Till, glacial deposits, Quaternary, Cainozoic of north-east Scotland

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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.

Introduction

Tills are the most widely distributed of the glacial deposits. They crop out over much of the district and also occur beneath younger superficial deposits. Tills are composed mainly of diamictons, materials that are characterised by a lack of sorting (in the geological sense). They are mostly matrix supported, dense, cohesive, commonly unstratified and comprise a mixture of rock fragments, gravel, sand, silt and clay (table below). Not all tills are diamictons and not all diamictons are tills. For example, many diamictons were formed as cohesive debris flows and mudslides, not necessarily in a glacial environment: others were formed by periglacial processes involving repeated freezing and thawing. The mapped tills probably include some of these nonglacial diamictons because it is not practical to delineate them separately.

Commonly occuring types of till and related sediments
Classification Genesis Typical lithology Common attributes
Lodgement till Formed beneath actively moving glaciers as a result of frictional retardation of debris particles and debris-rich ice masses against the glacier bed (Boulton and Deynoux, 1981) Extremely stiff, very stony, sandy, clayey diamicton with matrix support. Little stratification, but commonly with platy structure. Clasts typically less well dispersed than in deformation tills Small boulders with bevelled and striated surfaces. Subhorizontal fissures becoming more pronounced upwards. Concavo- convex discontinuities and shear planes lined with silt, clay or silty fine-grained sand. Fissure fillings commonly ferruginous due to passage of groundwater
Deformation till (Deforming-bed till) Formed by the disaggregation and homogenisation of sediments and weak rocks in the subglacial 'deforming layer' (Boulton, 1987; Hart and Boulton, 1991; Benn and Evans, 1996, 1998) As above, but more variable depending on nature of parent material. Clayey sediments commonly yield unstratified silty clays with well-dispersed pebbles. Far-travelled lithologies may be sparse As above, but with deformed inclusions and laminae of sand and decomposed rock that range in size from a few centimetres to large glacial rafts many tens of metres across. Sharp, planar, basal contacts with underlying penetrative glacitectonites
Glacitectonite Subglacially sheared and deformed sediment or weathered bedrock (Benn and Evans, 1996, 1998) Materials where primary features are replaced by tectonic lamination (`penetrative' glacitectonite) Lamination parallel to planar base of overlying deformation till. Strain decreasing downwards. Gradational basal contacts with non-penetrative glacitectonites below
Materials retaining some original sedimentary structures or igneous/metamorphic fabric (`non penetrative' glacitectonite) Various extensional structures e.g. boudins, low-angle shears, conjugate microfaults, breccia- don and folding. Strain decreasing downwards
Flow-till complex Formed as cohesive debris flows at the ice margin, amongst decaying ice or paraglacially from remobilised glacial deposits (Boulton, 1968; Boulton and Paul, 1976) Friable, sandy, matrix-supported diamicton interbedded with pebbly silty sand, clast-supported diamicton, and gravel and thinly laminated silt and clay Individual beds generally less than 50 cm thick (typically 5 to 20 cm), laterally impersistent with gradational contacts. 'Fold noses' may bound individual flows

Tills consist of ice-transported material laid down sub-glacially at the base of active glaciers and ice sheets, proglacially and supraglacially at the margins of retreating ice sheets, and paraglacially soon after glaciation when sediment deposited earlier is remobilised. Deposits formed in the proglacial, supraglacial and paraglacial environments commonly occur at the surface and comprise a metre or so of heterogeneous, very poorly sorted, crudely stratified, gravelly diamicton that is commonly intercalated with gravel, silty sand, silt and clay. These sediments accumulated at ice-sheet margins, mainly as debris flows that were modified and redeposited by ephemeral meltwater streams and sheet wash. They are generally permeable and include large boulders, up to several metres in diameter. In contrast, tills formed in the subglacial environment are relatively homogeneous (massive) and impermeable. In general, it is not practical to map out the various types of till separately. Locally, where proglacial, supraglacial and paraglacial deposits are several metres or more thick and form constructional mounds, they have been mapped out as ‘hummocky glacial deposits’.

File:P915251.png
Generalised flow-lines of ice during the Main Late Devensian glaciation. P915251.

