Site 25 Knockhill Wood, Glenbervie

A Late-glacial (Windermere) Interstadial peat, intercalated between red-brown diamictons at Knockhill Wood, Glenbervie (Map 10), provides important evidence of the age of slope modification in Strathmore by landslipping. The ¹⁴C age, pollen spectra and plant macrofossils obtained from the peat, and its stratigraphical contacts suggest that the last movement occurred either during the Loch Lomond Stadial or when the climate ameliorated, during the earliest Holocene.

A composite organic deposit was discovered beneath reddish brown diamicton of the Mearns Drift Group during the resurvey of Sheet 66E Banchory in 1989. The deposits were located in a drainage ditch [NO 7667 8012] on the north-eastern side of a forestry track through Knockhill Wood, on the southern side of the valley of the Bervie Water, upstream of Glenbervie (Figure A1.30). The organic sediments consisted largely of peat, with subordinate layers of fine peaty sand and laminated clayey silt (Table A1.15). They rested upon an unstratified bouldery till, also reddish brown in colour.

The organic deposits are exposed along about 7 m of the ditch, and appear to form a wedge in the side of Knock Hill (Figure A1.31). They overlie bouldery till (unit J of (Table A1.15)) and are interstratified with clayey silts and sands. At the north-eastern end of the section, the upper part of the organic sequence (units B–F) is truncated by the overlying clayey diamicton of unit A. This diamicton is hard, but plastic, and apart from flecks of more vivid red clay, it shows little sign of internal stratification. Its base is slightly uneven and gradational over about 1 cm. Other sections lower down the hillside show that the basal till (unit J) reaches up to 8 m in thickness and varies laterally from being stiff and clayey, to friable and sandy. It rests on decomposed feldspathic andesite of the Montrose Volcanic Formation in a river cliff of the Bervie Water [NO 7642 8024].

The base of unit A incorporates wisps and fragments of peat where it overlies the truncated stretch of the organic sequence. The organic sequence itself is penetrated to a depth of about 0.8 m by a vertical, downward-tapering crack filled with red-brown clayey diamicton (Figure A1.31). The crack is either an ice-wedge pseudomorph, or a vertical fracture that has opened up owing to down-slope gravitational movement. The latter explanation is perhaps more likely as there is no evidence of any reorientation of clasts within the fissure infill, as would be expected if it were of periglacial origin.

Six samples taken from a monolith (Figure A1.32) for pollen analysis by M J C Walker (University of Wales, Lampeter, 1989), yielded relatively sparse assemblages. Only one hundred pollen grains were counted from each sample (Table A1.16). The pollen assemblages suggest an open, essentially treeless landscape, dominated by grass and sedge, with a number of herbaceous taxa present (e.g. Compositae, Cruciferae, Rumex and Caryophyllaceae). Tree pollen was sparse, although willow pollen was notable towards the bottom of the profile. The treeless nature of the landscape suggested by these assemblages is clearly indicative of interstadial or tundra climate, rather than warmer interglacial conditions.

The climatic inferences from the pollen analyses were supported by the results of plant macrofossil analysis by M Field (Keele University, 1991) (see (Figure A1.32) for sampling intervals). Sample B9 yielded 34 compressed Carex (sedge) fruits. Sample B10 yielded a well-preserved assemblage of macrofossils, but revealed the presence of only three taxa. A single seed of Viola sp. (violet), 81 compressed Carex fruits and hundreds of seeds of Monitia fontana subspecies fontana, a herbaceous annual to perennial of the 'Blinks' family (Stace, 1991) were recorded. The latter form is widely distributed at present in northern Britain and reaches north-westwards into northern Scandinavia. It occurs in damp habitats and its occurrence with Carex suggests a damp open landscape.

An initial radiocarbon age (GX–14723) of 12 460 ± 130 14C years BP was obtained for an acid-washed bulk sample of the silty peat. A sample taken subsequently was pre-treated to separate the alkali soluble (humic) and alkali insoluble (humin) components for independent age measurement. This was undertaken in order to identify any younger contaminants not completely removed by the pre-treatment. An age of 12 305 ± 50 ¹⁴C years BP was obtained for the humic component (SRR–3687a) and 12 340 ± 50 ¹⁴C years BP for the humin component (SRR–3687b) (Table 8). All of the radiocarbon dates indicate a Late-glacial Interstadial age for the lower part of the sequence. It is possible that the upper parts of the organic sequence may extend into the succeeding Loch Lomond Stadial, but at present, the evidence from the flora is inconclusive.

The stratigraphical, sedimentological and palaeontological studies of the sequence at Knockhill Wood and the radiocarbon dates suggest that the organic sediments were laid down in a cool damp environment, during the Late-glacial (Windermere) Interstadial. The organic sequence overlies a basal bouldery lodgement till of probable main Late Devensian age, laid down on top of weathered andesite bedrock. The basal till is assigned to the Mill of Forest Till Formation (Chapter 8). The origin of the overlying matrix-supported clayey diamicton is more problematic. Its compact structure and absence of well-developed internal stratification are typical attributes of lodgement or deforming-bed tills, rather than flow tills. No tills of Loch Lomond Stadial age are known from the district. Indeed, dating and palynological evidence from sites such as the Loch of Park indicate that the low ground was deglaciated throughout the Loch Lomond Stadial.

The upper diamicton has a gradational, but apparently conformable contact with the underlying organic sediments in the south-western part of the Knockhill Wood section, but it sharply truncates the upper part of the same sequence at the north-eastern (downslope) end of the exposure. This cross-cutting relationship, together with incorporation of fragments of peat in the till near the truncation surface, suggest that the diamicton has been emplaced as part of a landslip. The slip occurred by downslope movement along a gently curved, low-angle plane at the top of the laminated clayey silts (Figure A1.33). It is likely that slipping also occurred at the till/bedrock contact. If the tapering feature cutting the organic sequence is an ice-wedge cast, its presence would suggest that the movement probably occurred immediately following the Loch Lomond Stadial when rapid climatic amelioration took place.

(Table A1.15) Stratigraphy of the Knockhill Wood site.

(Table A1.16) Outline pollen count from Knockhill Wood.

References