Adrian M. Hall

Luminescence dating and its application to key pre-Late Devensian sites in Scotland

Abstract Relatively few sites have been identified in Scotland that contain records that predate the peak of the Late Devensian glaciation (ca. 18 ka BP). Where such records have been identified they have assumed a key role in developing our understanding of climate change in Scotland. Radiocarbon dating has been the primary chronological tool applied to these sites, but the reliability of age determinations in excess of 30 ka is questionable. In this paper we describe the results of a programme which applies luminescence dating methods to eight key pre-Late Devensian sites in Scotland. Methods were developed and tested to enable the identification of samples for which luminescence dating was only able to provide an upper limit on the age because of the limited light exposure that the sample experienced at deposition. These new approaches were applied to four sites where independent age control was available; this meant that the choice of sediments from test sites was restricted to those sediments which were formed in the last 40,000 years and hence were amenable to radiocarbon dating. The luminescence age estimates for the control sites and the pre-Late Devensian sites are discussed within the context of any existing chronological control on a site by site basis.

Calculating Quaternary glacial erosion rates in northeast Scotland

Northeast Scotland is an area exhibiting selective erosion by Quatemary ice sheets. In this area both glacial and preglacial landforms exist in close proximity. The depths of erosion which this modification represents are calculated on the assumption of various depths of preglacial weathering. A depth of erosion of between 34 and 62 m per unit area is indicated. Calculated rates of erosion are 0.021 nun a- ’ for the entire 2.3 m.y. of the Quaternary, and between 0.1 and 0.5 mm aa1 on the assumption that glacial conditions existed in this area for 500,000 years and 100,000 years, respectively. These figures are compared to the offshore sedimentary record in the adjacent west-central North Sea. The volume of sediment deposited offshore is equivalent to a depth of erosion of 195 m per unit area, yielding an average erosion rate of 0.085 mm a-i over the entire Quatemary. Rates of erosion were low in the preglacial Pliocene (0.049 mm a- ‘1 and early Quatemary (0.063 mm a-‘). The expansion of ice sheets across the area in the middle Quatematy was associated with a sharp increase in the rates of erosion (> 0.13 mm a-‘) but the last (late Devensian) ice sheet in the area was less erosive (< 0.095 mm a-‘). The estimated rates of erosion represented by the offshore sedimentary record therefore exceed the estimated rates of glacial erosion from the onsho’re geomorphological reconstruction.

Age and significance of former low-altitude corrie glaciers on Hoy, Orkney Islands

Synopsis Geomorphological mapping provides evidence for two former low-level corrie glaciers on Hoy, both defined by end moraines. Five 10Be exposure ages obtained from sandstone boulders on moraine crests fall within the range 12.4 ± 1.5 ka to 10.4 ± 1.7 ka (weighted mean 11.7 ± 0.6 ka), confirming that these glaciers developed during the Loch Lomond (Younger Dryas) Stade (LLS) of 12.9–11.5 cal. ka bp, and demonstrate the feasibility of using this approach to establish the age of LLS glacier limits. The equilibrium line altitude (ELA) of one of the glaciers (99 m) is the lowest recorded for any LLS glacier, and the area-weighted mean ELA for both (141 m) is consistent with a general northward ELA decrease along the west coast of Britain. The size of moraines fronting these small (≤0.75 km2) glaciers implies that glacier termini remained at or close to their limits for a prolonged period. The apparent restriction of LLS glaciers to only two sites on Hoy probably reflects topographic favourability, and particularly the extent of snow-contributing areas.

Late Quaternary alluvial placer development in the humid tropics: the case of the Birim Diamond Placer, Ghana

Examination of the diamondiferous sediments of the Birim River floodplain in Ghana using radiocarbon dating allows the recognition of three chronostratigraphic units: 13000–11000, 9000–7000, and 2100 years BP to the present, while older sediments rest on bedrock benches and form terraces. The late Pleistocene sediments infill deeper, scour channels, are generally both thicker and coarser, and return higher diamond grades. One thousand nine hundred samples from 700 boreholes indicate that diamond grade is influenced by bedrock consistency, gravel thickness, gravel calibre, and stratigraphic position, but other external factors include supply of coarse quartz from bedrock, valley morphology, and the location of tributary junctions and diamond sources. The influence of palaeoenvironmental conditions on fluvial deposition and placer formation in W Africa appears widespread and relates to the known late Quaternary climatic changes established from Lake Bosumtwi, Ghana, and from the Sahara.

The altitude of the last ice sheet in Caithness and east Sutherland, northern Scotland

Synopsis On the mountains of southern Caithness and east Sutherland, geomorphological evidence for the upper limit of glacial erosion implies that, at the last glacial maximum (LGM), warm-based erosive ice extended up to altitudes of 410–600 m. Above the upper limit of glacial erosion is a cover of frost-weathered detritus, with tors on conglomerate bedrock. Kaolinite and gibbsite in soils on such terrain imply that it escaped significant glacial erosion at the LGM. This is confirmed by cosmogenic 10Be exposure ages of (>) 146.6±15.1 ka and (>) 178.2 ± 18.1 ka obtained for summit tors; sites below the upper limit of glacial erosion yielded deglacial ages of 16.6 ±2.1 to 18.5 ± 2.4 ka. It is unlikely that glacially unmodified summits represent LGM nunataks, as the implied ice thickness is inconsistent with an ice sheet that extended across high ground on Orkney onto the Atlantic shelf; under conservative assumptions at least 70 m of ice must have covered the highest ground in southern Caithness at the LGM. We infer that the absence of evidence for glacial erosion on some summits and plateaux reflects cover by cold-based ice, frozen to the underlying substrate, that preserved the underlying tors and blockfields. This inference calls into question the use of similar evidence in NW Scotland to identify the maximum altitude of the last ice sheet.

