Problems of Irish drumlins and Late Devensian ice sheetreconstructions
Jasper Knight
KNIGHT, J. 1999. Problems ofIrish drumlins and Late Devensian ice sheet reconstructions. Proceedingsofthe Geologists' Association, 110,9-16. Reconstructions of the Late Devensian ice sheet in Ireland arestrongly biased towards explaining data obtained by the Geological Survey of Ireland during the lastcentury, and observations undertaken by key workers in this century (i.e. J. K. Charlesworth). Modemfield observation and analytical techniques, however, show that many of these data, such as thedistribution of drumlins, are erroneous. Furthermore, models derived from these data, dealing with thesignificance and distribution of different bedform types, the location of ice centres and the direction ofice flow lines, can no longer be supported. New observations based on mapping from satellite imagery,and validated by detailed regional-scale field studies, show the complexity of subglacial bedformpatterns which record several phases of subglacial activity during the Late Devensian. The presence ofice flow-transverse Rogen moraines and patterns of overprinted bedforms in north central Ireland suggestthat subglacial environments and the location of ice centres changed dramatically during the overallglacial cycle.
School ofEnvironmental Studies, University of Ulster, Coleraine, Co. Londonderry, Northern IrelandBT521SA.
1. INTRODUCTION
Drumlins in Ireland have a long history of investigationwhich has focused mainly on their spatial distribution (i.e.Charlesworth, 1924, 1939, 1957; Warren, 1992) andinternal composition (i.e. Dardis, McCabe & Mitchell,1984; McCabe & Dardis, 1989). Together, these elementsprovide information on the ice flow regimes and subglacialenvironments associated with drumlinization (sedimentstreamlining). Models of principal ice flow vectors duringthe last (Devensian, also termed the Midlandian in Ireland)glacial event (21-13 ka) have been reconstructed from theflow-parallel orientation of drumlin long axes, together withother directional evidence such as striae alignment anderratic dispersal patterns (i.e. Synge & Stephens, 1960;Synge, 1969; Bowen, Rose, McCabe & Sutherland, 1986;Warren, 1992). An implicit assumption of these 'traditional'models is that drumlin formation is linked genetically toterminal moraines which can therefore be used to delimitthe location of ice margins during drumlinisation. Examplesof these linked drumlin-moraine systems include the'southern Irish end moraine' (Charlesworth, 1928) and the'drumlin readvance moraine' (Synge, 1969) (Fig. 1).However, the significance of these proposed major icelimits has been questioned on the basis of their compositemorphology and likely diachroneity, and the presence ofdrumlins outside of the 'drumlin readvance' limit (McCabe,1985; Warren, 1992).
Modem techniques of observing regional-scale bedformpatterns and assessing the significance of subglacialsystems include satellite imagery and AMS 14C dating ofmarginal sediment sequences. Recent work in Ireland using
Proceedings of the Geologists' Association, 110,9-16.
these methods concludes that:
(1) the external morphology of glacial bedforms is morediverse than has been previously recognized;
(2) the timing of drumlinization was different in differentice sheet sectors;
(3) drumlins are not the most spatially-dominant bedformtype;
(4) bedforms cannot always be linked genetically tocontemporaneous moraines;
(5) bedform patterns provide a record of ice activitypredating the final 'drumlin readvance' (i.e. McCabe,1996; Knight & McCabe, 1997a, b; McCabe, Knight &McCarron, in press).
In contrast to the 'traditional' models, these lines ofevidence suggest that the Irish ice sheet was highly dynamicand that periods of bedform formation coincide with thetiming of millennial-scale changes in amphi-North Atlanticclimate (McCabe, 1996; Knight, McCarron & McCabe,1997; McCabe & Clark, 1998). Therefore, in the light ofthese new data, it can be said that the 'traditional' view ofIrish drumlin formation, involving synchronous ice flowfrom stable ice centres, poses an interpretative problemparticularly concerning the timing and processes ofdrumlinization which occurred during multiple phases ofice activity between c. 17.0-14.5 14C ka (McCabe, 1996;Knight et al., 1997; McCabe & Clark, 1998).
