Welsh Ice Cap Research Articles

10Be and 26Al exposure history of the highest mountains in Wales: Evidence from Yr Wyddfa (Snowdon) and Y Glyderau for a nunatak landscape at the global Last Glacial Maximum

Twelve 10Be and five 26Al samples from the mountains of Yr Wyddfa (Snowdon) (1085 m) (n = 7 10Be) and Y Glyderau (the Glyders) (1001 m) (n = 5 paired 10Be/26Al) in Wales provide new insights into landscape evolution in the highest mountains in the British Isles outside of Scotland. The summits of Y Glyderau are characterised by intensely modified frost-shattered surfaces and have long been recognised as exemplars of mountain summit periglacial activity in the British Isles. However, glacially transported boulders on the highest ground indicate that ice overran the summits. Bedrock and boulder surfaces at altitudes >900 m yield 10Be and 26Al exposure ages of 61–78 ka, indicating that the last Welsh Ice Cap did not override and erode Y Glyderau summits at the global Last Glacial Maximum (LGM) and the summits stood as nunataks. Both the geomorphological and the exposure dating evidence indicate ice overran the summits earlier in the last glacial cycle during MIS 4, although erosion was only partial. Thick ice over Wales at this time is consistent with evidence of an extensive British-Irish Ice Sheet that reached the continental shelf to the west in MIS 4. The ice-scoured lower slopes of Y Glyderau and the arêtes of Yr Wyddfa were exposed as the ice cap rapidly thinned between 20-16 ka marking a transition from ice cap to alpine-style glaciation. On Yr Wyddfa, the highest mountain in southern Britain, local ice breached and abraded the central parts of the Crib Goch-Crib y Ddysgl arête as high as 874 m until c. 16 ka. However, some arête crests yielded Holocene ages (4.4–9.3 ka), which reflect continual post-glacial stripping.

Late Devensian deglaciation of south‐west Wales from luminescence and cosmogenic isotope dating

ABSTRACTThe Welsh Ice Cap was a dynamic component of the last British–Irish Ice Sheet at the Last Glacial Maximum, but there are few chronological constraints on the pace and timing of deglaciation. This paper presents new geomorphological and geochronological evidence that constrains the timing of the separation of the Welsh Ice Cap from the Irish Sea Ice Stream and the subsequent deglaciation of south‐west Wales; and allow these to be assessed in the context of late Pleistocene climatic events. Luminescence ages from glacial outwash sediments south of Cardigan demonstrate that the Irish Sea Ice Stream was receding by ∼26.7 ka. The subsequent recession of the Welsh Ice Cap is documented by cosmogenic ages from landforms and sediments in the Aeron and Teifi valleys and upland areas. Deglaciation of the Cambrian Mountains was underway by ∼19.6 ka. Cross‐valley moraines and associated deglaciation deposits show that minor re‐advances interrupted the recession of the Aeron Glacier twice, and the Teifi Glacier on at least 12 occasions. By ∼14.9 ka, the Aeron valley was probably ice‐free, but the northwards withdrawal of the Teifi glacier had halted in the Tregaron area. The final rapid recession of this glacier into the uplands of central Wales was completed during the Windermere Interstadial (13.5 cal ka BP) when, in common with much of the UK, the whole of Wales is believed to have become ice‐free. There is no evidence that the Cambrian Mountains contained ice‐free enclaves at the Last Glacial Maximum, as has been previously suggested. The new ages presented here support suggestions that there was rapid change in the configuration of the Welsh Ice Cap between 20 and 17 ka as upland areas became exposed and there was increasing topographic control on patterns of ice discharge.

New insights into the Quaternary evolution of the Bristol Channel, UK

ABSTRACTA synthesis of new publically available borehole and bathymetric data, combined with a wealth of other existing disparate data sources, reveals new insights into the Quaternary history of the Bristol Channel area. Sediment boreholes throughout the Bristol Channel confirm the area was glaciated in the Pleistocene. Till is present below marine deposits and, in some areas, is visible morphologically as submerged moraines. In the central and eastern Bristol Channel the submerged valley course of the palaeo‐Severn is very clear in new high‐resolution bathymetric surveys. This former river course and associated tributaries cross‐cut through glacial sediments in the Bristol Channel. At least three phases of glaciation are recorded in the Bristol Channel, one related to the southern limits of a Late Devensian Substage (∼Marine Isotope Stage (MIS) 2) Welsh Ice Cap which reached into Swansea Bay, an earlier Devensian (MIS 4–3) glaciation associated with Irish Sea ice, and another older glaciation that is associated with ice that filled the entire outer and central Bristol Channel. The age of the older Bristol Channel glaciation is still open, although it pre‐dates the Devensian (Late Pleistocene) and must date to the Middle Pleistocene. It is therefore evident that Pleistocene glacial and fluvial activity, combined with subsequent post‐glacial sea transgression, directly account for current morphometries of the Bristol Channel and Severn Estuary, and the current geography of the SW British Isles.

