sedimentary evidence for the formation mechanism of the armoy moraine and late devensian glacial...

GEOLOGICAL JOURNAL

Geol. J. 39: 403–417 (2004)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.964

Sedimentary evidence for the formation mechanism of the Armoymoraine and Late Devensian glacial events in the north of Ireland

JASPERKNIGHT*Department of Geography, University of Exeter, Exeter, UK

The internal sedimentology of the Armoy moraine, which marks a late Devensian (Weichselian) incursion of Scottish ice intothe north of Ireland, is described from a temporary exposure near Armoy village. The exposure (up to 30 m long, 10 m high)comprises folded gravel beds, massive clay grading up to pebbly diamicton, massive gravel, overfolded and deformed sand, anda gravel and massive diamicton cover. These sediments overlie glacially smoothed basalt bedrock. The sediment successionrecords the interaction of near-touching Scottish and Irish ice margins, and the formation, infilling and drainage of an ice-sup-ported proglacial lake. Scottish ice likely advanced into the north of Ireland by a short-lived glacier surge during the KillardPoint Stadial (coeval with Heinrich Event 1 at 14.5 14C ka BP), forming the Armoy moraine at its maximum extent. The presenceof rafted sediment blocks within the moraine suggests ice-marginal thrusting and deformation of proglacial sands, and syntec-tonic resedimentation into low points on the sediment surface. Asynchroneity between Scottish and Irish ice-marginal fluctua-tions may reflect differing responses to climate forcing during the Killard Point Stadial, or the effects of local factors such assubstrate type and basal ice thermal regime. Copyright # 2004 John Wiley & Sons, Ltd.

Received 24 January 2003; revised version received 15 May 2003; accepted 20 May 2003

KEY WORDS Irish ice; Scottish ice; surging; glaciolacustrine; British ice sheet; Killard Point Stadial; late Devensian

1. INTRODUCTION AND SIGNIFICANCE OF THE ARMOY MORAINE

The Armoy moraine, also termed the North Antrim moraine (Shaw and Carter 1980; McCabe et al. 1998) and the

Antrim coast moraine (Charlesworth 1939), has long been recognized as a major glacigenic feature in the north of

Ireland (Lewis 1894; Dwerryhouse 1923; Charlesworth 1939). The Armoy moraine refers to a series of interlinked

ridges, hummocks and kettleholes that extend discontinuously for a distance of 50 km between Articlave and

Ballycastle (Dwerryhouse 1923; Charlesworth 1939; Stephens et al. 1975; Shaw and Carter 1980; McCabe

1987; Gibson 1993; McCabe et al. 1998) (Figure 1). Based on its morphology, the moraine is generally agreed

to mark the limit of a late Devensian (Weichselian) age ice advance from southwest Scotland into the north of

Ireland (e.g. Stephens et al. 1975; McCabe 1987). The upstanding and preserved nature of the moraine, general

absence of deglacial sediments and landforms marking Scottish ice retreat, and absence of evidence for having

been overridden (Shaw and Carter 1980), suggest that the moraine represents the last ice advance event in the

region. The stratigraphic position of the moraine—overlying subglacial landforms that were formed during Irish

ice advance—is therefore central to the relative chronology of regional late Devensian glacial events. The moraine

is also significant because it records a major late-glacial readvance of part of the British ice sheet, with implications

for ice sheet driving mechanisms in relationship to North Atlantic climate (McCabe and Clark 1998).

Despite its importance, however, the moraine’s origins and internal composition are poorly known and are based

on only a few studies from isolated locations. In order to investigate in more detail events surrounding the

Copyright # 2004 John Wiley & Sons, Ltd.

* Correspondence to: J. Knight, Department of Geography, University of Exeter, Rennes Drive, Exeter, EX4 4KJ, UK.E-mail: [email protected]

Figure 1. (a) Map of generalized ice flow centres, vectors and ice limits during the maximum of the late Devensian glaciation in Ireland(Stephens et al. 1975) showing the location of the study area (shaded box). (b) Distribution of selected glacial geologic features and the positionof the Armoy moraine (after Charlesworth 1939; Stephens et al. 1975; McCabe et al. 1998), generalized Scottish ice flow vectors and maximalposition of the Scottish ice margin during the Killard Point Stadial (Heinrich event 1), and the locations of places named in the text. The boxindicates location of (c). (c) Detailed map of the Armoy area showing the location of the described exposure (star). Note the westward diversion

of the River Bush due to the presence of the Armoy moraine (shaded as in b).

404 j. knight

Copyright # 2004 John Wiley & Sons, Ltd. Geol. J. 39: 403–417 (2004)

moraine’s formation, this paper has two main aims: (1) to describe sediments within the moraine, from a new expo-

sure near Armoy village; and (2) to propose a new model for the formation of the moraine, emplacement of its

internal sediments, and the history of glacial events in the north of Ireland. The model suggests that Scottish and

Irish ice interacted dynamically before, during and after sediment deposition and moraine formation.

2. GEOLOGIC SETTING AND GLACIAL HISTORY

The north of Ireland is underlain dominantly by Tertiary basalt which overlies the Cretaceous-age Ulster White

Limestone Formation (Wilson and Manning 1978). The limestone is karstified and brecciated, and crops out

mainly along a narrow coastal zone. Inliers of Lough Neagh series Oligocene clays are present on the basalt sur-

face. The extreme northeastern part of Ireland is underlain by Dalradian-age metasediments including quartzose

schists, grits, phyllites and limestones (Wilson 1972). Glacial drift is generally thin and discontinuous; glacial dia-

micton (< 4 m thick) is present over most bedrock ridges. Glaciofluvial sand and gravel forms the arc shape of the

Armoy moraine (Figure 1b), and is also present in river valleys.

