Central Grampian Drift Group, Quaternary lithostratigraphy, Cainozoic of north-east Scotland
From: 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).
Central Grampian Drift Group
Deposits of the Central Grampian Drift Group occur on Sheet 95 Elgin and Sheet 96W Portsoy. They were laid down by ice that radiated outwards from a centre over Rannoch Moor in the western Highlands, carrying rock fragments from the Central Highland Migmatite Complex and Caledonian igneous rocks as well as the local, dominantly psammitic Grampian Group rocks (Figures 45; 47). The ice that entered the district did so by two routes (Figure 4). Some ice flowed down the Spey valley and merged with ice from the East Grampians. Other ice flowed northwards towards the Moray Firth where it abutted, and merged with a more powerful stream emanating from the north-west Highlands. These combined ice streams flowed into the Moray Firth and along its southern shores. The relative power of the merging ice streams varied through time, resulting in the interdigitation of deposits of the three drift groups. As in the other groups, some formations have been identified that predate the Devensian.
The interplay between ice streams has resulted in complicated glacigenic sequences in the Inverness area, on Sheet 84W (Fletcher et al., 1996). There, psammite-rich tills laid down by central Grampian ice overlie tills dominated by Old Red Sandstone lithologies. The latter were laid down by ice that flowed from the Central Highlands via the Great Glen towards Inverness, where it crossed Old Red Sandstone strata. The more powerful flow of ice from the north-west Highlands forced the ice from the Great Glen eastwards across high ground towards the Elgin area, where it laid down sandstone-rich tills and associated glaciofluvial deposits. Some Mesozoic erratics occur in the surficial tills around Burghead and Lossiemouth, indicating that some ice crossed the southern margin of the Moray Firth (Figure 45). Deposits have been assigned to the Central Grampian Drift Group on Sheet 96W Portsoy, but Sheet 95 Elgin and Sheet 84W Fortrose were published before the group was formally established.
These tills have been described in general terms in Chapter 6. No formations have been mapped out individually, but a local stratigraphy has been established in the vicinity of the Teindland site (Hall et al., 1995; Table 7; Appendix 1). In the Elgin area, brown sandy tills of the Central Grampian Drift Group contain many well-rounded clasts derived from Old Red Sandstone conglomerates cropping out to the west. They overlie dark grey clayey tills containing shell fragments and Mesozoic erratics derived from the bed of the Moray Firth to the north-west (Peacock et al., 1968; Aitken et al., 1979). Hall et al. (1995a) name the former unit the Tofthead Till and the latter the Alton-side Till (Banffshire Coast Drift Group). They also recognise a younger sandy diamicton, the Waterworks Till. They conclude that all three tills, together with locally intervening units of sand and gravel, were laid down in different phases of the Main Late Devensian glaciation, the Waterworks Till being the result of a minor re-advance. The Tofthead Till probably correlates with the Old Hythe Till at the Boyne Limestone Quarry (Peacock and Merritt, 2000).
None of the three Late Devensian tills occurring in the vicinity of Teindland appears to be present at the main site. Instead, another sandy till unit occurs that Hall et al. (1995a) name as the Teindland Till and assign to the Early Devensian. It overlies a package of units that includes the Teindland Buried Soil, which probably dates from the Ipswichian (OIS 5e). The soil lies stratigraphically above a unit that Hall et al. (1995a) name as the Teindland Gravel. It is composed predominantly of rounded clasts of quartzite and psammite that are probably largely derived from Old Red Sandstone conglomerates. Nearby, it overlies the Red Burn Till, the oldest known till in this area (Appendix 1). Although reddish brown in colour and containing clasts mostly of quartzite and psammite, this diamicton, like the Alton-side Till, contains some Mesozoic erratics and hence it is also assigned here to the Banffshire Coast Drift Group, albeit tentatively.
Another old till unit assigned here to the Central Grampian Drift Group occurs at the Boyne Limestone Quarry (Appendix 1). This very sandy diamicton, the Craig of Boyne Till Formation, contains much decomposed, easily weathered calc-silicate rock and therefore appears to be weathered to a greater extent than it is (Peacock and Merritt, 2000). Nonetheless, it contains some clasts that have been considerably weathered in situ and is therefore likely to be pre-Devensian in age, like the Red Burn Till at Teindland.
Although the sand and gravel occurring on Sheet 95 Elgin was mainly laid down at the margin of the Moray Firth ice stream as it retreated (Chapter 6), its composition suggests that much of the ice crossing the area was probably sourced in the central Highlands. Therefore, these glaciofluvial deposits should be assigned to the Central Grampian Drift Group, but no formal lithostratigraphical units have yet been set up. The only current named unit is the pre-Devensian Teindland Gravel.