Rapid thinning of the Welsh Ice Cap at 20–19 ka based on 10Be ages

New 10Be ages from the summits of three mountain areas of North Wales reveal a very similar exposure timing as the Welsh Ice Cap thinned after the global Last Glacial Maximum. Eight bedrock and one boulder sample gave a combined arithmetic mean exposure age of 19.08±0.80ka (4.2%, 1σ). Similar exposure ages over a 320m vertical range (824 to 581m altitude) show that ice cap thinning was very rapid and spatially uniform. Using the same production rate and scaling scheme, we recalculated six published 10Be exposure ages from the nearby Arans, which also covered a similar elevation range from 608 to 901m and obtained an arithmetic mean of 19.41±1.45ka (7.5%, 1σ). The average exposure age of all 15 accepted deglaciation ages is 19.21±1.07 (5.6%, 1σ). The complete dataset from North Wales provides very strong evidence indicating that these summits became exposed as nunataks at 20–19ka. This result provides important insight to the magnitude of ice surface lowering and behavior of the Welsh Ice Cap during the last deglaciation that can be compared to other ice masses that made up the British-Irish Ice Sheet.

The last Welsh Ice Cap: Part 1 - Modelling its evolution, sensitivity and associated climate

A high-resolution, three-dimensional, thermomechanical ice-flow model is used to investigate the glaciodynamics of the Last Glacial Maximum Welsh Ice Cap – a large, independent ice centre of the British–Irish Ice Sheet. The model uses higher-order physics to solve longitudinal stresses, and is coupled to climate via a distributed, positive degree-day mass-balance scheme. A suite of model experiments driven by the GISP2 δ18O curve was initiated from a climatic optimum at 38.3 ka BP through to the Devensian/Holocene boundary to identify an icecap configuration compatible with available empirical evidence. An enhanced cooling from present of 11.85°C and strong precipitation suppression are required between 27.4 and 23.5 ka BP for the modelled icecap to attain well-established empirical limits, a scenario probably associated with Heinrich Event-2 and the potential collapse of thermohaline circulation in the North Atlantic. The experiments indicate ice-dispersal centres located in North and Mid Wales, the latter being essential for forcing ice southwards of the Brecon Beacons during the Last Glacial Maximum. Deglaciation of the Welsh Ice Cap was relatively rapid, occurring within one millennium. Dynamic stability is governed largely by the dominance and vigour with which fast-flowing outlet glaciers drain the icecap interior, which in turn are linked to variations in the climatic forcing. The distribution of permanently cold-based ice across the uplands and summits indicates the probable preservation of relict landscapes in these areas throughout the full glacial cycle.

The last Welsh Ice Cap: Part 2 – Dynamics of a topographically controlled icecap

During the Last Glacial Maximum, the British–Irish Ice Sheet was dominated by a number of accumulation centres, including a terrestrially based, semi‐independent icecap centred on Wales. The dynamics of this Welsh Ice Cap (WIC) over the last glacial period are still relatively poorly understood, with few studies taking into consideration the dynamic evolution of the icecap as a whole. Here we contrast results from two modelled reconstructions of the WIC in conjunction with the wider glacial geomorphological record to elucidate understanding of its form, extent and dynamics. Model output was analysed to yield zones of high basal motion and the spatial distribution of potential glacial erosion. We conclude that coherent flowsets of streamlined bedforms are linked to fast‐flowing outlets dominated by basal sliding. Large‐scale changes in dynamics are discussed, with a number of possible major advances proposed over the glacial cycle. Maximum ice thicknesses of ∼1200 m in Mid Wales indicate that all mountain summits were probably ice‐covered during the Last Glacial Maximum, even if it was with a thin protective mantle of cold‐based ice, leading to landscape preservation of these upland zones. The distribution, dynamism and landscape modification related to the WIC are further discussed at the regional scale. Model predictions of glacier distribution through the Younger Dryas stadial accord well with geologically reconstructed limits at this time.

