mountains. Under this woodland many typical plants of the Late Glacial survived, for example Salix herbacea and Empetrum which are still found in the Mournes today. It is also at this time that many of the present raised bogs were reaching the reed-swamp or fen stage of their development (e.g. Sluggan, Fallahogy and Garry Bogs Key Sites 102, 104 and 103).
The next stage in the evolution of the forests was marked by the arrival of alder. Alder had been present in small numbers in the lowlands from before 8000 BP. At about 8000 BP it spread rapidly in the lowlands and more gradually into the uplands. Alder had expanded into the Antrim Plateau by about 7000 BP and into the Sperrins by about 6300 BP. There has been speculation about the explanations for the spread of alder at this time. Today we associate alder with fens, river banks and the wet margins of lakes, and it is tempting to associate the spread of alder with increasing wetness of the climate. Recent work (unpublished) on the sediments of Lough Neagh suggests that this may have been a time of low lake levels. If this is the case alder could have been expanding onto the lakeshores around Lough Neagh.
THE MID-HOLOCENE AND THE ARRIVAL OF PEOPLE
The first people to come to Ireland arrived shortly after the start of the Holocene. Their habitations were mainly coastal and along the major rivers such as the Lower Bann and they are thought to have had little impact on the vegetation. The same is not true of the next wave of invaders, the Neolithic people, who arrived about 5900 BP. They brought with them much new knowledge and technology from central Europe. They brought cattle and crop plants and started the process of forest clearance in Ireland. If we look at the pollen diagrams at the time of the arrival of the Neolithic people we see sudden changes in the composition of the forest and see an increase in grasses and plants of disturbed ground. This event in the pollen record has been extensively studied in NW Europe and one of the earliest investigations was by Smith (1958) and then by Smith and Willis (1964) at Fallahogy. Since that time evidence has accumulated that at least part of the vegetational changes may have been due to an outbreak of an elm pathogen like the recent Dutch Elm Disease. What part this may have played is still unclear, but there is a clear co-incidence in the dates of the reduction in elm pollen and the earliest Neolithic settlements (see e.g. Pilcher et al. 1971). After the initial elm decline and agricultural phase, the forest regenerated in most places. The composition of the regenerated forest was somewhat different with shrubs such as holly, rowan and the continued presence of weed and grass pollen even when there is no pollen of cultivated plants.
From this time on there are successive phases of degradation of the forest as waves of human populations or technological innovations reach Ireland. After the Neolithic came the Beaker people at about 4400 BP and then the Early Bronze Age with the first metal tools starting in about 3900 BP. These people had even greater impact on forest cover and forest regeneration was less complete.
At about 4500 BP pine pollen is greatly reduced in all Irish pollen diagrams and in most places it disappears soon after this. Just before its disappearance pine was colonising many of the large raised bogs. Pines from Garry Bog, Sluggan and Fallahogy have been studied from this time. They suggest that this was a period of drying out of the bog allowing pine to spread onto the bog surface. A similar phenomenon can be seen today where bogs have been drained. The bog at the Argory has extensive pine colonisation following drainage earlier in this century and there are a few young pines on the uncut, but rather dry surface of Fallahogy. The death of bog pines from water-logging seems to have contributed to the final demise of pine in Ireland. These events occurred at about the time of the Hekla 4 eruption in Iceland. Tephra (volcanic ash) from this eruption is present in many of the Irish bogs and has been used as a time marker for investigation of the pine decline (Hall et al. 1994a and b). Important sites for this work have been Sluggan, Garry and Fallahogy Bogs.
It is also at about this time (4500 BP) that profound changes were initiated in the uplands when blanket peats started to expand. There is no one single date that marks the start of blanket peat and there is no simple climatic explanation that can be used to explain its initiation. Much more constant, however, is the indication of burning and intensive grazing in the basal deposits under the blanket peat (Goddard 1971). Charcoal is almost always found under the blanket peat, with abundant pollen of grasses and weeds such as plantain. Current opinion would attribute the initiation of blanket peat to a largely man-made origin. It seems fairly certain that had the forest cover remained on the uplands, the bog would not have started to form. By 3000 BP we can see much of the present landscape features appearing. The forest had gone from the uplands and been replaced by blanket bog. The raised bogs had lost their capping of pine trees and were largely Sphagnum dominated. Most of the lowland forest was secondary and composed of alder, hazel, birch and ash with relatively few oak and elm.
Another 1000 years sees further reduction of the woodland cover and probably some deterioration in the climate. At this time there was the construction of trackways across bogs in various parts of Ireland.
The next 1000 years up to 1000 AD, sees the arrival of the horizontal mill and the mould-board plough. The latter opened up the heavier soils previously unsuitable for farming. Tree pollen percentages in the pollen diagrams fall below present-day levels. This is partly because there is in the present landscape some contribution of tree pollen from field hedges not extensive in Mediaeval times. There is a problem in reconciling the very low tree pollen values with the historical mediaeval accounts of vast areas of primeval forest full of "hostile natives" that was said to cover Ireland up to the 17th century. Until recently this part of the pollen record has been little studied.
The uppermost parts of lowland raised bogs contain the history of the climate and the development of the recent landscape of Northern Ireland. In many places the records of climate and landscape change are contained in the top metre of peat. These surface peats are most vulnerable to desiccation by drainage and removal by mechanised cutting. Problems with establishing a secure timeframe for the study of the recent past using radiocarbon dating have been overcome with the development and geographical expansion of tephrochronological studies in Northern Ireland. Volcanic ash from historic volcanic eruptions in Iceland has been detected at lowland sites. Tephra in peats from Sluggan and Garry Bogs is known to have originated from eruptions of Hekla in AD 1104 and AD 1510 with further finds of ash from the eruption of Oraefajokull in AD 1362. Dating by tephrochronology has revolutionised historic palaeoenvironmental research in Northern Ireland, but this work is only possible where the bog surfaces remain intact. One pass by a tractor-drawn peat extractor and the story is lost forever.
For site specific information on the Late Quaternary and Holocene of Northern Ireland see the following sites:
Key Site 102 - Sluggan Bog Key Site 103 - Garry Bog Key Site 104 - Fallahogy Bog Key Site 105 - Slieve Gallion Key Site 106 - Garvaghullion Bog Key Site 107 - Claraghmore Bog Key Site 108 - Glen West Bog
| Enlander, I., Dempster, M. & Doughty, P., 2024. Introduction to the Late Quaternary and Holocene of Northern Ireland, County Antrim, Fermanagh, Londonderry, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=101. Accessed on 2024-12-26 |
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