Review of past models for glaciation, 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.
A review of past models for the glaciation of NE Scotland
The systematic study of glacial deposits and landforms in NE Scotland began in the mid-Nineteenth Century with the pioneering work of Thomas Jamieson. There have since been several attempts to unravel the complicated record of glacial events, but although the glacial reconstructions that have been published are generally based on sound evidence, they were inevitably influenced by the contemporary knowledge of glacial processes and events.
The work of Jamieson
In a series of papers stretching over a period of fifty years, Thomas Jamieson developed modern concepts of Quaternary glaciation from previous theories of the great biblical submergence and erosion by floating ice (Jamieson, 1858; 1859; 1860a,b; 1862; 1865; 1874; 1882a,b; 1906; 1910). He published a wealth of stratigraphical detail, established sequences of glacial events and reconstructed former flow lines from the distribution of erratics, striae, and orientations of roche moutonees. He was one of the first to realise that the glaciation of Scotland involved the formation of a major ice-caps like in Greenland rather than Alpine-style valley glaciers.
In Jamieson’s final model (P915300) the oldest glacial event involved ice from the Moray Firth laying down ‘shelly indigo boulder clay’ in the Ellon district. The deflection was interpreted to have been caused by Scandinavian ice impinging on the eastern coast of Buchan. The indigo diamicton was largely stripped away by ice flowing from inland during a subsequent event, which laid down the locally-derived, non-shelly ‘Lower Grey Boulder Clay’ of Aberdeenshire. That till was subsequently overlain by the ‘Red Clay Series’ along the coast to the north of Aberdeen. These vivid red deposits with ORS erratics had been deposited by ice flowing northwards from Strathmore, but deflected onshore by Scandinavian ice in the North Sea. Jamieson noted that the Red Clay Series intermingled with, and thus was contemporaneous with the ‘Dark Blue Boulder Clay’ in the Peterhead area. The latter till contained many large erratics derived from Mesozoic rocks in the Moray Firth. Local ice had retreated far to west during this incursion, although the Lower Grey Boulder Clay was regarded as broadly similar in age to the red and dark blue coastal tills. There was local marine incursion following the decay of the coastal ice lobes leading to the deposition of clays. The final event involved an expansion of ice from inland. During this ‘Aberdeen Readvance’, a glacier descended the valley of the River Dee to reach the coast, where it scoured away the Red Clay Series deposits locally.
The work of Bremner and Read
The mantle of Jamieson was taken over by Alexander Bremner in the early Twentieth Century. He also published a wealth of detail on the Quaternary of NE Scotland, paying particular attention to the pattern of supposed ice-marginal glacial meltwater channels formed during glacial retreat (Bremner, 1912; 1915; 1917; 1921; 1925a,b; 1931; 1934; 1938; 1939; 1943). Bremner developed the triple glaciation model, although he disagreed with Jamieson about the sequence of events and many details. Unlike Jamieson, Bremner claimed to have found evidence confirming that the main glacial episodes were separated by warm, non-glacial periods.
In Bremner’s final (1943) model (P915301), the first glaciation involved ice sourced in Sutherland, Ross and Cromarty and the Great Glen flowing into the region from the W and NW. It picked up Mesozoic rocks and shells from the Moray Firth and laid down dark blue shelly till along the northern coast. The Moray Firth ice stream coalesced with Scandinavian ice at the angle of Buchan and the combined ice masses flowed southwards leaving a train of boulders of Peterhead Granite and sparse erratics of rhomb-porphyry and larvikite. Peat preserved in the vicinity of the Burn of Benholm site (Table *) supposedly formed in the subsequent interglacial period.
Ice traversed Buchan from the SW and S in Bremner’s second glaciation. He used evidence cited by Read (1923) to show that it flowed northwards into the Moray Firth along the northern coast. Ice from Strathmore deposited red tills along the eastern coast and laminated clays were deposited in freshwater lakes ponded against the landward margin of that ice stream during its retreat.
Tills resulting from Bremner’s third glaciation were supposedly difficult to identify owing to the glacial reworking of older deposits. Patterns of ice flow were mainly determined from the orientation of supposed ice-marginal drainage channels and other morphological evidence. During this glaciation, the ‘Aberdeen Readvance’ was supposedly contemporaneous with another advance of Moray Firth ice that affected the northern coast as far east as Fraserburgh. Widespread glaciofluvial deposits and morainic deposits in the lower Dee Valley were attributed to the Aberdeen Readvance and another spread at Dinnet, in the Tarland Basin, was attributed to a subsequent readvance.
Bremner disagreed with H.H.Read regarding the evidence for the number of glacial episodes that had affected the northern coast. However, he acknowledged the careful observations that Read (1923) had made during the detailed geological survey of Banffshire, in particular Read’s wide knowledge of the source rocks from which the glacial erratics were derived. Read deduced that the earliest ice-movement to affect the Banffshire coast came from the west and north out of the Moray Firth, conditioned by the presence of Scandinavian ice at the mouth of the firth. Glaciolacustrine deposits of his ‘Coastal Series’ formed in lakes following the partial withdrawal of the Moray Firth ice stream. Inland ice then ‘crept’ northwards towards the coast, redistributing erratics, but depositing little recognisable till.
