Subglacial bedforms of the Irish Ice Sheet

Journal of Maps ISSN: (Print) 1744-5647 (Online) Journal homepage: http://www.tandfonline.com/loi/tjom20 Subglacial bedforms of the Irish Ice Sheet Sarah L. Greenwood & Chris D. Clark To cite this article: Sarah L. Greenwood & Chris D. Clark (2008) Subglacial bedforms of the Irish Ice Sheet, Journal of Maps, 4:1, 332-357, DOI: 10.4113/jom.2008.1030 To link to this article: https://doi.org/10.4113/jom.2008.1030 Copyright Taylor and Francis Group, LLC View supplementary material Published online: 23 Jan 2012. Submit your article to this journal Article views: 368 View related articles Citing articles: 37 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tjom20 Download by: [37.44.199.49] Date: 02 December 2017, At: 10:46

Subglacial bedforms of the Irish Ice Sheet SARAH L. GREENWOOD 1,2 and CHRIS D. CLARK 1 1 Department of Geography, University of Sheffield, Winter Street, Sheffield S10 2TN, UK; sarah.greenwood@natgeo.su.se 2 Department of Physical Geography and Quaternary Geology, Stockholm University, 10691 Stockholm, SWEDEN. (Received 7 th July 2008; Revised 30 th October 2008; Accepted 4 th November 2008) Abstract: The last Irish Ice Sheet has a long history of investigation, but its most basic properties are still debated. A palaeo-glaciological inversion of the glacial geomorphological record has elsewhere proven an effective means to reconstruct former ice sheets at ice sheet scale. To this end, this paper describes new glacial geomorphological mapping of the Irish landscape, using Digital Elevation Models, Landsat ETM+ and SPOT satellite images, with the objective of achieving a thorough and systematic documentation of the form, spatial distribution and arrangement of glacial landforms in Ireland. A resulting map, presented here at a scale of 1:425,000, identifies 33,000 subglacial bedforms. Glacial lineations (drumlins, mega-scale glacial lineations and crag and tails) and ribbed moraine comprise the bulk of the bedform population, although a wide spectrum of bedform types, shapes and sizes is observed. In particular, a suite of enigmatic bedforms were mapped which were not classified either as streamlined lineations or transverse ridges; rather, they are quasi-circular in form. This map is the first to document the subglacial bedform record of Ireland at the scale of individual landforms, and countrywide. In so doing, it reveals the extent and complexity of superimposed and cross-cutting bedform relationships, which demand an ice sheet reconstruction depicting marked changes in ice sheet geometry, configuration and behaviour through its evolution. This map forms the basis for a palaeo-glaciological reconstruction which addresses and attempts to incorporate the complexity of ice flow history revealed by the subglacial bedforms of Ireland. ISSN 1744-5647 http://www.journalofmaps.com 332

1. Introduction A fundamental problem in understanding contemporary ice sheet dynamics is the slow evolution of an ice sheet. We can only observe a short snapshot of activity in a much longer cycle of evolution. However, continental-scale ice sheets have waxed and waned throughout the Quaternary period, particularly across the northern hemisphere mid-latitudes, leaving a record of their evolution in the landscape. Quaternary ice sheet reconstruction therefore provides an opportunity to understand ice sheet behaviour through a full glacial cycle. An inversion of the glacial geomorphological record provides an effective means (though not the sole route of enquiry) to reconstruct former ice sheets at ice sheet scale (e.g. Dyke and Prest, 1987; Boulton and Clark, 1990a;b; Clark, 1997; Kleman and Borgström, 1996; Kleman et al., 1997; 2006). An essential pre-requisite for such an approach is a glacial landform map. This paper presents an attempt to fully and systematically document the subglacial geomorphology of Ireland (the Republic of Ireland and Northern Ireland), in order to yield the desired elements of the palaeo-glaciology of the Irish Ice Sheet. The Irish landscape is rich with evidence of former glaciation, and has been extensively studied for well over 100 years (e.g. Close, 1867; Hull, 1878; McCabe, 2008; Ballantyne et al., 2008; Brooks et al., 2008). Indeed, the terms drumlin and esker are of Irish origin. A comprehensive map of Irish glacial landform distribution has existed since Synge and Stephens (1960) and has been much replicated and updated subsequently (e.g. Synge, 1970; 1979; McCabe, 1985; 1987; 1991; 2008; Knight et al., 2004; Farrell et al., 2005, see Figure 1). Whilst this map (and its subsequent iterations) informs us of the main distribution of landform types, and provides a sense of the main ice flow paths or ice-marginal zones, it does not record individual landforms, and generalises information at the landscape level. This disguises the detailed field observations of glacigenic landforms and deposits which, for many areas, have locally yielded highly detailed information of past glacial environments (e.g. Kinahan and Close, 1872; Farrington, 1936; Synge, 1950; Hill, 1973; Thomas and Summers, 1983; Dardis et al., 1984; McCabe et al., 1984; Warren, 1988; Ó Cofaigh and Evans, 2001b; Hegarty, 2002; Knight, 2003). However, such detailed local observations are spatially fragmented, by virtue of chosen field areas, across the former ice sheet bed. At ice sheet scale, when compared to what is required to constrain a dynamically evolving ice sheet, the documented evidence-base for ice sheet reconstruction is internally inconsistent, patchy and incomplete. 333