In response to these limitations of interpreting glacialbedforms and modelling glacial events in Ireland, a 'new'ice sheet model is proposed in order to reconstruct changesin subglacial environments and the location of ice centres.These changes are inferred from the spatial patterns and
50 km-Fig. 1. 'Traditional' view of Late Devensian ice flow vectors inIreland (redrawn after McCabe (1985,1987 and references therein),and the position of the southern Irish end moraine (SIEM)(Charleworth, 1928) and drumlin readvance moraine (DRM)(Synge, 1969), and places mentioned in the text. The generaldistribution of Rogen moraines resulting from recent satellite andfield observations is also shown.
morphology of subglacial bedform s, and from sedimentsequences described from key marginal areas of the icesheet. In order to compare the 'traditional' and 'new'models, the aims of this paper are to: (I) discuss thedifferent methodologies and assumptions involved insetting up these models; (2) compare and contrast themodels ' major characteristics; and (3) highlight twomarginal areas of the Irish ice sheet in which the two modelsconflict. More widely, this comparison emphasizes the wayin which glacial geology has evolved from a restricted,observation-based chorological science to one examiningintegrated subglacial systems, and involving time- andspace-dependent subglacial processes.
2. 'TRADITIONAL' IRISH GLACIAL MODELS
Models of this type derive directl y from field mappingundertaken by officers of the Geological Survey of Ireland(GSI) during the later part of the last century (i.e. Close,1867; Hull, 1878; Kilroe, 1888). These investigations were
concerned with exarmrnng regional-scale land scapephysiography and did not involve the detailed investigationof landform relationships or their internal composition.Despite regional mapping exercise s undertaken in the northofIreland by Dwerryhouse (1923) and Charlesworth (1924,1939), successive workers (i.e. Warren, 1990, 1992) havenot questioned the field data of the GSI. These models (Fig.I) are based on the specific assumption s that:
(I ) drumlins are ubiquitous bedforms (Charlesworth,1939);
(2) drumlin fields are geneticall y related to, and can bedelimited by, frontal moraines which mark maximal icepositions (Synge, 1969);
(3) Late Devensian drumlinization was synchronousbeneath all ice sheet sectors and erased all possibleevidence for previous ice flows; and thus
(4) drumlins and related subglacial lineations such as striaeare an accurate record of Late Devensian ice flowvectors.
Characteristics of the models derived from theseinterpretations include: (I ) 'static' domes or ice centrespresent in north-central and west-central Ireland with localcentres in Kerry/Cork and Donegal (Warren, 1992; Warren& Ashley, 1994); and (2) unchangin g ice flow vectors andice centre locations throughout the entire glacial cycle.Additionally, these models do not describe Late Devensianice extent or llow direction prior to drumlinization, and donot link ice dynamics with rapid changes in subglacialenvironments which are known to have occurred before ,during and after drumlinisation (McCabe, 1993, 1996;Knight & McCabe, 1997a,b; McCabe & Clark, 1998;McCabe et al., in press).