During the late Devensian (Weichselian) glaciation (c. 25–13 ka BP) in western Britain, ice flowed outwards

from upland dispersal centres towards ice margins located on the exposed shelves of adjacent seas (Bowen et

al. 1986). The landscape of northeastern Ireland records the history of ice advance, retreat and interaction from

separate centres in western Scotland and the north of Ireland. Four ice events in the north of Ireland can be recon-

structed from regional geomorphic, sedimentary and dating evidence (McCabe 2002). These are named according

to Knight et al. (in press).

1. During the last glacial maximum (LGM, Glenavy Stadial; c. 18–22 ka BP) (Bowen 1999) Irish ice advanced

northwards from an ice centre located in the Lough Neagh basin (50 km to the south of the study area),

converging with Scottish ice in the Malin Sea. Offshore sediments deposited at this time are grouped into the

Jura Formation, observed seismically in the Hebrides Sea (Fyfe et al. 1993). Ice limits during this period are not

well known, but may correspond with submarine moraines identified off the western Scottish Highlands (e.g.

Stoker and Holmes 1991).

2. Ice retreat onshore took place following a short period of drumlinization which shaped bedforms across the

north of Ireland (McCabe et al. 1998). Evidence for northward ice advance and drumlinization in the study area

(Belderg Stadial; 18–17 ka BP) comes mainly from drumlin orientation and erratic carriage (Dwerryhouse 1923;

Charlesworth 1939; Hill and Prior 1968; McCabe 1987) and deposition of the local Antrim Coastal Till (Hill

and Prior 1968). Onshore geologic signatures for ice retreat following drumlinization (Cooley Point

Interstadial; 17–15 ka BP) include transgressive marine muds underlying or contained within gravelly moraines

or morainal banks which are preserved in some coastal embayments (McCabe 1996, 2002; McCabe and Clark

1998). Inland, a 36Cl exposure age on a rock surface at Rasharkin, 10 km south of the Armoy moraine, dates

downwasting of Irish ice at this location to 17.5� 2.6 ka BP (Bowen et al. 2002).

3. Reactivation of ice activity in Ireland, and elsewhere in the British Isles, took place around the time of Heinrich

event 1 (H1) in the North Atlantic (c. 14.5 14C ka BP). This period (Killard Point Stadial; 15–13 ka BP) was

characterized by ice streaming in eastern Ireland (Knight et al. 1999), renewed drumlinization in eastern and

western Ireland, and likely incursion of Scottish ice into the north of Ireland, although this latter event is not

dated radiometrically (McCabe et al. 1998). Reactivation of several British ice sectors at the time of H1, which

is radiometrically dated from sediments at the termini of drumlin/moraine fields, suggests that this represents a

regional-scale ice sheet response to temporary northern hemisphere cooling (McCabe and Clark 1998). The

nature of the Scottish ice event is the focus of this paper.

Evidence for the south-going Scottish ice event during the Killard Point Stadial comes from a range of field data

including the arcuate shape of the Armoy moraine (reflecting the shape of the incursive ice lobe), directional

indicators within the moraine including south-going thrust planes, and presence of Scottish erratics (Shaw and

Carter 1980; McCabe 2002). Located to the north and south of the moraine, however, are streamlined bedrock

sedimentology of armoy moraine, northern ireland 405


ridges which mark the earlier northward advance of Irish ice during the Belderg Stadial (Figure 1b). The later

ice advance from Scotland is believed to have stripped away most sediment deposited during this earlier event,

leaving the bedrock ridges exposed (McCabe 2002).

4. In situ downwasting (stagnation zone retreat) of ice margins in northern and eastern Ireland following H1

(Rough Island Interstadial; 13–11 ka BP) led to transgression of marine waters in some marginal locations, and

the deposition of a mud drape over earlier subglacial landforms (McCabe and Clark 1998). At Portballintrae,

located inside the Armoy moraine limit, delicately rippled sand–mud couplets overlain by raised beach gravel

(McCabe et al. 1994) were formed during a period of higher relative sea-level (RSL) following Scottish ice

retreat. A Killard Point Stadial-age for Scottish ice incursion is further supported by radiocarbon ages on the

marine Clyde Beds of southwest Scotland (e.g. Peacock 1981)—which may be correlative to the sand–mud

beds at Portballintrae—which show that Scottish ice retreated rapidly onshore during the Rough Island

Interstadial.

3. GEOMORPHOLOGY AND SEDIMENTOLOGY OF THE ARMOY MORAINE

Morphologically the Armoy moraine comprises a series of west–east aligned ridges (Figure 1b) which generally lie

parallel to overall moraine margins (Stephens et al. 1975; Gibson 1993). Drift is thickest, and the ridges of greatest

relief (in total< 35 m high, 4 km wide), in lowland areas around Armoy and Stranocum and near the lower River

Bann (Figure 1b). Some isolated ridges in these areas (e.g. Seacon and Dervock) are separated from the main mor-

aine by kettled topography or outwash spreads which may be associated with deep postglacial infills (e.g. Garry

Bog). Moraine ridges in higher, drift-poor areas, as around Articlave, are smaller and associated with outsized

surficial boulders (McCabe 2002). The southern margin of the Armoy moraine is generally oversteepened by melt-

water channels such as near Ballymoney (Charlesworth 1939).

Sediments within the Armoy moraine have been described occasionally. A large, temporary exposure near Bal-

lymoney, at the centre of the moraine complex, was described by Shaw and Carter (1980). Here, major WNW-

dipping shear planes separate imbricated and interbedded units of sand, gravel, silt and laminated clay. Detailed

sedimentary structures were not recorded. The tectonically thickened sediment sequence (backstripped to <60 m

original thickness) was interpreted as formed by a southward advance of Scottish ice, bulldozing into shallow-

water proglacial sediments which were cleanly thrust up at the ice margin with little or no internal deformation

(e.g. Croot 1987). Formation by ice-marginal thrusting supports the Armoy moraine limit as the maximum ice

extent reached during Scottish ice advance (Stephens et al. 1975). Ahead of this southern ice limit, around Agha-

dowey and Vow (Figure 1b), are found sand and gravel deposits which are hummocky or have flat upper surfaces.

