JOURNAL OF QUATERNARY SCIENCE (1997) 12 (6) 519–524 CCC 0267-8179/97/060519–06$17.50 1997 by John Wiley & Sons, Ltd.

Identification and significance of ice-flow-transverse subglacial ridges (Rogen moraines)in northern central IrelandJASPER KNIGHT* and A. MARSHALL McCABESchool of Environmental Studies, University of Ulster, Coleraine, Co. Londonderry BT52 1SA, Northern Ireland

Knight, J. and McCabe, A. M. 1997. Identification and significance of ice-flow-transverse subglacial ridges (Rogen moraines) in north central Ireland. J.Quaternary Sci., Vol. 12, pp. 519–524. ISSN 0267-8179

Received 17 April 1997; accepted 4 June 1997

ABSTRACT: In the Omagh Basin, north central Ireland, subglacial diamict ridges lie transverseto southwestward Late Devensian (ca. 23–13 ka) ice flow. These ridges (0.5–2.5 km long, 100–450 m wide, 15–35 m high), are similar morphologically to Rogen moraines, which have notbeen described previously from the British Isles. The crests of some transverse ridges arestreamlined, cross-cut or overprinted by drumlins, whereas other ridges are unmodified andwere not affected by later drumlinisation. At Kilskeery, west–east trending eskers overlyingunmodified transverse ridges post-date drumlinisation (17–14 14C ka). Esker formation showsthat the subglacial thermal regime changed from cold-based, favouring bedform preservation, towarm-based with meltwater flowing through enclosed subglacial channels. Patterns of flow-transverse-ridges and spatial variations in the degree of bedform modification record dynamicchanges in regional subglacial environments during the last deglacial cycle. This ice-massvariability cannot be reconciled with current Irish glacial models, which are based on immobileice centres and ordered stages of ice retreat. In a wider context, these changes in bedformpatterns and basal ice regimes have a similar signature to millennial-scale ice-mass oscillationsrecorded by dated proxy evidence elsewhere in the amphi-North Atlantic. 1997 by JohnWiley & Sons, Ltd.

KEYWORDS: Ireland; Rogen moraines; drumlinisation; glacial bedforms.

reveals complex deglacial-stage bedform patterns acrossIntroductionnorth central Ireland and suggests that (i) ice masses werehighly mobile, and (ii) episodic events of fast ice flow werelinked to changes in subglacial thermal regimes, bedformOverprinted subglacial bedform sets can be used to evaluate

the relative timing and palaeoenvironmental regimes of dif- shaping events, and a high sediment flux to ice margins(McCabe, 1993, 1996; McCabe and Haynes, 1996; Knight,ferent ice-flow events (Clark, 1993; Shaw, 1994). Glacial

landscapes formed in north central Ireland during the last 1997; Knight and McCabe, 1997). This evidence also impliesthat the general glaciation models cannot describe or(Late Devensian) glacial event (ca. 23–13 ka) show a range

of subglacial bedform types with variable directional signa- account for complex bedform patterns, which are linked toregional shifts in ice-flow direction and ice-sheet oscillationstures at ice sheet scales (Dardis, 1982; McCabe, 1987,

1993). However, general glaciation models derived from during the last deglacial cycle.The aim of this paper is to describe and interpret patternsthese distributions (i.e. Synge and Stephens, 1960; Warren,

1992) do not record landform complexity and recognise of ice-flow-transverse subglacial ridges from specific areasof the north central Irish lowlands. Transverse ridges areonly one main period of substrate streamlining

(drumlinisation). These models are based on immobile ice similar in overall morphology to Rogen moraines, whichhave not been described previously from the British Isles.centres in the Omagh and Lough Neagh Basins, and ice

flow directed radially through lowland corridors towards Some transverse ridges have streamlined crests generatedduring drumlinisation, or are overlain by eskers. Thesemarine margins located on the Irish Sea and North Atlantic

seaboards respectively (Fig. 1). relationships, and dated evidence elsewhere in north centraland eastern Ireland, show that millennial-scale episodes ofRecent work using satellite, field and dating evidencerenewed ice activity punctuated the deglacial period (ca.17–13 ka; McCabe, 1996). Spatial variability in the preser-

**Corresponding author: Jasper Knight, School of Environmental Studies, Uni-vation and modification of transverse ridges shows that theseversity of Ulster, Coleraine, Co. Londonderry, Northern Ireland, BT52 1SA,episodes are related to highly dynamic changes in subglacialUK. Email: s067fKsperrin.ulst.ac.uk; Fax 01265 324911.

