Eastern delights: following the retreat of the Irish Sea Ice Stream

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By Rich Chiverrell

The transect 4 team of BriticeChrono converged for the last time on land in a small pad just outside Gorey on the Wexford coast of southeast Ireland to complete the terrestrial sampling. The team comprised Chiverrell, Burke, Scourse (and van Landeghen) sailing Holyhead to Dublin, the OSL crew of Duller and new Britice-Chrono postdoc Rachel Smedley taking the Fishguard-Rosslare, cosmogenic nuclide team Small sailing Stranraer-Belfast, and Ó Cofaigh jetting into Dublin from the northeast. The challenge facing us was to augment the already excellent geochronology that existed for this sector of the former British-Irish Ice Sheet, ~4 young (out of ~30) radiocarbon ages on shells reworked into Irish Sea tills documenting the advance of ice south of Ireland. Optically stimulated luminescence ages for sand grains in outwash deposits on the south coast and in the Screen Hills north of Wexford constrain the retreat of ice marginal positions. However there are then gaps in the geochronology to the north of Ireland, and the region is devoid of low altitude cosmogenic ages with most effort so far expended in the Blackstairs and Wicklow Mountains.

In the south our efforts (Day2) focused on the coastal exposures at Kilmore Quay where stratified outwash sands fill depressions in Irish Sea tills and have been deformed during over-ride by inland ice out of Ireland. We recovered four OSL samples targeting shallow and possible slack water lithofacies. On the south-eastern corner of Ireland between the Irish Sea and Lady’s Island Lake we encountered numerous granite boulders, sourced from local bedrock but transported short distances by ice. Though slightly sub-optimal, three of these were sampled and make an excellent cluster of age determinations with the OSL samples ~4 km to the west.

Up-ice, north of Wexford the coastline from Blackwater Harbour to Cahore Point offers 7.8km of difficult to rival exposure of the Quaternary stratigraphy. The winter storms and tides had rendered the cliff-line along the coast both unstable and very well exposed in equal measures. Drawing on the stratigraphical logs of Geoff Thomas and John Summers the OSL team made short work of the sequences (Day 2). Working north-to-south, accessing the coast northwards from Blackwater Harbour, after the tide allowed access, our sampling targeted rippled sands within stacked planar cross-stratified sands, probable bar-forms, in a thick >30m packages extending south of a marginal position (2) to the rear of Blackwater Head. The locations has been OSL dated previously to ~23.6 kyrs, with sampling targeting equivalent deposits exposed in quarry faces ~1km inland.

Earlier, the OSL team had split up for more rapid sampling, with Chiverrell and Burke accessing the sections from Knocknasilloge, and sampling south of the access point in a thick 25m package of outwash sands lying south of four ice marginal positions (4-7) identified by Thomas and Summers. Sampling targeted distal style deposition, with low energy rippled samples and falling flow regime finer drape laminations. Approximately 2.4km further north, around Ballyvaldon, Rachel Smedley (taking her first BriticeChrono sample) and Geoff Duller sampled the extensive 600-800m sequence of outwash sands fronting ice limit 9 north of the access point. The final samples in this section, were sampled north of Tinaberna, where outwash sands front limit position 11, and sit directly on consolidated Irish Sea till.

On the final morning (Day 5), heading north to Dublin, Chiverrell, Burke and Small visited Wicklow Point, to sample a subglacial channel system on the tip of the headland. The channel 20m wide in places and >10m in depth, has an undulating floor and exits to the sea and both end, thus can only really have formed beneath ice covering the headland. On the walk south from the golf course, two granite erratics were encountered sat on schist bedrock, but the contexts were poor for sampling. Instead we sampled quartz veins in the walls of the channel for cosmogenic nuclide dating. From here David headed to the north of Ireland and a ferry, with Rich and Matt sprinting west for some unfinished transect 5 business in County Clare and the Burren coastline.

Day 4 saw an assault on the north of the region, with half the team (Chiverrell, Ó Cofaigh, Duller and Smedley) focused on the embayment south of Bray Head yielding OSL samples from the top-set sands of inland delta near Kilpedder from a now dormant quarry. The coast at Greystones proved tricky to access with engineering works and erosion producing excellent exposures once we reached the beach from the northern end. The exposures described previously by McCabe and Ó Cofaigh, were for the most part similar to the published work, with a thinning wedge of subaqueous diamict lapping-off Bray Head. The notable differences were a 5-6m thick sequence of steep angle delta foreset sands and gravels, which were a capped by a more horizontally stratified set of sand and gravel described previously, but here interpreted as a topset deposit. Thus the only differences to the previous interpretation are for the sequence to include a sub-aerial component as the delta broke the water surface of the subaqueous basin. OSL sampling targeted a very precariously located sand unit at the base of the top-set. In the afternoon, our team merged again and attention then turned to Bray Head and the search for glacially scoured bedrock and erratic boulders. One of the highlights of the trip! A delightful granite erratic sat on quartzite bedrock, within 100m we located striated and smoothed quartzite on the stoss side of a roche moutonnée and adjacent plucked surfaces on the lee side. Together the three locations have to potential to tie down the retreat chronology northwards.

Finally, north of Dublin at Howth, (Day 4) three of the team (Scourse, Burke and Small) an ice proximal delta is exposed in a former quarry and road cut showing shallow angle sand and gravel fore-sets capped by gravel top-sets. OSL sampling targeted sandy units towards the top of the fore-set. Within 1km at the top of Howth Hill ice-scoured quartzite bedrock yielded two samples, and an apparently excellent fish and chip shop yielded lunch before they returned south to Bray Head.

Spaced over >100km south to north the samples hopefully will advance understanding of ice retreat on the west flank of the Irish Sea Ice Stream, parallel equivalent efforts in the east where sites extend from Scilly, Pembroke, Cardigan Bay, Llyn and Anglesey. Other activities on our travels in Ireland include planning and discussions in advance of the marine sampling in this sector in July 2014. Not satisfied with finishing off one transect, a delta sequence had been overlooked on the south coast of the Galway Bay Icestream (Transect 5) at Doolin, so Chiverrell and Burke completed a 500km round trip to box that one off. A very happy team ticked off a whole load objectives on this trip……

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