The subglacially formed diamictons include lodgement tills and deformation (deforming-bed) tills, although the two types may be difficult to distinguish, resulting as they do from a continuum of processes. There is commonly a sharp boundary between deformation till and underlying units of pervasively sheared and deformed sediment or weathered bedrock. The amount of deformation in such units, which are now commonly referred to as glacitectonites (table above), generally decreases downwards. Some sandy deformation tills are weakly stratified on a scale of a few millimetres to a few centimetres and the stratification is formed of laterally impersistent laminae and wisps of pale, very fine-grained sand and silt. The presence of these laminae may indicate that some resedimentation in running water has occurred at the ice–till interface during melt-out, in which case the deposit may be classified as a melt-out till (Boulton, 1970; Shaw, 1979). However, the stratification can also result partially, if not wholly, from subglacial shearing and associated comminution of granular material (Boulton, 1979; Boulton et al., 1974). True melt-out and ablation tills are rare because they have poor preservation potential (Paul and Eyles, 1990).

Subglacial till of the Banchory Till Formation. P266691.
Mill of Forest Till containing rounded cobbles of Devonian rocks at its type locality. P104107.

Tills vary considerably in thickness and lithology across the district where they tend to underlie poorly drained ground of low relief and smooth slopes (P266691; P104107). The tills of the district are described in terms of the five lithostratigraphical groups to which they are assigned (P915251). The groups relate to the major bodies of ice responsible for depositing the sediments. However, the groups have been recognised only on the more recently published 1:50 000 maps. Diamictons belonging to different groups interdigitate locally, especially towards the coasts. The following account describes tills that are most likely to be encountered at the surface and in shallow excavations. They are predominantly Late Devensian in age, although much of the material forming them has probably been reworked from older glacial deposits and weathered bedrock. Information on the sparse, but stratigraphically important occurrences of older till is contained within Quaternary lithostratigraphy and correlation and Important localities.

Central Grampian Drift Group

This group of deposits, which mainly lies to the west of the River Spey (P915251), includes some of the thickest tills in the district. Sequences of glacigenic deposits with individual till units up to 15 m or so in thickness are common, and similar to successions described in detail nearer to Inverness by Fletcher et al. (1996). Tills occurring at the surface to the north of a line roughly between Rothes and Portsoy are predominantly reddish brown to brown sandy lodgement tills derived from sandstones and conglomerates of the Old Red Sandstone Supergroup cropping out to the west (Peacock et al., 1968; Aitken et al., 1979). However, a significant proportion of the clasts in these diamictons are ‘Moine’ (Grampian Group) psammitic granulites and quartzites together with minor amounts of local Permo–Triassic rocks, granite from Nairnshire, quartz porphyry and Jurassic limestone derived from the bed of the Moray Firth.

East Grampian Drift Group

Grain size plots for four of the drift groups in north-east Scotland. P915258.

Tills of this group are generally less than 5 m thick. They become patchy and are rarely thicker than 2 m across central Buchan. They are generally very sandy because they incorporate much decomposed, grussified rock. Many are deformation tills derived largely from local decomposed rock, but with well-dispersed pebbles derived from farther afield. Rubbly, clast-supported diamictons up to about 1.5 m thick are common where the ice has overridden relatively fresh, quartzitic rocks. The colour, matrix and clast composition of the tills of this group, more than any other, reflect the nature of the underlying bedrock, or rocks cropping out within a kilometre or so ‘up-glacier’ (generally west or south-west). They are generally yellowish brown in colour, but reddish brown, orange-brown and olive-grey tills occur locally depending on the colour of the local bedrock. The tills are pale grey to white, kaolinitic and contain many well-rounded pebbles of quartzite, vein-quartz and flint where ice has crossed outcrops of the Buchan Gravels Formation. The tills are very gritty inland from Aberdeen, where they include much decomposed granitic and psammitic bedrock (P915258). Where medium- to coarse-grained igneous rocks have been overridden, there are numerous boulders strewn across the surface that were probably formed as ‘core-stones’ within bedrock weathering profiles before being glacially transported.

Banffshire Coast Drift Group

The tills of this group are typically clayey, bluish grey to brown in colour and contain erratics, fossils, microfossils and shell fragments derived from the Moray Firth, as well as more local rocks. They crop out extensively at the surface only between Fraserburgh and Peterhead, but they occur patchily at depth beneath tills of the Central and East Grampian Drift groups between Elgin and Aberdeen (P915251). Thicknesses of 10 m or more are common. In the Elgin area, the tills are typically dark olive-grey to greyish brown and in addition to psammitic granulites and quartzites, include clasts of calcareous sandstone, glauconitic sandstone, black shale and shelly limestone from offshore (Peacock et al., 1968; Aitken et al., 1979). Fossils of Rhaetic to Early Cretaceous age are common. These dark tills of the Elgin area are almost certainly equivalent to diamictons within the Whitehills Glacigenic Formation, which has been recognised from Cullen eastwards. Clasts of Jurassic mudstones and shales are common in these latter deposits, whereas the matrix has been formed partly from comminuted Mesozoic shales and partly from reworked Quaternary marine deposits from which the comminuted shell fragments have been derived. Fragments of Lower Cretaceous rocks and fossils are common in tills that outcrop farther east, towards Fraserburgh, where they include sparse clasts of chalk.

