Describe the spatial characteristics of drumlins in Ireland and explain their relationship to the evolution of the last lowland ice sheets in Ireland.

Drumlins are undoubtedly among the most intensively studied of all glacial landforms and have bee particularly widely used as ice-directional indicators. They frequently occur in ‘fields’ or ‘swarms’ in lowland areas where there was little obstruction to the passage of ice, or in piedmont zones where flow was radiative or dispersive. They are also occasionally found on the floors of glacial troughs. Many are ellipsoidal in form, some kilometres in length have been observed (Lemke 1958). Most possess a prominent stoss end with a trailing distal slope. It is generally agreed with the direction of the drumlin long axis reflects local direction of ice movement with the stoss end usually pointing up-glacier. The ice moulded or streamlined form appears to be produced by variations in stress levels at the base of the ice, although the precise mode of formation of the features is far from clear. The Kingscourt drumlins mentioned below gives a detailed account of the spatial characteristics and it’s internal components. ‘In addition to the regional ice flow trends displayed by their long axes, the overall shape of drumlins can provide information on former glacial dynamics, such as indications of basal ice pressure and rate and type of ice flow (Doornkamp and King 1971)’[1].

The recent work on drumlins has tended to concentrate on three aspects: drumlin shape and distribution, stone orientations within drumlins and theoretical considerations of pressure distributions within the ice and the till. Vernon (1966), in a study of drumlins in County Down, noted that they are concentrated in bands both perpendicular and parallel to ice flow but that their spacing is variable. Doornkamp and King (1971) point out that where drumlin density is high, the drumlins themselves were small. This is not as obvious as it seems, because small drumlins, being spread sparsely, could result in low-density values. It also seems likely that conditions that are conducive to regular spaced but few in number are also conducive to large drumlins. ‘In contrast, drumlins are more numerous in those areas where conditions were marginal for their formation and the ice moved less consistently and slowly’[2].

McCabe has helped our understanding of the drumlinising of the landscape substantially, which had been analysed mainly using morphological criteria, and to determine ice direction. Sedimentological analyses have indicated a complex set of depositional units within the drumlins: 1. Forms with a core of older till facies. 2. Subglacial lodgement/ meltout till facies. 3. Subglacial channel stratified facies. 4. Lee-sided stratified facies. 5. Re-mobilised/ superimposed till facies. 6. Overridden ice-marginal subaquatic facies.

‘These facies have led to the conclusion that the large drumlin fields of Ireland are the result of high basal water volumes leading to uncoupling of the glacier from the underlying sediments, resulting in downdraw into the surrounding ocean and a surge of the glacier. Such surging, involving high-pressure water, would streamline the underlying deposits and the large volumes of water and low basal shear stresses would allow the preservation of the resulting drumlins’[3].

This abstract details precisely a drumlin and it’s internal, external and importance to understanding drumlin’s significance in studying their evolution from the last glacial maximum. ‘An exposure in a Late Pleistocene drumlin near Kingscourt, Ireland, provides a good insight into some of the processes that give rise to such subglacial bedforms. The drumlin is located on a ridge, cored by red sandstone of Carboniferous (Namurian) age, which rises to 150 m above m.s.l. The drumlin itself is about 380 m long by 170 m wide and is 20 m in height. It is orientated VVNW-ESE. Ice flow direction in the area, as inferred from general drumlin orientation, striae, and erratic dispersal, was NW-SE.
The drumlin is composed of diamicton (with four constituent facies), containing large-scale (up to 20 m long and 1.5 m high) slabs of the sandstone bedrock, which have been displaced tens of metres by ice dragging. The thin diamicton matrix is sand-rich with few clasts greater than pebble size. It contains green sandstone erratics from west and northwest of the study site along with clasts of weathered Namurian sandstone, which have been sheared from local bedrock. The diamicton attains a maximum thickness of 5.4 m. The slabs are confined to the basal 2.5 m of the diamicton. Thus the drumlin is essentially rock cored. This is unusual as most drumlins are soft unconsolidated landforms.
Parts of the matrix are interpreted as injection sediments that have been squeezed into fractures and voids in the bedrock, and between rock slabs, under high porewater pressures, when the ice began to displace fractured substrate. Deformation structures, at a variety of scales are seen in the matrix. Deformation, erosion and deposition were all important in the formation of the drumlin. Much of the upper part of the diamicton has been sheared, subsequent to initial deposition, by ice action. The sheared units are uppermost in the stratigraphic sequence. The shearing is the last glacial process apparent in the drumlin sediment and may have been contemporaneous with drumlinisation. Both the squeezing process, which deposited the injections within the matrix, and the deflection in flow of ice were influenced by the obstructing bedrock ridge which, in this case, is also responsible for the anomalous orientation of the feature’[4].

An important relationship between the phases of drumlinisation and millennial scale discharge event in the North Atlantic has been suggested by McCabe (1996) and crucial new dates suggest that the period of drumlin formation can be constrained to c. 14 ka. The possible link between drumlinisation in Ireland and Heinrich event 1 is of considerable importance and the evidence from Ireland of strong links between the behaviour of terrestrial ice masses and the timing of palaeoenvoirnment changes in the North Atlantic has attracted much attention. The date of 14ka for deglaciation in Ireland may explain the enigma of the biologically barren nature of lacustrine sediments that have been analysed from below Late-glacial sequences that start at 13ka. ‘During and subsequent to the formation of the large drumlin fields the ice decayed rapidly, leaving its extensive legacy of glacial features including the ice-contact, ice-proximal, and the ice-distal glacifluvial and glacilacustrine deposits that mantle many areas (McCabe 1985). The climate remained cold until the sudden amelioration heralded the last flicker of the global ice age’[5].

