PRISTINE MIRE LANDSCAPES

Preparing the ground

William P. Warren

Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland Telephone: +353 1 6782865 Fax: +353 1 6681782, E-mail: [email protected]

Summary The concentration of raised peat bogs across the centre of Ireland is often attributed simply to the poorly drained nature of the Shannon basin and the former occurrence of a broad interconnecting network of lakes across the postglacial landscape. This describes the conditions that obtained, but not the cause. The nature of deglaciation, the disposition of the retreating ice sheets relative to one another and the isostatic lowering of the land surface were critical factors in determining the conditions that lead to the rapid growth of basin peat in the central midlands. Fundamental to this was the way in which the ice sheet, which had been composed of a number of coalescing ice domes broke up into its constituent parts while the domes retreated back to their centres of dispersion.

Key index words: Irish midlands, deglaciation, glaciolacustrine, postglacial, isostacy

Introduction most of these, in particular the esker complexes, to the final The bogs of the midlands of Ireland are founded almost acts of deposition during deglaciation (Warren and Ashley, exclusively on late Quaternary glacial, glaciofluvial, and 1994). most particularly, glaciolacustrine deposits. Commonly, The overall pattern of the glaciation of Ireland has been the raised bogs are easily seen to be separated by ridges or understood in outline since Maxwell Close (1867) carried out hills of relatively well-drained mineral soil: coarse esker his seminal work more than 140 years ago. It is unfortunate gravels, fans of well-bedded sands and gravels, and a that this work was very little understood at the time, and that mixture of low permeability and medium permeability a model of glaciation later emerged, largely influenced by glacial tills. The immediately sub-peat substrate in the peat Charlesworth (1928), which postulated a simple ice sheet that basins was until recently poorly known and very little extended from north to south and receded again from south understood. This uncertainty as to the sub-peat geology to north, came to be established as the working model of our raised the obvious question as to the influence of this understanding of the events of the last glaciation for a period substrate on the evolution of the basin peats that charac- of more than fifty years (e.g. Synge, 1970). More recently a terise the midlands. This presentation attempts to model that relates directly to the available evidence for ice summarise the current understanding of the sub-peat movement and to the deglacial processes has been developed Quaternary geology of the midlands and to consider the (Warren, 1992). This is very close to the model presented by possible influence of isostatic depression on the evolution Close and has been rigorously tested in the midlands with of the raised bogs of the midlands of Ireland. It focuses on regard to the pattern of deglaciation and to esker genesis (see work carried out in investigating the raised bogs at Clara Warren and Ashley, 1994). According to this model the Irish and Raheenmore in (see Schouten, 2002), ice sheet was composed of a number of discrete ice domes, on investigations as to the characterisation of the eskers of initially separate ice sheets, which coalesced to form a single the midlands (see Warren and Ashley, 1994) and on body, but retained a flow pattern driven by the operation of ongoing geological mapping by the Geological Survey of the snow collection areas as individual centres of ice dispersal Ireland. (Fig. 1). Thus the important elements of the Irish ice sheet were the Northern Dome, the Central Dome, the Southern Glaciation and deglaciation of the Irish Dome and the Irish Sea ice lobe. During deglaciation, all of these bodies separated one midlands from the other as they shrank back to their respective There is unequivocal geological evidence for no more than centres of dispersion. As the Northern and Central Domes two Pleistocene glacial events in Ireland, and in by far the separated, they did so initially along the east to west line of greater part of the country we see evidence of no more than suture, and the area between the two separating domes was one, the most recent, termed the Fenitian Glaciation flooded by the emerging meltwater. The Shannon/Boyne (Warren, 1985). The important glacial features (the eskers, interfluve held up this lake at its eastern end and controlled subaqueous fans, deltas and drumlins) that, together with its depth. All of the deglacial sediments, the extensive esker the raised peat bogs, dominate the landscape of the systems and the great fans and deltas, of the central midlands of Ireland all relate to this final glaciation, and midlands were deposited into this lake. These are the readily

