W2A-029 Lowland blanket bog-0.txt
ORIGINS OF IRISH LOWLAND BLANKET BOG

M. O'Connell

Department of Botany, University College Galway

ABSTRACT

Lowland or Atlantic blanket bog is largely confined to the western seaboard counties of Ireland, where its eastern limit closely coincides with the 1200mm isohyet and number of rain days per annum exceeding 200. The underlying solid geology consists of base-poor metamorphic, sedimentary and igneous rock types. These rock types also provide the parent material for the drifts which, in parts, are patchy or absent. This combination of factors results in edaphic conditions favourable to podzolization and bog growth.

Within the region occupied by blanket bog, minerotrophic peat began to accumulate in small basins early in the post-glacial. Succession to ombrotrophic peat was effected at some sites in the Atlantic period, but blanket bog, as a dominant feature in the landscape, is essentially a phenomenon of the later post-glacial. While climatic factors and time-dependent pedogenic processes form the background against which the spread of blanket bog took place, it is argued that human impact played a major role in its widescale expansion in the second and first millennia B.C.

INTRODUCTION

Blanket bog constitutes the main landscape feature and the only remaining extensive unit of natural vegetation in the western seaboard counties of Kerry, south-west Clare, Galway, Mayo and Donegal, and in upland regions with elevation exceeding 200m (cf. Fig. 1). Some 10% of the total land surface of the country, or almost 900,000ha, is covered by blanket bog, 37% of which is classified as lowland or Atlantic blanket bog, not only on the basis of relief but also in recognition of its distinctive floristic composition (Hammond 1979; Doyle 1982; 1990).

While the main purpose of this paper is to provide a chronological framework and to reconstruct the factors that may have led to blanket bog growth in the lowlands, the origins of upland blanket bog are also considered, particularly since research by others provides information relevant to the better evaluation of the lowland data.
A substantial body of data on blanket bog initiation derives from research carred out in Northern Ireland in the Palaeoeocology Laboratory, Queen's University, during the 1960s. In this region, it is not so much location vis-\a-vis the rain-bearing westerlies but rather an upland relief in the form of the Sperrin mountains in mid-Ulster and the Antrim Plateau that results in a precipitation/evaporation regime favourable to blanket bog growth (Cruickshank 1970). Smith (1975), reviewing the radiocarbon and palaeoenvironmental evidence for blanket bog initiation, shows that a wide time-span is involved and that, apparently, altitude is not an important factor. The dates from the Sperrin uplands centring on 4000 B.P. are, however, older than those from the Antrim Plateau, where most of the dates lie within the range 3700 to 3300 B.P. (all dates in uncorrected years B.P.)
In County Donegal, the initiation of lowland blanket peat probably dates to the earlier fourth millennium B.P. Investigations by Telford (1977) in the Glenveagh National park indicate that peat growth was underway by 4000 B.P. (pine stumps dating to 3850 ± 100 and 3880 ± 110 (Birm-845, -844)), though in a pollen profile from Lough Nadourcon, in the same area, the transition to blanket peat does not take place until c. 2500 B.P.