These sediments show northward-dipping gravel foresets, interbedded with sand and mud layers, which grade dis-

tally into rhythmically bedded muddy toesets (Dardis 1990). The sediments are interpreted to have formed in a

proglacial lake, impounded by an Irish ice margin to the south and Scottish ice margin located at the Armoy mor-

aine limit to the north (Creighton 1974; Dardis 1990; McCabe 2002). Feeder eskers at Vow record the subglacial

component of this system (McCabe 2002). The relationship of these lake sediments to the formation of the Armoy

moraine is unclear.

3.1. New sedimentary evidence

A new temporary exposure in the Armoy moraine, 1 km northeast of Armoy village (Figure 1c), lies at the flank of

a north-going meltwater channel which exploits a pre-existing fault in the underlying Lower Basalt Formation. The

meltwater channel (<15 m deep, 30–100 m wide, 800 m long) cuts directly through part of the moraine; bedrock is

exposed intermittently on channel flanks. The exposure (up to 30 m long, 10 m high) comprises a succession of

gravel, sand and diamicton beds (Figure 2). These sediments overlie a glacially smoothed and scoured basalt bed-

rock surface from which bedrock blocks have not been displaced. The bedrock surface forms a 7-m-high dome,

406 j. knight


and sediments are stacked against the northern side of this dome. The lowermost bedrock–sediment contact is not

observed except over the crest of the bedrock dome. Five major sedimentary units are observed (Figure 2). These

are described using the facies codes of Eyles et al. (1983). Sedimentary units lower in the succession are eroded out

along the sides of the meltwater channel, indicating that channel formation postdates deposition of (most likely)

the entire sediment pile.

(1) The basal gravel (Gcs facies, <3.5 m thick) onlaps onto the bedrock dome and comprises dipping and folded

massive gravel beds individually up to 0.5 m thick (Figure 3a). Clasts are edge- to well-rounded granules to

boulders, variously clast- to matrix-supported, and are arranged in normally graded to massive layers which

have variably sharp to gradational bed boundaries. Chevron folds are observed in the gravel beds (Figure 3b).

These folds (n¼ 3, limbs up to 1.5 m high) are tight and subvertical with the axial plane striking to 15� and

dipping to the west-northwest, indicating ice pressure from that direction. Two bulk sediment samples from the

gravel, located stratigraphically above (sample B) and below (sample A) the fold limbs show a dominance of

schists derived from the Dalradian rocks to the northeast (75% and 85% respectively; Figure 2). This

provenance is also confirmed by the presence of vein quartz which is found within the schist. Isolated lens-

shaped intraclasts (<40 cm high, 1 m long) of well-sorted sand are sometimes contained within the gravel beds

(Figure 3a). These sand lenses have sharp outer margins and have been folded and contorted with the

surrounding gravels. Small contractional faults may be present within the lenses but are not observed to

penetrate the gravel beds.

(2) A muddy diamicton (Fm/Dmm to Sl facies), which has a sharply conformable (occasionally graded and

contorted) contact with the underlying gravel, thickens across the exposure from a few centimetres to over 1 m

thickness. The base of the diamicton unit (Figure 4) comprises massive red to black silty clay (0.5–10 cm

thickness) which is overlain sharply by, or interbedded with, laminated to massive silt (10–50 cm thickness).

Clay/silt interbeds are generally swaley in geometry and laterally discontinuous. Upwards, sediment coarsens

Figure 2. Composite facies log of the exposure at Armoy showing palaeoflow direction and clast lithology source regions (left-hand column),and reconstructed chronology of the main glacial events in northeastern Ireland (right-hand column, stages numbered 1–6 according toFigure 7). Advance and retreat of Scottish and Irish ice is shown on the left and right, respectively. The likely temporal extent of the glacial lake

is shown by the vertical arrow.



to medium sand, occasionally with granule lenses (a few centimetres high, <10 cm long). Small pebbles are

found dispersed throughout. The pebbles may occupy depressions or scours within the sand beds, or may lie

parallel to bedding. Both clast frequency and grain size range increase upwards. The boundary between the

diamicton unit and the overlying gravels is generally sharp and conformable.

Figure 3. (a) Photo of the gravel unit (unit 1) showing clast-supported nature and presence of deformed sand lenses. The section is 4 m high. Thefold limb shown in (b) is arrowed. (b) Detailed photo of the gravel fold within unit 1 with the fold limb (shown in a) arrowed. The trowel (28 cm

long) is located within the centre of the fold. This figure is available in colour online at http://www.interscience.wiley.com/journal/gj

Figure 4. Photo of the massive clay layer (base of unit 2), grading up into gravelly diamicton. The trowel is 28 cm long. This figure is availablein colour online at http://www.interscience.wiley.com/journal/gj

408 j. knight


(3) The gravel unit (Gcm/Gcs facies; 1.5–3.0 m thick) comprises sub- to well-rounded pebbles to boulders

(<40 cm diameter) arranged in normally graded beds 0.2–1.0 m thick (Figure 5). The gravel beds are internally

variable and may include massive granule to pebble lenses, clast-supported pebble beds that may be normally

or inversely graded, and lens-shaped openwork boulder clusters (<1 m high, 2 m wide). Individual clasts have

no preferred long axis alignment. Some pebble beds have a minor (<15%) matrix component of medium to

coarse sand. Lithologically, clasts within the gravel facies are mainly (56%) from the north and northeast

(including flint derived from the Ulster White Limestone series), and a high local basalt component (34%).

