Periglacial trimlines and the extent of the Kerry-Cork Ice Cap, SW Ireland

Quaternary Science Reviews 30 (2011) 3834e3845

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Periglacial trimlines and the extent of the Kerry-Cork Ice Cap, SW Ireland Colin K. Ballantyne a, *, Danny McCarroll b, John O. Stone c a

School of Geography and Geosciences, University of St Andrews, Fife KY16 9AL, Scotland, UK Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, Wales, UK c Department of Earth and Space Sciences and Quaternary Research Center, University of Washington, Box 351310, Seattle, WA 98195-1310, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 May 2011 Received in revised form 13 October 2011 Accepted 15 October 2011 Available online 10 November 2011

Periglacial trimlines on the mountains of SW Ireland demarcate a contrast between glacially scoured bedrock on lower ground and thick periglacial regolith covers on summits and plateaus, and have previously been interpreted as marking the maximum altitude of the Kerry-Cork Ice Cap (KCIC). Glacially eroded bedrock extends to the highest points on the Beara peninsula, but the highest summits on the Iveragh Peninsula support thick regolith covers. Trimline altitudes on the Iveragh Peninsula decline radially from a former ice divide near the head of Kenmare River, from w700 m altitude near the divide to
470 m near Dingle Bay. Contrasts in the clay mineral content of above- and below-trimline soils and cosmogenic isotope exposure ages indicate a LGM age for the trimlines. Assumption that the trimlines represent the maximum thickness of the KCIC implies an ice divide altitude of w825 m. However, modelling of ice-surface profiles demonstrates that an ice divide at 825 m is irreconcilable with stratigraphic evidence for extension of the KCIC at least as far as Garryvoe, 115 km east of the divide. We infer that the Iveragh trimlines represent a former englacial transition between warm-based sliding ice on low ground and cold-based ice on high ground rather than delimiting nunataks as previously interpreted. Modelling of ice-surface altitudes suggests that during the LGM the KCIC probably overtopped all of the mountains in SW Ireland. The trimlines of SW Ireland thus define the minimum rather than maximum altitude of the KCIC at the LGM, as well as the sites of former ‘cold patches’ within the KCIC. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Trimlines Cold-based ice Ice-surface profiles Exposure dating Clay-fraction mineralogy

1. Introduction A periglacial trimline represents the upper level to which glacial erosion has removed or ‘trimmed’ a pre-existing regolith cover in glaciated mountain environments. Such trimlines can be interpreted in two ways: they may be of ice-marginal origin, and represent the upper limit of a warm-based glacier or ice sheet at its maximum thickness; or they may represent an englacial transition within a former ice sheet from erosive warm-based ice occupying low ground to cold-based ice occupying upper slopes, summits and plateaus (Ballantyne, 1997, 2007). In the latter case, it is assumed that the cold-based ice was frozen to the underlying substrate and that the adhesive strength of the rockesubstrate interface exceeded basal shear stresses (Kleman and Glasser, 2007), permitting the survival of tors, shattered rock outcrops, blockfields and other regolith covers under a ‘protective’ cover of cold-based ice on high ground whilst adjacent lower slopes experienced bedrock abrasion and quarrying by warm-based (wet-based) sliding ice.

* Corresponding author. Tel.: þ44 0 1334 463907; fax: þ44 0 1334 463949. E-mail addresses: [email protected], [email protected] (C.K. Ballantyne). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.10.006

A contrast between ice-scoured valley-side slopes and regolithmantled plateaus is widespread on the mountains of Great Britain (e.g. Ballantyne et al., 1998; Lamb and Ballantyne, 1998; McCarroll and Ballantyne, 2000). Trimlines on British mountains were originally interpreted as being of ice-marginal origin, and thus indicative of the maximum altitude reached by the last BritisheIrish Ice Sheet during the last glacial maximum (LGM) of c. 26e21 ka. However, inconsistencies in trimline altitudes, the presence of above-trimline erratics and offshore evidence indicating a much more extensive ice sheet than previously believed all favour reinterpretation of trimlines on British mountains as being of englacial origin (Ballantyne and Hall, 2008; Ballantyne, 2010). Recent proxyclimate driven models of the BIIS also imply that ice covered the highest ground in Scotland, England and Wales during the LGM, consistent with an englacial interpretation of trimlines in these areas (Boulton and Hagdorn, 2006; Hubbard et al., 2009). In Ireland, similar contrasts between regolith covers on plateaus and ice-abraded lower slopes have been mapped on the Wicklow Mountains in the east, Donegal in the northwest and MayoConnemara in the west, and interpreted as indicating the minimum LGM ice altitude in these areas (Ballantyne et al., 2006, 2007, 2008). Some of the most conspicuous periglacial trimlines

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in Ireland occur on the sandstone mountains of Kerry in the southwest of the country (Fig. 1). Trimlines on mountains in the Iveragh Peninsula (Fig. 2) were first mapped by Wright (1927), and interpreted by him as demarcating the altitude of former ice cover during the period of ‘maximum glaciation’ in this area. Additional ‘unglaciated’ areas of high ground in the same area were identified by Farrington (1947), and Bryant (1968) depicted several further ‘unglaciated’ mountains in SW Iveragh. Warren (1979) subsequently identified areas in north and west Iveragh that were apparently free of glacier ice when the ice margin stood at the Kilcummin moraine, which several authors (e.g. Synge and Stephens, 1960; McCabe, 1985, 1987; Bowen et al., 1986, 2002) have considered to delimit the maximum lateral extent of ice in the area during the LGM. More recently, Rae et al. (2004) focused on trimlines flanking the Gap of Dunloe in eastern Iveragh (Fig. 2), and demonstrated significant differences in rock and soil weathering between above- and below-trimline sites, leading them to conclude that the Gap of Dunloe trimlines mark the upper limit of ice at the LGM. Although all the above accounts interpret the trimlines of SW Ireland as representing the upper limits of former ice cover, they differ markedly in terms of the size and locations of inferred palaeonunataks (Rae et al., 2004). Here we employ a combination of geomorphological evidence, analysis of clay-fraction mineralogy and 10Be exposure dating (1) to establish the altitude and locations of trimlines across the entire area of the Iveragh and Beara Peninsulas in SW Ireland, and (2) to examine the significance of the trimline evidence for the vertical and lateral dimensions of the ice that occupied this area during the LGM. 2. The mountains of SW Ireland The mountains of SW Ireland occupy three peninsulas (the Dingle, Iveragh and Beara Peninsulas), and the area inland to the east. This study focuses on the mountains of the Iveragh and Beara Peninsulas and their eastern neighbours within an area defined by latitudes 51330 Ne52 050 N and longitudes 09130 e10 260

