The Irish Ice Sheet

By Dr Sam Roberson, British Geological Survey, Belfast

Glaciation of Ireland during the Devensian

The island of Ireland has been glaciated many times during the Quaternary period.  The last glaciation in Ireland is referred to historically as the ‘Midlandian’, but now, the British-Irish term ‘Devensian’ is more commonly used. During the Devensian, the British-Irish Ice Sheet was confluent, with an ice stream flowing south down the Irish Sea.

This change in terminology reflects a dramatic revision of the views held about the maximum extent of the Irish sector of the British-Irish ice sheet (Hegarty, 2004; Clark et al., 2012).  Prior to the acquisition of high-resolution sea bed bathymetry, evidence for the last glaciation in Ireland was strictly limited to onshore geological records. The advent of marine geological data (such as submarine landforms and sediment cores taken from the bed of the Irish Sea) has changed our views about the glaciation of Ireland.

LGM of the British-Irish Ice Sheet
The Last Glacial Maximum. A confluent ice sheet covered both Britain and Ireland. The Irish Sea Ice Stream flowed southwards in the Irish Sea.

The GIF below, from Hughes et al. (2016) shows the evolution of the European Ice Sheets through time. You can see how the British-Irish Ice Sheet remained confluent with ice flowing down the Irish Sea until around 16,000 years ago!

DATED Database, by Hughes et al. 2016
The evolution of the Eurasian Ice Sheet. DATED-1 reconstruction, from Hughes et al., 2016.

During the Last Glacial Maximum, the British-Irish Ice Sheet expanded onto the continental shelf, west of Ireland and Britain (Roberts et al., 2020). There were a number of fast-flowing ice streams that delivered ice and sediment to the continental shelf edge during these periods of maximum ice extent.

Irish Ice Sheet at the Last Glacial Maximum
The configuration of the BIIS at the Last Glacial Maximum (grey shading). Maximum ice extent offshore from western Ireland is likely to have been reached at ~26,000 to 24,000 years ago. Ice retreat and re‐advance across the mid‐continental shelf is dated to between 21,000 and 18,500 years ago, and is marked by the Galway Lobe Grounding Zone Wedge (GLGZW) and the Galway Lobe Re‐advance Moraine (GLRM). Deglaciation of the inner shelf back to the Galway and County Clare coast is poorly constrained. From Roberts et al., 2020.

Onshore geological record of the glaciation of Ireland

The onshore geological record takes the form of (i) landforms and (ii) stratigraphy. Glacial landforms include: moraines, drumlins, subglacial ribs, eskers, meltwater channels, and streamlined bedrock features (Greenwood and Clark, 2008).  Click here to see a map of the glacial landforms in Ireland.

In fact, drumlins are so common in Ireland, that the word ‘drumlin’ comes from the Irish word droimnín (“littlest ridge”)!

Stratigraphic evidence of glaciation includes: widespread till, glaciofluvial outwash, ice-marginal lake deposits, erratics, and organic remains. Erratics (glacially transported boulders of a different lithology to where they now lie) record the passage of ice and can be used to reconstruct ice-flow directions.

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Carboniferous limestone pavement on Inis Meáin. Perched granite erratics on the pavement were transported from the Galway mainland. From Roberts et al., 2020.

Moraines at the modern Irish coast (e.g. Dundalk Bay and Galway Bay) are consistent with ice flowing offshore, but until comparatively recently the full extent of this ice was mainly speculation (Haflidason et al., 1997; Ballantyne et al., 2007).

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Geomorphological map of SW County Clare and the Shannon Estuary, showing the Kilkee-Kilrush moraine complex and the Scattery Island Moraine. The top panel also shows mapped drumlins (long axis mapped as a black line). From: Roberts et al., 2020.

You can explore all of the landforms yourself using the BRITICE Glacial Map. Click the image below. Once the map loads, press shift and use the mouse to draw a box over Ireland. This will zoom in over Ireland and the landforms will appear. You can also view the BRITICE map (Ireland Sheet).