(4) The sand unit (Sm/Ss facies; 1.5–6.0 m thick) is of variable geometry, generally thickens upslope, has a sharply

planar and dipping lower boundary (forming an angular unconformity to the underlying gravel unit), and a

variably sharp to graded and contorted upper boundary (Figure 6). Internally the sand unit comprises beds of

well-sorted massive to laminated sand and silt (0.2–0.5 m thick). The beds are generally laterally continuous

and conformable to one another. No rippled beds are observed. Towards the eastern part of the exposure the

sand and silt beds are folded downwards. Beds lowermost in this unit form a tight recumbent fold with an axial

plane that plunges at a low angle to the southeast. Fold strike indicates ice pressure from 310� (Figure 6a). This

ice pressure direction is also supported by the orientation of tight, cylindrical folds (<0.5 m high, 0.2 m wide at

their base), developed in the sand beds above this recumbent fold, which verge towards the southeast (Figure

6b). Associated with some of these folded sand beds are ball-shaped sediment intraclasts or rafts (<0.3 m

wide) that are located stratigraphically towards the base of the sand unit. Sediments within the intraclasts are

identical to those found in pinched-out and downward-dipping sand beds located immediately above them.

Towards the top of the unit, beds are less tightly folded and dip southeastwards. In the upper 0.5–1.0 m of the

unit, sand beds are thinner (<10 cm thickness), contorted, and interbedded with pebble layers (Figure 6c).

Occasionally, normal faults (<1 m long) are found within the sand unit. These faults strike at 80� and have a

vertical displacement of a few centimetres.

(5) A massive, gravelly diamicton (0.3–2.0 m thick) caps the sediment succession (Figure 6c). This unit is variable

in thickness across the exposure and grades upwards from a matrix-poor massive to poorly stratified gravel

(Gmc facies) to a clast-poor diamicton (Dmm facies). The basal gravel component infills the syntectonic low

formed above the underlying sand unit. Clasts within this gravel component are derived mainly (68%) from the

underlying basalt bedrock.

Figure 5. Photo of the gravel unit (unit 3) showing clast-supported local boulders. Note the very sharp and erosionally unconformable boundarybetween the gravels and the overlying sands (unit 4). The trowel in the centre of the photo is 28 cm long. This figure is available in colour online

at http://www.interscience.wiley.com/journal/gj



Figure 6. (a) Detailed photo of overfold in the sand unit (unit 4). (b) Detailed photo of cylindrical folds developed in the laminated sand andmassive silt beds. Trowel is 28 cm long. (c) Photo of units 4 (sand) and 5 (gravelly diamicton, to right). Note the presence of gravel interbedstowards the top of the sand unit. The section is 5 m high. This figure is available in colour online at http://www.interscience.wiley.com/journal/gj

410 j. knight


3.2. Interpretation of sedimentary processes and depositional setting

The well-sorted and interbedded sands and gravels at Armoy suggests deposition in a proglacial (forefield and

glaciolacustrine) environment associated with variations in water depth and velocity (Maizels 1989, 1993;

Marren 2002). The smoothed basalt bedrock surface is interpreted to have been scoured subglacially (e.g.

Evans 1996) by an earlier northward advance of Irish ice. This is supported by the presence of such surfaces

regionally both inside and outside the Armoy moraine limit, and by the orientation of bedrock striae (Hill and

Prior 1968).

The basal gravel (unit 1) is interpreted to have been deposited in a high-energy, ice-proximal outwash environ-

ment characterized by shifting, braided rivers. This interpretation is supported by the clast-supported nature of the

gravels, indicating bypassing of finer sediment, and presence of discrete, massive sand lenses which may be inter-

preted as waning flood or slack-water deposits formed along bar flanks (Church and Gilbert 1975; Nemec and

Steele 1984; Maizels 1997). Alternatively, the gravel beds may reflect deposition by mass flows into a proglacial

standing water body. Both fluvial and lacustrine sediments are found in proglacial, forefield environments (Marren

2002). These interpretations cannot be assessed easily because of the later folding of the gravel beds which reflects

ice advance from the west-northwest, and proglacial bulldozing and deformation of the sediment pile to a depth of

at least several metres, possibly forming a moraine.

The overlying muddy diamicton unit (unit 2) reflects sediment deposition in a quiet-water environment increas-

ingly disturbed by debris flows and (likely) bottom current circulation. The basal clays, which are not rhythmically

bedded, record sediment rainout from suspension, possibly in shallow water or as distal muddy bottomsets (Ashley

1995). Upwards, interbedded clays and coarser sediments may reflect increased influence of bottom currents,

dense underflows, or a change to seasonal (rhythmic) sedimentation. These processes, reflecting rapid water dee-

pening and changes in the location of water inflow, are consistent with changes in lake geometry as by a blocking

of lake outlets. More open-water conditions and vigorous lake water circulation are also evidenced by the upward

transition to muddy diamicton. The presence of isolated clasts within the diamicton suggests rafting by lake ice,

deposition by debris flows or mass flows, or as outrunners from dense underflows. Discrimination of these deposi-

tional mechanisms cannot be fully evaluated on the basis of the evidence available.

The overlying gravel unit (unit 3) is interpreted as an efflux jet deposit (Powell 1990) associated with episodic

point-source sediment input into a standing water body. The basal unconformity of the gravel unit may be the

erosional signature of this efflux. The large clast size, chaotic, massive nature and clast organization into lens-

shaped clusters suggest turbulent, high-energy flows with the larger clasts moving in traction or supported by

grain–grain interaction. Absence of a sandy matrix indicates sediment bypassing during maximal flow stages (Car-

ling and Glaister 1987). The presence of a granule and sand matrix at the top of the unit may reflect sediment from

turbulent suspensions settling in between the larger clasts during waning flow stages. These flows likely exited

subglacially into a proglacial lake, with variations in flow velocity resulting in beds of different grain size and

lateral extent. The efflux jet may have switched input location within the glacial lake, or been switched off as

ice activity decreased.

