Hummocky glacial deposits, Quaternary, Cainozoic of north-east Scotland
|Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul.
Hummocky glacial deposits form a distinctive sedimentlandform association that has been identified on some of the more recently published maps. The deposits are highly variable lithologically and include complex interdigitations of matrix- and clast-supported diamicton, stratified and unstratified silty boulder gravel, and lenses of sand, silt and clay (P104108). Most are formed primarily of very poorly sorted materials that were deposited during deglaciation. They spilled from the ice-fronts as mud-slides and debris flows, either subaerially, or into standing water. Some of the deposits are constructional moraines formed at active ice margins, but most examples in the district apparently formed when large masses of ice stagnated. This situation commonly occurred towards the margin of the East Grampian ice sheet as it retreated westwards across north–south-trending ridges. For example, sandy morainic deposits derived largely from disaggregated granite occur quite extensively on Sheet 76E Inverurie in the lee of the Hill of Fare, between Banchory and West Cullerley (NJ 766 030) (P915378). These mounds are commonly strewn with large boulders and blocks, but locally contain better-sorted materials at depth.
Similar deposits occur within a col at Tillyfourie and between the Hill of Fare and Cairn William, where they form irregular mounds and ridges with intervening peat-filled hollows (P915378). These morainic deposits contain a high proportion of silty, matrix-supported bouldery diamicton as well as stratified sandy boulder gravel. Those at Tillyfourie are well seen from the A994 Alford–Aberdeen trunk road.
Hummocky glacial deposits are also common within the East Grampian Drift Group on Sheet 66E Banchory, where ice stagnated in the lee of high ground (P915380). They occur extensively on the southern side of the Water of Feugh catchment and on the interfluve between the Dee, Carron and Cowie waters. Notable examples are present in the valley of Burn of Greendams, south-east of Strachan and in the valleys of the burns of Knock and Curran, to the south of Banchory. Most of the deposits form boulder-strewn lateral moraines, up to 10 m high in places, resting on granite bedrock. They are composed of boulder gravel with a coarse sandy matrix of disaggregated granite, derived from the underlying bedrock. Good examples of lateral moraine ridges, occur between Powlair (NO 621 912) and Green-dams (NO 649 900). Similar landforms are present on the south-eastern side of the col between Craigbeg (NO 770 913) and Cairn-mon-earn (NO 783 920). They were visible for many years from the A596 Banchory–Stonehaven trunk road (Slug Road), where it crosses the Dee–Strathmore watershed, but most of these ridges are now obscured by coniferous forest.
Few cross-valley retreat moraines have been recognised, but a good example is the Lady’s Moss Moraine (NO 782 901) in the valley of the Black Burn, south of Cairn-mon-earn (Auton et al., 1988). Other examples include north-east-trending morainic ridges in the col between Shillofad (NO 724 888) and North Dennetys (NO 710 877), and a north-west-trending ridge up to 18 m high, at Rouchan (NO 640 897) (Auton et al., 1990). The cross-valley moraine at Lady’s Moss and the morainic ridges between Shillofad and Kerloch lie close to the limit of the Aberdeen–Lammermuir Re-advance of Sissons (1967; P915286). The former moraines occur at elevations of 170 to 200 m OD, the latter at 275 to 320 m OD. They appear to mark still-stands in the late-stage retreat of small glaciers in the cols rather than the limit of a regionally significant ice advance. Although the concept of Sissons’ Aberdeen–Lammermuir Re-advance is now largely discredited, it was the recognition of morainic features such as these that first led to its proposition.
Constructional moraines were formed locally at the margin of the East Grampian ice sheet, especially where it ‘actively’ retreated from high ground and bedrock ridges trending at right angles to ice flow. In central Buchan, this situation led to the cutting of groups of north–southtrending meltwater channels, but the ice was locally sufficiently active to form low morainic ridges parallel to the channels. Good examples of such ridges occur on Sheet 76E Inverurie in the vicinity of Burnhelvie (NJ 724 198) and Moss-side (NJ 710 183) (P915378).
Push moraines appear to be rare in the district, but to the south of Ellon, two concentric ridges in the vicinity of Cross Stone Wood (NJ 955 278) are examples (P915297; P915376). The ridges stand up to about 8 m high and are apparently formed mainly of sand and gravel with a capping of red diamicton. The ridges are asymmetrical in cross-section, with steeper slopes facing south-westwards. They were probably formed at the margin of the coastal ice stream that was responsible for laying down the deposits of the Logie-Buchan Drift Group. Other moraines of this type lie at the southern end of the Den of Boddam (NK 102 408), where the coastal ice pushed northwards towards the Hill of Longhaven (P915377). The form of all these features strongly suggests that the coastal ice was pushing towards ice-free ground.