Selective glacial erosion on the Norwegian passive margin

Glaciated passive margins display dramatic fjord coasts, but also commonly retain plateau fragments inland. It has been proposed recently that such elevated, low-relief surfaces on the Norwegian margin are products of highly efficient and extensive glacial and periglacial erosion (the glacial buzzsaw) operating at equilibrium line altitudes (ELAs). We demonstrate here that glacial erosion has acted instead to dissect plateaus in western Norway. Low-relief surfaces are not generally spatially associated with cirques, and do not correlate regionally with modern and Last Glacial Maximum ELAs. Glacier dynamics require instead that glacial erosion is selective, with low-relief surfaces representing islands of limited Pleistocene erosion. Deep glacial erosion of the coast and inner shelf has provided huge volumes of sediment (70,000 km3), largely resolving apparent mismatches (65–100,000 km3) between fjord and valley volumes and Pliocene–Pleistocene sediment wedges offshore. Nonetheless, as Pleistocene glacial valleys and cirques are cut into preexisting mountain relief, tectonics rather than isostatic compensation for glacial erosion have been the main driver for late Cenozoic uplift on the Norwegian passive margin.

Late Devensian glaciation of southern Caithness

Synopsis A formal stratigraphy is proposed for the Quaternary deposits of south-east Caithness. A shelly till from the Moray Firth, the Forse Till, overlies an early till from inland, the Balantrath Till, which demonstrates an expansion of inland ice on to the edge of the plain of Caithness. Along its south-eastern margin the Forse Till is found interbedded with the Balantrath Till in the Dunbeath Water valley and a second inland till, the Ladies’ Tent Till, in the Langwell Water valley. The stratigraphy indicates that Moray Firth ice was contiguous with inland ice at this stage and evidence from striations shows that inland ice was diverted to run northwards parallel to the margin of the inland ice. After retreat of Moray Firth ice there was a minor advance of inland ice in the Berriedale area and formation of hummocky moraine. There is no evidence of any subaerial exposure within till sequences and the tills are therefore related to a single, complex phase of glaciation. The Forse Till and the Ladies’ Tent Till are overlain by peats of Windermere Interstadial age and by gelifluctates of Loch Lomond Stadial age. A variety of relative age criteria indicate that almost all the Quaternary deposits of south-east Caithness are of Late Devensian age.

Middle Devensian ice-proximal gravels at Howe of Byth, Grampian Region

Synopsis Ice-proximal glaciofluvial gravels at Howe of Byth, Grampian Region, have given luminescence ages of 45 ± 4 and 37±4 ka BP. The gravels were laid down by meltwater draining from an ice margin in the Moray Firth and provide the first firm evidence of extensive glaciation in NE Scotland during the Mid Devensian.

Pre‐glacial landform inheritance in a glaciated shield landscape

Abstract We seek to quantify glacial erosion in a low relief shield landscape in northern Sweden. We use analyses of digital elevation models and field mapping of glacial erosion indicators to explore the geomorphology of three granite areas with the same sets of landforms and of similar relative relief, but with different degrees of glacial streamlining. rea 1, the arkajoki district, shows no streamlining and so is a type area for negligible glacial erosion. Parkajoki retains many delicate pre‐glacial features, including tors and saprolites with exposure histories of over 1 . rea 2 shows the onset of significant glacial erosion, with the development of glacially streamlined bedrock hills. rea 3 shows extensive glacial streamlining and the development of hill forms such as large crag and tails and roches moutonnées. Preservation of old landforms is almost complete in rea 1, due to repeated covers of cold‐based, non‐erosive ice. In rea 2, streamlined hills appear but sheet joint patterns indicate that the lateral erosion of granite domes needed to form flanking cliffs and to give a streamlined appearance is only of the order of a few tens of metres. The inheritance of large‐scale, pre‐glacial landforms, notably structurally controlled bedrock hills and low relief palaeosurfaces, remains evident even in rea 3, the zone of maximum glacial erosion. Glacial erosion here has been concentrated in valleys, leading to the dissection and loss of area of palaeosurfaces. Semi‐quantitative estimates of glacial erosion on inselbergs and palaeosurfaces and in valleys provide mean totals for glacial erosion of 8 ± 8 m in rea 1 and 27 ± 11 m in rea 3. These estimates support previous views that glacial erosion depths and rates on shields can be low and that pre‐glacial landforms can survive long periods of glaciation, including episodes of wet‐based flow.

Devensian organic interstadial deposits and ice sheet extent in Buchan, Scotland

Pre-Late Devensian organic deposits in the Buchan area of northeast Scotland were investigated for their geomorphological and palaeoecological (pollen, plant macrofossils, coleoptera) properties. Close ecological agreement exists between fossil indicators and allows the inference that the environment in the vicinity of the deposits was a dwarf shrub tundra of the type met today in high latitude areas of Scandinavia and arctic Russia. The latest in a series of radiocarbon dates from the site produced determinations beyond the limits of the method, although the geomorphological and fossil evidence appears to point to an interstadial date within Oxygen Isotope Stages 5a or 5c. The site has special significance for arguments concerning the much-debated concept of ‘Moraineless Buchan’; indeed, evidence is presented which supports the concept of extensive ice sheet glaciation during the Late Devensian for this crucial geographical area. If Buchan is to be seen as a further casualty amongst other disputed ice-free enclaves, then a return to earlier models of extensive ice sheet glaciation in the Late Devensian of Scotland would seem to be necessary.