The massive to stratified sediments comprising the sand unit (unit 4) reflect deposition in a more ice-distal pro-

glacial environment by density flow and grainflow processes. Sediment may have been contributed by both Scot-

tish and Irish ice in a quiet-water location on glacial lake margins. The finer silty sediments may have accumulated

partly through suspension settling. The lack of rippled beds suggests absence of bottom currents and sediment

input by dense underflows from lake basin margins. Deformation of these sediments occurred by Scottish ice

advance from the west-northwest and was characterized by shunting or shearing ahead of the ice margin, evidenced

by the basal angular unconformity of the sand unit. That the sediment layers remained largely intact despite intense

folding suggests they were semi-consolidated; no evidence for water escape associated with this tectonic deforma-

tion is observed. The presence and disposition of sediment intraclasts towards the base of the folded sand beds

suggest that, during deformation, lumps of the sand beds were detached and incorporated in subjacent sediment

down a descending slope ahead of the fold nose, possibly by turbulent flow processes. Little or no new sediments

derived from the Scottish ice margin were included within the deposit. This may attest to clean basal ice or indicate



that folding was distal from the ice front. Likewise, the presence of gravelly interbeds towards the top of the unit

suggests more proximal sediments were reworked and incorporated along with the uppermost sand beds during

Scottish ice advance. These sediments infilled syntectonic palaeolows, which can account for the upward decrease

in fold amplitude.

The diamicton (unit 5) that caps the sediment succession likely records the transition from a subaqueous (basal

gravel) to subaerial (diamicton) depositional setting. The gravels, grading upwards from the underlying sand unit,

reflect deposition in shallow water ahead of the advancing ice front. Sediments were reworked by subaerial mass-

and debris-flows following ice retreat, forming the diamicton cover. The high percentage of local basalt bedrock

recorded in this unit may suggest that ice-marginal shearing brought sediment higher in the profile. Sediments may

have been exposed subaerially in association with glacial lake drainage and the formation of meltwater channels.

3.3. Comparison with regional sedimentary evidence

The sediment succession at Armoy is very similar to that observed at Dervock (8 km to the west of this site) by

Dwerryhouse (1923, p. 358) who described a ‘ridge of gravel covered by contorted sands, with a thin layer of

boulder-clay’. For this reason the depositional processes and environmental setting of sediments at Armoy are

likely of regional significance. However, it is notable that the sand and gravel-dominated Armoy sediments are

rather different to the thick, laminated (rhythmically bedded) clays with dropstones recorded within the moraine

complex at Ballymoney, 14 km to the southwest (Shaw and Carter 1980). The laminated clays at Ballymoney, in

total 8–10 m thick, likely represent distal bottomsets deposited in the lowest part of the proglacial basin. The mas-

sive clay located at the base of the muddy diamicton unit at Armoy may be a localized shallow-water drape and

may be the lateral equivalent to these bottomsets.

4. DEPOSITIONAL MODEL

The sediments at Armoy, and inferred depositional setting and ice flow direction, can be used to construct a six-

stage depositional model which involves the interaction of Irish and Scottish ice margins and the development and

drainage of a proglacial lake (Figures 2, 7).

(1) Northward advance of Irish ice off the north coast of Ireland during the Belderg Stadial smoothed bedrock

surfaces and formed north-going drumlins and rock ridges across the region (stage 1 on Figure 7). This period

may correlate with the formation of subglacial diamicton offshore while Scottish ice was present in the

Hebrides Sea (Binns et al. 1974). At Armoy this stage is evidenced by the smoothed and scoured basalt surface

indicating subglacial abrasion under warm basal ice conditions. The general absence of sediments marking

Irish ice retreat during the later Cooley Point Interstadial (McCabe 2002) suggests the ice may have retreated

by in situ stagnation and downwasting of the ice margin, which may have resulted from the longitudinal

extension associated with drumlinization. Alternatively, basal ice may simply have been clean.

(2) Southward advance of Scottish ice into the north of Ireland during the Killard Point Stadial (McCabe et al.

1998) overrode and deformed the coarse proglacial outwash (stage 2 on Figure 7). Synsedimentary folding of

sediments at Armoy—and the likely formation of moraine ridges along the lateral margins of the ice lobe near

Ballycastle and Articlave—was probably accentuated by ramping against the rising basalt bedrock slopes to

the south. Based on erratic content, the gravels (unit 1) located above the bedrock surface at Armoy may have

been deposited from both Irish (basal part) and Scottish (upper part) ice. Stephens et al. (1975) argued that the

Scottish and Irish ice margins were located close to one another (a few kilometres distance) at this time.

Regionally, sediments derived from the Malin Sea are found uncommonly onshore, which may indicate clean

basal ice advanced over a soft, deformable substrate offshore (cf. O Cofaigh and Evans 2001) which did not

permit a high basal sediment flux towards the ice margin. This may be evidenced regionally by the so-called

‘North-Antrim Scottish Till’ which contains only 0.7% Ailsa Craig erratics (Hill and Prior 1968), similar to the

412 j. knight


Figure 7. Model illustrating ice event stages 1–6 (shown on Figure 2) during formation of the Armoy moraine. See text for discussion.



Scottish erratic content recorded in unit 1 gravels in this study. The North-Antrim Scottish Till thins landwards

and interbeds with local Irish tills (Hill and Prior 1968). The presence of Arran granite erratics at Armoy also

suggests a northerly rather than northeasterly source area.