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W (Fig. 2). Within this area all high ground is underlain by nonmarine arkoses of Devonian age, gently folded into WSW-trending synclines and anticlines. The mountains of the Iveragh Peninsula fall into two groups. The northern group extends 40 km eastwards from Knocknadobar (2 in Fig. 2) to Purple Mountain (21) and incorporates the Iveragh Mountains and Macgillycuddy’s Reeks, the latter culminating in Carrauntoohil (1039 m), the highest summit in Ireland. The southern group stretches 62 km ENE from Cahernageeha (22) to East Pap (43), and incorporates the Coomcallee-Mullaghanattin chain and the Mangerton Mountain massif. On the Beara Peninsula the Caha and Shehy Mountains reach their highest altitudes on Hungry Hill (685 m; 48), and Knockboy (706 m; 52) respectively. The passes and cols that interrupt these mountain chains are floored by ice-moulded bedrock, reflecting abrasion by glacier ice passing between mountain barriers. On many mountains, icescoured bedrock extends to summit levels, but on others the highest ground is occupied by a continuous regolith cover comprising slabby sandstone clasts overlying and embedded within a sandy matrix, with no evidence for over-riding by warmbased erosive ice (Figs. 1 and 3). 3. The Kerry-Cork Ice Cap Abundant striae and roches moutonnées indicate that the Iveragh and Beara Peninsulas were occupied by a locally-nourished ice cap, the Kerry-Cork Ice Cap (KCIC), which radiated outwards from an ice divide near the head of Kenmare River (Fig. 2), the inlet that separates the Iveragh and Beara Peninsulas (Wright, 1927; Farrington, 1936, 1954; Bryant, 1968; Warren, 1979, 1991a, 1991b). Similar evidence and the pattern of moraines on the low ground north of the Macgillycuddy’s Reeks (Warren, 1979, 1991a) demonstrates that the latter formed a major barrier to northwards ice movement, which was deflected around the western and eastern flanks of the Reeks to coalesce on low ground in northern Iveragh. The outermost moraine in this area, the Kilcummin moraine,

Fig. 1. Broachnabinnia (735 m) on the Iveragh Peninsula, SW Ireland, photographed from the north. A trimline at 690e700 m is defined by the boundary between a lower zone of ice-abraded sandstone outcrops and the periglacial regolith cover on the plateau.

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Fig. 2. The Iveragh and Beara Peninsulas, SW Ireland, showing ground above 300 m, the locations of the 53 mountains on which observations were carried out and the Kilcummin moraine complex. Individual mountains are identified in Table 1.

defines an ice limit that sweeps round in a broad arc from the head of Dingle Bay to a point 8 km east of Killarney (Fig. 2; Warren, 1979, 1991a). The lateral extent of the KCIC during the LGM is contentious. There are three conflicting interpretations. 1. Synge and Stephens (1960) depicted an ice cap of limited extent that terminated across the Iveragh Peninsula in the west, terminated at the Kilcummin moraine limit in the north and extended eastwards to the Kilumney moraine west of Cork city. A similar LGM ice limit, though with ice cover extending seawards from the Iveragh Peninsula, is depicted in several later publications (e.g. McCabe, 1985, 1987; Bowen et al., 1986, 2002; Knight et al., 2004; Fig. 4a). 2. Warren (1985, 1991a, 1991b, 1992) argued that the extent of the KCIC coincides with the lateral extent of a till of local provenance (Garryvoe till) that overlies a periglacial slope deposit, which in turn overlies a raised beach (Courtmacsherry Formation). This view implies that KCIC ice extended at least

115 km eastwards from the Kenmare River ice divide to the type site at Garryvoe on the south coast, and was confluent with ice moving southwards from the Irish midlands (Fig. 4b). Ó Cofaigh et al. (2010) have confirmed confluence of KCIC ice with ice from the Irish midlands in this area, and employed OSL dating to show that the Courtmacsherry raised beach was deposited during marine isotope stages 3e4, demonstrating that the Garryvoe till was deposited during the LGM. Warren (1991a) conceded that the northern limit of the KCIC is difficult to determine, but observed that till of southern provenance occurs north of the Slieve Mish mountains on the northern part of the Dingle Peninsula. He inferred from absence of till overlying raised beach deposits in the SW part of the Dingle Peninsula and north of Tralee Bay that part of the Dingle Peninsula remained ice-free, and that an unglaciated enclave separated the KCIC and inland ice to the north (Fig. 4b). Ó Cofaigh et al. (2008) have shown that tills exposed on the northern shore of the Dingle Peninsula reflect northwards ice movement beyond the limit depicted by Warren (1992),

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Fig. 3. (a) Glacially-abraded sandstone slabs and roches moutonnées extend to the summit of Hungry Hill (685 m) on the Beara Peninsula, indicating that the mountain was overrun by glacier ice. (b) Thick regolith cover on Teermoyle Mountain (760 m) in the Iveragh Mountains. (c) Glacially-moulded sandstone bedrock below 600 m on the SW spur of Caher (1001 m) near the western end of Macgillycuddy’s Reeks. (d) Cover of coarse debris above 700 m on the SW spur of Caher; compare with (c).

though it is unclear whether this ice originated on the mountains of the Dingle Peninsula or represents northwards extension of ice from the Kenmare River ice centre. 3. The third interpretation envisages offshore extension of Irish ice to the Atlantic shelf edge in all sectors. This interpretation implies that all of the present land area of SW Ireland was completely ice covered at the LGM, and that the KCIC was confluent to the north with ice from Galway and to the east with ice from the Irish midlands (Fig. 4c). This view, advocated by Sejrup et al. (2005), is supported by the presence of large north-south aligned linear drift ridges on the shelf west of Ireland. These features are undated, but were interpreted by Haflidason et al. (1997) and King et al. (1998) as representing retreat positions of the last ice sheet as it withdrew from the shelf edge. There is strong evidence that during the LGM a major ice stream from the Irish Sea basin extended southwards across the Celtic Sea to a grounding line SW of the Scilly Isles at latitude 49 300 N (Scourse et al., 1990; Scourse and Furze, 2001; Hiemstra et al., 2006; McCarroll et al., 2010). The LGM ice limit on the shelf SW of Ireland depicted in Fig. 4c was drawn by Sejrup et al. (2005) along the 200 m isobath to link the outermost (undated) drift ridge west of Ireland with the southward extent of ice in the Celtic Sea. However, analysis of exposures along the southern coast of Ireland (Ó Cofaigh et al., 2010) has shown that the southwards advance of the Irish Sea Ice Stream occurred before inland ice from the Irish midlands reached the area, suggesting that the ice limit depicted in Fig. 4c is asynchronous.

We consider below the validity of these three models of KCIC extent from the evidence of former ice altitudes implied by trimline evidence, then employ the eastwards lateral extent of the KCIC to test the validity of the ‘ice-marginal’ and ‘englacial’ interpretations of the trimline evidence in SW Ireland. Evidence for the later development of glacier ice on the Macgillycuddy’s Reeks during the Younger Dryas Stade (c. 12.9e11.7 ka) is immaterial in this context, as such glaciers were confined to corries (Anderson et al., 1998; Harrison et al., 2010).