10Be and 26Al exposure‐age dating of bedrock surfaces on the Aran ridge, Wales: evidence for a thick Welsh Ice Cap at the Last Glacial Maximum

AbstractThis paper presents results of the analysis of paired cosmogenic isotopes (10Be and 26Al) from eight quartz‐rich samples collected from ice‐moulded bedrock on the Aran ridge, the highest land in the British Isles south of Snowdon. On the Aran ridge, comprising the summits of Aran Fawddwy (905 m a.s.l.) and Aran Benllyn (885 m a.s.l.), 26Al and 10Be ages indicate complete ice coverage and glacial erosion at the global Last Glacial Maximum (LGM). Six samples from the summit ridge above 750–800 m a.s.l. yielded paired 10Be and 26Al ages ranging from 17.2 to 34.4 ka, respectively. Four of these samples are very close in age (10Be ages of 17.5 ± 0.6, 17.5 ± 0.7, 19.7 ± 0.8 and 20.0 ± 0.7 ka) and are interpreted as representing the exposure age of the summit ridge. Two other summit samples are much older (10Be ages of 27.5 ± 1.0 and 33.9 ± 1.2 ka) and these results may indicate nuclide inheritance. The 26Al/10Be ratios for all samples are indistinguishable within one‐sigma uncertainty from the production rate ratio line, indicating that there is no evidence for a complex exposure history. These results indicate that the last Welsh Ice Cap was thick enough to completely cover the Aran ridge and achieve glacial erosion at the LGM. However, between c. 20 and 17 ka ridge summits were exposed as nunataks at a time when glacial erosion at lower elevations (below 750–800 m a.s.l.) was achieved by large outlet glaciers in the valleys surrounding the mountains. Copyright © 2011 John Wiley & Sons, Ltd.

Connectivity analyses of valley patterns indicate preservation of a preglacial fluvial valley system in the Dyfi basin, Wales

Coastal valleys in the west part of Mid-Wales, such as the Mawddach, Dysynni, Tal-y-llyn and Dyfi, acted as corridors for ice which drained the Welsh Ice Cap during the Devensian. Analyses of detailed digital elevation models, and interpretation of satellite images and aerial photographs, show the existence of large variations in the amount of glacial modification between these valleys. Although all the valleys are glacially over-deepened along late Caledonian fault lines, only the Dyfi basin exhibits a dendritic pattern, with V-shaped cross-profiles and valley spurs typical of valleys formed by fluvial processes. Connectivity analysis of the Dyfi basin shows that it exhibits an almost completely dendritic pattern with connectivity α and β values of 0.74 and 1.01, respectively, with little glacial modification of the preglacial fluvial valley pattern in the form of glacial valley breaching. Several examples of glacial meltwater incision into a well-developed pre-existing river valley system, causing river capture across watersheds, have been identified in the Dyfi basin. The degree of preservation of the preglacial fluvial valley system within the Dyfi basin indicates limited modification by glacial processes, despite the area being subjected to glacier activity during the Late Devensian at least. It is possible that major parts of the basin were covered by cold-based or slow-moving ice, close to, or under, a migrating ice-divide, with the major ice drainage occurring along the weaker zone of the Pennal Fault along which teh Dyfi valley is located, causing minor adjustments to the surrounding interfluves and uplands. It is proposed here that the general river valley morphology of the Dyfi basin is of a pre-Late Devensian age.

Ice-marginal sedimentation associated with the Late Devensian Welsh Ice Cap and the Irish Sea Ice Stream: Tonfanau, West Wales

This paper provides the first comprehensive description and interpretation of Late Devensian glacigenic deposits exposed in a 2 km-long cliff section at Tonfanau, Mid-West Wales. The section offers crucial insight concerning glacier dynamics and sedimentation on the eastern margin of the Irish Sea Basin. Six lithofacies are identified: diamicton, sandy gravel, well-rounded gravel, well-sorted sand, muddy sand, and laminites. Two diamicton horizons are present: an upper brown clast-rich sandy diamicton, and a lower grey clast-poor muddy diamicton, the latter being only sporadically exposed by wave action. Sand and gravel occur in close association, as well as minor mud and gravel successions. The sedimentary succession at Tonfanau records the dynamics and deposition of both Welsh and Irish Sea Ice. The lowermost part of the exposed section records Welsh ice extending into Cardigan Bay, depositing a diamicton originating from the adjacent hinterland to the West. As the Irish Sea Ice Stream flowed south, it impinged upon the present coastline at Tonfanau, depositing sediment with a northern provenance on top of predominantly Welsh material, as indicated by clasts from the Irish Sea Basin, Anglesey and the Llŷn Peninsula. The depositional environment for the Irish Sea materials is interpreted as one where a fluctuating ice margin generated a grounding-line fan complex in an ice-contact lake that merged laterally with lake sediments derived from suspension and turbidity currents. Succeeding facies indicate over-riding by Irish Sea Ice. Finally, the upper few metres of the succession was subjected to cryoturbation under a periglacial climate.