(3) Irish ice advanced northward while the Scottish ice margin was stable or static in position, and an impounded

proglacial lake formed through the closure of southerly meltwater outlets (stage 3 of Figure 7). The Scottish

ice margin at this time is likely to have been positioned between Dervock and Armoy, evidenced by gravelly

moraines at these locations, and the closing off of northward meltwater outlets near Coleraine (Charlesworth

1939). Lacustrine sediments, including muddy bottomsets, are recorded in the Tow valley near Ballycastle

(Charlesworth 1939) which may be considered a topographically controlled marginal lake unrelated to

the formation of the Armoy moraine. The Irish ice margin advanced to approximately the present limit of the

Armoy moraine, with a lobe present in the Bann valley to the area around Castleroe. Deposition of massive

clay draping the gravel facies at Armoy indicates a period of lake deepening and/or more distal inflow.

(4) Northward advance of Irish ice (stage 4 of Figure 7) is associated with the deposition of more ice-proximal

sediments (unit 3). An Irish ice source for the efflux jet deposit is supported by the high percentage of basalt

clasts, derived mainly from the south. The feeder esker at Glarryford (McCabe et al. 1998), composed entirely

of basalt clasts, may have developed at this time, and indicates a period of renewed Irish ice activity. Scottish

ice advance at this time, and formation of the tectonically thickened Ballymoney ridge at its maximum extent,

may have forced an increase in lake height, with high-level overflow channels formed at the western and

eastern ends of the glacial lake, towards Articlave and Ballycastle, respectively (Charlesworth 1939).

(5) Scottish ice retreated from Ballymoney to a position near Dervock (5 km distance; Figure 1) and the sand unit

(unit 4) was emplaced. Directional indicators within the sand unit at Armoy suggest it was deformed by

Scottish ice pressure from the north (stage 5 of Figure 7), evidenced by the direction of overfolding and

thrusting. It is possible that an extension to the glacial lake, with a lowered water surface of 30–40 m OD,

opened out in the River Bann valley between Castleroe and Vow, depositing glaciolacustrine clays

(Charlesworth 1939). These sediments are presently concealed by postglacial diatomite (Wilson and Manning,

1978). The Ballymoney ridge would therefore have formed an ‘island’ amid this shallow glacial lake. This

interpretation can also account for the evidence for water shallowing in the Armoy sediment profile, and the

development of south-going meltwater channels around Ballymoney (Charlesworth 1939).

(6) Deposition of the capping diamicton (unit 5) took place possibly in either a subaerial or subaqueous

environment in which sediment was reworked down unstable slopes. This period was likely associated with ice

retreat, glacial lake drainage and meltwater channel incision (stage 6 of Figure 7). Lack of evidence north of

the Armoy moraine limit for Scottish ice retreat suggests either very rapid active retreat of clean ice, or in situ

ice-marginal stagnation. The presence of kettleholes around Castleroe (Hamilton et al. 1985) may record

active Scottish ice retreat from this area. Garry Bog, north of the moraine, may also be an infilled kettled

landscape. The Castleroe kettleholes are infilled with sediments from Irish ice, which is considered coeval with

esker activity at Vow. During glacial lake drainage therefore, Scottish ice probably retreated actively, leaving a

fringe of stagnant ice blocks among small outwash surfaces. Irish ice in the Bann valley retreated actively to a

position near Vow. Hummocky topography south of the Armoy moraine near Dunloy may record ice

stagnation associated with a shut-down of the Glarryford esker. Meltwater channels in topographically low

areas through the Tow valley towards Ballycastle, and in the Bann valley towards Coleraine, postdate final

sediment deposition.

5. DISCUSSION

The model for the progressive opening, infilling and closure of a glacial lake basin (Figure 7) infers the

dynamic interaction of two opposing ice margins, and is different to the models of Charlesworth (1939) and

Shaw and Carter (1980) which advocate a Scottish ice advance into a pre-existing lake, and napping of pre-

existing sediments. The arcuate morphology of the Armoy moraine, and the presence of deformed and thrusted

414 j. knight


ridge sediments, is consistent with landform–sediment assemblages found associated with modern surge-type

glaciers in Iceland, Alaska and Svalbard (e.g. Benn and Evans 1998). Landform–sediment associations of sur-

ging glaciers include glaciotectonized and overridden sediment, crevasse-fill ridges, end moraines containing

thrusted sediment blocks, and kettled moraines formed during post-surge quiescence. Most of these landforms

are found in the Armoy moraine region. The periodicity of modern surging glaciers is variable, up to a decade

(Dowdeswell et al. 1991; Kamb et al. 1985), thus the Armoy moraine landform–sediment association could

contain several such cycles.

The behaviour of some present-day surging glaciers is linked to basal thermal regime, whereby surging takes

place as a result of basal ice warming (Murray et al. 2000). Further, surging may be associated with meltwater

lubrication at the bed rather than pervasive deformation deep into the sediment pile (Fuller and Murray 2002),

and is therefore associated with a relatively low basal sediment flux. A surging mechanism for Scottish ice

advance into Northern Ireland (Figure 8) can be explained as overriding of fine-grained glacial sediment of

the Jura Formation in the Malin Sea region (Fyfe et al. 1993), generating a combination of shallow-depth sedi-

ment deformation and sliding at the ice–bed interface due to high pore-water pressure. The absence of Jura

Formation sediments transported onshore, and the absence of evidence for ice retreat stages, also argue against

basal ice thrusting and sediment incorporation, and in favour of clean basal ice. Increased pressure melting of

basal ice over the elevated basalt plateau of northeastern Ireland may have acted as a meltwater source for the

glacial lake. The smoothed bedrock ridges and absence of large basalt erratics suggests the dominance of

subglacial abrasion over plucking and therefore warm-based ice. The clean ice-marginal thrusting of lacustrine

clays (seen at Ballymoney), and thrusting with syntectonic folding of sands seen at Armoy, demonstrate a pro-

glacial rather than a subglacial origin. High subglacial pore-water pressure over low-permeability clays may

lead to both fast ice flow (surging) behaviour, and clean thrust sheets at the ice margin where pore-water pres-

sure is released (Boulton and Caban 1995).