In evaluating the data from Northern Ireland, Smith (1975) recognizes the possible role of deteriorating climate in bog initiation. At the same time, he emphasizes the coincidence of bog initiation and the advent of cultures with a knowledge of metallurgy, and also the pollen analytical evidence for a general opening-up of woodland from early Bronze Age times, over and above that which took place in the preceding Neolithic. The possible key role of Townland, County Antrim, where depth of cultivation in the soil during the later Neolithic is regarded as determining the position at which iron pan formation occurred (Proudfoot 1958). This, and the subsequent formation of an impervious humus layer, are regarded as the main factors leading to blanket bog formation (Case et al. 1969; Mitchell 1972).
In north County Mayo, where some of the most extensive tracts of low-land blanket bog occur, investigation of early field systems preserved beneath the bog has provided pertinent information on the timing of lowland blanket bog formation. In the Behy/Glenulra area, a reliable terminus post quem for the formation of blanket bog over a wide area is provided by stone wall enclosures extending over c. 1000ha that have been recorded beneath the bog (Caulfield 1978; 1983; Lynch et al. 1987). As far as can be ascertained by probing through the often deep overlay of peat, the stone walls of this enclosure system lie directly on mineral ground. Caulfield (1987; 1983) has shown, on the basis of both 14C and archaeological evidence, that the walls were laid out in the Neolithic, i.e. during the fifth millennium B.P. The 14C evidence points to widespread blanket bog in the early fourth millennium B.P. (cf. Smith et al. 1973). Two summary pollen diagrams have been published by Moore (1979), but, unfortunately, these are not 14C-dated. The earliest Landnam may be represented in Moore's Behy II profile, which would suggest a pre-Neolithic age for the basal peat. Macrofossil and pollen evidence suggest that the basal peat, while containing fossil remains of blanket bog plants such as Sphagnum and Potentilla erecta, may only represent the precursor of blanket bog. Juncus seeds, for instance, are frequent in the basal 50cm. More detailed investigations to determine the age and the nature of the basal peats in this area are obviously desirable.
At Belderg, 7km to the west, a more complex pattern of blanket bog initiation is recorded. The main enclosure system situated on the leeward side of a gently sloping hillside was laid out in the Neolithic (Caulfield 1978), when peat growth had not yet commenced, at least in the area with stone walls. Radiocarbon dates of 4220 ± 95 for pine with lateral roots overlying mineral ground and 3835 ± 85 B.P. for oak growing in peat suggest that at the end of the fifth millennium bog was present in the area. Part of the enclosure remained free of peat or, at most, had only a thin covering until after an occupation phrase of late Bronze Age date (3200 B.P.; Caulfield 1978). This suggests that human activity, which is often implicated in bog initiation, may in some circumstances inhibit peat growth. In this connection, it is of interest that some of the latest dates for bog growth in the Sperrin uplands are those from the archaeological site at Beaghmore, County Tyrone (195m O.D.), where, though heath taxa were prominent in the pollen record from 3800 B.P. onwards (Pilcher 1969, diagram 1), local blanket bog initiation begins no earlier than 2700 B.P. (cf. also Cashelkeelty, county Kerry, Lynch 1981).

There is evidence to suggest that immediately to the west of Belderg blanket bog formation dates to the Atlantic period. Near Bellanaboy Bridge, to the west of Glenamoy, wood from a pine root growing in the mineral soil beneath peat gave a date of 7110 ± B.P., while another stump from the same areas, but with roots c. 20cm above the mineral soil, yielded the date 4340 ± 65 B.P. (Hakansson 1974). This, and the lack of evidence for pre-bog settlement in an extensive area about Glenamoy (Fig. 2), prompted Caulfield (1983) to suggest that blanket bog was already widespread here at the beginning of the Neolithic.

At Carrownaglogh, situated at 150m O.D. on the lower western slopes of the Ox Mountains, peat accumulation commenced in a shallow basin at or shortly after 4000 B.P. (O'Connell 1986). Lateral extension was slow, so that it was not until 3200 B.P. that peat began to accumulate near the base of the knoll (Location 5 in O'Connell 1986). Over 500 years later, blanket bog began to envelop the knoll. Here the mineral soil, which had been formed into cultivation ridges, showed strong iron pan development. Tillage, carried on to the point of soil exhaustion, and the subsequent abandonment of the site are envisaged as the factors giving rise to iron pan formation which in turn led to waterlogging, humus accumulation and bog growth (O'Connell 1986).