So u t h ern Ir e la nd E nd M Journal of Maps, 2008, 332-357 or e ai n striae kames drumlins eskers glacial limits outwash gravels Figure 1. Glacial landform map of Ireland, redrawn from Synge (1979). 334

Furthermore, interpretation of the Irish evidence of glaciation seems to be invariably associated with debate and controversy: an offshore extent versus a maximum limit at the Southern Ireland End Moraine (see Figure 1; e.g. Warren, 1991; 1992; Synge, 1969; 1970); major readvances vs steady retreat (e.g. Synge, 1970; 1979; McCabe et al., 1998; Meehan, 2004; 2006); thick ice vs thin ice (e.g. McCabe, 1997; Lambeck, 1996); glaciomarine vs glaciolacustrine or subglacial sedimentation around the present-day Irish coast, particularly in the Irish Sea Basin (e.g. Eyles and McCabe, 1989; Knight, 2001; McCarroll, 2001; Ó Cofaigh and Evans, 2001b;a); and differing representations of the overall ice sheet configuration and history (e.g. Warren, 1992; McCabe, 2008; Brooks et al., 2008). Consequently, despite many recent advances, consensus regarding many characteristics of the last (and previous) ice sheet(s) to engulf Ireland is yet to be achieved. Here, for the first time, we present a countrywide glacial map of Ireland which details individual landforms and their spatial relationships. We take a top-down approach to systematically map glacial landforms throughout Ireland, using a variety of high resolution satellite images and digital elevation models. The Subglacial Bedform Map forms the primary basis for interpreting a new palaeo-glaciological model of the last Irish Ice Sheet. We publish the map here as a basic dataset that underpins our interpretations of ice sheet history which will be presented elsewhere (Greenwood, 2008, and forthcoming papers). 2. Data and Methods 2.1 Primary data The glacial geomorphology of Ireland was mapped digitally from a variety of remotely sensed data. The primary imagery used comprised two Digital Elevation Models (DEMs) and two suites of satellite images; their coverage and basic properties are outlined in Table 1 and Figure 2. A variety of imagery was used in order that the relative merits of each data source (resolution, image visualisation, data quality etc.) were maximised and the limitations mitigated (Clark, 1997; Jansson and Glasser, 2005; Smith and Clark, 2005; Smith and Wise, 2007). Data from the various sources were geocorrected, DEM tiles stitched together as necessary, and processed to 335