4. Methods To establish the upper altitudinal limits of evidence for former glacial erosion, observations were made on 53 mountains: 43 on the Iveragh Peninsula and neighbouring areas to the east, 9 on the Beara Peninsula, and the isolated summit of Nowen Hill (53) the highest point east of Bantry Bay (Fig. 2 and Table 1). Features recorded included landforms indicative of former ice cover (ice-abraded bedrock, roches moutonnées, glacially-transported ’perched’ boulders and the upper limit of glacial drift) and features indicative of high ground that escaped glacial erosion (autochthonous blockfields and other forms of in situ regolith cover, and angular, shattered bedrock outcrops; Figs. 1 and 3). To reduce the complications introduced by downslope movement of debris since deglaciation, mapping was focused on summit plateaus, cols and other areas of level terrain unaffected by mass movement. Although on some mountains a distinct trimline provides a well-defined boundary

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a

b

c

Fig. 4. Models of the extent of glacier ice on Ireland and the adjacent shelf during the LGM. (a) Restricted ice cover, with the KCIC terminating at the Kilcummin moraine in the north and Kilumney moraine in the east, as depicted by McCabe (1987). (b) More extensive KCIC, based on stratigraphic evidence, with unglaciated enclaves on the Dingle Peninsula and north of Tralee Bay as depicted by Warren and Ashley (1994). (c) Extensive ice cover extending offshore to the shelf edge (200 m isobath) from SW Ireland, as depicted by Sejrup et al. (2005).

between these two zones (Fig. 1), on most high ground only minimum and/or maximum altitudes for the former upper limit of glacial evidence could be determined, particularly on steep slopes or where unbroken peat cover obscures the underlying substrate. To test the validity of the mapping and establish trimline age, differences in weathering characteristics above and below the mapped upper limit of glacial evidence were assessed by extracting samples from the base of soil pits for analysis of clay-fraction mineralogy. Twelve samples were obtained from regolith cover at altitudes of 495e795 m and nine from soils below upper limit of glacial evidence from altitudes of 405e700 m. For each sample the clay fraction was isolated by centrifugation, then samples were submitted for X-ray diffraction (XRD) analysis using a PW 1049/ DG2 Philips/Hiltonbrooks diffractometer. As a further test of trimline age, we collected samples for surface exposure dating using cosmogenic 10Be. A total of 12 samples were obtained, three from rock outcrops above mapped trimlines and nine from ice-moulded rock outcrops below trimlines. Only four samples, however, permitted extraction of sufficient quartz for AMS assay, two from above mapped trimlines and two from ice-abraded bedrock slabs. Pure quartz was obtained from these samples by flotation in dense liquids and selective dissolution of other minerals with dilute HF (Kohl and Nishiizumi, 1992). Beryllium-10 was separated from samples weighing 10e20 g in the presence of w250 mg 9Be carrier, using conventional methods (Ditchburn and Whitehead, 1994; see also; Stone, 2005). Details of the isotopic analyses are given in the footnotes to Table 3. Blank corrections amounted to <1% in all cases. 5. Geomorphological mapping 5.1. Iveragh Peninsula: Cahernageeha (22) to East Pap (43) Along the mountain chain that runs ENE from Cahernageeha to East Pap (Fig. 2), all summits below 650 m support ice-abraded

bedrock, but higher summits carry a thick regolith cover and exhibit no convincing evidence for the passage of glacier ice (Table 1). The summits of the westernmost hills (Cahernageeha (499 m; 22) and Eagles Hill (549 m; 23)) are obscured by peat, but ice-moulded bedrock up to altitudes of 467 m and 500 m respectively suggests that both were probably over-run by ice. Regolith cover under peat on the summit of Coomcallee (675 m; 24) suggests that the highest parts of this hill escaped glacial erosion. On Knockmoyle (684 m; 25), striated bedrock and perched boulders occur up to 628 m, but both are absent from higher ground. On the twin summits of Mullaghanattin (752 m and 773 m; 26 and 27) whalebacks and perched boulders extend up to w670 m, but above 680 m the terrain consists of angular bedrock outcrops, with a thick cover of gelifluctate on moderate slopes. A well-defined trimline occurs on Broaghnabinnia (735 m; 30), where ice-scoured rock ribs extend up to 690e700 m but upper slopes and the summit plateau are covered by a blockfield (Fig. 1). Similar evidence places the upper limit of glacial erosion between 670 m and 710 m on neighbouring Stoompa Duloch (784 m; 29). Between Stoompa Duloch and Mangerton Mountain (839 m; 39) all eight intervening summits (509e645 m; 31e38) exhibit ice-abraded rock slabs. The summit plateau of Mangerton Mountain itself supports a thick regolith mantle above w700 m, but ice-moulded rock extends up to w680 m on a subsidiary summit to the north. On Stoompa (705 m; 40) icemoulded rock outcrops extend up to 540 m and the lowest in situ regolith cover occurs at w660 m. Crohane (655 m; 41) supports a regolith mantle at its summit, but ice-scoured slabs up to at least 530 m. Peat obscures much of East Pap (694 m; 43), but perched boulders occur up to 530 m and rock outcrops above 680 m are shattered and display no evidence for the passage of ice. A massive cairn on the summit has been constructed from angular sandstone clasts stripped from the summit blockfield. Collectively, the evidence from this mountain chain indicates that the upper limit of glacial erosion rises gently from Coomcallee to Broaghnabinnia near

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Table 1 Investigated mountains and inferred minimum ice surface altitude.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53.

Mountain

Summit altitude (m)

OIS grid reference (summit)

Upper limit of glacial evidence (m)

Lower limit of in situ regolith cover (m)

Inferred minimum ice surface altitude (m)

Bolus Knocknadobar Knocknadobar NE Drung Hill Beenmore Been Hill Mullaghnarakill Teermoyle Mountain Coomacarrea Caunoge Meenteog Colly Beenreagh Seefin Knocknacusha Caher Skregmore Skregbeg Carrauntoohil Cnoc na Dtarbh Purple Mountain Cahernageeha Eagles Hill Coomcallee Knockmoyle Mullaghanattin SW Mullaghanattin Knockaunanattin Stoompa Duloch Broaghnabinnia Knocklomena Graighnagower Boughill Peakeen Mountain Knockanaguish Knockrower Knockbrack Dromderalough Mangerton Mountain Stoompa Torc Mountain Crohane East Pap Knockgour Maulin Coomacloghane Lackabane Hungry Hill Knockowen Coomnadiha Sugarloaf Mountain Knockboy Nowen Hill