Bedrock lineations and drumlins in the central sector of Ireland in the BRITICE Glacial Map.

Offshore geological record of the Irish Ice Sheet

Landforms on the sea bed around Ireland has provided clear geomorphological evidence that ice extended onto the continental shelf during the Quaternary (Benetti et al., 2010; Clark et al., 2012; Ó Cofaigh and Ballantyne, 2017, Ó Cofaigh et al., 2019, Roberts et al. 2020).  

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Continental shelf offshore NW Ireland showing core locations and seismic profiles taken as part of the BRITICE-CHRONO project (From: Ó Cofaigh et al., 2019). The blue shows mapped moraines on the continental shelf.

These data were collected by (i) processing navigational data from boats (e.g. Olex), as well as (ii) via bespoke sea bed surveys, (e.g. the Irish National Sea Bed Survey and INFOMAR). 

As part of the BRITICE-CHRONO consortium, research cruises have also collected sub-bottom data using seismic surveys and borehole corers (Peters et al., 2016; Callard et al., 2018; Ó Cofaigh et al., 2019). These studies have documented the nature and timing of the glacial processes that led to the formation of moraines, drumlins, grounding-zone wedges, meltwater channels, iceberg plough marks on the continental shelf.

Beyond the continental shelf itself, trough-mouth fans and debris rafted by icebergs provide further evidence of the nature of the last glaciations.  Importantly, the majority of the data gathered indicate that the ice arrived at the continental shelf edge after 27,000 years ago and had departed by 21,000 years ago.

Timing of glaciation

Within this general pattern is a considerable amount of asynchronicity, both in terms of ice advance onto the shelf, and retreat back onshore.

The most northerly Donegal and Malin Sea ice streams were heavily influenced by ice flowing westwards from Scotland, while the western and south-western ice streams were more heavily influenced by oceanic forcing.

Dating of shell remains from offshore cores indicates that the most northerly ice streams advanced to the shelf edge around 27,000 years ago, while the more southerly Galway ice stream and ice from the Cork-Kerry ice centre advanced later, 24,000 years ago (Callard et al., 2020).

Irish Sea Ice Stream

On the eastern and southern side of Ireland, ice advance and retreat of the ice sheet was dominated by the behaviour of the Irish Sea ice stream; a monster of an outlet glacier that at one point drained the majority of the British-Irish Ice Sheet.

This Irish Sea Ice Stream coupled the eastern part of the Irish Ice sheet with western part of the British Ice Sheet, reaching the edge of the continental shelf and the Isles of Scilly ~25,000 years ago (Smedley et al., 2017). 

https://ars.els-cdn.com/content/image/1-s2.0-S0037073815002651-gr2.jpg
British–Irish Ice Sheet with reported palaeo-ice streams, extent at Last Glacial Maximum (LGM) and simplified bedrock geology. Named palaeo-ice streams: 1. Norwegian Channel; 2. Orkney; 3. Minch; 4. Moray Firth; 5. Hebrides, 6. North Channel—Malin Shelf; 7. Irish Sea; 8. Tyne Gap; 9. North Sea lobe. Locations described in this paper: Ul = Ullapool; SJ = Sound of Jura; TG = Tyne Gap. From: Krabbendam et al., 2015.

The huge extent of the Irish Sea ice stream made it inherently unstable, leading to rapid retreat.  By ~23,000 years ago the ice stream had retreated as far north as the St George’s Channel, while ~1000 years later it was level with Brae, Co. Wicklow and the Llyn Peninsula in north Wales (Chiverell et al., 2013).  The final collapse of the Irish Sea ice stream is not well understood and remains a point of contention.

The Younger Dryas in Ireland

The Younger Dryas was an abrupt period of intense cold that drove the readvance of glaciers from 12,900 to 11,700 years ago. This was the Younger Dryas Stadial; in Ireland, this is commonly referred to as the Nahanagan (Colhoun and Synge, 1980). 