5.1. Glacial lake development

Evidence for water-sorted sand and gravel, and waterlain silts and clays, argues strongly for the presence of a

standing water body. However, it is unlikely that a large glacial lake ever existed, rather, that water was temporarily

impounded between opposing ice margins, between ice and emergent bedrock slopes, and between ice and emer-

gent moraine ridges (stages 3–5 of Figure 7). A dynamic depositional setting with rapidly changing lake levels,

lake geometry, and inflow and outflow locations is therefore more likely. The thicker clay beds, and biggest mor-

aines, seen around Ballymoney may reflect the deepest or most permanent lake cover. Elsewhere, any glacial lake

Figure 8. Schematic north–south cross-section from the Malin Sea to the Armoy moraine region showing the relationship between substratetype and deposition of ice-marginal sediments. See text for discussion.



was of varying depth and thus sedimentologically transitional to proglacial outwash (Church and Gilbert 1975).

The Armoy moraine therefore represents a composite succession of sediments formed and deformed in a range of

water-depth environments (e.g. Rovey and Borucki 1995; Marren 2002). The morphological signature of Scottish

ice advance varies across the length of the moraine depending on original sediment thickness and presence of bed-

rock highs which acted as a thrust ramp.

It is also likely that two opposing ice margins (Scottish and Irish) were required in order to dam this lake, rather

than just a Scottish ice margin forming a temporary marginal lake against high ground (Figure 7). Input from north-

going Irish ice into a proglacial lake is evidenced by lacustrine sediments at Vow. Deposition of sediments here

also requires a northern ice boundary (i.e. from Scottish ice). If opposing ice margins were not present, extensive

waterlain sedimentation could not have occurred from either ice sheet.

6. AGE OF THE ARMOY MORAINE AND CONCLUDING REMARKS

The Armoy moraine likely formed diachronously, over up to a few hundred years and over several small ice

advances (Figure 7), but it has not been geochronometrically dated and any discussion of age or correlation is

tentative. Based on correlation with dated sites in western Scotland, Stephens et al. (1975) suggested Scottish

ice advanced to the Armoy limit around 13.5 14C ka BP. McCabe et al. (1998) correlated Scottish ice advance

with the Killard Point Stadial ice limit (in eastern Ireland dated to 13.8 14C ka BP, cal. 16.0 ka BP). However,

interpretation of the sedimentary evidence from Armoy suggests that Scottish and Irish ice advanced and

retreated asynchronously (Figures 2, 7). If climate was the primary driver of ice sheet activity from both cen-

tres, then this asynchronous behaviour can be interpreted in terms of the greater distance from the Scottish ice

centre compared to the Irish ice centre, differences in ice sheet size, and different response times to external

forcing. If climate was not the primary driver of ice sheet activity, asynchroneity can be interpreted in terms of

the different substrates the ice traversed, and likely variations in basal thermal and hydraulic regime. The

absence of dated Killard Point Stadial-equivalent ice sheet limits in western Scotland means that these alter-

native explanations are as yet untested.

ACKNOWLEDGEMENTS

I thank Peter Wilson for bringing this exposure to my attention. Danny McCarroll and an anonymous reviewer are

thanked for their comments.

REFERENCES

Ashley GM. 1995. Glaciolacustrine environments. In Modern Glacial Environments: Processes, Dynamics and Sediments, Menzies J (ed.).Butterworth-Heinemann: Oxford; 417–444.

Benn DI, Evans DJA. 1998. Glaciers and Glaciation. Arnold: London.Binns PE, Harland R, Hughes MJ. 1974. Glacial and postglacial sedimentation in the Sea of the Hebrides. Nature 248: 751–754.Boulton GS, Caban P. 1995. Groundwater flow beneath ice sheets: part II—its impact on glacier tectonic structures and moraine formation.

Quaternary Science Reviews 14: 563–587.Bowen DQ (ed.). 1999. A Revised Correlation of Quaternary Deposits in the British Isles. Geological Society Special Report 23. Geological

Society: London.Bowen DQ, Rose J, McCabe AM, Sutherland DG. 1986. Correlation of Quaternary glaciations in England, Ireland, Scotland and Wales.

Quaternary Science Reviews 5: 299–340.Bowen DQ, Phillips FM, McCabe AM, Knutz PC, Sykes GA. 2002. New data for the Last Glacial Maximum in Great Britain and Ireland.

Quaternary Science Reviews 21: 89–101.Carling PA, Glaister MS. 1987. Rapid deposition of sand and gravel mixtures downstream of a negative step: the role of matrix-infilling and

particle-overpassing in the process of bar-front accretion. Journal of the Geological Society 144: 543–551.Charlesworth JK. 1939. Some observations on the glaciation of north-east Ireland. Proceedings of the Royal Irish Academy 45B:

255–295.

416 j. knight


Church M, Gilbert R. 1975. Proglacial fluvial and lacustrine environments. In Glaciofluvial and Glaciolacustrine Sedimentation, Jopling AV,McDonald BC (eds). Society of Economic Paleontologists and Mineralogists Special Publication 23: 22–100.

Creighton RJ. 1974. A Study of the Late Pleistocene Geomorphology of North-Central Ulster, Northern Ireland. PhD thesis, Queen’sUniversity, Belfast.