Structural and depositional evidence for repeated ice-marginal oscillation along the eastern margin of the Late Devensian Irish Sea Ice Stream

Late Devensian glacigenic sediments and landforms along the north-west coast of Wales document the advance and subsequent retreat of the eastern margin of an Irish Sea Ice Stream that met, coalesced and ultimately uncoupled from ice radiating outwards from the adjacent Welsh Ice Cap centred over Snowdonia. Across the boundary between the two former ice masses is a set of sediment–landform assemblages that reflect rapidly changing erosional and depositional conditions during ice interaction. From the inner part of the ice-stream the assemblages range outwards, from a subglacial depositional assemblage, characterised by drumlin swarms; through a subglacial erosional assemblage, marked by prominent bedrock scours and large subglacial rock channels; through an ice-marginal assemblage, identified by closely spaced, glaciotectonised push moraines and intervening marginal sandur troughs; into a freely expanding proglacial sandur and lacustrine delta assemblage. The ice-marginal assemblage provides evidence for numerous oscillatory episodes during retreat and at least 20 ice-marginal limits can be identified. At least 11 of these display multiple criteria for identifying readvance and, in the ideal case, is characterised by a moraine form built by localised tectonic stacking of diamict to the rear, fronted by a clastic wedge of ice-front alluvial fan gravel and intercalated flow till. The distribution of sediment–landform assemblages suggests a highly dynamic, convergent ice-stream flow pattern, with high ice velocity, a sharply delineated lateral shear margin, pervasive ice-marginal glaciotectonic deformation and a tightly focused ice-marginal sediment delivery system; all signature characteristics of contemporary ice streams.

Modification of peripheral mountain ranges by former ice sheets: The Brecon Beacons, Southern UK

The Brecon Beacons in southern Wales is a large upland area (900 km 2) close to the periphery of the former British–Irish Ice Sheet. The geomorphology of the Brecon Beacons highlands and surrounding areas was mapped using satellite imagery and a digital elevation model (DEM). A fragmentary pattern of glacial lineations is inferred to represent ice build-up in the mountains. A more coherent set of glacial lineations formed by southward ice flow across the area from the Welsh Ice Cap represents recession from the Last Glacial Maximum (LGM). During this phase the Brecon Beacons acted as an obstacle, deflecting the ice around higher terrain. Most of the higher terrain in the Brecon Beacons therefore appears to have escaped significant glacial erosion during both phases of ice-flow due to cold-based ice or low basal ice velocities beneath a local ice cap. Landforms previously interpreted as cirques in fact have v-shaped profiles and lack evidence of major over-deepening. They are interpreted to have been formed primarily by processes other than glacial erosion. The overall impact of glaciation in the Brecon Beacons appears to be limited primarily to the time when the area was over-run by the Welsh Ice Cap and the influence of local glaciation has been minimal. This study has two main implications for landscape evolution in mountain ranges that lie at the periphery of former ice sheets. First, due to the presence of cold-based ice or low basal ice velocities beneath a local ice cap it may be possible to find fragmentary traces of pre-LGM ice flow. Second, the pattern of glacial lineations in and around the Brecon Beacons indicates that peripheral mountain ranges may have deflected ice flow at the LGM, resulting in low basal ice flow velocities and possible frozen-bed conditions over higher terrain which consequently resulted in ineffective glacial erosion within the mountains themselves.

Palaeoglaciology of the Welsh sector of the British–Irish Ice Sheet

This palaeoglaciological reconstruction, based on the distribution of glacial lineations interpreted from satellite imagery, considers the extent, thickness and dynamics of the Welsh Ice Cap and its interaction with the British–Irish Ice Sheet. Two major ice-flow events were identified. The oldest phase of ice flow (Event I) is characterized by ice flowing from an ice dispersal centre situated over the higher terrain in north–central Wales and by ice thick enough to cover the mountain summits. At this time the interior of the ice cap is inferred to have been dominated by cold-based ice. During the growth of the Welsh Ice Cap it became confluent with the British–Irish Ice Sheet. The Welsh Ice Cap at this time was again sufficiently thick to submerge the highest mountain summits. The youngest detected phase of ice flow (Event II) was marked by an abrupt change in the dynamics of the Welsh Ice Cap. During Event II the Welsh Ice Cap was drained by at least four ice streams, which followed major troughs in northern and eastern Wales.