In south-west Ireland, the data of Jessen (1949) relating to blanket bog initiation have been augmented by investigations of Lynch (1981) into the palaeoenvironment of stone alignments and circles of the south Kerry/south-west Cork region (cf. Cashelkeelty, Fig. 1). These investigations confirm the relatively late date for widespread blanket bog initiation suggested by Jessen's data. The 14C dates from the basal peat at the archaeological sites span a wide range, from 3260 ± 55 B.P. at Maughansilly, south-west Cork (213m O.D.) to early first millennium B.P. dates at other sites. The pollen content of the buried mineral soils shows, however, that heath vegetation, and possibly bog, were present and that regional podzolization was taking place from early in the fourth millennium B.P. Pollen analytical investigations by Welten (1952), at altitudes of 270m and higher on Carrauntoohil (1041m), County Kerry, show blanket bog initiation taking place quite late in the sub-Atlantic, though Hakansson (1974) gives a date of 4600 ± 65 B.P. for a pine stump in basal peat from an altitude of 250m. The overall lateness of the dates for bog expansion from south-west Ireland is surprising in view of rainfall in excess of 1600mm per annum, the high altitude, and the long history of agriculture, which at Cashelkeelty extended back to the earliest Neolithic. These results, together with those reviewed earlier, serve to emphasize the difficulties in deciphering a pattern and establishing the causes of blanket bog growth.

The results of investigations which allow reconstruction of the environmental conditions and timing of blanket bog growth in Connemara, County Galway, are now considered.
PALAEOECOLOGICAL INVESTIGATIONS INTO BLANKET BOG GROWTH AT CONNEMARA NATIONAL PARK, LETTERFRACK, NORTH-WEST CONNEMARA
The sites chosen for investigation lie to the south-east of the Park Centre (grid ref. L 7157) where blanket bog, now much cut over, dominates the treeless landscape (Fig. 3).

The deepest peats are recorded in a basin bounded to the south by the Owengarve River and its tributary, the Sruffaunnanoon, and to the north by elevated ground that rises stepply at the north-eastern side. A feature of this basin peat is an extensive intercalated layer of pine stumps. At the location of the main profile (FRKII), this pine stump layer rested on 2.4m of peat and was overlain by a layer 1.2m thick. Pollen analysis and radiocarbon dating showed that the profile spanned most of the post-glacial (Bowler 1986). An account of the woodland history is published in O'Connell et al. (1988). Here the history of mire development at the sampling site will be considered in the light of the pollen analytical results (Fig.4), supplemented by macro-fossil analysis carried out at selected levels (Heijnis 1987), a summary of which is provided in Fig. 5.

The macro- and microfossil analyses indicate that, though terrestrial peat as distinct from limnic deposits began to accumulate in the pre-Boreal (zone IV, local posslen assemblage zones FRKII-1 (this pollen analytical zone is omitted in Figs 4 and 5) and FRKII-2, it was not until the mid-Atlantic period (post 6000 B.P., FRKII-7), that a mire vegetation with at least some floristic affinities to present-day blanket bog communities was established locally. Macrofossil analysis has not been carried out on the peat relating to the lower part of FRKII-7, but the pollen evidence indicates that grasses (probably Molinia), Narthecium, sedges, Calluna and Potentilla (most likely P. erecta) were frequent on the mire surface. Remains of the rhizopod Assulina are frequent at these levels, indicating that wet habitat conditions prevailed (Grospietsch 1972).
Across the elm declin (FRKII-7/8 boundary), Gramineae values decrease from 42% to 8%, Sphagnum sect. acutifolia leaves are frequent, and Sphagnum spores are well represented. This suggests that the grass component of the mire vegetation declined while Sphagnum expanded. Assulina is now rare so that a drying out of the bog surface may have occurred, but this cannot have been severe as Narthecium continues to be well represented for some time. In the middle of FRKII-8, centres on 4000 B.P., a pine woodland became established on the mire surface. In this woodland, heathers (Erica tetralix and also Calluna) were important in the ground layer. As Pinus representation begins to decline, Sphagnum sect. acutifolia leaves are again recorded, suggesting that Sphagnum growth contributed to the demise of the bog woodland.