best visualise the landscape for geomorphological mapping. The Irish National Grid (Transverse Mercator projected coordinate system; spheroid: Airy Modified 1849; datum: Ireland 1965; units: metres) was selected as the base projection for all work and data were geocorrected accordingly. To improve landform detectability and minimise illumination bias (Clark and Meehan, 2001; Smith and Clark, 2005) a standard protocol was employed for landform mapping from the DEMs, using at least three differently illuminated visualisations for any area: a NW solar shading, NE, and a view illuminated from directly overhead ( gradient visualisation of Smith and Clark, 2005). Other directions of solar shading were often explored to best highlight features for accurate mapping. Vertical exaggeration between 3x and 5x was used. Satellite imagery was typically viewed using either solely the panchromatic band, a false colour composite using a variety of band combinations (5,3,2 [R,G,B] typically proved most useful for Landsat ETM+ images, for example), or a hybrid (pan-sharpened) image which transferred the information in a colour composite onto the higher resolution panchromatic image. Draping satellite imagery over a shaded DEM was further found to be a useful tool for landform visualisation (cf. Jansson and Glasser, 2005). 2.2 Ancillary data A number of extra sources of information assisted landform detection and identification of features, providing a degree of quality control over the mapping and ensuring only features with a glacial origin were included. Quaternary geology maps and digital data ( 1:50,000 Republic of Ireland National Soil and Subsoil Survey digital data; 1:250,000 map of the Quaternary geology of Northern Ireland; and 1:625,000 UK surficial sediment digital data) which distinguish till, glaciofluvial material, lake or marine sediments, alluvium, peat, made ground, modern water bodies and bedrock exposed at the surface, make them an invaluable accompaniment to glacial landform mapping (e.g. Figure 3). Solid and structural geological datasets and Ordnance Survey Ireland (OSi) maps at a range of scales helped to ensure a glacial feature was not interpreted where there was clearly an alternative explanation, either geological, structural or due to modern-day land cover or use. In select areas, higher resolution digital elevation data (e.g. OSi 10 m DEM) were used to enhance the clarity of features observed in the primary imagery. Finally, published literature was a key element of mapping quality control and support. For consistency, all 336

Data source Technical properties Spatial resolution Landmap DEM DEM built from ERS 1 and 2 SAR images via interferometry Initial projection 25 m Irish National Grid Useful data coverage Acknowledgement Countrywide MIMAS (University of Manchester-UCL) http://www.landmap.ac.uk Shuttle Radar Topographic Mission (SRTM) DEM DEM built by interferometry from shuttle mission 3 arc seconds ( 58 m long, 92 m lat at 51 N) Geographic (lat/long) Countrywide Global Land Cover Facility (University of Maryland) http: //glcf.umiacs.umd.edu/index.shtml Landsat ETM+ Winter scenes (path 207, rows 022, 023, 024) Bands 2,3,4,5 (visible to mid-ir) Panchromatic band (visible and IR) UTM Zones 29-30 3 images, 185 x 185 km (see Figure 2) Global Land Cover Facility (University of Maryland) 30 m http: //glcf.umiacs.umd.edu/index.shtml 15 m SPOT Winter scenes (see Fig 2) Bands 1-4 (visible to mid-ir) Panchromatic band (visible) 10-20 m 2.5 m UTM Zones 29-30 16 images, 60 x 60 km (see Figure 2) OASIS programme for SPOT image distribution, c CNES Table 1. Properties of data sources used for landform mapping. 337

a b 207,022 013,240 (SPOT 2) 014,240 (SPOT 1) 207,023 013,240/7 (SPOT 5) 013,242 (2) 013,242 (4) 014,240/6 (SPOT 5) 018,241/6 (5) 018,241/3 (5) 207,024 013,244 (5) 013,245 (5) 013,243 (5) 014,245 (5) 013,242/7 (5) 014,244/1 (5) Figure 2. Data coverage of imagery used for geomorphic mapping in Ireland. Two DEMs cover the whole land area (i.e. black coastline). All available winter, cloud-free Landsat ETM+ (a) and SPOT (b) imagery was employed (full coverage unavailable). Path and row numbers as labelled. previously mapped areas were remapped, but published literature provided a useful guide and check on the mapping conducted here. 2.3 Mapping A repeat-pass procedure was adopted in order to ensure the end mapping product is as thorough and consistent as possible. Countrywide mapping was conducted landform type by landform type, and data source by data source, which ensured there were multiple passes of the same area and the same imagery. Any additional details could be added or refinements made to the initial pass of mapping as necessary. DEMs were the primary source, given their full coverage and greater visualisation flexibility; satellite imagery consulted independently of or in combination with DEM data supplemented initial mapping. Imagery was consulted at a range of scales to ensure the full spectrum of landforms could be identified. Mapping was typically conducted between about 1:8,000 and 1:150,000 depending on the 338