410 690 633 640 655 651 665 760 772 502 715 679 495 493 545 1001 848 573 1039 655 832 499 549 675 684 752 773 569 784 735 641 595 639 555 509 554 605 645 839 705 535 655 694 481 621 599 602 685 658 644 574 706 509

V 399635 V 507845 V 530858 V 602879 V 597868 V 590854 V 601851 V 604833 V 611826 V 582800 V 638826 V 650808 V 661853 V 688900 V 676782 V 792839 V 792839 V 787874 V 804844 V 862850 V 980808 V 539613 V 583631 V 624677 V 665750 V 726765 V 739772 V 770790 V 787793 V 801813 V 787766 V 823766 V 833767 V 904765 V 919769 V 937785 V 953779 V 959789 V 980808 W 007818 V 955839 W 050830 W 133855 V 615450 V 713506 V 733549 V 752536 V 761498 V 809555 V 847600 V 873530 W 005620 W 128520

260 310 300 300 e e e 500 480 502 565 635 495 410 545 670 550 460 680 560 580 499 500 640 628 670 650 569 670 690 641 595 639 555 509 554 605 645 680 540 535 540 530 470 555 599 602 685 658 644 574 706 480

e 470 e 500 535 600 535 585 700 e 640 660 e e e 700 700 560 770 600 600 e

>260 310e470 >300 300e500 <535 <600 <535 500e585 480e700 >502 565e640 635e660 >495 >493 >545 670e700 550e700 450e560 680e770 560e600 580e600 >500 >500 640e650 >628 670e680 650e680 >569 670e710 690e700 >641 >595 >639 >555 >509 >554 >605 >645 680e700 540e680 >535 540e640 530e680 >481 >555 >599 >602 >685 >658 >644 >574 >706 >509

the axis of the former ice divide, and descends gently via Mangerton Mountain, Stoompa and Crohane to East Pap. 5.2. Iveragh Peninsula: Macgillycuddy’s Reeks (16e21) Wright (1927) and Warren (1979) demonstrated that northwards-moving ice split around the main ridge of Macgillycuddy’s Reeks, and the latter estimated the maximum elevation of ice on the southern flanks of Carrauntoohil (1039 m; 19) at 700 m, similar to the trimline altitude (690e700 m) on neighbouring Broaghnabinnia. Ice-abraded rock outcrops on the floors of cols at 640 m and 595 m near the eastern end of the Reeks provide a minimum altitude for glacial erosion in this area. At the western end of the Reeks, ice-moulded bedrock extends up to 670 m on the SW spur of Caher (1001 m; 16; Fig. 3c), but debris completely

650 e 680 680 e 710 700 e e e e e e e e 700 680 e 640 680 e e e e e e e e e e

mantles terrain above 700 m (Fig. 3d). Trimlines and drift limits on the western flanks of Skregmore (848 m; 17) and Skregbeg (573 m; 18) at 550e700 m and 450e560 m respectively indicate northwards decline in the upper limit of glacial evidence across the western flanks of the Reeks. A similar northwards decline in the upper limit of glacial evidence is evident in the Pass of Dunloe, which separates Cnoc na Dtarbh (655 m; 20) from Purple Mountain (832 m; 21) at the eastern end. Ice-scoured bedrock extends up to at least 550 m on Cnoc na Dtarbh, and a trimline at 580e600 m on the SW spur of Purple Mountain descends both northwards through the Pass of Dunloe and more gently ENE across the southern flank of Purple Mountain itself (Rae et al., 2004). Icescoured bedrock extends to the summit of Torc Mountain (535 m; 41), 7 km east of Purple Mountain, providing a minimum altitude for ice moving eastwards around the Reeks towards Killarney.

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Table 2 Soil clay-fraction mineralogy. Site

Altitude (m) Grid reference I

Samples above the mapped weathering limit A1 Mangerton Mountain 795 V 980808 A2 Purple Mountain 720 V 897858 A3 Stoompa Duloch 765 V 788792 A4 Stoompa Duloch 765 V 788792 A5 Meenteog 625 V 635833 A6 Meenteog 670 V 635827 A7 Meenteog 660 V 621825 A8 Teermoyle Mountain 680 V 605852 A9 Teermoyle Mountain 750 V 604834 A10 Teermoyle Mountain 755 V 605833 A11 Knocknadobar 680 V 669882 A12 Knocknadobar 495 V 521850 Samples below the mapped weathering limit B1 Hungry Hill 685 V 761498 B2 Knockboy 700 W 005620 B3 Knockboy 695 W 004620 B4 Purple Mountain 560 V 881844 B5 Purple Mountain 555 V 915858 B6 Mullaghanattin 665 V 729769 B7 Seefin 405 V 669882 B8 Seefin 410 V 670883 B9 Knockmoyle 553 V 645735

X X X X X X X X X X X X

Cl G Ca L

K

X X X X X X X X X X X X

X

X X X X X X X X X X X X X X X X X

X

X X X X X X X

X X X X X X X X

X X

X

X X X

X X

X X

All samples were obtained from sandstone soils. Quartz was detected in all samples. I ¼ Illite; Cl ¼ clinochlore; G ¼ gibbsite; Ca ¼ calcite; L ¼ lepidocrocite; K ¼ kaolinite.

Collectively, these observations show that evidence for glacial erosion reaches its maximum altitude on the southern flanks of the Reeks then descends northwards at their western end and through the Pass of Dunloe. 5.3. Iveragh Peninsula: the Iveragh Mountains (4e15), Bolus (1) and Knocknadobar (2,3) On the Iveragh Mountains, the higher parts of the ridge that extends southwards from Drung Hill (640 m; 4) to Meenteog (715 m; 11) supports a thick regolith cover, extensively covered by peat (Fig. 3b). It supports no unequivocal evidence for over-running by ice though large (>1 m long) tabular sandstone boulders resting on or embedded in frost-weathered regolith near the summit of Teermoyle and elsewhere on the ridge may have been emplaced by glacier ice, though it is not possible to exclude a frost-heave origin. On Drung Hill sandstone erratics extend up to 300 m (Warren, 1991a), but ground above 500 m supports a cover of frostweathered regolith. On Teermoyle Mountain (760 m; 8) near the centre of the ridge, ice-abraded bedrock slabs occur up to 500 m, and in situ periglacial regolith cover (Fig. 3b) could be traced down to 585 m. At the southern end of the main ridge, roches moutonnées and perched boulders at 555e565 m on the col between Meenteog (715 m; 11) and Colly (679 m; 12) demonstrate overrunning by westwards moving ice. Flow of ice across the Meenteog-Colly col is consistent with the presence of glaciallyabraded bedrock slabs at the summits of Caunoge (502 m; 10) and Knocknacusha (545 m; 15), respectively 6.5 km WSW and 4.5 km SE of the col, but the summit of Colly itself is covered by in Table 3 Exposure dating sample locations and Sample Samples from Purp-01 Mullagh-01 Samples from Der-01 MC-01 10

