Geomorphological features of glacial or glaciofluvial deposition, 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.
Features of glacial or glaciofluvial deposition
Eskers, kames and moraines
Descriptions of individual eskers and kames are given in the notes on glaciofluvial ice-contact deposits, and their resource potential is discussed in Bulk mineral resources. The most prominent eskers in the district are located on maps:
- Elgin district. P915371.
- Sheet 96W Portsoy. P915372.
- Sheet 96E Banff. P915373.
- Sheet 97 Fraserburgh. P915374.
- Sheet 86E Turriff. P915375.
- Sheet 87W Ellon. P915376.
- Sheet 87E Peterhead. P915377.
- Sheet 76E Inverurie. P915378.
- Sheet 77 Aberdeen. P915379.
- Sheet 66E Banchory. P915380.
- Sheet 67 Stonehaven. P915381.
Moraines are not common within the district (Charlesworth, 1956; Synge, 1956). Those that do occur have been described in ‘hummocky glacial deposits’. Most moraines are of a rather chaotic nature and formed when large masses of ice stagnated in the lee of major topographical barriers. There are some examples of recessional moraines formed during ‘active’ glacial retreat, particularly on Sheet 66E Banchory and Sheet 67E Stonehaven, and a significant terminal moraine close to the limit of the Logie-Buchan Drift on Sheet 87W Ellon.
It has long been recognised that the occurrence of boulders in areas far removed from their source is good evidence of the former direction of glacial transport (P015377; P104115). At times too much emphasis has been placed on the distribution of distinctive ‘indicator erratics’ in early reconstructions of former ice sheets and transport pathways (e.g. Jamieson, 1906; Read, 1923; Bremner, 1934a, 1938; Synge, 1956). In addition, some of the observations made in the older literature are questionable. However, with caution, the known distribution of erratics still provides valuable evidence for determining the extent and interrelationships of former ice sheets in the district (Sissons, 1967). They also form an important element in the lithostratigraphical division of the Quaternary sequence at group level. In more modern work, the complete description of clast types is made as ‘stone counts’ rather than relying on single far travelled clasts that may have complicated transport histories.
The paths determined for the transport of erratics in the Elgin area are reasonably consistent, revealing a predominantly easterly flow of ice, although there is also evidence of an older movement towards the south-east (P915292). The distribution of erratics of the granitic ‘Inchbae’ augen gneiss from central Ross-shire is particularly important, indicating the dominant ice flow into the Moray Firth from the north-west Highlands. The picture that emerges in the rest of north-east Scotland is more difficult to understand (P915294). The south-easterly movement of erratics both from the Boyndie metagabbro and the ‘lower zone’ cumulate basic and ultrabasic rocks of the West Huntly and Barra Hill intrusions, is consistent with the distribution of other erratics and rafts of sediment and fossils derived from the bed of the Moray Firth (see below and Whitehills Glacigenic Formation). The radial movement of erratics away from the Netherly Diorite and West Huntly intrusion is more difficult to explain. One explanation could be that the outcrops of indicator rocks may be more extensive than thought hitherto because of poor exposure. For example, concealed masses of gabbro have been identified by geomagnetic surveys between Huntly and the coast (Munro, 1970; Munro and Gallagher, 1984). Another explanation involves the probable cold-based nature of former ice sheets over the interior of north-east Scotland, and their inability to scour away the products of previous glaciations. The interrelationship of East Grampian ice with more powerful ice streams occupying the Moray Firth basin may also be responsible for such patterns (Peacock and Merritt, 2000).
The analysis of ice-movement direction is further complicated by the reported south-westward carry of erratics of the Peterhead granite (Bremner, 1928, 1943; Synge, 1956; P915294). However, the occurrence of little known sheets and small bodies of granite, including red granite, between Fraserburgh and Peterhead on Sheet 97 Fraser-burgh and Sheet 87E Peterhead casts some doubt on the observation. The boulders do, however, lie along the margin of the ice that moved onshore from the North Sea basin giving rise to the Logie-Buchan Drift Group. It is difficult to account for this direction of movement unless ice crossed the North Sea basin from Scandinavia. Some foundation to this argument has been derived from the discovery of erratics of rhomb-porphyries and larvikites also thought to have been carried from southern Norway (Read et al., 1923; Campbell, 1934; Bremner, 1939, 1943; Ehlers, 1988; Hall and Connell, 1991, fig. 74). These include a boulder on the beach at Portsoy, and several findings around Ellon and Aberdeen (P915294; Sissons, 1967), including Nigg Bay. However, the majority of confirmed Scandinavian erratics have been found inland of where deposits of the Logie-Buchan Drift Group occur at surface and are thus likely to have been transported onshore during a pre-Devensian glaciation.
Large rectangular blocks of hornfelsed sillimanitebearing pelitic gneiss, many weighing several tons, form an erratic train eastward from its outcrop around Green Hill (NO 638 162), on the western margin of Sheet 76E Inverurie (P915378). The erratic train extends as far as Monymusk, and isolated erratics many metres in diameter have been found farther east, in the vicinity of ‘The Horner’ (NO 748 156), east of Kemnay.