Contract grant sponsor: DENI CAST thermal and hydraulic regimes.

520 JOURNAL OF QUATERNARY SCIENCE

Figure 1 Sketch map of north central Ireland showing general ice flow directions, and the location of Figs 2 and 3.

their continuity, relative size, shape and alignment. BedformsSatellite evidence and ridge locationswere grouped into three distinct geomorphic types on thebasis of bedform scale, morphological characteristics andalignment. Regionally these types are (i) kilometre-scale bed-The lowlands of north central Ireland (,120 m a.s.l.) extend

west to east from Lower Lough Erne to Dundalk Bay (110 km rock scallops located on the highest hill summits, such asSlieve Beagh and Slieve Croob (.300 m a.s.l.); (ii) flow-distance), and north to south from the Lough Neagh Basin

to the north Irish Midlands (80 km distance) (Fig. 1). Geologi- transverse ridges located at intermediate elevations (ca. 120–160 m a.s.l.) around Ballybay; and (iii) streamlined bedformscally, the Omagh Basin is dominated by Devonian Old Red

Sandstone. The area between Lower Lough Erne and Cavan including drumlins located in lowland areas (,120 m a.s.l.),such as south of Monaghan.is underlain by Carboniferous limestones and shales. Block-

faulted Devonian and Carboniferous sandstones form the Generally, zonal patterns of transverse ridges in northcentral Ireland closely reflect ice flow patterns reconstructedFintona Hills and Slieve Beagh massif respectively. Tertiary

basalts and clays are present around Lough Neagh, which from striation and drumlin alignment. Transverse ridges rec-ord a clear northeast to southwest ice flow across the Omaghis a tectonically controlled basin. Silurian grits and shales

dominate to the east of Cavan and Armagh, and intrusive Basin (Knight, 1997). Similar patterns of transverse ridgescan be traced across the Cavan and Armagh lowlands andigneous bodies underlie the Mourne and Carlingford Moun-

tains (Wilson, 1972). record southeasterly ice flow towards the Irish Sea Basin.Transverse ridges are largely absent in upland areas, andThe distribution and alignment of subglacial bedforms

were recorded from Landsat Thematic Mapper (TM) imagery ridge orientations in lowland areas are unrelated to localbedrock strikes. More complex ridges showing several over-(scale 1:250 000). Overall bedform patterns were dis-

tinguished by the nature of and changes in reflectance printed directional elements are also identified in ice-mar-ginal areas near Dundalk, and around Upper Lough Erne,(Ronnert and Nyborg, 1994) (Fig. 2). Bedform types, such

as drumlins and transverse ridges, were distinguished by which is close to former ice dispersal centres (Fig. 2).

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521ROGEN MORAINES IN NORTH CENTRAL IRELAND

Figure 2 Black and white Landsat TM image of the southern part of Upper Lough Erne (location on Fig. 1) showing ice-flow-transverseridges which are highlighted by intervening loughs. Note that some ridges show streamlined crests (A) whereas others are unmodifiedmorphologically (B). Ice-flow direction is generally from the top margin of the image to the bottom margin, i.e. approximately northeast tosouthwest. Image courtesy of ERA-Maptec, Dublin.