Croot DG. 1987. Glacio-tectonic structures: a mesoscale model of thin-skinned thrust sheets? Journal of Structural Geology 9: 797–808.Dardis G. 1990. Vow. In North Antrim and Londonderry, Wilson P (ed.). Field Guide 13. Irish Association for Quaternary Research: Dublin;

85–88.Dowdeswell JA, Hamilton GS, Hagen JO. 1991. The duration of the active phase on surge-type glaciers: contrasts between Svalbard and other

regions. Journal of Glaciology 37: 388–400.Dwerryhouse AR. 1923. The glaciation of north-eastern Ireland. Quarterly Journal of the Geological Society 79: 352–421.Evans IS. 1996. Abraded rock landforms (whalebacks) developed under ice streams in mountain areas. Annals of Glaciology 22: 9–16.Eyles N, Eyles CH,Miall AD. 1983. Lithofacies types and vertical profile models: an alternative approach to the description and environmental

interpretation of glacial diamict and diamictite sequences. Sedimentology 30: 393–410.Fuller S, Murray T. 2002. Sedimentological investigations in the forefield of an Icelandic surge-type glacier: implications for the surge

mechanism. Quaternary Science Reviews 21: 1503–1520.Fyfe JA, Long D, Evans D. 1993. United Kingdom Offshore Regional Report: The Geology of the Hebrides-Malin Sea Area. HMSO: London

(for the British Geological Survey).Gibson PJ. 1993. Geological and geomorphological applications of low-angle illumination satellite imagery in Northern Ireland. Irish

Geography 26: 58–64.Hamilton A, Dalzell LR, Kenehan BP, McDonagh BJ. 1985. A palynological investigation of kettle-hole sediments at Mount Sandel. In

Excavations at Mount Sandel 1973–77, Woodman PC (ed.). Northern Ireland Archaeological Monographs 2. HMSO: Belfast; 185–192.Hill AR, Prior DB. 1968. Directions of ice movement in north-east Ireland. Proceedings of the Royal Irish Academy 66B: 71–84.Kamb B, Raymond CF, Harrison WD, Engelhardt H, Echelmeyer KA, Humphrey N, Brugman MM, Pfeffer T. 1985. Glacier surge

mechanisms: 1982–1983 surge of Variegated Glacier, Alaska. Science 227: 469–479.Knight J, McCarron SG, McCabe AM. 1999. Landform modification by palaeo-ice streams in east central Ireland. Annals of Glaciology 28:

161–167.Knight J, Coxon P, McCabe AM,McCarron SG. In press. Pleistocene glaciations in Ireland. In Chronology and Extent of World Glaciations,

Ehlers J, Gibbard PL (eds). Elsevier: Amsterdam.Lewis HC. 1894. Papers and Notes on the Glacial Geology of Great Britain and Ireland. Longman: London.Maizels J. 1989. Sedimentology, paleoflow dynamics and flood history of jokulhlaup deposits: paleohydrology of Holocene sediment sequences

in southern Iceland sandur deposits. Journal of Sedimentary Petrology 59: 204–223.

Maizels J. 1993. Lithofacies variations within sandur deposits: the role of runoff regime, flow dynamics and sediment supply characteristics.Sedimentary Geology 85: 299–325.

Maizels J. 1997. Jokulhlaup deposits in proglacial areas. Quaternary Science Reviews 16: 793–819.Marren PM. 2002. Fluvial—lacustrine interaction on Skeidararsandur, Iceland: implication for sandur evolution. Sedimentary Geology 149:

43–58.

McCabe AM. 1987. Quaternary deposits and glacial stratigraphy in Ireland. Quaternary Science Reviews 6: 259–299.McCabe AM. 1996. Dating and rhythmicity from the last deglacial cycle in the British Isles. Journal of the Geological Society 153: 499–502.McCabe AM. 2002. Quaternary events and history on the northern coastal fringes of Ireland. In Field Guide to the Coastal Environments of

Northern Ireland, Knight J (ed.). University of Ulster: Coleraine; 83–104.

McCabe AM, Clark PU. 1998. Ice-sheet variability around the North Atlantic Ocean during the last deglaciation. Nature 392: 373–377.McCabe AM, Carter RWG, Haynes JR. 1994. A shallow marine emergent sequence from the northwestern sector of the last British ice sheet,

Portballintrae, Northern Ireland. Marine Geology 117: 19–34.

McCabe AM, Knight J, McCarron SG. 1998. Evidence for Heinrich event 1 in the British Isles. Journal of Quaternary Science 13: 549–568.Murray T, Stuart GW, Miller PJ, Woodward J, Smith AM, Porter PR, Jiskoot H. 2000. Glacier surge propagation by thermal evolution at

the bed. Journal of Geophysical Research 105: 13491–13507.NemecW, Steel RJ. 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits. In

Sedimentology of Gravels and Conglomerates, Koster EH, Steel RJ (eds). Memoir 10. Canadian Society of Petroleum Geologists: 1–31.O Cofaigh C, Evans DJA. 2001. Deforming bed conditions associated with a major ice stream of the last British ice sheet. Geology 29: 795–

798.Peacock JD. 1981. Scottish late-glacial marine deposits and their environmental significance. In The Quaternary in Britain, Neale JW, Flenley J

(eds). Permagon Press: Oxford; 222–236.Powell RD. 1990. Glacimarine processes at grounding-line fans and their growth to ice-contact deltas. In Glacimarine Environments: Processes

and Sediments, Dowdeswell JA, Scourse JD (eds). Special Publication 53. Geological Society: London; 53–73.Rovey CW, Borucki MK. 1995. Subglacial to proglacial sediment transition in a shallow ice-contact lake. Boreas 24: 117–127.Shaw J, Carter RWG. 1980. Late-Midlandian sedimentation and glaciotectonics of the North Antrim End Moraine. Irish Naturalists’ Journal

20: 67–69.Stephens N, Creighton JR, Hannon MA. 1975. The Late Pleistocene period in north-eastern Ireland. Irish Geography 8: 1–23.Stoker MS, Holmes R. 1991. Submarine end-moraines as indicators of Pleistocene ice limits off NW Britain. Journal of the Geological Society

148: 431–434.Wilson HE. 1972. Regional Geology of Northern Ireland. HMSO: Belfast.Wilson HE, Manning PI. 1978. Geology of the Causeway Coast. HMSO: Belfast.

Scientific editing by Brian Williams.



sedimentary evidence for the formation mechanism of the armoy moraine and late devensian glacial...

Documents