A glaciological approach for the discrimination of Loch Lomond Stadial glacial landforms in the Brecon Beacons, South Wales

The interpretation of small ridge systems observed within cirques is often ambiguous and glacial, periglacial and mass-movement origins have been proposed. Much of this ambiguity centres around the interpretation of purely geomorphological data, with little consideration of the glaciological constraints upon the dimensions of small snowpatches and niche glaciers. Application of a glaciological approach to reconstruction, using an estimation of former glacier ablation gradient and the temperature at the reconstructed equilibrium line allows the calculation of glacier mass-balance, total velocity and basal slippage, which may be compared with observed data from contemporary glaciers. The approach outlined is applied to five sites in the Brecon Beacons, South Wales and suggests that four sites reflect upland glaciation during the Loch Lomond Stadial. A fifth site is rejected as being of glacial origin at this time and is suggested to reflect earlier glaciation during different environmental conditions, probably during final wastage of the Last Glacial Maximum Welsh ice cap.

Middle Pleistocene glaciation in Herefordshire: sedimentological and structural evidence from the Risbury Formation

Herefordshire has been subject to glaciation during at least two cold stages of the Pleistocene Epoch. During Middle Pleistocene glaciation, an ice-sheet, emanating from a Welsh ice-cap, extended across Herefordshire as far as the Cradley Brook Valley, on the western margins of the Malvern Hills. The sediments deposited by this ice-sheet, the Risbury Formation, occur as a sequence of small remnants outside of the Late Devensian limit in northeast Herefordshire. Kinetostratigraphical members of this formation accumulated during recessional phases on the retreat of the Middle Pleistocene ice-sheet. Their sedimentological and structural characteristics record a variety of ice-marginal, glaciotectonic, glaciolacustrine and glaciofluvial regimes. Observations demonstrate that active retreat occurred, with minor, localized stagnation in association with a polythermal ice-margin.

Re-evaluation of the Middle Pleistocene stratigraphy of Herefordshire, England

Lithostratigraphical analysis of remnants of high-level, poorly sorted gravel remnants in northeast Herefordshire has allowed the revision and expansion of the existing regional Middle Pleistocene stratigraphical sequence to incorporate two new formations. The oldest Pleistocene sediments in the region, the Humber Formation, represents part of a major, northerly derived, cold-stage fluvial system, which incorporated the ancestors of the rivers Lugg and Wye and tributaries of the Severn. The Risbury Formation consists of glacigenic sediments, which were deposited by a large ice-lobe emanating from a Welsh ice-cap. The Risbury Formation is correlated with glacigenic deposits in the Cradley Brook Valley on the west side of the Malvern Hills, which were formerly ascribed to a pre-Hoxnian, Severn Valley ice-sheet. © 1998 John Wiley & Sons, Ltd.

The late devensian and holocene evolution of Barmouth Bay, Wales

A marine seismic and sedimentological investigation of the Late-Devensian and Holocene sediments of Barmouth Bay, North Wales, was undertaken to reconstruct late-glacial and Holocene sedimentary environments, and to evaluate post-glacial sedimentation rates. In the last glacial, the Devensian, many Welsh estuaries were the sites of valley glaciers flowing from the Welsh Ice cap onto the modern inner-shelf. At Barmouth, the Mawddach valley glacier flowed into the present inner-shelf of Cardigan Bay. The modern courses of the Mawddach Estuary and the river Gwril were overdeepened by erosive sub-glacial flows, and are later infilled by a coarse-grained sub-glacial and pro-glacial infill sequence. With late-glacial relative sea-level rise and retreat of the Welsh Ice, the Mawddach valley tidewater glacier became grounded and confined to the valley. Re-equilibration of the ice profile occurred, and a morainal complex was formed at its toe. In the early Holocene, the morainal complex formed the seaward boundary of a sheltered nearshore basin where fine-grained, partly organic, sediments were deposited in a lagoonal or estuarine environment. Holocene reworking of the sediments by shallow-marine processes has formed a lag surface to the morainal complex, and has introduced gravel of Irish Sea Ice character from offshore. Wave action has formed a shingle coastal barrier, narrowing the mouth of the modern estuary. A shoreface-attached sand wedge, which attains a maximum thickness of 4 m beneath the modern ebb-tidal delta, overlies the lag surface and onlaps the coastal barrier. Since 8–9 ky BP, the mean sediment accumulation rate of estuarine and shallow-marine sediments has been 2.3-3.1 mm/yr, roughly equivalent to the long-term mean rate of sea-level rise for Cardigan Bay. This mean rate masks rapidly increasing accumulation rates in the Mawddach Estuary in the last 5–6 ky, which now may be 82 mm/yr, and which are due to reworking of shoreface and barrier sands into the estuary under a dynamic tidal regime which has a marked time-velocity asymmetry.