Most theories and the evidence suggest that drumlins were formed during the termination of the last (Devensian) glaciation (22 000–13 000 radiocarbon (14C) years BP). This glaciation saw a southwesterly flow of Scottish ice across eastern Ireland.
During the glacial maximum between 25000-22000 years BP, ice has been recorded to have a south-easterly flow from independent dispersal centres in parts of Ireland like Lough Neagh and Sperrin mountains in Derry as seen below in Fig 1. This resulted in sediment streamlining or drumlinisation by ice streams drawn into the Irish Sea Basin. ‘This flow stage is constrained by AMS radiocarbon dates from marine microfaunas to around 14 500 14C years BP. Flow stage D records drumlinisation from an inland centre in the Lough Neagh Basin’.[6] Drumlinisation was followed closely by rapid ice. There is still the discussion about the formation of drumlins, and many geologists from Chamberlain in 1883 to today’s geologists such as McCabe and others claim that the most common explanation is deposition. However (Price 1973) believes ‘it is highly unlikely that deposition is responsible for drumlins with its’ polygenetic internal structure’. And he also states that ‘the facies analysis suggests that most clastic drumlins are erosional landforms’[7], which were produced by active ice during the last deglacial period. Neither subglacial deformation of soft sediments or the squeezing of material into basal cavities is likely to have produced drumlins. There is a sequence of events leading to drumlinisation i.e. 1. Erosion of pre-existing till and bedrock. 2. Local lodgement and subglacial channel sedimentation. 3. Regional basal deposition associated with reduced basal hydraulic transmissibility due to blocking of arterial channels by sediment (i.e. an aggrading sequence).
[pic]Fig.1 shows the direction of ice flow and consequently drumlin field location.

As drumlins are usually not hard sediment landforms but are preserved in conditions of low basal shear stress in fast ice flow during surges. These surges are when ice sheets, due to a number of reasons, normally as a response to isostatically controlled marine transgressions or tectonic uplift or other factors, and begin to move rapidly, normally shearing off huge chunks of ice. Drumlins form near trigger zones (robin and Weertman 1973), separating cold based thermal regimes down ice from warm based regimes up ice. Streamlining near a trigger zone during basal uplift of the cold thermal regime ice produces similar drumlin types in linear belts normal to ice flow. Up ice migration of trigger zones due to ice front melt-back will produce large drumlin swarms. Therefore one must concede that those complex depositional processes and environments rather than a series of mass balance driven ice oscillations form drumlins.

In conclusion, drumlins are long, wide ellipsoidal shaped landforms that occur predominantly in large swarms in lowland areas. These features are normally unconsolidated but with its’ internal components have a stratified facies of both lithos and various soil types. Drumlins were formed in the late Devensian period around thirteen thousand years ago. The drumlins that stretch from the north east coast of Ireland to the west coast form the longest drumlin belt in the world. They are formed through a complex depositional process from older till cores to overridden ice-marginal subaquatic facies. Drumlins form in particular trigger zones where cold based thermal regimes are separated from warm based thermal regimes. Surging of ice sheets has a huge part to play in the formation of drumlins as it because of this increase in the basal water and uplift of the ice sheet, the shear stress decreases and drumlinisation can occur. The inside of a drumlin is an intricate system of depositional and erosional processes and the rocks that lie inside normally lie at an interesting angle of 23 and a half degrees. Drumlins are so well researched as they provide so much information about the last ice sheets that were in the area, like the direction it moved, as drumlins always relay the direction of the moving ice sheet, whether it was a cold or warm based ice sheet and like the Scottish ice sheet that spread into Ireland, the ice sheets origins can be analysed from the deposited clasts and soil.
Bibliography:

Ehlers, J. Gibbard, P. Rose J. (1991). Glacial Deposits in Great Britain and Ireland. Rotterdam. Balkema,

Hepworth Holland, C. (2001). The Geology of Ireland. Edinburgh. Dunedin Academic.

Lowe, J. J. Walker, M.J.C. (1984). Reconstructing Quaternary Environments. London. Longman.

Price, R.J. (1973). Glacial and Fluvioglacial Landforms. Edinburgh. Oliver and Boyd.

http://jgs.geoscienceworld.org/cgi/content/abstract/156/1/63

http://ppg.sagepub.com/cgi/content/abstract/20/1/1

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6X-3SWK0F5-K&_user=103682&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000007921&_version=1&_urlVersion=0&_userid=103682&md5=83cbeaee79ca22f4b792e2cacdce75ca.
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[1] Lowe, J. J. Walker, M.J.C. (1984). Reconstructing Quaternary Environments. Pp 34.
[2] Price, R.J. (1973). Glacial and Fluvioglacial Landforms. Pp 80.
[3] Coxon in Hepworth Holland, C. (2001). The Geology of Ireland. Pp 416.
[4] Gallagher,C on http://www.sciencedirect.com/science.
[5] Coxon in Hepworth Holland, C. (2001). The Geology of Ireland. Pp 419.
[6] Coffaigh (1996) in http://ppg.sagepub.com/cgi/content/abstract/20/1/1
[7] Price, R.J. (1973). Glacial and Fluvioglacial Landforms. Pp 81.