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Figure 1. A model of the Irish ice sheet during the last Pleistocene Figure 2. A section of core from laminated clay and silt be- glaciation, the Fenitian, in Ireland. neath . The sample is 73 mm wide.

seen Pleistocene deposits of the midlands. They imply the A 5m long undisturbed sample taken from the clays in the existence of fine distal muds and silts, but these were Clara Bog basin permitted a close analysis of their structure by generally not readily obvious. means of thin-sectioning and microscopical study (van der Meer and Warren, 1997). This study revealed that the clays were for the most part laminated, displaying particularly The sub-peat sediments of Clara and continuous rhythmic sets of laminae in the lower part and Raheenmore Bogs considerable disturbance due to sediment loading towards the Substantial drilling and electrical resistivity surveys carried top (Fig. 2). The largely normally graded sets of laminae were out at both Clara Bog and in County interpreted as rhythmites deposited by density underflow Offaly during the 1990s (Warren et al., 2002) significantly currents in a proglacial lake. This is not a surprising refined the understanding we previously had of midland conclusion, given the nature of the adjacent esker deposits, peat substrate from Bord na Móna records and very limited particularly at Clara Bog and near (Fig. 3). exposure in areas of cutaway bog. Generally the lake clays were seen to be underlain by These surveys revealed a general sub-peat sequence of glacial sediments, although the sampling process made it glacial sediments, overlain by rhythmically laminated sterile very difficult in places to determine whether these were clays and silts containing isolated pebbles, overlain in turn by direct glacial deposits or were deposited in a glaciofluvial or more massive sterile muds. These muds are directly overlain glaciolacustrine environment. by gyttja containing small carbonate grains, plant fragments, snail shells and shell fragments as well as quartz grains. The broader local context The lake clays underlying Clara bog achieve a thickness The basinal clays of Clara and Raheenmore can be of approximately 6m in the centre of the basin and thin considered as an integral part of a glaciolacustrine suite that towards the margin where they phase out entirely. At characterises much of the midlands. The Clara Bog basin is Raheenmore the clays are up to 8m thick, but are largely broadly defined by the Clara Esker ridge to the north and confined to two deep depressions in the centre of the basin. The Tullamore or Ballyduff Esker to the South. At Clara