The work by Eric Colhoun and Francis Synge remains the only one to have successfully dated this period of glacial activity in Ireland. However, other researchers have argued on the basis of stratigraphic and geomorphological evidence, that mountains across Ireland hosted small cirque glaciers during this time (Synge, 1968; Rae et al., 2004; Barr et al., 2017).

About the Author

Sam Roberson is a Quaternary geologist at the Geological Survey of Northern Ireland. He has a PhD in glaciology and is interested in the impact of Pleistocene ice sheets in the UK and Ireland. Sam helped to create the first Quaternary Geological Map of Ireland and is currently working to update of the UK Superficial Deposits map.  He is involved in field mapping and geological modelling for the survey and likes using geostatistics and scientific programming as part of his applied research into the subsurface. Sam is an avid cyclist and in 2019 rode the length of Ireland to promote awareness of Quaternary geology.

Key Publications

  • S. Roberson and Weltje, G.J. 2014. Inter-instrument analysis of particle-size analysers. Sedimentology. 61, 1157-1174.
  • Merritt, J.W., Roberson, S. and Cooper, M.R., 2018. A critical review and re-investigation of the Pleistocene deposits between Cranfield Point and Kilkeel, Northern Ireland: Implications for regional sea-level models and glacial reconstructions of the northern Irish Sea basin. Proceedings of the Geologists’ Association, 129, 583-609.
  • S. Roberson, Hubbard, B., Coulson, H. and Boomer, I. 2011. Physical properties and formation of flutes at a polythermal valley glacier: Midre Lovénbreen, Svalbard. Geografiska Annaler Series A, Physical Geography. 93, 71-88.
  • Barr, I.D., Roberson, S., Flood, R. and Dortch, J., 2017. Younger Dryas glaciers and climate in the Mourne Mountains, Northern Ireland. Journal of Quaternary Science, 32, 104-115.
  • Roberson, S. and Hubbard, B., 2010. Application of borehole optical televiewing to investigating the 3-D structure of glaciers: implications for the formation of longitudinal debris ridges, midre Lovenbreen, Svalbard. Journal of Glaciology, 56(195), pp.143-156.

Further Reading

Videos

Glacial deposits at Killiney Beach, Co. Dublin, Ireland

https://youtu.be/JxHR1HvCgGs

References

Ballantyne, C.K., McCarroll, D. and Stone, J.O., 2007. The Donegal ice dome, northwest Ireland: dimensions and chronology. Journal of Quaternary Science22(8), pp.773-783.

Ballantyne, C.K. and Ó Cofaigh, C., 2017. The last Irish Ice Sheet: extent and chronology. In Advances in Irish Quaternary Studies (pp. 101-149). Atlantis Press, Paris.

Barr, I.D., Roberson, S., Flood, R. and Dortch, J., 2017. Younger Dryas glaciers and climate in the Mourne Mountains, Northern IrelandJournal of Quaternary Science32(1), pp.104-115.

Benetti, S., Dunlop, P. and Ó Cofaigh, C., 2010. Glacial and glacially-related features on the continental margin of northwest Ireland mapped from marine geophysical data. Journal of Maps6(1), 14-29.

Callard, S.L., Ó Cofaigh, C., Benetti, S., Chiverrell, R.C., Van Landeghem, K.J., Saher, M.H., Gales, J.A., Small, D., Clark, C.D., Stephen, J.L. and Fabel, D., 2018. Extent and retreat history of the Barra Fan Ice Stream offshore western Scotland and northern Ireland during the last glaciation. Quaternary Science Reviews201, pp.280-302.

Callard, S.L., Ó Cofaigh, C., Benetti, S., Chiverrell, R.C., Van Landeghem, K.J., Saher, M.H., Livingstone, S.J., Clark, C.D., Small, D., Fabel, D. and Moreton, S.G., 2020. Oscillating retreat of the last British-Irish Ice Sheet on the continental shelf offshore Galway Bay, western IrelandMarine Geology420, p.106087.