In the Omagh Basin, a continuum of ridge morphological flow-transverse ridges and their drumlinised variants recordice mass variability in the north of Ireland during the lasttypes exist between the end-members of intact ridges (no

alteration of the primary bedform pattern and alignment) deglacial cycle.and streamlined ridges (complete reorientation of the primarybedform pattern and alignment). Between these end-mem-bers are overprinted and cross-cut ridge types, reflectingpartial sediment remoulding and reorientation (cf. Clark,

Field evidence and ridge morphology in the1993). Overprinted ridges are those in which both earlierOmagh Basinand later ice-flow directions can be distinguished geomorph-

ically (i.e. Rose and Letzer, 1977). Cross-cut ridges arethose which have been segmented by sediment removal andtransport through discrete subglacial channels (cf. Dardis From morphological field mapping, drumlins represent about

85% of all diamict bedforms in the Omagh Basin. They areand McCabe, 1983), and may or may not show surficialstreamlining. Streamlining involves the net transfer of sedi- generally of classical morphology (65% of all drumlins) and

record northeast to southwest ice flow (Charlesworth, 1924;ment from up- to down-ice ridge flanks. If the nature ofbedform modification reflects the degree or duration of fast- Dardis, 1982; Knight, 1997) towards a marine-based margin

located in Donegal Bay (McCabe, 1993; Knight and McCabe,ice-flow activity, intact ridges must be a priori located out-side the zone of fast ice flow. Intact and unmodified ridges 1977). Ridges (15% of all diamict bedforms), aligned gener-

ally northwest–southeast, are transverse to this ice-flow direc-therefore contrast morphogenetically with cross-cut, stream-lined and overprinted ridges that are affected by fast ice tion and occupy the lowest topographic positions (50–90 m

a.s.l.) in the basin. Some transverse ridges (35%) have aflow, and sediment removal and remoulding processes. How-ever, the suggestion that some landforms may be preserved pristine morphology and do not show evidence of surficial

streamlining; other ridges (65%) possess drumlinised andintact beneath cold-based ice (Lagerback, 1988; Kleman,1994; Kleman et al., 1994) means that some bedform pat- streamlined crests or are overprinted by drumlins. This field

relationship demonstrates clearly that drumlinisation post-terns might pre-date the most recent phase of ice activity.Bedform patterns therefore need to be evaluated carefully dates transverse ridge formation.

Generally, transverse ridges are 0.5 to 2.5 km long, havein order to assess why bedforms in some areas are preservedmorphologically whereas others show cross-cut or stream- undulating crest profiles, and occur within a northeast to

southwest orientated zone, 10 km wide and 35 km long,lined signatures.In this paper it is suggested that variable bedform patterns extending from Omagh to Enniskillen (Fig. 3). The regional

diamict surface declines towards the southwest from 110 mand overprinting relationships, together with inferred changesin subglacial thermal and pore-water regimes, show that (Omagh) to 50 m a.s.l. (Enniskillen). Ridge dimensions

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Figure 3 Sketch map showing the crest-line of flow-transverse ridges in the Omagh Basin. Ice-flow direction is also shown (arrowed). Maplocation given in Figure 1.

increase along this northeast–southwest transect: average (individually up to 100 m long and 20 m high). Esker ridgesridge length increases from 800 m to 1500 m, width from climb from west-southwest to east-northeast up the regional100 m to 450 m, and height from 15 m to 35 m. Throughout topographic gradient (rise of 60 m over 10 km), and arethis zone, individual ridges are spaced between 30 m and associated closely with flanking fans and kettleholes. Near100 m apart and are separated by lowland raised peat bogs, Kilskeery village, some eskers also lie across the flanks andwetlands and orientated, dispersed lakes. margins of the underlying transverse ridges (Fig. 4). A subgla-

In detail, ridge morphology is highly variable and reflects cial setting for esker deposition is evidenced by the discretethe degree of ridge modification by later ice activity. A and discontinuous nature of individual ridge segments, pres-continuum of modification styles exists (producing intact → ence of associated landforms such as flanking fans, andoverprinted → cross-cut → streamlined ridges), but no single position within a Nye-type channel. Outwash and morainicstyle dominates spatially and ridges with different modifi- sediments at the eastern terminus of the esker system nearcation styles are juxtaposed. Intact ridges are generally sym- Trillick (Fig. 3) show that this active subglacial conduitmetric in cross-profile with rounded crests. Modified ridges operated under a west to east hydraulic gradient. This depo-may be segmented into several (three to eight) weakly drum- sitional and morphological trend is directionally opposed tolinised mounds separated by shallow saddles (2–5 m deep). the earlier northeast to southwest ice flow associated withOccasionally, deep meltwater channels cut ridge cross-pro- both the underlying transverse ridges and their variablyfiles to the level of the ridge basal contour. Modified ridges streamlined crests.are generally asymmetric in cross-profile with steeper nor- A critical conclusion from this overprinted subglacial pat-theast-facing (stoss) slopes. Southwest-facing ridge margins