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Figure 3. Sand and gravel foreset beds in a subaqueous fan (Tullamore Esker) deposited from the south and built up against the core of coarser tunnel-fill gravels, now removed. the esker is not the classical narrow winding ridge of Discussion – the deglacial sediments of subglacial river bottom sediments that are commonly the midlands thought of as characterising such landforms. Rather the vast Deglaciation of the midlands of Ireland took place in the bulk of the sediments immediately to the north of Clara context of a widening interdomal lake that filled the area Bog, and similarly in the Tullamore esker ridge some 4km between the Northern and Central Domes of the Irish ice to the south, are composed of well sorted and relatively sheet as these domes pulled apart from one another. With steeply-dipping foreset beds of sand and gravel (Fig. 3). the exception of some of the tills, almost all of the sediments These foresets relate to a series of coalescing sediment lobes are glaciolacustrine in character. Where studies have been and make up the ridges which are up to 10 km long at a carried out the distal sediments, corresponding to the stretch and up to 400 m wide. The overlapping sediment proximal esker and fan deposits, are found within the basins lobes dip transverse to the overall alignment of the ridges that are now largely, or were until recently, covered by raised and are interpreted as subaqueous fan deposits (generally peat bogs (van der Meer and Warren, 1979; Long and well stratified medium gravel to fine sand) These are the O’Riordan, 2001; Delaney, 2008). proximal deposits of a subglacial drainage systems that fed The large esker complexes of Clara and Tullamore are subaqueously into an ice marginal lake and were deposited each composed of a central ridge composed of very coarse at, or very close to, a retreating ice margin (Warren, 1991; gravels, which were laid down in subglacial arterial tunnels Smyth, 1994; Warren and Ashley, 1994). The finer running along the suture of the Northern and Central sediments, the lake muds and silts in the bog basins are Domes and draining both domes, very shortly before the interpreted as the distal equivalents of these and other domes separated. These sediments young from east to west, similar sediments elsewhere in the midlands. reflecting the zipper-like opening of the ice sheet. Once the Ongoing geological mapping has revealed and/or domes had separated, the sediment reaching the lake expanded on our knowledge of sequences of subaqueous fan supported between them formed huge fans which were and delta sediments and associated mud drapes that can be banked up against the coarse gravel, subglacially formed traced throughout the midlands. Notable localities include ridges, and the fines were carried away into the deeper south-east of Tullamore, where coarse esker gravels basins, where they were deposited as rhythmic couplets. As and associated fan sediments are draped by up to 1m of the ice fronts further receded the fans formed at the mouths apparently massive clay, Screggan where an extensive fan- of these drainage tunnels that once were feeders into the to-delta complex extends over an area of approximately 18 great west – east drainage arteries. These ice marginal fans 2 km , Kilbeggan and Split Hill where there are extensive ice or fan moraines marked positions of ice-front still-stand. marginal fan sediments, Kildallan some 8 km west- Although these gravel fans are ice-marginal deposits and northwest of Mullingar where there are extensive lacustrine each marks a point of meltwater egress into the lake defined sands and gravels, and west of the Shannon from Athlone as by the separating margins of the two ice domes, the precise far north as Ballyhaunis where there are complex ice pattern of retreat of the ice fronts remains to be determined. marginal subaqueous sands and gravels (see Warren and The general pattern is however clearly reflected in both the Ashley, 1994; Ashley and Warren, 1995; van der Meer and disposition of the core esker ridges and flow-direction Warren, 1997; Long and O’Riordan, 2001; Delaney, 2001; indicators (cross lamination, cross-bedding, clast Warren et al., 2002; Delaney, 2008). imbrication and dip of foreset beds) in the fan sediments. When viewed within the context of the current model Thus it is clear that the margin of Northern Dome in the of glaciation and deglaciation for the midlands area central midlands retreated to the northwest as reflected in (Warren, 1992), these sediments offer considerable insight the alignment of the eskers of Westmeath and Longford, into the geological conditions that obtained both during while the margin of the Central Dome retreated to the and immediately following deglaciation and allow us to southwest as reflected in the alignment of the Kilcormack estimate with a high degree of confidence what the general Esker which extends from Birr to Tullamore in County nature of the sub-peat geology is. Offaly. Both the eskers of the Northern Dome, for example

97 PRISTINE MIRE LANDSCAPES the Split Hill Esker, and that of the Central Dome, the Mayo, each a legacy of a subglacial stream that fed into this Kilcormack Esker, show clear evidence in their associated great midlands glacial lake. marginal fans of having been deposited in a lacustrine Because we have not yet been able to reconstruct a con- environment (Warren and Ashley, 1994). temporaneous chronology for the recession of the two main In the western midlands, in the area around Ballyhaunis ice domes, it is not possible to determine either the for example, the ice of the Central Dome pushed forward a maximum extent of the interdomal lake or to date its final little to occupy some of the space that had been occupied by drainage. However some at least of the fine clays and silts are ice of the Northern Dome, so that eskers deposited in the rhythmic (van der Meer and Warren,1997; Long and area of suture between the two domes immediately prior to O’Riordan, 2001) in character and may in places be varves, separation are seen to run over drumlins deposited by an that is to say, each couplet may represent an annual accu- earlier strong movement of the Northern Dome. Around mulation. Recent research (Delaney, 2008) indicates that Ballyhaunis most of the coarse glaciolacustrine sediments some at least are indeed varves. A varve chronology, if one (generally ice-marginal subaqueous fan sediments), are can be established, would have the potential to relatively associated with ice of the Central Dome (Warren and date the retreat of the ice fronts during deglaciation and Ashley, 1994). ultimately possibly provide an absolute chronology. Apart from the major features, such as those mentioned The configuration of the topography is such that a very hitherto, small mounds of subaqueous fan sediments, often large portion of the midlands would have remained flooded no more than a few hundred metres across, occur dotted even after the ice domes had been reduced to very small ice across the central midlands from eastern Counties Offaly bodies. Figure 4 is a schematic diagramme designed to and Westmeath into Counties Galway, Roscommon and illustrate this rather than to define a specific deglacial pattern.