Work post the Second World War
Detailed, accurate stratigraphical work in the Aberdeen area was continued by Simpson (1948; 1955). He demonstrated that the moraines and meltwater channels attributed by Bremner (1938; 1943) to the Aberdeen and Dinnet readvances of his third glaciation were simply produced during periods of stagnation (still-stands) that interrupted the decay of contemporaneous coastal and inland ice masses. Simpson (1955) supported Jamieson (1865; 1906) in that he believed the dark shelly tills of the northern coast were contemporaneous with the red tills of eastern Aberdeenshire, an interpretation disputed earlier by Bremner (1943).
Charlesworth (1956) and Synge (1956) both concluded that a substantial portion of Buchan had not been glaciated during the last glaciation. This resulted in the concept of ‘Moraineless Buchan’. They supported Jamieson (1865; 1906) that ice from the Moray Firth and Strathmore had coalesced in the vicinity of Peterhead whilst there was no ice immediately inland. Charlesworth concluded that many valleys became blocked by ice at the coast causing a series of proglacial lakes to form. These lakes were supposedly interconnected by glacial spillways. The concept of ice free enclaves during the Devensian and earlier glaciations was supported by the recognition of widespread saprolites, weathered tills and periglacial phenomena (FitzPatrick, 1958, 1972; Galloway, 1961a,b,c).
Synge (1956; 1963) recognised two major glaciations in his model (P915302), with two readvance stages occurring during the decay of the final ice sheet. Ice first moved into Buchan from the E or NE, leaving Scandinavian erratics along the coast. There followed a ‘Greater Highland Glaciation’ (‘Wolstonian’ or Anglian) in which the Scandinavian ice caused Scottish ice to bifurcate over Buchan, thus preserving the unique Tertiary flint and quartzite Buchan Gravels in an enclave of minimal glacial erosion. Extensive weathering of bedrock supposedly occurred in the region during the succeeding interglacial period. Ice reoccupied the coasts during the second (Devensian) glaciation, but a substantial area of central Buchan purportedly remained ice free. The decay of the Strathmore ice lobe was accompanied by the deposition of estuarine red clays. Two short cold intervals lead to readvances of Grampian ice at Aberdeen and Dinnet, the latter being correlated incorrectly with the Younger Dryas Loch Lomond Readvance of western Scotland.
Sissons (1965, 1967) assigned Synge’s Greater Highland Glaciation to the Devensian and correlated the Aberdeen and Dinnet events with his Aberdeen-Lammermuir and Perth readvances respectively (P915303). He later abandoned the concept of the first readvance and reluctantly agreed that there was no irrefutable evidence for the latter at Perth (Paterson, 1974, Sissons, 1974b,c). Peacock et al. (1968) proposed that another ‘oscillation’ occurred in the Elgin area and that earlier in the deglaciation, ice-dammed lakes developed against the Moray Firth ice lobe along the Banffshire coast.
Work in the late1960’s to the early 1980’s
The evidence for multiple glaciations and active, phased retreat of the last ice sheet became seriously questioned during this period of research. This stemmed from the general recognition that complex depositional sequences could result from rapidly changing processes and environments within single glacial events. Most drift deposits in the region were assigned to the last glaciation (Murdoch, 1975; Maclean, 1977; Clapperton and Sugden, 1975, 1977), although doubts remained about its timing and the main directions of ice flow (Sissons, 1981).
The spatial continuity of the pattern of glacial landforms and deposits, particularly of glaciofluvial origin, led Clapperton and Sugden (1975, 1977) to conclude that the entire area of NE Scotland had been covered by the Late Devensian ice sheet (P915304). They concluded that the ice sheet was largely wet-based, and it was composed of three distinct regional ice streams with a triple junction zone located over central Buchan. The ice sheet decayed comparatively quickly leading to widespread stagnation in many lowland sites and no major readvances were recognised. Central, northern and eastern Buchan became free of ice ‘early’ in the deglaciation, allowing time for proglacial lakes to form against the more persistent coastal ice streams and for previously deposited sediments and saprolites to become modified periglacially. It was subsequently widely believed that the whole of Scotland, together with the northern and central North Sea basin, was glaciated during the Late Devensian (Sissons, 1976; Boulton et al., 1977; Price, 1983).
