GLWG 11: Anglesey, 29th - 31st October, 2010
Organisers
Dr Amanda Williams (University of Chester) & Dr Emrys Phillips (British Geological Survey).
Introduction
This was the 11th meeting of the Glacial Landsystems Working Group but the first as a formal working group of the Quaternary Research Association. As the evening of the 29th October approached, 39 people from far and wide were making their way to The Outdoor Alternative, Holy Island, with the intention of a 2030 hrs introductory talk in the common room, but as traffic and winding Welsh roads took their toll the night crept on. Once the group had collected we were treated to a brief overview of the weekend to come, including possibilities of bedrock control on subglacial bedforms and some new theories for stratified diamicts within drumlins. All in all it promised to be a weekend of lively discussion. With that in mind the group proceeded to the local ale house for some much needed sustenance.
Saturday 30th October
Firmly prepared with waterproofs, just in case, the group headed out in convoy to the first field site at Cemlyn Bay. Before moving into the field, the group was treated to a look at a NEXTMap digital elevation model of Anglesey showing distribution of landform assemblages and underlying bedrock geology, inspiring conversation about possible bedrock control on the development of elongate subglacial bedforms and the ‘footprint’ of the Irish Sea Ice Stream.
Moving on to Cemlyn Bay itself, the group was presented with a cliff section through a drumlin that displayed distinct stratification including brown and grey diamicts and relatively well sorted sands exhibiting internal sedimentary structures. It was suggested that the sands may have been emplaced by mechanism of hydrofracturing; that water under intense pressure had been forced along the contact between units effectively splitting them apart and then depositing sands as pressure decreased in a manner that was dubbed ‘split, squirt and seal’. While the hydrofracture theory came under much discussion there was a moment of silence as it was suggested that the sequence may not in fact represent tills but may have formed in a subaqueous environment thereby eliminating any need to explain the stratification within the drumlin itself. Whether the internal structure of drumlins need be explained by the drumlin formation process or whether it can be attributed to deposition prior to glacial overriding was to become a much debated feature of the weekend.
After a brief but pleasant stroll, the group reached Hen Borth, a drumlin truncated parallel to its long axis by wave action. The basal unit rests on bedrock and is described as a transitional sheared ‘bedrock-diamict’ unit. In the northern part of the section, sheets of bedrock can be seen to have sheared up into the overlying diamict. This conforms to ice flow direction established from striae in the local bedrock, which suggests ice flow was from the North East. This sequence provided more evidence for the previously discussed hydrofracture theory, with most agreeing that there was at least evidence of water transport along unit boundaries before the formation of water escape structures into the overlying diamicts.
After a slightly nervous drive through torrential rain, the skies cleared over the next field site at Penrhos. This drumlin section shows an internal structure consisting of crude bedding with stacked sheets of diamict interbedded with fine sand. The discussion here broadened to encapsulate the wider region, debating single and multiple event theories and the concept of a large scale surge type advance of ice south down the Irish Sea followed by a period of stagnation and eventual swift retreat. This provided an argument for the apparent lack of sediment deposited and evidence of a prominent retreat phase over Anglesey more generally.
The last stop of the day was unusual for the group in that it involved ‘hard rock geology’ rather than the Quaternary sediments most were used to. At the Rhoscolyn anticline the structural similarities and differences between sediments and hard rock were discussed, specifically in the context of deformation. The placement of S-type flanking folds in compressional or shear related environments was discussed with a view to establishing pressure direction and core features. Links were made to analogous glaciotectonic features that are often observed in diamicts.
A retreat was then made to the safety of the lodge to gather thoughts and prepare for the dark and winding route to the pub. During the course of the night it became obvious how stimulating the day had been as discussions and theories were all abound, ranging from ideas for a unifying theory of drumlinisation to how much importance should be ascribed to colour changes in the sedimentary sequence.
Sunday 31st October
During a bright morning, despite all forecasts for the area, a slightly relieved group made their way, although somewhat indirectly, to Lleiniog. At this site there were red-brown sands overlying a red-brown till and red-brown sands and gravels. The group were assured that several feet beneath us lay a blue-grey diamict and, as nobody seemed inclined to dig, this was generally accepted. The issue of Welsh ice versus Irish ice influence was discussed; the bulk of material within the section having being sourced from northern Britain and the Irish Sea Basin but with a small continuous influence of Welsh material taken to mean a confluence of the two ice streams somewhere to the North-east of the site.
While at this site, a structure previously identified as glaci-tectonic in origin was discussed in detail. However, as debate continued, overwhelming support for a syn-sedimentary process came to the fore, though exactly which process was still under question. The group fell predominantly into two camps; either a buried ice block with subsequent melting, slumping and water escape (the argument against being a lack of evidence for the processes necessary for ice block emplacement of the size required), or a density driven slump scenario eliminating the need for an ice block (the argument against being lack of evidence or space for movement in the underlying material). This debate remains unresolved and will, I am sure, feature in future communiqué.
After this site, those of the group eager to avoid being trapped in north Wales for the witching hour began the long drive home. In summary, the field sites inspired not only interesting discussion but also a basis for further research in the area. Overall, an excellent meeting and the organisers, Amanda Williams and Emrys Phillips, are thanked for all their hard work. The GLWG is going strong and plans are already in the making for next year’s event.
Iain Leighton
Department of Geography
Swansea University