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.
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!
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.
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.
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).
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).
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).
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).
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
- Liverpool Uni blog about Irish Sea Ice Stream
- Explore the landforms on Ireland using the BRITICE map.
- Download the PDF of the BRITICE Map (Ireland Sheet).
- Drumlins of Clew Bay, Ireland
- Geological Society: Drumlins of Clew Bay
Videos
Glacial deposits at Killiney Beach, Co. Dublin, Ireland
Hi Sam!
Lovely to stumble upon your page! I’m gathering material for a lecture in glaciology and really liked your animation of Anna Hughes et al’s ice sheet reconstruction. May I use it (with credits of course)?
All the best,
Eva, your long lost PhD pal from Aber