a b c d Figure 3. Image pairs a&b and c&d reveal how Quaternary geological data considerably supports landform mapping and interpretation. a) linear landforms (drumlins) oriented NW-SE overlie smaller-scale features, of uncertain origin, oriented perpendicularly. b) Quaternary geology data reveals that the NW-SE landforms are characterised by till at the surface and the underlying structures are in fact bedrock. c) lineations oriented NW-SE are revealed by the distribution of till and bedrock at the surface in (d) to be a mix of drumlins and crag and tails (all shown as mapped black lines). Surficial geology data from the Irish National Soil and Subsoil Survey, 2006, Teagasc/GSI/EPA. Projected on the Irish National Grid. data type and the landform scale in question. Three groups of landforms were mapped to achieve the overall objectives of the research and derive a palaeo-glaciological reconstruction: subglacial bedforms (glacial lineations and ribbed moraine, see Table 2 for diagnostic characteristics), meltwater landforms (meltwater channels and eskers), and moraines. The identified features were digitised directly on-screen in a GIS framework, either as single lines (arcs) or as closed shapes (polygons) depending on their scale relative to data resolution: typically lineations were mapped by their crestlines, ribbed moraine as polygons around their lower break-of-slope. This paper presents the subglacial bedform record of Ireland. 339

Landform group Glacial lineations: drumlins Glacial lineations: crag and tails Characteristics and diagnostic criteria Subglacial bedforms streamlined in the direction of ice flow. Classically an elongated ellipse, or egg shape, but may encompass a range of forms including more spindle-like, parabolic or barchanoid. Lengths are typically on the order of 100s-1000s metres. Features tend to develop in fields, i.e. as a suite of landforms rather than as individuals in isolation. Lineations with such characteristics, and a smooth surface appearance indicative of a drift composition, were mapped as drumlins (Menzies, 1987; Rose, 1987; Benn and Evans, 1998). As above, but with an identifiable bedrock crag at the head of the feature and drift tail. Glacial lineations: mega-scale glacial lineations Ribbed moraine Particularly long and elongate glacial lineation, with lengths on the order of kilometres. Typically found in highly parallel arrangements (Clark, 1993; Stokes and Clark, 1999). Tabular or ripple-like ridges of sediment formed transverse to the ice flow direction. Ridges possess a wide range of sizes and morphologies, from minor ridges (order of metres - 10s m) to mega-ridges (order kms). Occur in fields (Dunlop and Clark, 2006). Table 2. Characteristics of subglacial bedforms which underpin their identification in the imagery used. 3. Subglacial bedform map: description of glacial geomorphology The map presented here reveals three main groups of subglacial bedform identified by the mapping programme: glacial lineations, ribbed moraine, and a group of other bedforms. These are considered in turn, below. In total, the bedform population of Ireland documented by our mapping comprises 32,725 features and, having examined the whole landmass of Ireland, bedforms appears to characterise 80% of its area, illustrating the ubiquity of such landforms. 3.1 Glacial lineations Drumlins, crag and tails and mega-scale glacial lineations (MSGLs) comprise the bulk of the bedform population: 23,500 features. Purely drift lineations are symbolised on the map by a black line, with crag and tails in grey arrows indicating the interpreted direction of ice flow. Lines are mapped along the feature s crestline, and the line length shown on the map is therefore representative of the individual feature s length. Glacial 340

lineations identified in Ireland occupy a large size range, from the smallest identified drumlins at 131 metres in length (Figure 4a) to lineations which are more suitably interpreted as MSGLs, reaching lengths of over 10 km (Figure 4b). These longer lineations are found within landform patterns which have long been known (the north-westerly directed lineation pattern across Counties Roscommon and Mayo, towards the Ox Mountains and County Sligo coast; and the south-westwards pattern through the broad valley in County Clare constricted by the high terrain of the Burren to the west and the Slieve Aughtys to the east (see Close, 1867; Synge, 1970; McCabe, 1985; Warren, 1992)), these features have not previously been identified as MSGLs. This interpretation suggests these landform patterns are the legacy of faster flowing ice (Clark, 1993; Stokes and Clark, 1999; 2001). Overall, the range of observed morphologies of glacial lineations varies from classic elliptical drumlins to spindle shapes, barchan forms, curvilinear landforms, to quasi-circular forms which are less straightforward to classify (see other bedforms, below). The map clearly reveals the glacial lineations to dominate the lowlands, almost fully occupying the wide open plains and broad valleys which characterise the macro-topography of Ireland. A striking characteristic of the lineation distribution is the complex arrangement of individual features. Instances of lineation superimposition upon other bedforms, either ribbed moraine or other lineations, are found in abundance (Figure 5). Such arrangements suggest changing subglacial conditions for bedforming, frequently associated with a change in the requisite ice flow direction. The lineation record therefore contributes a relative chronology of palaeo-ice flow configuration. Interpretations of ice flow directions from lineation data, and their palaeo-ice sheet implications, will be addressed in forthcoming papers. 3.2 Ribbed moraine Ribbed moraine represent 20% of the number of features mapped. These ridge-like bedforms, which form transverse to the ice flow direction, were not widely recognised in the Irish landscape until the last 10 years or so (e.g. Knight and McCabe, 1997; McCabe et al., 1999; Clark and Meehan, 2001). The ribbed moraine population documented by our mapping confirms and extends the previously known distribution. The individuals within this population represent the largest forms of ribbed moraine identified hitherto in the world, and our mapping confirms their shape and scale and is consistent with the detailed morphometric analyses of Dunlop 341