A south-eastward movement of erratics of andalusite schist and an eastward carry of mafic and ultramafic rocks from the Insch intrusion have been recorded in the northwest part of Sheet 77. The Insch erratics result from the final eastward flow of the East Grampian ice sheet, but the schists were probably carried by an earlier movement. At the coast, erratics of sandstone and conglomerate have been carried north-westwards by the onshore movement of ice that laid down deposits of the Logie-Buchan Drift Group. This movement possibly also carried erratics of the mafic and ultramafic Belhelvie intrusion, which extends offshore, rather than the north-eastward movement depicted on P915294. An earlier south-east movement is required to account for the Belhelvie erratics found in the lowest exposed gravels at Nigg Bay (Bremner, 1928).
Bremner (1920a) reported the presence of erratics of quartzite and sandstone that had been carried north-east from Strathmore onto the watershed between the Dee and Cowie Water (P915380). In particular, a quartzite boulder was found in the col between Craigbeg (NO 769 911) and Mongour (NO 757 900), over 330 m above OD. This north-eastward transport has been confirmed by the recent discovery of a rounded erratic of porphyritic andesite, many metres in diameter, ‘wedged’ in the bottom of a small glacial drainage channel occupied by the Burn of Anaquhat. The erratic was found (at NO 746 881), 260 m above OD and almost 5 km inland of the nearest outcrop of andesite in Strathmore. Bremner (1920a) postulated that the Old Red Sandstone erratics had been transported by an advance of Strathmore ice, prior to the arrival of ‘Dee Valley Ice’ (East Grampian ice sheet), which ‘completely cleared out or covered up all traces of Strathmore drift, in places where one is safe to infer that it was once present’. The age of the early advance of Strathmore ice is not known, but it possibly occurred during the maximum development of the Main Late Devensian glaciation of the district, prior to 22 ka BP (Balnakettle).
A distinctive suite of erratics occurs in the deposits of the Logie-Buchan Drift Group, including Permian and Mesozoic limestones and dolomites in addition to Old Red Sandstone and conglomerates, indicating a source to the south and offshore (Kippet Hills and Sandford Bay).
Glacial rafts or ‘megablocks’ are dislocated slabs of rock and unconsolidated strata that have been transported from their original position by glacial action (Aber et al., 1989). Erratics and large glacial rafts of Mesozoic rocks derived from the floor of the Moray Firth and the western North Sea are widespread in coastal areas, where they occur in deposits of the Banffshire Coast Drift Group and Logie-Buchan Drift Group respectively (P915251). Rafts are particularly common in the White-hills Glacigenic Formation of the former group, where they include reddish brown Permo–Triassic marls, dark grey to black Jurassic mudstones, Cretaceous glauconitic sandstones and Quaternary marine clays, silts and sands (P104123; P104124). The rafts range upwards in dimension from fist-size fragments to slabs that are probably several tens of metres thick and a kilometre or more in width. Several very large rafts of mudstone, silt and clay were worked for making bricks and tiles.
The known localities of rafts are concentrated along the northern coast of the district where glacial striae also indicate that ice moved onshore towards the south-east at one stage. This suggests that the rafts were emplaced in zones of longitudinal compression, either proglacially or subglacially, as ice occupying the Moray Firth passed from deformable marine sediments on to bedrock, and was forced to override the high coastline and interior valleys (Peacock and Merritt, 1997, 2000). The rafts were detached in the Moray Firth basin, probably as high pore-water pressures built up in confined aquifers (sands interbedded with clays), a situation that has been invoked for the emplacement of similar rafts at Clava, near Inverness (Merritt, 1992b). Rafts were once assumed to have been moved while ‘frozen on’ to the base of cold-based ice sheets, but their intimate association with deformation tills suggests that failure may also have occurred along décollement surfaces in a subglacial deforming layer (Benn and Evans, 1998).
Rafts have been described at several localities in the district (Boyne Limestone Quarry, Gardenstown, Oldmill and Moss of Cruden). At the time of writing, several rafts could be seen in a quarry at Ardglassie (NK 012 617), 5 km south-south-east of Fraser-burgh on Sheet 97 (Merritt and Connell, 2000). The rafts include sheared pale yellowish brown fine-grained sands with traces of ripple cross-lamination and dark grey clay. One raft of black lignitic clay contained a rich Late Jurassic palynoflora (information from Dr L A Riley, 1998). The rafts rest on relatively fresh bedrock and are capped locally by a thin unit of brown till. The quarry is situated on a south-west–north-eastorientated ridge, and most of the shear planes preserved in the rafts dip towards the north-north-west, suggesting ice push from that direction against the ridge. Many characteristic features of rafts are displayed, including shear zones, conjugate microfaulting, brecciation and folding.