3. NEW EVIDENCE FOR ICE SHEETDYNAMICS
Observations from 1:100 OOO-scale Land sat thematicmapper images and 1:10 OOO-scale aerial photographs wereintegrated with detailed field mapping and sediment faciesanalysis in order to reconstruct glacial bedform patterns andsubglacial environments in north-central Ireland (i.e.McCabe, 1996; Knight, 1997; Knight & McCabe, 1997a, b;McCabe et al., in press). The main results of theseinvestigations are that:
(I ) ice-flow transverse subglacial ridges, similar in outlinemorphology to Rogen moraines, extend across a largetract (> 1500 krn-) of the north central Irish lowlandsbetween Banbridge and Roscommon (Fig. I) (Knight &McCabe, 1997b);
(2) drumlins are concentrated locally in specific lowlandareas near coastal embayments such as Dundalk Bayand Done gal Bay, and include a wide range ofmorphological types (Knight & McCabe, 1997a;McCabe et al., in press);
(3) Rogen moraine ridges were formed prior to drumlinization phases because ridge crests are streamlined in the
IRISH GLACIAL M ODE L S II
same direction as, and merge spatially into, drumlins(Knight & McCabe, 1997b; McCabe et at ., in press);
(4) Rogen moraine ridges were modified and partiallystreamlined by headward-eroding ice streams whichterminated at tidewater margins around the easternIreland coast (McCabe et at., in press). Rogen ridgesoutside lateral margins of the ice streams were largelyunmodified;
(5) ice stream activity is dated by AMS 14C evaluation oftests of Ihe cold-water foraminifera Elphidium clavatumembedded in marginal morainic muds to between14 705 ±130 BP (sample from Cranfield Point; Fig. I)and 13 955 ±105 BP (sample from Killard Point)(McCabe & Clark, 1998). This time interval, whichcorresponds to a major North Atlantic ice-rafting event(Heinrich event 1) from the Laurentide ice sheet (Bond& LOlli, 1995; McCabe, 1996; McCabe & Clark, 1998),is temporally distinct from the ice advance anddrumlinization episode recorded by conventional 14Cdates from marine molluscs at Belderg, western Ireland(17 370 ±100 BP and 16940 ±120 BP; McCabe,Haynes & Macmillan, 1986).
These studies conclude that the Irish ice sheet marginexperienced repeated oscillations (periods of rapid iceadvance and retreat) during the Late Devensian glacialcycle, and assume that ice oscillations are recorded by thealignment and outline morphology of subglacial bedforms,and by the nature of their internal sediments. On astratigraphic basis, complex patterns of overprinted, crosscut and superimposed bedforms (Knight & McCabe,1997b), which are related to multiple ice oscillations(McCabe et al., in press), record major shifts in the locationof ice centres and changes in subglacial thermal andhydraulic regimes. These factors of the subglacialenvironment also affected the ways in which bedformsignatures were preserved or modified by later ice flows(Kleman, 1994). The overall reasons for this dynamic icebehaviour are unknown, but may be related to externalforcing by high frequency changes in amphi-North Atlanticclimate and relative sea-level (Bond & Lolli, 1995;McCabe, 1996; Knight et at., 1997; McCabe & Clark,1998).
4. NEW GLACIAL MODEL
From the distribution and relative-age relationships ofindividual bedforms, a new glacial model can be proposedwhich describes shifts in ice centres and changes in glacialand subglacial characteristics over time (Fig. 2). Numerically, drumlins make up only a small part (about 20%) ofthe ice flow indicators used to construct the model.Principal assumptions of the model are that:
(1) drumlins, Rogen moraine ridges and other landformsare accurate indicators of regional ice flow direction;
(2) changes in ice flow direction are recorded in bedformand sediment stratigraphies;
(3) the preservation potential of bedform signatures in anyone area is related closely to temporal changes insubglacial environmental regimes.
These assumptions are valid because the direction ofindi vidual ice flow events and their stratigraphicrelationships (i.e. relative timing) can be identified clearlyon the satellite imagery used, and that these are supportedby existing data on striae patterns , diamict fabrics, erraticcarriage, and the dating of marginal sediment sequences(summarized by McCabe, 1985, 1987, 1993; McCabe et at.,in press). The timing of bedform formation inland can alsobe estimated through its genetic link to these dated marginalsequences such as in Dundalk Bay (McCabe & Clark,1998). This therefore results in a 'dynamic' model of LateDevensian ice events in Ireland with good spatial andtemporal control (Fig. 2).