200000 202000 204000 a 160000 160000 162000 162000 164000 164000 Figure 4. Glacial lineations observed in Ireland range in size from (a) small drumlins (arrowed) of <500 m in length, to (b) MSGLs (mapped crestlines shown) several kilometres long (note the scale bar differences). Image in (a): SPOT, Slieve Aughty mountains; b): SRTM, NE visualisation with 4x vertical exaggeration, Counties Roscommon and Mayo. Projected on the Irish National Grid. and Clark (2006). The form and distribution of the Irish ribbed moraine appear to be largely unaffected by either geological or topographic variability, rather covering a large swath of the Irish midlands and occupying the core zone of the palaeo-ice sheet (Figure 6). At the other end of the scale, small transverse ridges were found in clusters across Counties Meath, Westmeath, Roscommon and Mayo (Figure 7). The size of these features is almost at the resolution limit of the available imagery and it is therefore likely that the mapped population of minor ridges is an under-representation of their true number and distribution. The interpretation of these minor transverse ridges clearly has implications for ice sheet reconstruction: subglacial or ice-marginal? Meehan (1999) first suggested the ridges in Co. Meath were push moraines and therefore ice-marginal. However, they were re-interpreted as ribbed moraine by Clark and Meehan (2001) on the basis of their relation to the underlying macro-topography: ridge disposition and distribution shows little sign of deflection by topography, suggesting ice was thick and ridges are subglacial, i.e. minor ribbed moraine. Following the same logic (Figure 7), two of the three major clusters of minor ridges are interpreted as subglacial in origin (Figure 7b and 7c) and are presented on the map. By contrast, the other cluster in north Co. Mayo (Figure 7d) feeds into a topographically confined bay; an ice-marginal context is plausible for the development of these ridges, b 130000 130000 140000 140000 10 km 280000 290000 300000 342

290000 300000 a 120000 130000 b 176000 174000 172000 324000 326000 320000 c 210000 22000 d 250000 260000 270000 Figure 5. Superimposition and cross-cutting of landforms overwhelms the subglacial bedform record in Ireland. a) drumlins (SW-NE) cross-cut MSGLs (SE-NW) in Co. Mayo; SRTM image, NW illuminated, 4x vertical exaggeration. b) cross-cutting drumlins (left-right and top left - bottom right) in Co. Leitrim; Landsat ETM+, image rotated through 180 for better visualisation of landforms. c&d) drumlins superimposed perpendicularly on ribbed moraine; the ice flow direction revealed by the two landform types is the same. c: Landmap, NW illuminated, 4x vertical exaggeration; d: SRTM, NW illuminated, 4x vertical exaggeration. Projected on the Irish National Grid. 2km 328000 290000 300000 310000 recording recession of an ice lobe through Killala Bay. This is consistent with previously interpreted retreat patterns for this area (e.g. Charlesworth, 1928; Synge and Stephens, 1960; Synge, 1969). Whereas larger ribbed moraine are frequently found to be overprinted by lineations, minor ribbed moraine display the reverse landform association, and are observed superimposed upon, or actively ribbing underlying glacial lineations. 343

Figure 6. Large transverse ribbed moraine ridges occupy a vast swath of the Irish lowlands, and additionally overrun some areas of higher terrain such as Slieve Beagh (immediately NE of image centre), parts of the Cuilcagh mountains (west of centre) and Slieve Gullion (centre east). Cross-cutting between ribbed moraine groups, and the patterns of lineation superimposition indicate that the ribbed moraine population is the product of multiple phases of ice flow with different geometries. SRTM image, illuminated from the NW with 4x vertical exaggeration. Projected on the Irish National Grid. 344