OIS Grid Reference

situ frost-shattered debris above 635 m. East of the Iveragh Mountains, ice moving to the northwest crossed the ridge that extends SSW-NNE from Beenreagh (495 m; 13) to Seefin (493 m; 14) and merged with locally-nourished glaciers flowing northeast along Glen Behy from five corries on the northeast side of the main ridge of the Iveragh Mountains (Warren, 1988). Much of the Beenreagh-Seefin ridge exhibits ice-moulded rock and perched boulders indicating northwards transfluence into Glen Behy. Regolith cover is absent from the summit of Beenreagh (495 m; 13) and a sandstone slab resting on steeply-dipping sandstone strata implies over-running by glacier ice. On Seefin (493 m; 14) at the NNE end of the ridge, ice-scoured bedrock and scattered glaciallydeposited boulders occur up to 410 m and the summit lacks thick regolith cover, implying removal by glacial erosion. Over-running of the entire ridge by ice is consistent with Warren’s (1988) observations of erratics extending to summit level (274 m) on low hills to the north, on the south side of Dingle Bay. Around Bolus (410 m; 1) on the SW tip of the Iveragh Peninsula, drift limits and ice-scoured rock indicate a minimum ice altitude of 260e300 m, confirming that the KCIC extended offshore. On Knocknadobar (690 m and 633 m; 2 and 3), ice-scoured rock ribs extend up to 310 m and perched boulders up to 360 m, but the col at 495 m between the two summits supports a blockfield, and debris occupies the flanks of the mountain down to 400 m. A minimum ice altitude of 310e400 m is implied, indicating that ice extended northwestwards into Dingle Bay. 5.4. Beara Peninsula (44e52) and Nowen Hill (53) All mountains investigated on the Beara Peninsula exhibit evidence of over-running by warm-based sliding ice, in the form of ice-scoured bedrock slabs and perched boulders on or near summits. Regolith covers are absent from even the highest ground, confirming observations made by Rae (2002) on the summits of Coomnadiha (644 m; 50) and Knockboy (706 m; 52). The alignment of striae, roches moutonnées and ice-moulded bedrock on summits and cols confirms radial ice movement across the peninsula from the Kenmare River ice divide (Warren, 1991a). Glacial over-running of Knockboy (706 m; 52) in the Shehy Mountains and of Nowen Hill (509 m 53) in the SE of the study area implies that ice cover extended east of the study area. Ice-moulded rock on the summit of Hungry Hill (685 m; 48; Fig. 3a) implies extension of ice cover >700 m thick into Bantry Bay. Near the SW extremity of the Beara Peninsula, ice-abraded bedrock slabs at 470 m on Knockgour (481 m; 44) indicate that the KCIC extended offshore. Collectively, this evidence implies that all of the Beara Peninsula was over-run by the KCIC and that the ice-surface altitude exceeded 650e700 m across all of the Caha and Shehy Mountains. 5.5. Geomorphological evidence: synthesis Fig. 5 depicts the areas of high ground that have apparently escaped glacial erosion together with contour lines depicting the approximate altitude of the ice surface if we assume that the

10

Be analytical data.

Latitude ( N)

Longitude ( W)

Altitude m OD

sandstone outcrops above the upper limit of glacial erosion V 896859 52.0149 9.609 740 V 601851 52.0011 10.0382 665 vein quartz on glacially-abraded bedrock surfaces V 770512 51.7007 9.7802 549 V 647819 51.9735 9.9700 564

Be concentrations are all 07KNSTD3110 standard, normalized to the KNSTD

10

Thickness mm

Density g cm3

Shielding correction

[10Be] 105 atoms g1

52 45

2.60 2.60

1.000 1.000

6.595  0.155 7.670  0.191

20 14

2.65 2.65

1.000 1.000

1.129  0.023 2.027  0.047

Be/9Be standard, traceable to the ICN

10

Be standard.

C.K. Ballantyne et al. / Quaternary Science Reviews 30 (2011) 3834e3845

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Fig. 5. Summit areas that apparently escaped glacial erosion, showing generalised directions of former ice movement derived from various sources (Wright, 1927; Bryant, 1968; Warren, 1979, 1991a) and contours of minimum ice surface altitude inferred from trimline data and directions of ice movement.

trimlines represent the maximum elevation of former ice cover. ‘Icesurface’ contours are interpolated between trimlines and extrapolated beyond trimlines by assuming that such contours lie approximately at right angles to directions of ice movement indicated by striae and roches moutonnées. The 700 m contour is primarily constrained by trimline evidence on Mullaghanattin (26 and 27; 670e680 m), Stoompa Duloch (29; 670e710 m), Broaghnabinnia (30; 690e700 m) and Mangerton Mountain (39; 680e700 m), the 600 m contour by the trimline altitudes on Coomcallee (24; 640e650 m), the Meenteog-Colly col (11 and 12; 565e635 m) the western flank of the Reeks between Skregbeg (18; 450e560 m) and Skregmore (17; 550e700 m), Cnoc na Dtarbh (20; 560e600 m), the SW shoulder of Purple Mountain (21; 580e600 m) and, with less confidence, between Crohane (42; 540e600 m) and East Pap (43; 530e680 m). The approximate position of the 500 m contour is constrained by the trimline altitude of 310e470 m on Knocknadobar (2 and 3) and the lower limit of regolith cover on Drung Hill (4) at

500 m. The general northwards decline in contour altitudes in Fig. 5 is consistent with the evidence for radial ice movement from an ice divide centred near the head of Kenmare River (Wright, 1927; Farrington, 1954; Bryant, 1968; Warren, 1979, 1991a, 1991b). A feature of the ice-surface reconstruction is the southward dip of the 600 m contour where northwards ice movement was impeded by the Macgillycuddy’s Reeks, and a slight dent in the 700 m contour where northwestwards ice movement was impeded by high ground between Mullaghanattin (26) and Broaghnabinnia (30). It should be emphasised that the contours depict only the general configuration of the inferred ice surface, and that a more complicated ice configuration is likely to have occurred in local centres of ice build-up such as the corries of the Iveragh Mountains (Warren, 1988) and those on the north side of the Reeks. On Iveragh Peninsula, the areas of high ground inferred to have escaped glacial erosion (Fig. 5) correspond particularly closely with the palaeonunataks (‘pre-glacial uplands’) mapped by Wright

