Ed Anderson

Holocene alluvial-fan development in the Macgillycuddy's Reeks, southwest Ireland

Holocene alluvial landforms in the Macgillycuddy9s Reeks, southwest Ireland, were investigated to determine the controls and timing of postglacial geomorphic activity. Detailed geomorphologic analysis of three alluvial-fan and debris cones within high-level cirque basins demonstrates evidence of episodic phases of late Holocene surface aggradation and incision. Radiocarbon dates from peat horizons above and below inorganic units show that phases of aggradation cluster into two distinct periods, the first after 230–790 calibrated (cal.) yr A.D. and the second from 1510 cal. yr A.D. to the present. An additional phase of fan aggradation at one site is dated after 1040–1280 cal. yr A.D. All three phases coincide with episodes of enhanced late Holocene valley-floor alluviation and debris-flow activity from upland Britain. Alluvial fans and debris cones have developed primarily as a result of the resedimentation of late Midlandian (Wisconsin) drift and talus slopes, and mobilization of materials involved flooding, transitional-flow, and debris-flow processes. Pollen analysis of peat horizons interbedded with alluvial-fan and debris-cone sediments indicates that land-use changes were an important factor in lowering the threshold for local slope erosion. Phases of aggradation also coincide with well-documented episodes of climate change, and, hence, fan development is probably a function of both anthropogenic and climatic forcing. A sequence of events may have involved initial slope destabilization due to overgrazing and removal of vegetation that was followed by debris mobilization and fan aggradation during intense rainstorms associated with climate change.

A small glacial cirque basin on Exmoor, Somerset

Geomorphological and sedimentological investigations at the Punchbowl on Exmoor, north Somerset, have demonstrated evidence of cirque basin glaciation, some 30 km south of the accepted southern limit of Anglian ice cover in this area. The Punchbowl is a steep-sided, north-facing basin that has a subdued arcuate ridge at its mouth. Analysis of macrofabrics, clast form and roundness and particle size on sediments forming the ridge indicate it to consist of glacial diamict deposited as a terminal moraine. Further evidence supporting the presence of a former cirque basin glacier at this site is derived from basin morphology and a striated sandstone boulder. Palaeoglacier reconstruction gives a surface area of 0.38 km 2 , a maximum thickness of 46.5 m and a maximum basal shear stress value of 51 kPa. The ELA of the former glacier has been calculated at 334 m OD. There is no evidence for multiple glaciations at this site, and, in the absence of dating controls on the moraine, the glacier is provisionally assumed to have formed during severe climatic conditions associated with the southerly advance of ice sheets during the Anglian cold stage or during the Dimlington Stadial. Development of a glacier at this location appears to have been a rare event during the Quaternary, and may have been facilitated by accumulation of windblown snow from the adjacent plateau.

Nature, culture, clitter: Distinguishing between cultural and geomorphological landscapes; The case of hilltop tors in south-west England

This article addresses the problem of how to distinguish between natural and humanly modified features of the cultural landscape with reference to clitter (boulder and stone) masses in the south-west of England using the example of Leskernick hill, Bodmin Moor with its well-preserved Bronze Age settlement. We first set out a series of criteria for distinguishing between natural and humanly placed stones on the basis of a series of formal geomorphological criteria. We then discuss the stones from an archaeological perspective setting out a series of archaeological criteria by means of which we can recognize the presence of humanly modified stones. From this basis we discuss four examples in detail. Finally we attempt to interpret the significance of the cultural modification of stone masses, previously regarded by both archaeologists and geomorphologists as being entirely natural in origin, by challenging the very culture/nature distinction for ascribing meaning on which the previous considerations are made. Whilst acknowledging that the distinction between a stone that has been moved by human agency, and one that has not, is important for interpretation this does not make it more or less culturally significant.

Late quaternary paraglacial sedimentation in the Macgillycuddy's reeks, southwest Ireland

Talus slopes in the Curraghmore cirque basin in the Macgillycuddys Reeks have developed since the end of the Glenavy Stadial (Last Glacial Maximum; MIS 2). The site lay outside Younger Dryas glacier limits and, as a result, thick rockfall and debris flow accumulations form a significant landform assemblage draping lower hillslopes. Rockwall retreat rates since icesheet deglaciation average 0.08mm per year, and 0.2mm per year up to the beginning of the Holocene.

THE EASTERN MARGIN OF GLACIATION IN THE BRITISH ISLES DURING THE YOUNGER DRYAS: THE BIZZLE CIRQUE, SOUTHERN SCOTLAND

ABSTRACT. Geomorphological and sedimentological evidence of former glaciation in the Bizzle valley in the Cheviot Hills of northern England and southern Scotland was used to reconstruct the dimensions of a small topographically constrained glacier with an equilibrium line altitude (ELA) of 535 m. This was interpreted as having formed during Younger Dryas cooling; this is the only glacier to have been described from the area and is the most easterly site of Younger Dryas glaciation in the British Isles. Whilst glaciation at this time was extensive in the Lake District to the southwest, the restricted nature of Cheviot ice cover suggests that a steep west–east precipitation gradient existed in this region during the Younger Dryas.

Further glacial tills on Exmoor, southwest England: implications for small ice cap and valley glaciation

This paper discusses glacial deposits down valley of the Punchbowl cirque basin on Winsford Hill, Exmoor. Geomorphological mapping, lithostratigraphic logging and sedimentological analyses have differentiated basin floor landform assemblages lying down valley of the Punchbowl terminal moraine ridge to include: subglacial diamict, periglacial solifluctate and fluvial deposits. Local ice cover at some stage during the Quaternary appears to have been more extensive than hitherto believed, and most probably extended to a small ice cap on the broad plateau surface of Winsford Hill at just over 400 m OD. It is concluded that other plateau surfaces on Exmoor at similar altitudes may also have nurtured ice caps at certain times during the Quaternary. These findings further suggest that the long-held view of Dartmoor as an unglaciated upland region may need re-evaluating.

Optical Dates from British Isles ‘Solifluction Sheets’ Suggests Rapid Landscape Response to Late Pleistocene Climate Change

Abstract The most widespread depositional landforms in many of the upland valleys of the British Isles which have undergone glaciation during the Last Glacial Maximum (LGM) are so-called solifluction sheets. These form a landform-sediment association consisting of smoothed-slopes underlain by poorly-sorted deposits. The timing and nature of sediment deposition is unclear in the absence of a dating framework within which solifluction sheet development can be placed. This paper presents the first dates from these landforms in the British Isles. We used optical dating on deposits exposed at Linhope Burn in the Cheviot Hills of North East England. The results suggest that the bulk of sediment deposition at this site happened over a period spanning the Younger Dryas. We argue that both paraglacial and periglacial processes can account for the main periods of deposition and that the timing of sediment deposition reflects the rapidity with which landform development responded to climatic shifts.