Figure 4. A schematic representation of postulated ice retreat and interdomal lake growth in central Ireland.

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Figure 5. Mushroom stones near the western shore of Lough Ree. Taken from Foot et al. (1865).

Conclusions – the immediate a former lake level about 4 m above the level of Lough Ree postglacial landscape in the nineteenth century (Foot et al., 1865) as witness to Following final deglaciation, not only was the newly its former greater height and extent. emergent land surface quite bare of peat and alluvium, but The combination of an extensive drape of impermeable sea level was far lower and as a consequence the total land glaciolacustrine muds in the basins and low ground of the area or Ireland much larger. A large proportion of the midlands, coupled with the much greater extent of lake Shannon basin would have been composed of an extended basins in the immediate postglacial period will have greatly shallow lake or a network of interconnected lakes that is facilitated peat initiation in extensive lake-marginal fen reflected broadly in the modern pattern of basins areas. This in turn will have impeded the development of a (see Mitchell, 1976). Furthermore, one must consider that better drained area as isostatic recovery was completed, the land surface remained isostatically depressed as it some time after 5000 BP. Thus the nature and extent of recovered slowly from the weight imposed by the former ice what became the raised peat basins owe much to both the cover. Full recovery was probably not achieved until some disposition of the late Pleistocene ice sheets and to the time after 5,000 BP (see Synge, 1977a). The isostatic nature of the deglaciation of the midlands isobases for the east coast are well known (Synge, 1977b) and demonstrate a northward lag in isostatic recovery. This References suggests that ice was generally thicker to the north in the Ashley, G.M. and Warren, W.P. (1995). Irish Eskers – Origins of Ice-Contact east coast area, and this fits with the known disposition of Stratified Deposits. Field guide for the INQUA Commission on the ice domes and the Irish Sea ice sheet (Warren, 1992). Formation and Properties of Glacial Deposits. Dublin, Geological Survey of Ireland, 59pp. The relative thickness of the ice domes with respect to the midlands is less well known. However, by analogy with Charlesworth, J.K. (1928). The Glacial retreat from Central and Southern Ireland. Quarterly Journal of the Geological Society of London 84, 293- the east coast area it is likely that overall the uplift curve 342. would rise in a generally northerly direction, reflecting a Close, M.H. (1867). Notes on the general glaciation of Ireland. Journal of greater isostatic depression and thicker former ice cover to the Royal Geological Society of Ireland 1, 207-242. the north. Thus it is likely that the upper Shannon basin was depressed relative to the lower part. This being the case Delaney, C. (2001). Morphology and sedimentology of the Rooskagh Esker, Co. Roscommon. Irish Journal of Earth Sciences 19, 5-22. there would have been a prolonged period of greater general flooding in the Shannon basin than its present configura- Delaney, C. (2008). Seasonal controls on deposition of Late Devensian glaciolacustrine sediments, Central Ireland. In M. Hambrey, P. tion might suggest. Christoffersen, N. Glasser, P. Janssen, B. Hubbard and Siegert, M. The absence of a continuous horizontal marker, as is (eds.), Glacial Sedimentary Processes and Products, 146-163. Special provided by the east coast littoral, makes it difficult to Publication, International Association of Sedimentologists. Oxford, reconstruct the isostatic history of the area. A clue to the Blackwells. former extent of a postglacial lake in the midlands is Dunne, L. and Feehan, J. (2003). Ireland’s Mushroom Stones. Dublin, En- provided by the so-called mushroom stones (Fig. 5) that are vironmental Institute UCD, 28pp. particularly common around Lough Ree (Foot et al., 1865; Farrington, A. and Synge, F.M. (1970). The Eskers of the Tullamore Dunne and Feehan, 2002). These are limestones, the lower district. In N. Stephens and R.E. Glascock (eds.), Irish Geographical parts of which were eroded by solution below and up to the Studies in honour of E. Estyn Evans, 49-52. Belfast, The Department of Geography, The Queen’s University. then level of the lake, but which now at Lough Ree indicate