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(1927) in NW Iveragh, exhibit some correspondence with the generalised ‘unglaciated areas’ on the Iveragh Mountains and Knocknadobar depicted by Farrington (1947), and are in agreement with the mapping of Rae et al. (2004) of the Gap of Dunloe area. In SW Iveragh, however, our mapping indicates little correspondence with that of Bryant (1968), who identified several large ‘unglaciated areas’ where our mapping confirms over-riding by ice. 6. Trimline age 6.1. Clay-fraction mineralogy Table 2 summarises the XRD analyses of clay-fraction mineralogy. Of the secondary clay minerals identified, only gibbsite is more strongly represented above this limit (8/12 samples), than below it (1/9 samples), a contrast significant at p < 0.05 (c2 test). On Purple Mountain (21), Rae et al. (2004) detected gibbsite in 4/10 of samples from pits above the trimline, but in none of seven samples from below-trimline pits. Combining the two sets of data gives gibbsite representation in 12/22 samples from pits excavated in soils above trimlines and 1/16 of samples from below this limit, a difference significant at p < 0.005. As gibbsite represents advanced alteration of silicate minerals by chemical weathering, its absence at all but one below-trimline site is consistent with removal of weathered soils at lower altitudes by glacial erosion. Gibbsite is significantly better represented in soil samples from summits that escaped LGM glacial erosion elsewhere in Ireland (Ballantyne et al., 2006, 2007) as well as in Great Britain (e.g. Ballantyne, 1994, 1997; Ballantyne et al., 1998; McCarroll and Ballantyne, 2000) and on mountains in Québec and Labrador (Marquette et al., 2004). As studies of gibbsite occurrence in montane soils in Scotland have shown that it is a product of pre-LGM weathering (Wilson, 1985; Hall et al., 1989; Mellor and Wilson, 1989), the significantly greater representation of gibbsite in above-trimline samples suggests that the trimlines in SW Ireland represent the upper limit of glacial erosion during LGM, as previously inferred by Rae et al. (2004) from a range of weathering indicators for sites above and below the trimline on Purple Mountain. 6.2. Surface exposure dating Cosmogenic 10Be exposure ages were obtained for four rock samples chiselled from near-horizontal bedrock outcrops, two from above-trimline sites near summits of Mullaghnarakill (7) and Purple Mountain (21), and two from ice-abraded rock outcrops, at 549 m on Derryclancy near Hungry Hill (48) and at 564 m on the Meenteog-Colly col. Site, sample and analytical details are given in Table 3, and exposure ages in Table 4. The above-trimline ages of (>) 76.3  6.8 ka obtained for the Purple Mountain sample and (>)

94.7  8.5 for the Mullaghnarakill sample are consistent with our interpretation that these sites escaped glacial erosion during the LGM, but do not preclude the possibility of cover by cold-based ice at that time. These ages should be regarded as minimal, in view of the likelihood of significant erosion of the sampling surfaces under periglacial conditions. The 10Be exposure age of 14.9  1.3 ka to 16.0  1.9 ka obtained for the Derryclancy sample is consistent with over-running by warm-based ice and removal of >2 m of bedrock at the sampling site during the LGM. That of 26.3  2.3 ka to 28.3  3.5 ka obtained from the sample obtained from a roche moutonnée on the Meenteog-Colly col predates the duration of the LGM (26e21 ka; Peltier and Fairbanks, 2006) and is inferred to reflect nuclide inheritance due to removal of an insufficient thickness of bedrock (cf. Stone and Ballantyne, 2006; Ballantyne et al., 2008). Two samples obtained by Bowen et al. (2002) from two low-level sites occupied by the KCIC (near Kenmare and on Sheep’s Head Peninsula south of Bantry Bay) produced 36Cl exposure ages of 22.0  3.4 ka and 21.3  1.3 ka. Both ages confirm occupance by ice during the LGM and imply early deglaciation of low ground, though ice may have persisted longer at higher altitudes, as suggested by the exposure age of the Derryclancy sample. Four 10Be exposure ages obtained by Harrison et al. (2010) from boulders on moraines in corries north of Macgillycuddy’s Reeks (22.9  1.5 ka, 17.1  1.3 ka, 15.1  1.0 ka and 14.9  1.8 ka) are consistent with this conclusion, particularly if (as Harrison et al. argue) their oldest age reflects nuclide inheritance. Collectively, the evidence provided by secondary clay minerals and the exposure age data indicate a LGM age for the trimlines in SW Ireland, consistent with previous inferences regarding the age of the KCIC based on weathering evidence (Rae et al., 2004) and stratigraphic arguments (Warren, 1985, 1991a, 1991b; Ó Cofaigh et al., 2010). 7. Interpretation of the trimlines in SW Ireland On the assumption that the mapped trimline altitudes represent the maximum altitude achieved by the KCIC during the LGM, our ‘ice-surface’ reconstruction (Fig. 5) indicates that ice 400e500 m thick crossed the northern shore of the Iveragh Peninsula into Dingle Bay, and that ice 500 m thick crossed the site of the Kilcummin moraine near Killarney. These considerations invalidate the model of a KCIC defined to the northwest by the Kilcummin moraine (Fig. 4a), and imply that the moraine represents a postLGM readvance of the KCIC ice, as favoured by Warren (1991a). It is also clear from the trend of the trimline contours that the KCIC extended WSW across the western coasts of the Iveragh and Beara Peninsulas on to the adjacent shelf. As outlined in the introduction, however, trimlines may be interpreted either: (1) as being of ice-marginal origin, defining the

Table 4 10Be exposure ages. Sample

Lal/Stone (Lm) scaling,

3

¼0

Exposure age (ka) Samples from sandstone outcrops above the upper limit of glacial Purp-01 76.34 Mullagh-01 94.67 Samples from vein quartz on glacially-abraded bedrock surfaces Der-01 14.93 MC-01 26.31

Dunai (Du) scaling, External uncertainty (ka)

3

¼ 1 mm ka1

Exposure age (ka)

External uncertainty (ka)

erosion 6.81 8.52

85.48 107.81

11.22 14.54

1.30 2.32

16.02 28.31

1.94 3.49

Scaling from CRONUS online calculator (Balco, 2007; Balco et al., 2008): wrapper script version 2.2; main calculator version 2.1; constants version 2.2.1; muons version 1.1. External errors (1s) incorporate analytical uncertainty on 10Be measurements and uncertainties in the calibration and scaling procedures. Exposure ages and associated external uncertainties were calculated using the Lm scaling (which assumes a high-latitude sea-level 10Be production rate of 4.37  0.37 atoms g1 yr1 and yields the youngest ages) assuming zero erosion (3 ¼ 0) and using the Du scaling (which assumes a high-latitude sea-level 10Be production rate of 4.43  0.52 atoms g1 yr1 and yields the oldest ages), assuming an erosion rate of 1 mm ka1. The two results bracket the probable exposure ages nominally implied by the 10Be concentrations of the samples.