Figure 2 shows the new model describing the timing andgeometry of the Late Devensian Irish ice sheet. The modelis broken down into two temporal stages related to (1) aprominent ice oscillation at c. 19-17 ka, corresponding tothe period of drumlinization and sediment fluxes to theBelderg ice margin in western Ireland (Fig. 2a), and (2) icestreaming in eastern Ireland at 14.7-13.7 ka when highsediment fluxes built ice-marginal subaqueous moraines(McCabe, Dardis & Hanvey, 1984; McCabe & Clark, 1998)(Fig. 2b). Characteristics of the model include:
( I) the linked relationship between periods of bedformformation and temporal changes in ice margins and icecentre locations;
(2) ice downdraw into marine embayments and headwarderosion along ice streams;
(3) evolution of bedform patterns during stages of iceadvance.
5. COMPARISON OF AND CONTRASTSBETWEEN THE GLACIAL MODELS
These models can be compared on a like-for-like basisbecause they both attempt to account for field evidenceinside the major ice limits (southern Irish end moraine anddrumlin readv ance moraine) identified by Synge &Stephens (1960) and Synge (1969, 1970). Use of thesemoraine limits facilitates comparison between the twogeneral models, and contrasts with the ice cover models ofWarren (1992) and Warren & Ashley (1994) which aregeomorphically uncon strained by this field evidence.However, as re-investigation shows that bedform pallernsare generally more complex than assumed in 'traditional'models (i.e. McCabe et at., in press), detailed bedformrelationships and reconstructed ice flow events in keymarginal areas can be used to illustrate differences betweenthe two models. This is important because it provides amethodology for assessing ice dynamics elsewhere inwestern Britain, and shows that ice margins did notgenerally experience a stately retreat towards core dispersalareas throughout the last deglaciation (cf. Charlesworth,1924, 1939; Synge, 1969).
12
(a)
•
J. KNIGHT
SOlan-Ice flow vectors
Location of dated sections
(b)North Atlantic
Ocean
SOlan-Fig. 2. 'New' glacial model of ice flow vectors in Ireland during the Late Devensian: (a) during the main period of drumlinization(19-17 ka), as constrained by 14Cdates from Belderg, western Ireland (McCabe et ai., 1986); (b) during ice streaming (14.7-13.7 ka), asconstrained by dates from the foraminifera Elphidium clavatum from mud beds at Killard Point and Cranfield Point, eastern Ireland(McCabe & Clark, 1998). Undated ice flows in other areas can be matched with dated flows on the basis of shared morphological andsedimentary signatures.
Field examples
In Donegal Bay, 'traditional' models state that classicaldrumlins present in lowland areas reflect a single episodeof northeast to southwest ice flow (Wright, 1912;Charlesworth, 1924; Hanvey, 1988). However, detailedsatellite and field evidence shows that bedforms aremorphologically complex and vary in size and alignmentfrom north to south (Fig. 3). This morphological variabilitysuggests the bedforms can be better explained by twoseparate ice flow events derived from different directions(Knight & McCabe, 1997a) (Figs 3, 4). In the northeasternsector of the bay ridges « 35 m high, 3 krn long), which arealigned northeast-southwest and feed towards the coastfrom higher ground, lie parallel to the first ice flow event. Inthe southwestern sector of the bay these ridges areprogressively dissected and remoulded into drumlin-likeshapes with an east-west orientation. More complexhooked and barchanoid forms are also present (Fig. 3).These bedforms record a second ice flow event whichinvolved east-to-west flow from centres in the Omagh Basinand western Sperrin Mountains (Knight, 1997). Sediments
within the Mullinasole drumlin reflect these shifts in iceflow direction and associated changes in subglacialenvironments (Fig. 4). The first ice flow event (units 1-6 inFig. 4) is recorded by a glaciomarine diarnict (units 1, 3, 5)containing striated boulder pavements of faceted locallimestone clasts (Knight & McCabe, 1997a). The diamict,deposited by rainout and resedimentation processes, isinterbedded with massive gravel (unit 4) and laminated mudfacies (unit 2) which reflect the maximal and waning phasesof subglacial meltwater pulses respectively. The second iceflow event is recorded by a sandy diarnict facies (unit 7)which unconformably overlies the glaciomarine sedimentsand contains igneous clasts derived from the north andnorthwest (> 10 km distance). This second period of iceactivity, during which final drumlinization (shaping) of thesediment pile occurred, belongs to the same ice flow stageas that dated at Belderg (McCabe et al., 1986). Thereforethe bedform and sediment record in Donegal Bay reflectstwo distinct ice flow events, the first of which predatesdrumlinization. This contrasts with 'traditional' models(Charlesworth, 1924; Hanvey, 1988) which advocate asingle unidirectional (east-west) ice flow event,
IRISH GLACIAL MODELS 13
~Flrst ice
flow.. . . . . . .Secondice flow
- - 100m contour
2km ~
Fig. 3. Bedform patterns and ice flow shifts in Donegal Bay asmapped from satellite imagery. Bedforms are identified anddepicted on the basis of their outline shapes, i.e. image tone.