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Foot, F.J., O’Kelly, J. and du Noyer, G.V. (1865). Explanation to accompany Synge, F.M. (1977a). The coasts of Leinster (Ireland). In C. Kidson and sheets 98, 99, 108 and 109 of the one-inch map of the Geological Survey M.J. Tooley (eds.), The Quaternary History of the Irish Sea, 199-222. of Ireland illustrating parts of the Counties of Westmeath, Roscommon, Geological Journal Special Issue No. 7. Liverpool, Seel House Press. Galway, Longford and King’s County. Dublin, Geological Survey of Synge, F.M. (1977b). Records of sea levels during the Late Devensian. Ireland, 39pp. Philosophical transactions of the Royal Society of London, B, 280, 211- Long, M.M. and O’Riordan, N.L. (2001). Field behaviour of very soft 228. clays at the Athlone embankments. Géotechnique, 51, 293-309. Warren, W.P. (1985). Stratigraphy. In K.J. Edwards and W.P. Warren (eds.), van der Meer, J. J.M. and Warren, W. P. (1997). Sedimentology of Late The Quaternary History of Ireland, 39-65. London, Academic Press. Glacial Clays in Lacustrine Basins, Central Ireland. Quaternary Science Warren, W.P. (1991). Ireland 1991, INQUA Commission on formation and Reviews, 16, 779-791. properties of glacial deposits. Field guide for excursion. Dublin, Geological Mitchell, F. (1976). The Irish Landscape. London, Collins, 240pp. Survey of Ireland, 65 pp. Schouten, M.G.C. (ed.) (2002). Conservation and Restoration of Raised Warren, W.P. (1992). Drumlin orientation and the pattern of glaciation in Bogs. Geological, Hydrological and Ecological Studies. Dublin and Ireland. In A.M. Robertson, B. Ringberg, U. Miller and L. Brunnberg Driebergen, Dúchas, Staatsbosbeheer and the Geological Survey of (eds.), Glacial Morphology and Morphological Changes. Sveriges Ireland, 220pp. Geologiska Undersökning, Ser. Ca 81, 359-366. Smyth, M. (1994) Quaternary Geology and Geophysical Studies of Clara and Warren, W.P. and Ashley, G.M. (1994). Origins of the ice-contact stratified Raheenmore Bogs, Co. Offaly, Ireland. Ph.D. Thesis, National University ridges (eskers) of Ireland. Journal of Sedimentary Research 64, 433-449. of Ireland Galway. Warren, W.P., Smyth, M., van der Meer, J.J.M. and Hammond, R.F. Synge, F.M. (1970). The Irish Quaternary: current views 1969. In N. (2002). Chapter 2: Geology. In M.G.C. Schouten (ed.), Conservation Stephens and R.E. Glascock (eds.), Irish Geographical Studies in honour and Restoration of Raised Bogs. Geological, Hydrological and Ecological of E. Estyn Evans, 34-48. Belfast, The Department of Geography, The Studies, 16-31. Dublin and Driebergen, Dúchas, Staatsbosbeheer and Queen’s University. the Geological Survey of Ireland.

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