a b 250000 260000 d c b 270000 280000 High : 1020m 290000 300000 c 2 km Low : 0m 140000 150000 d 120000 130000 Figure 7. Classification of minor ridges (a) into subglacial (b, c) or ice-marginal (d) or contexts on the basis of their relationship with the underlying macro-topography: a lack of deflection by the underlying gross topography suggests thick ice and a subglacial ridge origin; distribution restricted to low ground is more indicative of an ice-marginal context. Projected on the Irish National Grid. 2km 2km 320000 330000 3.3 Other bedforms The mapping programme revealed a significant number of landforms which we interpret as bedforms, but which do not fit into the classical and clear-cut categories of lineations or ribbed moraine. These are usually quasi-circular in form, with diameters on the order of 500 metres (Figure 8), and were recorded as other since their genetic origin and palaeo-glaciological significance is less well known than for lineations or ribbed moraine. These bedforms are usually found within larger 345

assemblages of either drumlins, ribbed moraine or a mixed population, lending them an appearance of a hybrid bedform between the two end members. Such bedform morphologies have been previously observed in Ireland (e.g. Hill, 1973; Finch and Walsh, 1973; Dardis et al., 1984; McCabe, 1991; Smith and Clark, 2005); other authors have typically described as drumlins some of the features we record as quasi-circular. These enigmatic landforms have been little discussed regarding either their genetic (i.e. geomorphological process) or palaeo-glaciological significance. 98000 97000 96000 48000 48000 47000 46000 47000 46000 1 km 98000 97000 96000 Figure 8. Quasi-circular drift landforms, interpreted as subglacial bedforms. Such landforms constitute 6% of the mapped subglacial bedform record and are frequently found in spatial association with drumlins or ribbed moraine. SPOT image from Bantry Bay, SW Ireland. Note this image is rotated through 180 to achieve a sun angle illuminating the image from the top. Projected on the Irish National Grid. 346

3.4 Map completeness and accuracy The imagery available to this research has been thoroughly examined through a number of repeat passes, and it is with confidence that the bedform dataset presented in the map is as complete a record of subglacial bedforms as revealed by this imagery. It is most unlikely that this is an entirely complete bedform record, i.e. truth. Submerged landforms would likely fill gaps in the spatial record of bedforms were they visible under the larger and deeper lakes: Lough Neagh, Lower Lough Erne, Lough Mask and Lough Corrib. The bedform record also extends to the present-day coastal zones and it is likely they pass offshore, where imagery was lacking at the time of mapping. We highlight the nearshore zone as a prime region for future mapping from bathymetric data. Data resolution (e.g. Landmap 25 m; Landsat ETM+ up to 15 m horizontal resolution) inevitably limits the number and accuracy of landforms mapped at the smaller end of the scale, although such under-representation within the mapped population is thought to be minimal. Bedform size is typically on the order of 100s - 1000s of metres, with relief up to 10s metres (Rose, 1987; Benn and Evans, 1998; Clark, 1993); indeed, the population of lineations identified here captures a range from 131-10,522 m. Only the smallest and most subdued features should have escaped detection. It is anticipated that the mapping effort here has captured the range of shapes, scales, and most of the patterns of bedforms in Ireland. The new, emerging generation of even higher resolution data (i.e. sub 5 m) will likely yield higher populations of the smallest features. The accuracy of the mapping output is dependent upon the initial location accuracy of the source imagery. Minor geo-positional errors between data sources, and their deviation from true positioning (i.e. compared to OSi products) is estimated to be on the order of 100 m. 4. Implications and Conclusions Interestingly, whilst most Irish Ice Sheet models now depict offshore ice cover for the maximum period of glaciation (e.g. Warren, 1992; Bowen et al., 2002; Sejrup et al., 2005; Ó Cofaigh and Evans, 2007; McCabe, 2008), the overall bedform distribution is reminiscent of the restricted 347