C.K. Ballantyne et al. / Quaternary Science Reviews 30 (2011) 3834e3845

extent of former nunataks and the approximate maximum altitude of former ice cover; or (2) as having an englacial origin, defining high ground that supported non-erosive cold-based ice and thus demarcating the minimum altitude of former ice cover. The evidence provided by soil clay mineralogy and exposure ages is consistent with both interpretations. Previous interpretations of trimlines or ‘unglaciated areas’ in SW Ireland have concluded that they represent the maximum altitude of former ice cover and define palaeonunataks, either during ‘maximum glaciation’ (Wright, 1927; Farrington, 1947) or the LGM (Bryant, 1968; Rae et al., 2004); the ice-free areas depicted by Warren (1979) relate to the Kilcummin moraine limit, and hence to a post-LGM readvance (Warren, 1991a). Below we test the validity of the ‘ice-marginal’ (maximum LGM ice thickness) and ‘englacial’ hypotheses of trimline formation by using the Profiler v.2 programme of Benn and Hulton (2010) to establish the compatibility of the trimline-based ‘ice surface’ depicted in Fig. 5 with the lateral extent of the KCIC. Because the northern and offshore extents of the KCIC are unknown, we base our test on a flowline eastwards from the ice divide at the head of Kenmare River (Fig. 5) to Garryvoe, the type site of the Garryvoe till, which has been shown by Warren (1991a) to represent deposition by KCIC ice and by Ó Cofaigh et al. (2010) to be of LGM age. We assume a maximum possible altitude of 825 m for the Kenmare River ice divide on the basis of trimline altitudes (Fig. 5). Termination of KCIC ice at Garryvoe, 115 km east of the ice divide, represents the minimum eastwards lateral extent of KCIC during the LGM; Warren (1991a) suggested that KCIC ice may have extended farther, and Ó Cofaigh et al. (2010) depicted KCIC ice extending w100 km farther to the SE on to the shelf beyond the south coast of Ireland. The Profiler v.2 programme of Benn and Hulton (2010) permits two-dimensional reconstruction of ice-surface configuration along a former flowline based either on a known terminus position (assumed to be Garryvoe) or an ice divide of known or assumed altitude (assumed to be 825 m over Kenmare River) or both. It requires as inputs both data on bed topography along the former flowline, obtained from contour data, and an estimated yield stress, for which we assume a very conservative value of 50e100 kPa for

3843

ice moving over a rigid, non-deforming bed (Cuffey and Paterson, 2010). To test the compatibility of an assumed ice-shed altitude of 825 m at the Kenmare River ice divide with an ice margin 115 km farther east at Garryvoe we ran the Profiler programme for five models, each with different constraints (Fig. 6). Models 1e3 assume an ice divide altitude of 825 m. Model 1 represents the ice-surface profile for a constant yield stress (s) of 50 kPa, assumed to be the minimum for ice movement over a rigid bed. Under these assumptions the ice terminates only 45 km east of the ice divide and 70 km west of Garryvoe (any higher yield stress produces an ice cap of even more restricted extent). Model 2 assumes a 50 kPa yield stress to a point 40 km east of the ice divide, where bedrock is succeeded eastwards by drift-covered lowlands interrupted by low bedrock knolls, to mimic the effect of ice extending eastwards under very low driving stresses under conditions of subglacial bed deformation (cf. Boulton and Hindmarsh, 1987; Alley, 1991; Kamb, 1991). We progressively reduced the value of s in this zone until the glacier terminus reached Garryvoe. This model meets the two critical boundary conditions (ice extending to Garryvoe from a 825 m high ice divide) but introduces several anomalies, notably (1) an implausibly thin ice thickness of 29 m at a distance of 6.5 km from the terminus; (2) the ice-surface intersects a bedrock obstruction 18 km from the terminus (i.e. a ‘negative ice thickness’ is produced); (3) ice-surface gradient declines only 74 m over 53 km in the area of the assumed bed deformation; and (4) this solution requires reducing s in the zone of assumed bed deformation to 2 kPa, much lower than the yield stress calculated for extant land-based glaciers (e.g. Cuffey and Paterson, 2010, Table 8.2). It is also questionable whether deforming-bed conditions can be assumed for drift-covered lowlands studded with bedrock knolls, as the latter provide ‘sticky patches’ at which subglacial bed deformation would have been impossible (Stokes et al., 2007). Model 3 also pins the ice terminus at Garryvoe and the ice divide at 825 m, but assumes a constant yield stress throughout. This solution also satisfies the boundary conditions but can only be met if the yield stress is reduced to 20 kPa, which is implausible for the bedrock-floored areas that extend 40 km eastwards from the ice divide.

Fig. 6. Two-dimensional models of ice-surface configuration constructed using the Profiler v.2 model of Benn and Hulton (2010). Model 1: ice divide altitude 825 m and a yield stress of 50 kPa imply ice termination 45 km east of the ice divide. Model 2: ice divide altitude 825 m but yield stress reduced over low ground to permit ice extension to Garryvoe, 115 km from the ice divide. This model requires implausibly low yield stresses (2 kPa) over low ground. Model 3: ice divide altitude 825 m and terminus at Garryvoe, modelled using a uniform yield stress. The implied yield stress (20 kPa) is incompatible with ice moving over bedrock for 40 km east of the divide. Model 4: Ice terminus at Garryvoe with s ¼ 50 kPa indicates an ice divide altitude of w1200 m. Model 5: Ice terminus at Garryvoe with s ¼ 100 kPa indicates an ice divide altitude of w1635 m.

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Models 1e3 demonstrate that the KCIC cannot have extended 115 km eastwards from a 825 m high ice divide over Kenmare River unless unrealistic assumptions are made regarding the yield stress of the ice. A corollary of this finding is that the trimlines on which the 825 m ice divide is predicated cannot represent the maximum altitude of the KCIC during the LGM. To obtain an indication of the ice-shed altitude implied by an ice terminus at Garryvoe, we ran models 4 and 5, which assume respectively minimum (50 kPa) and maximum (100 kPa) yield stresses for ice moving over a rigid bed. Model 4 yields an ice divide altitude of w1200 m and model 5 an ice divide altitude of w1635 m. Even the lower of these estimates indicates that ice overtopped the highest summit (1039 m) in the area. There are three possible explanations for the discrepancies between trimline altitudes and projected maximum ice surface altitudes (models 4 and 5) noted above. The first is that the KCIC terminated far short of the assumed terminus at Garryvoe (model 1), but this explanation not only contravenes the evidence of striae aligned west-east as far as Garryvoe (Warren, 1991a) but also the recurrent outcrop of the Garryvoe till eastwards along the south coast of Ireland as far as the type site (Warren, 1991a; Ó Cofaigh et al., 2010). The second possible explanation is that the trimlines relate not to the maximum extent and thickness of the KCIC during the LGM, but to a later readvance to the Kilcummin moraine, as suggested by Warren (1979). However, not only are the trimline altitudes much too high to relate to this readvance (Fig. 5), but also the evidence provided by secondary clay minerals and the exposure age data indicate a LGM age for the trimlines. The third and most plausible explanation for the discrepancies noted above is that the trimlines of SW Ireland, like those elsewhere in the British Isles (Ballantyne and Hall, 2008; Hubbard et al., 2009; Ballantyne, 2010) are of englacial origin, and reflect an altitudinal transition from warm-based ice occupying low ground to summits and plateaus occupied by cold-based ice that was frozen to the underlying substrate and thus preserved pre-LGM regolith covers. Preservation of periglacial blockfields and other regolith covers on plateaus formerly occupied by cold-based ice has been demonstrated in a wide range of glaciated mountain environments (e.g. Fabel et al., 2002; Marquette et al., 2004; Staiger et al., 2005; Sugden et al., 2005; Fjellanger et al., 2006; Phillips et al., 2006) and, far from being exceptional, is emerging as the most common explanation of high-level trimlines in areas formerly occupied by ice sheets or ice caps. This interpretation implies that the ice-surface contours depicted in Fig. 5 represent only the minimum possible ice surface altitude. It also implies that the above-trimline localities depicted in Fig. 5 represent ‘cold patches’ within the former ice cap (Kleman and Glasser, 2007), and thus have potential for tuning climateproxy-driven models of ice build-up and decay.