corresponding to the second event described above, whendrumlins in this region were formed (Synge & Stephens,1960).
In Clew Bay, 'traditional' models state that a singleeast-west ice flow during the Late Devensian wasresponsible for shaping a unidirectional suite of drumlins(Synge, 1968; Hanvey, 1988). However, satellite and fieldinvestigations show that bedforms are morphologicallycomplex and have a number of directional components (Fig.5). Typically bedforms comprise composite elongate ridges« 60 m high, 4 krn long) which are aligned from east towest. Towards the west bedforms change in nature fromrock-cored ridges and drumlins of classical and barchanoidshape to drift-dominated bedforms with complex basaloutlines, sometimes with hooked margins that extendtowards the northwest (Fig. 5). Sediments within the morecomplex bedforms such as at Rosmore Point comprisemassive glaciomarine diamict facies (units 1 and 2 of Fig.6) interbedded with poorly-sorted sand and gravel lensesand boulder pavements made up of local limestone clasts.Clast fabrics and lithology change up-sequence fromeast-west orientated and limestone-dominant (unit 1), tosoutheast-northwest orientated and more lithologicallydiverse, including granite and sandstone clasts (unit 2). Thisreflects the combination of a change in sediment inputdirection, and sediment reworking by debris flow processesdown the lee-slopes of subglacial ridges. A thin anddiscontinuous covering of massive sand (unit 3) caps theexposure and may relate to final ice retreat from the bay.This overall facies sequence is similar to that observed bySynge (1968) at Askillaun and Old Head (Fig. 5) wheresubaqueous morainal banks mark stillstands of the icemargin during retreat (Hanvey, 1988). Well-marked thrustmoraine ridges are present near Furnace Lough (Fig. 5).These moraines, containing disorganized, angular boulders,
field relationships, linking glacial bedforms. tee margmalpositions and sediment characteri stics, s~ggest two maj.orice flow events (rather than one) occurred in Clew Bay (FIg.5). The first (east- west) event is marked by the formation ofrock-cored drumlins and ridges inland , glaciomarine faciesdeposition, and sediment transport to terminal morainesincluding those at Askillaun and Old Head. The second(southeast-northwest) event is characte~z~d by bedfo~m
remoulding, the reworking of pre-existing subglacialsediments by subaqueous mass flows, and the formation ofnorthwe st-trending 'hooks' on bedform leeside flanks. Alow marginal sediment flux during this second flow event isevidenced by the pattern of thrust moraines near Furn~ce
Lough which have not incorporated the underl yingglaciomarine sediments during ice passa~e across the bay.This two-stage ice flow model more effectively acc~unts f~r
detailed field evidence than models advocatmg UnI
directional ice flow alone. The timing of these events isunknown in the absence of radiometric dating evidence, butit is reasonable to suggest that they correspond broadly tothe (very similar) glacial anatomy in Donegal Bay.