tern is that the hydraulic potential and ice-flow gradientsmay also be hooked or crescentic and show horns that trend

must have shifted over time from southwesterly to easterly.down-ice.Four sequential stages of ice activity describe this directionalAn interconnected suite of subparallel unmodified trans-change: (i) Rogen formation by southwesterly ice flow;verse ridges near Kilskeery (Fig. 4) have long axes aligned(ii) partial modification of some Rogen ridges during south-northwest to southeast, associated with southwesterly icewesterly drumlinisation. Spatial variability of modificationflow. Ridges are up to 2 km long, 0.75 km wide and 35 msignatures, including zones of streamlining and preservation,high. Morphologically, ridges are comprised of intercon-suggests some ridges were modified whereas others werenected diamict mounds and hummocks that are symmetricleft untouched by drumlinisation; (iii) westward migration ofin cross-profile, have undulating crestal profiles, and dothe ice centre across the Omagh Basin towards the Lowernot generally show down-ice (southwesterly) streamlining.Lough Erne Basin. There is no evidence for drumlinisationMounds and hummocks give ridges a highly intricate basalassociated with this period of ice centre shifting; (iv) eskeroutline. The flanks of the ridge long axes are generallyformation by a west-to-east hydraulic gradient directed fromoversteepened by later proglacial meltwater. Thick (,4–5 m)the Lower Lough Erne ice centre. Variable bedform signa-post-glacial raised peat in inter-ridge hollows masks the basetures resulting from these shifting ice flows are also relatedof the ridges, suggesting they were formerly more significantto changes in subglacial thermal and hydrological regimes.features and had a stronger local relief.No sedimentary exposures are known near Kilskeery thatAt Kilskeery, an esker (in total 10 km long) is presentmay document the switching of ice-flow gradients that isalong the axis of a meltwater-incised valley (300–500 mrecorded by regional morphological evidence.wide, ,35 m deep) that cleanly dissects the transverse ridges

(Fig. 4). The meltwater-incised valley is occupied almostentirely by discontinuous and bifurcated esker ridges

J. Quaternary Sci., Vol. 12(6) 519–524 (1997) 1997 by John Wiley & Sons, Ltd.

523ROGEN MORAINES IN NORTH CENTRAL IRELAND

Figure 4 Morphology of flow-transverse ridges and overlying eskers in the Kilskeery area, Omagh Basin. Location given in Fig. 3.

1989, 1997). Triggers for subglacial reorganisation may haveDiscussion and conclusionsincluded changes in basal ice thermal and hydrologicalregimes associated genetically with overall ice mass dynam-ics. For example, bed warming and meltwater generationMorphologically, ice-flow-transverse ridges described from

satellite and field evidence in north central Ireland are similar may lead to enhanced ice flow, or result from the frictionalheat generated by fast-moving basal ice. A genetic linkto Rogen moraines described commonly from the last Scand-

inavian (Lundqvist, 1969, 1989, 1997) and Laurentide ice between subglacial hydraulic and thermal conditions andindividual ice flow events can account for the local conti-sheets (Bouchard, 1989; Fisher and Shaw, 1992). Rogen

moraine ridges are an important bedform type and cover an nuity of intact Rogen ridges in the north central Irish low-lands, and the absence of ice-flow-directional signatures atarea of at least 1500 km2 in north central Ireland.