Work in the last two decades of the Twentieth Century
The discovery of thick glaciomarine sequences with apparently no lodgement tills beneath most of the northern North Sea basin had a profound influence on modelling work during the 1980’s for it was generally assumed that this indicated that Scandinavian ice had not crossed the North Sea during the Late Devensian glaciation (eg. Cameron et al., 1987). This led to Sutherland’s (1984) reconstruction of an independent British ice sheet of restricted size in which ice terminated at the eastern margin of the Wee Bankie Formation off the eastern Scottish coast (similar to P915305, D). Sutherland (1984) also cited evidence from the Crossbrae and King Edward sites (Table *) to argue that the northern coast to the east of Portsoy had not been glaciated during the Late Devensian. The non-glaciated enclave he proposed differed slightly from that envisaged by Synge (P915302) and enclaves of various extent subsequently appeared in several reconstructions of the north-eastern sector of the last British ice sheet, including Bowen et al. (1986), Sejrup et al. (1987), Nesje and Sejrup (1988), Lambeck (1993, 1995) (P915305). Although Sutherland’s evidence was seriously questioned by Peacock (1985), Hall and Bent (1990) and Hall (1997), it was still accepted by Bowen et al. (2001). Ehlers and Wingfield (1991) also recognised an unglaciated enclave in Buchan, but suggested Scottish ice expanded farther east than the Wee Bankie Moraine, coalescing in places with Scandinavian ice.
Modified versions of the single stage model of Clapperton and Sugden (1977) for the last glaciation were followed by BGS (Merritt, 1981; BGS, 1992), but although stratigraphical evidence confirmed that the ‘inland’, Moray Firth and Strathmore ice streams were confluent, both Hall (1983, 1984) and Hall and Connell (1991) reverted to a two stage model similar to that originally proposed by Jamieson (1906). This was because there was apparent conflicting dating evidence for the age of the ‘inland’ tills in Buchan. The whole of Scotland was ice-covered during the first phase of Hall’s 1983 model, but ice from Strathmore reached only as far as Stonehaven. Following Wilson and Hinxman (1890), Hall proposed that ice flowed down the Dee valley and Insch depression to cross the coast between Aberdeen and the Ythan Estuary, depositing basic igneous erratics at Nigg Bay (Table *). In Buchan, the carry of pebbles of the distinctive Buchan Gravels from Whitestones Hill and of erratics from the quartz-biotite norite of the Maud intrusion noted by Wilson (1886), both indicated flow towards the NE and E. ‘Inland’ ice may have been confluent with Moray Firth ice along the valley of the North Ugie Water. Ice flowed NE across Banffshire to join the Moray Firth ice stream at the coast, west of the mouth of the Deveron, as proposed by Read (1923). East Grampian ice retreated to an unknown position in the west during Hall’s second phase. Strathmore ice flowed northwards until it met the more vigorous Moray Firth ice stream, which diverted ‘inland’ ice eastwards into the lower Spey valley (Read, 1923) and pushed across northern parts of Buchan.
New information from offshore allowed Hall’s model to be refined (Hall and Bent, 1990; Hall, 1997). The maximum limit of the Late Devensian ice sheet was placed at the Bosies’ Bank Moraine (P915306), where a tidewater margin was identified some 25 km NE of Buchan. East Grampian ice had already retreated before the maximum offshore extent was attained, allowing incursion of Moray Firth ice across northern Buchan. This timing of maximum expansion was not known, but deglaciation was well advanced by 15.3 ka, when ice had retreated from the eastern Buchan coast at St.Fergus (Table *). Glaciomarine sedimentation occurred to the west of Bosies’ Bank, at the retreating ice margin (Bent, 1986).
By the 1990’s it has been demonstrated that low gradient ice sheets that crossed wet, deformable beds, especially in marine areas, were unlikely to lay down widespread lodgement till (refs). Instead, thin units of fine-grained diamict (deformation/deforming bed tills) would be produced that could be mistaken for undeformed glaciomarine deposits, especially where they were too thin to be detected on contemporary seismic records. This knowledge, together with new AMS 14C dates on individual molluscs and hand-picked foramiferids, led Sejrup et al. (1994) to conclude that the British and Scandinavian ice sheets had indeed coalesced in the northern North Sea basin during the early part of the Late Devensian, between 28 and c.22 ka BP. He considered that the central part of the northern North Sea was deglaciated by 20 ka BP. The Tampen Readvance followed in the eastern North Sea, between 18.5 and 15.1 ka BP, when a contemporaneous readvance probably affected NE Scotland, reaching the limits that Sejrup et al. (1987) had used in their earlier reconstruction of the Late Devensian maximum (P915306). Peacock (1997) argued that the glacitectonic disturbance of the St.Fergus Silts Formation at St Fergus (Table *) resulted from a subsequent readvance of the Moray Firth ice stream between 15 and 14 ka BP. Micromorphological examination of sediment thin sections from boreholes subsequently enabled Carr (1998) to demonstrate that ice had crossed the North Sea basin on three occasions during the Devensian/Weichselian. Sejrup et al. (2000) concluded that it did so during the Karmoy, Skjonghelleren and main Late Weichselian glaciations of Fennoscandia (Figure 5.16).
The glacial record in Banffshire was re-appraised by Peacock and Merritt (2000). Ice first flowed south-eastwards from the Moray Firth during the first phase of the Late Devensian glaciation, as originally deduced by Read (1923). The Moray Firth ice stream then withdrew from the coast, allowing ice from the East Grampians to creep northwards until being steered eastwards by the more powerful coastal ice-stream. The inland ice subsequently withdrew, allowing glacial lakes to form against the Moray Firth ice stream, which remained active and locally expanded southwards (Figure 5.15)