6. DISCUSSION
Reinvestigation of bedform pattern s in north-central Irelandsuggests that classical drumlins (Charlesworth, 1939) arenot as common a bedform type as previous workers havebelieved. This is especially the case in key marginal areas(i.e. Donegal Bay, Clew Bay) where ice osci llation~ weremost dynamic, and therefore questions the validity ofglacial models derived from 'drumlin' spat ial patt~rn~ . Thepresence of Rogen moraines near ice sheet centres in inlandareas of Ireland also questions these previous models(Knight & McCabe, 1997b). For example , ridges in theOmagh Basin, which were interpreted by Charlesw~rth
( 1924) as icc- margina l end moraines forme.d dur ingdeglaciation. can be better explained as subglacial Rogenmoraines. These ridges are morphologi cally similar to, andmerge laterally into. drumlin complexes. and some ridgecrests are streamlined in the same direction as drumlins nearDonegal Bay into which Omagh Basin ice was fed (Knight,1997).
The proposed new glacial model has advantage over the'traditional' models for several reasons.
(1) It can more accurately account for the complexity of
Rosmore Point. C lew S av
IRISH G LACIAL M O D ELS
Units and fac ies type Process and flow eve nt
3 Massive sand Grain flow
Massive 10 SubaqueousSecond
2 stratified diamict debri s flows
15
m
F G
Massive, tabulardiarnict beds with
boulder pavement s,occas ional sand Icnscs
Massive gravellydiarnict
Rainout,mass flows
Debri s flows
First
Fig. 6. Generalized sediment stratigraphy at Rosmore Point, Clew Bay.
bedform patterns in different areas and on differentscales.
(2) It can integrate effectively both geomorphic andsediment stratigraphic data.
(3) It emphasizes the role of complex subglacial systems,including variations in sediment deposition processes.
(4) It is temporally constrained by radiometric dates fromeastern and western Ireland.
The complex spatial pattern of partially-preservedbedform sets (i.e. Rogen moraines), drumlin swarmsfeeding into coastal emba yment s, and erosion alongheadcutting marine-based ice streams in eastern Ireland,suggests there were complex and interlinked controls onrepeated ice oscillations during the Late Devensian inIreland, which changed over both time and space (McCabe,1996; Knight et aI., 1997; McCabe et al., in press). Thesimilar timing of eastern Ireland ice streaming and NorthAtlantic Heinrich event 1 at c. 14.5 14C ka suggestsmillennial-scale oscillations of the Irish ice sheet may havebeen related to changes in temperature and precipitationregimes in the eastern Atlantic (Bond & LOlli, 1995;McCabe, 1996; McCabe & Clark, 1998). Possible processesof drumlinization, or factors leading to fast ice flow, are notspecified in 'traditional' Irish glacial models (i.e. Warren,1992) despite drumlins providing much (probably 80-90%)of the directional data upon which the models are based.The 'new' glacial model (Fig. 2), identifying a millennialscale ice oscillation signature in Ireland, proposes thatinteractions with the wider environment, as by sedimentfluxes to marine termini, are genetically related to bedform
generation and changes in ice sheet characteristics inland,and provide a 'dynamic' approach to problems of Irishdrumlinization and bedform development.
7. CONCLUSIONS
(1) 'Traditional' Late Devensian ice sheet models inIreland (i.e. Charlesworth, 1924; Synge & Stephens,1960; Synge, 1969, 1970; Warren , 1992) cannotaccount for detailed observations of the distribution ofbedforms such as drumlins, and the presence ofpreviously-unrecorded bedform types such as Rogenmoraines.
(2) A 'new ' ice sheet model links the complex spatial andtemporal relationships of these bedform sets to otherevidence such as local-scale sedimentology and datedice limits in order to reconstruct ice sheet dynamics.
(3) Irish ice events correspond chronolog ically with datedevidence for millennial-scale Laurentide ice sheetadvances (North Atlantic Heinrich events) and changesin amphi-North Atlantic climate. This suggests the Irishice sheet may have been sensitive to changes in thecoupled ice-ocean- atmosphere system.
ACKNOWLEDGEMENTS
I thank Steve McCarron for commenting helpfully on adraft of the paper. Mark Millar and Kilian McDaid drewseveral diagrams. The comments of referee Professor JimRose are appreciated.
16 J. KNIGHT
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Manuscript received 21 January 1998; revised typescript accepted 12 May 1998.