A Rogen moraine interpretation for diamict ridges in north Kilskeery during ice centre shifting that was not associatedwith drumlinisation.central Ireland is supported by (i) the regional pattern of

ridge long axes lying transverse to early deglacial-stage ice Changes in basal ice conditions during drumlinisationresulted in a high subglacial sediment flux and the buildingflow direction; (ii) the presence of streamlined, cross-cut and

overprinted bedform sets that reflect regional shifts in ice- of marginal subaqueous moraines (McCabe et al., 1984;McCabe, 1993, 1996). Drumlinisation was also associatedflow direction during later deglacial stages; and (iii) the pres-

ence of a subglacial esker overlying these ridges at Kilskeery. with ice streams erosively headcutting from marine-basedmargins, such as the Armagh ice stream which extendedThe presence of Rogen ridges is significant because it

impacts on assessments of basal ice dynamics and the rela- 60 km up-ice from its Dundalk Bay terminus (McCabe andKnight, submitted) (Fig. 1). Bedforms within this ice streamtive chronology of bedform development during the last

deglacial cycle. Spatial variability in the pattern of ridge are modified and streamlined. Outside the margins of theice stream, most bedforms including Rogen moraines aremodification in the north central Irish lowlands also suggests

that responses to ice-flow shifts during deglaciation were unmodified (McCabe and Knight, submitted). This meansthat modified and unmodified Rogen ridges can be closelydifferent in different areas.

Bedform relationships in north central Ireland, south of juxtaposed. In the Omagh Basin, absence of an ice-marginalmorainic signature implies that Rogen formation was intern-the main ice dispersal centres (McCabe, 1987), record the

sequence of ice-flow stages and basal ice conditions leading ally driven and contained within the subglacial environment,rather than associated with a high marginal sediment flux.up to, during and following drumlinisation (McCabe and

Knight, submitted). Streamlined crests of transverse ridges Therefore temporal and spatial switches between non-streamlining/static (forming Rogen ridges) andand cross-cutting bedform relationships suggest that Rogen

ridges were formed relatively early in the deglacial cycle streamlining/erosive (drumlinising) ice regimes may reflectchanges in ice–sediment interface conditions related to dis-during a period in which previously deposited subglacial

sediments were reorganised morphologically (cf. Lundqvist, tinct phases of ice activity and periods of subglacial bed reor-ganisation.

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CLARK, C. D. 1993. Mega-scale glacial lineations and cross-cuttingAccelerator mass spectrometry 14C assessment of in situice-flow landforms. Earth Surface Processes and Landforms, 18,marine microfaunas from mud beds located beneath frontal1–29.subaqueous moraines in Dundalk Bay, which were deposited

DARDIS, G. F. 1982. Sedimentological aspects of the Quaternaryduring drumlinisation, yields dates of 15.5–15.8 14C kageology of south central Ulster, Northern Ireland. Unpublished(McCabe, 1996). This suggests Rogen moraine formationPhD thesis, Ulster Polytechnic, Jordanstown.

occurred during or just after the Last Glacial Maximum DARDIS, G. F. and McCABE, A. M. 1983. Facies of subglacialaround 23–18 ka when bedforms were preserved beneath channel sedimentation in late-Pleistocene drumlins, Northern Ire-cold (dry based) ice (Knight, 1997; McCabe and Knight, land. Boreas, 12, 263–278.submitted). The spatial variability of Rogen moraine ridge FISHER, T. G. and SHAW, J. 1992. A depositional model for Rogenpreservation, and genetic relationship between fast ice flow moraine, with examples from the Avalon Peninsula, Newfound-and a high subglacial sediment flux, suggest that temporal land. Canadian Journal of Earth Sciences, 29, 669–686.

FRONVAL, T., JANSEN, E., BLOEMENDAHL, J. and JOHNSEN, S.switches between cold (dry based) and warm (wet based)1995. Oceanic evidence for coherent fluctuations in Fennoscand-regimes controlled patterns of bedform modification. Theian and Laurentide ice sheets on millennium timescales. Nature,near-opposing directional signatures of the Kilskeery esker374, 443–446.and the Rogen ridges over which it lies record one such

KLEMAN, J. 1994. Preservation of landforms under ice sheets andswitch when the ice centre present in the northeasternice caps. Geomorphology, 9, 19–32.Omagh Basin shifted westwards during later deglacial stages