8. Conclusions 1. On the mountains of the Iveragh Peninsula, trimlines define a contrast between ice-scoured bedrock on low ground and periglacial regolith covers on high ground. On the neighbouring Beara Peninsula, ice-abraded bedrock extends to the highest summits and periglacial regolith covers are absent. The altitude of the trimlines on the Iveragh Peninsula declines radially from a former ice divide at the head of Kenmare River, from w700 m on mountains nearest the ice divide to
470 m near the southern shore of Dingle Bay. Mapped trimline altitudes near the ice divide correspond closely with previous depictions by Wright (1927) and Rae et al. (2004), but thicker ice cover is implied than previously inferred for western Iveragh by Bryant (1968).

2. The occurrence of gibbsite in 55% of above-trimline soil samples contrasts with the near-absence of gibbsite in belowtrimline soils, and is consistent with a LGM age for the trimlines. This is confirmed by 10Be and 36Cl exposure ages. Rock outcrops above trimlines yielded apparent minimum exposure ages exceeding 76.3  6.8 ka, but outcrops or boulders below trimlines have (with one exception) yielded post-LGM exposure ages. 3. The altitude of trimlines on the Iveragh Peninsula implies that the Kerry-Cork Ice Cap extended over the site of the Kilcummin moraine complex, northwards and northwestwards into Dingle Bay and westwards on to the Atlantic shelf. Extrapolation of trimline altitudes under the assumption that the trimlines represent the maximum LGM thickness indicates a maximum ice divide altitude of w825 m over Kenmare River. However, two-dimensional reconstructions of ice-surface configuration from an assumed eastern terminus 115 km from the ice divide show that an ice divide at 825 m cannot be reconciled with the minimum possible eastern extent of the KCIC without the assumption of impossibly low yield stresses. 4. We infer from the discrepancy between inferred ice divide altitude and the minimum lateral extent of the KCIC that the Iveragh Peninsula trimlines represent a former englacial transition between warm-based sliding ice on low ground and cold-based ice on high ground, rather than delimiting former nunataks as previously interpreted. Modelling of ice-surface altitude on the basis of assumed rigid bed conditions and a yield stress of 50 kPa suggests that during the LGM the KCIC reached an altitude of at least w1200 m and overtopped all mountain summits. Acknowledgements This work was supported by UK NERC Grant NER/B/S/2001/ 0919. We thank Pyrs Gruffuth and Neil Loader for assistance in the field, Doug Benn for assistance in reconstructing lateral ice extent, Graeme Sandeman for preparing the maps, Angus Calder for XRD analyses and Joy Laydbak for preparing samples for exposure dating. We also thank three anonymous reviewers for their critical and insightful comments, all of which proved useful in improving the paper. References Alley, R.B., 1991. Deforming bed origin for southern Laurentide till sheets? Journal of Glaciology 37, 67e76. Anderson, E., Harrison, S., Passmore, D.G., Mighall, T., 1998. Geomorphic evidence of Younger Dryas glaciation in the Macgillycuddy’s Reeks, South West Ireland. Quaternary Proceedings 6, 75e90. Balco, G., 2007. CRONUS-Earth Online Calculators, Version 2.0. http://hess.ess. washington/edu/math/ (November 2010). Balco, G., Stone, J.O., Lifton, N.A., Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology 3, 174e195. Ballantyne, C.K., 1994. Gibbsitic soils on former nunataks: implications for ice-sheet reconstruction. Journal of Quaternary Science 9, 73e80. Ballantyne, C.K., 1997. Periglacial trimlines in the Scottish Highlands. Quaternary International 38 (39), 119e136. Ballantyne, C.K., 2007. Trimlines and palaeonunataks. In: Elias, S. (Ed.), Encyclopedia of Quaternary Science. Elsevier, Amsterdam, pp. 892e903. Ballantyne, C.K., 2010. Extent and deglacial chronology of the last BritisheIrish Ice Sheet: implications of surface exposure dating using cosmogenic isotopes. Journal of Quaternary Science 25, 515e534. Ballantyne, C.K., Hall, A.M., 2008. The altitude of the last ice sheet in Caithness and West Sutherland. Scottish Journal of Geology 44, 169e181. Ballantyne, C.K., McCarroll, D., Nesje, A., Dahl, S.O., Stone, J.O., 1998. The last ice sheet in North-West Scotland: reconstruction and implications. Quaternary Science Reviews 17, 1149e1184. Ballantyne, C.K., McCarroll, D., Stone, J.O., 2006. Vertical dimensions and age of the Wicklow Mountains ice dome, Eastern Ireland, and implications for the

C.K. Ballantyne et al. / Quaternary Science Reviews 30 (2011) 3834e3845 extent of the last Irish ice sheet. Quaternary Science Reviews 25, 2048e2058. Ballantyne, C.K., McCarroll, D., Stone, J.O., 2007. The Donegal ice dome, NW Ireland: dimensions and chronology. Journal of Quaternary Science 22, 773e783. Ballantyne, C.K., Stone, J.O., McCarroll, D., 2008. Dimensions and chronology of the last ice sheet in Western Ireland. Quaternary Science Reviews 27, 185e200. Benn, D.I., Hulton, N.R.J., 2010. An ExcelÔ spreadsheet program for reconstructing the surface profile of former mountain glaciers and ice caps. Computers and Geosciences 36, 605e610. Boulton, G.S., Hagdorn, M., 2006. Glaciology of the British Isles Ice Sheet during the last glacial cycle: form, flow, streams and lobes. Quaternary Science Reviews 25, 3359e3390. Boulton, G.S., Hindmarsh, R.C.A., 1987. Sediment deformation beneath glaciers: rheology and geological consequences. Journal of Geophysical Research 92, 9059e9082. 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