glacial limits originally proposed by Synge (Synge and Stephens, 1960; Synge, 1970). However, it would be erroneous to return to this ice sheet interpretation on the basis of the subglacial bedform distribution. The absence of bedforms in southern-most Ireland is not necessarily indicative of the absence of ice, merely conditions in the subglacial environment which were not conducive to bedform generation, or, indeed, could indicate their subsequent removal or burial. The value of the subglacial bedform map of Ireland is twofold: the documented distribution and form of a large population of subglacial bedforms allows us to pursue a greater understanding of the geomorphological processes which produce subglacial bedforms; and it reveals elements of the geometry and behaviour of ice flow routes which generated the observed geomorphological record, thereby providing the basis for extracting palaeo-glaciological properties of the last Irish Ice Sheet. We have documented here the morphological and spatial properties of almost 33,000 bedforms. Such a large dataset is better able to examine and test theories of bedform genesis than smaller sample sizes from specific topographic, geologic or glaciological settings. Any bedforming theory must be able to account for the diversity of characteristics this record displays. This includes not only the classic drumlin and ribbed moraine properties, but the atypical shapes, sizes, arrangements, settings and degrees of reworking we have observed in the Irish record (e.g. Figure 9). Detailed geomorphological mapping and analysis of morphometric and spatial parameters (e.g. Clark et al., in press) enables quantitative bedforming models to be tested (e.g. Dunlop, 2004; Dunlop et al., in press) and will enable comparisons with potential controlling variables to be examined: topographic properties (shape, slope, elevation), bedrock and surface sediment lithologies, and glaciological properties (e.g. ice thickness). The overall coverage and majority of flow patterns revealed by the bedform record confirms and replicates what the existing literature reports and, in many places, adds considerable details of morphology, pattern, and the complexities of landform arrangements. The bedform record can therefore be interpreted in terms of what it reveals regarding ice flow directions, the necessary flow geometry and flow behaviour to explain the record, the ice sheet configuration and configuration changes, and should be integrated with other components of the glacial record for a full palaeo-glaciological reconstruction. The most significant new insight yielded by our mapping, and the most powerful indicator of the need for a new model of ice sheet history for Ireland, is the prevalence of bedform cross-cutting and 348

superimposition throughout the Irish landform record (Figure 10). The overwhelming dominance of cross-cutting bedform patterns has significant implications for the positions and movements of ice divides over Ireland. The bedform record demands an ice sheet model characterised by significant, and possibly repeated re-organisations of its geometry and structure. The map we present in this paper is the foundation for interpreting and reconstructing such a model of Irish Ice Sheet history. Software All imagery for geomorphological mapping was imported into, geocorrected and processed in ERDAS Imagine 8.7 software. Mapping was conducted in both ERDAS Imagine 8.7 and ESRI ArcMap 9.1, recording all mapped features in separate shapefiles according to landform type. The SRTM DEM (for terrestrial areas) and GEBCO DEM (bathymetry) used for the map background were processed in ArcMap, in which the final map production was undertaken. Figures within this paper were created in ArcMap and manipulated for aesthetic purposes in Adobe Illustrator 9. Acknowledgements The Geological Survey of Ireland, Environmental Protection Agency, Ordnance Survey Ireland and the OASIS programme for dissemination of SPOT imagery kindly provided data, on request, for free or at a considerably reduced cost. S.L.G. acknowledges a University of Sheffield studentship from 2004-2007. Mike Smith, Jasper Knight and Tim Fisher are thanked for their reviews which have improved the clarity of the paper. Figure 9 (next page). The new Subglacial Bedform Map reveals a wide range of bedform shapes and sizes which provide a test of bedform genesis theories: any theory must be able to account for these morphologies. a) barchan-type drumlins intermingled with more classic types (County Monaghan), SRTM, NE illumination with 4x vertical exaggeration; b) crag and tails (County Westmeath), Landmap, NE illumination with 4x vertical exaggeration; c) quasi-circular bedforms (Bantry Bay, County Cork), SPOT; d) complex ridges with horns and drumlinised parts; we interpret this particular suite as a composite product of more than one ice flow (Clew Bay, County Mayo), SRTM, NW illumination with 4x vertical exaggeration; e) ribbed moraine with branching ridges displaying no coherent orientation (Omagh Basin, County Tyrone), SRTM, overhead (gradient) illumination with 4x vertical exaggeration; f) drumlinised mega-scale ribbed moraine (Counties Monaghan & Cavan), Landmap, NW illumination with 4x vertical exaggeration. Projected on the Irish National Grid. 349

Journal of Maps, 2008, 332-357 2 km Figure 9. Caption on previous page. 350

Journal of Maps, 2008, 332-357 Figure 10. Cross-cutting and superimposed bedform patterns are widespread throughout the bedform record of Ireland. On a 2 x 2 km grid, black cells show the distribution of bedforms; green cells indicate where superimposition or cross-cutting of bedforms occurs. There is not a simple snapshot pattern of ice flow in Ireland but, rather, the prevalence of superimposition and cross-cutting relationships demands an ice sheet model characterised by frequent changes of flow configuration to repeatedly rework the bed in changing directions. 351

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