KLEMAN, J., BORGSTRO¨

M, I. and HA¨TTESTRAND, C. 1994. Evi-

into the Lower Lough Erne Basin (Knight, 1997). Morphologi- dence for a relict glacial landcape in Quebec–Labrador. Palaeoge-cally, this period of a shifting ice centre is recorded by a ography, Palaeoclimatology, Palaeoecology, 111, 217–228.break in the morphological record of bedform directional KNIGHT, J. 1997. Glacial sedimentology and the conservation ofsignatures preserved at Kilskeery. This break may have been sand and gravel resources in the Omagh Basin, southwesterngenerated as the cold and dry-based ice divide passed over Northern Ireland. Unpublished DPhil thesis, University of Ulster,

Coleraine.the site, and is supported by the intact and non-streamlinedKNIGHT, J. and McCABE, A. M. 1997. Drumlin evolution and icenature of the Rogen ridges. Similar ice-centre shifts and

sheet oscillations along the NE Atlantic margin, Donegal Bay,periods of ice rejuvenation are recorded by morphologicalwestern Ireland. Sedimentary Geology, 111, 57–72.and sedimentary evidence elsewhere within the north central

LAGERBA¨CK, R. 1988. The Veiki moraines in northern Sweden—Ireland Rogen moraine zone (McCabe and Knight,

widespread evidence of an early Weichselian deglaciation. Boreas,submitted).17, 469–486.

Changes in ice-flow direction, ice-mass dynamics and LUNDQVIST, J. 1969. Problems of the so-called Rogen moraine.basal thermal regimes in north central Ireland may be related Sveriges Geologiska Undersokning, C648, 1–32.closely to the operation of the coupled ice–ocean–atmos- LUNDQVIST, J. 1989. Rogen (ribbed) moraine—identification andphere system (McCabe, 1996). Patterns of superimposed and possible origin. Sedimentary Geology, 62, 281–292.cross-cut bedforms may be a proxy record of rapid climate- LUNDQVIST, J. 1997. Rogen moraine—an example of two-step

formation of glacial landscapes. Sedimentary Geology, 111, 27–related oscillations of sensitive ice masses in the amphi-40.North Atlantic (Fronval et al., 1995; McCabe, 1996). Because

McCABE, A. M. 1987. Quaternary deposits and glacial stratigraphyboth unmodified and streamlined Rogen ridges are closelyin Ireland. Quaternary Science Reviews, 6, 259–299.juxtaposed in the north central Irish lowlands, spatially dis-

McCABE, A. M. 1993. The 1992 Farrington Lecture: drumlintinct episodes of subglacial activity must have occurred latebedforms and related ice-marginal depositional systems in Ireland.in the last deglacial cycle (ca. 15–14 ka). This degree ofIrish Geography, 26, 22–44.

bedform complexity and the inferred chronology of bedform McCABE, A. M. 1996. Dating and rhythmicity from the last deglacialdevelopment cannot be reconciled with current Irish glaci- cycle in the British Isles. Journal of the Geological Society ofation models, which are based on unidirectional flow signa- London, 153, 499–502.tures derived mainly from drumlin orientations, and describe McCABE, A. M., DARDIS, G. F. and HANVEY, P. M. 1984. Sedi-‘immobile’ ice centres and ordered stages of ice retreat (i.e. mentology of a late Pleistocene submarine-moraine complex,Synge and Stephens, 1960; Bowen et al., 1986; Warren, County Down, Northern Ireland. Journal of Sedimentary Petrology,

54, 716–730.1992). Therefore a new ice-flow model for north centralMcCABE, A. M. and HAYNES, J. R., 1996. A Late PleistoceneIreland is required in order to account for evidence of

intertidal boulder pavement from an isostatically emergent coast,dynamic ice activity and rapid changes in subglacial environ-Dundalk Bay, eastern Ireland. Earth Surface Processes and Land-ments during the last deglacial cycle.forms, 21, 555–572.

McCABE, A. M. and KNIGHT, J. submitted. Ice flows, glacialAcknowledgements We thank Kilian McDaid for drafting, andbedforms and ice mass migration in north central Ireland: aNigel McDowell for photography, and acknowledge the useful com-record of rapid environmental change during the last glacial cycle.ments of the referees Jan Lundqvist and Wishart Mitchell. This workGeological Society of America Bulletin.was supported partly through a DENI CAST award (to JK).

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J. Quaternary Sci., Vol. 12(6) 519–524 (1997) 1997 by John Wiley & Sons, Ltd.