Chiverrell, R.C., Thrasher, I.M., Thomas, G.S., Lang, A., Scourse, J.D., van Landeghem, K.J., Mccarroll, D., Clark, C.D., Ó Cofaigh, C., Evans, D.J. and Ballantyne, C.K., 2013. Bayesian modelling the retreat of the Irish Sea Ice Stream. Journal of Quaternary Science28(2), pp.200-209.

Clark, C.D., Hughes, A.L., Greenwood, S.L., Jordan, C. and Sejrup, H.P., 2012. Pattern and timing of retreat of the last British-Irish Ice Sheet. Quaternary Science Reviews44, pp.112-146.

Colhoun, E.A. and Synge, F.M., 1980, January. The cirque moraines at Lough Nahanagan, County Wicklow, Ireland. In Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science (pp. 25-45). Royal Irish Academy.

Greenwood, S. L., & Clark, C. D. (2008). Subglacial bedforms of the Irish Ice Sheet. Journal of Maps, 4(1), 332–357. https://doi.org/10.4113/jom.2008.1030

Haflidason, H., King, E.L., Kristensen, D.K., Helland, E., Duffy, M., Scourse, J.D., Austin, W.E.N. and Sejrup, H.P., 1997. Marine geological/geophysical cruise report on the western Irish margin: Donegal Bay, Clew Bay, Galway Bay, Irish Shelf and Rockall Trough. University of Bergen, Bergen.

Hegarty, S., 2004. Limits of Midlandian glaciation in south‐eastern Ireland. Irish Geography37(1), pp.60-76.

Hughes, A. L. C., Gyllencreutz, R., Lohne, Ø. S., Mangerud, J., & Svendsen, J. I. (2016). The last Eurasian ice sheets–a chronological database and time‐slice reconstruction, DATED‐1. Boreas, 45(1), 1–45.

Krabbendam, M., Eyles, N., Putkinen, N., Bradwell, T., & Arbelaez-Moreno, L. (2016). Streamlined hard beds formed by palaeo-ice streams: A review. Sedimentary Geology, 338, 24–50. https://doi.org/https://doi.org/10.1016/j.sedgeo.2015.12.007

Ó Cofaigh, C., Weilbach, K., Lloyd, J.M., Benetti, S., Callard, S.L., Purcell, C., Chiverrell, R.C., Dunlop, P., Saher, M., Livingstone, S.J. and Van Landeghem, K.J., 2019. Early deglaciation of the British-Irish Ice Sheet on the Atlantic shelf northwest of Ireland driven by glacioisostatic depression and high relative sea level. Quaternary Science Reviews208, pp.76-96.

Peters, J.L., Benetti, S., Dunlop, P., Ó Cofaigh, C., Moreton, S.G., Wheeler, A.J. and Clark, C.D., 2016. Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland. Quaternary Science Reviews140, pp.101-124.

Rae, A.C., Harrison, S., Mighall, T. and Dawson, A.G., 2004. Periglacial trimlines and nunataks of the Last Glacial Maximum: the Gap of Dunloe, southwest Ireland. Journal of Quaternary Science: Published for the Quaternary Research Association19(1), pp.87-97.

Roberts, D. H., Ó Cofaigh, C., Ballantyne, C. K., Burke, M., Chiverrell, R. C., Evans, D. J. A., … Callard, S. L. (2020). The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin. Boreas, 49(3), 438–460. https://doi.org/10.1111/bor.12448

Smedley, R.K., Scourse, J.D., Small, D., Hiemstra, J.F., Duller, G.A.T., Bateman, M.D., Burke, M.J., Chiverrell, R.C., Clark, C.D., Davies, S.M. and Fabel, D., 2017. New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly. Journal of Quaternary Science32(1), pp.48-62.

Synge, F.M., 1970. The Irish Quaternary: current views 1969. Irish Geographical Studies in honour of E. Estyn Evans (Eds., Stephens, N. and Glasscock, R.), Queens University Belfast, pp.34-48.

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