Tectonic lamination in a raft of sand (possibly glaciomarine) disrupted by lowangle shears at Ardglassie. P104123.
Attenuated, fractured and sheared lenses of sand within the Whitehills Glacigenic Formation at the Boyne Limestone Quarry. P104124.

The diamictons of the Whitehills Glacigenic Formation are mostly deformation tills and glacitectonites. They commonly include lenses (boudins) and partially disaggregated masses of a range of Permo–Triassic, Jurassic and Cretaceous lithologies and Quaternary sediments (P104123; P104124). They include dark grey to black mudstones, shales, clays and lignite, and yellowish brown fine-grained sands. Lenses of white, friable, kaolinitic sandstone are found sporadically in the west of the district, whereas vivid reddish brown and orange marls occur to the east of Troup Head. The lenses of all the aforementioned rock-types range in size from a few centimetres up to large glacial rafts several hundreds of metres across. The latter have been confused with bedrock in some site investigations. Contact relationships between the rafts and enclosing diamictons are locally highly complex and in places confusing owing to considerable glacitectonic disturbance (Boyne Limestone Quarry and Gardenstown).

Logie-Buchan Drift Group

Distribution of the Logie-Buchan Drift Group and related features. P915297.

This group underlies the coastal lowlands to the north of Aberdeen, east of Ellon and south of Peterhead (P915297). It comprises a complex sequence of relatively poorly consolidated, thinly interbedded, vivid reddish brown, clayey, calcareous diamictons, waxy clays, muds, sands and gravels. A few metres of stiff, stony lodgement till commonly occur at the base of the sequence, locally resting on yellowish brown lodgement tills of the East Grampians Drift Group. Otherwise the sequence is composed largely of laterally impersistent, and mainly cohesive, debris flow deposits (flow tills) that collectively may reach over 25 m in thickness, especially over hollows in the bedrock surface (Merritt, 1981; Munro, 1986) and close to contemporary ice sheet margins. The beds and lenses of sand within the sequence are typically fine to medium grained, silty and micaceous. Shell fragments are common. In addition to locally occurring rock-types such as amphibolite, psammite, quartzite and metagreywacke, the diamictons include rocks derived from the sea bed to the east, including dolomite, limestone, calcareous siltstone, both white and red friable sandstones and red-stained, rounded quartzite pebbles from Old Red Sandstone conglomerates to the south. Palynological analysis of the diamicton matrix has revealed complex assemblages, including Permo–Triassic pollen and Early Pleistocene dinoflagellate cysts (information from R Harland, 1980).

Mearns Drift Group

Tills of this group are invariably reddish brown in colour and contain a significant proportion of clasts derived from andesitic volcanic rocks, red sandstones, siltstones and mudstones of the Old Red Sandstone Supergroup cropping out in Strathmore. Unlike the Logie-Buchan Drift Group described above, the matrices are only weakly calcareous. Characteristically, they also contain rounded quartzite boulders, derived from Old Red Sandstone conglomerates. These clasts of sedimentary and volcanic rocks predominate in tills developed within the outcrop of the Old Red Sandstone, but they become less numerous where the tills extend onto the outcrop of adjacent Dalradian and Highland Border Complex rocks. These latter diamictons, which commonly abut and, in places, interdigitate with glacigenic sediments of the East Grampian Drift Group, contain clasts derived from local Caledonian igneous and Dalradian metamorphic rocks in addition to Old Red Sandstone material.

The tills are generally less than 5 m thick and are typically clayey, silty, pebbly diamictons that are matrix supported. Extremely overconsolidated clayey and silty lodgement tills are developed abundantly at the base of thick glacigenic sequences. More friable, sandy diamictons, generally less than 2 m thick, occur as flow tills capping deltaic spreads of glaciofluvial sand and gravel. Both of these types of till resemble diamictons within the Logie-Buchan Drift Group, but the presence of offshore-derived shell fragments and exotic clasts of dolomite, limestone and calcareous siltstone, which are only present in these deposits, serve to distinguish them from diamictons within the Mearns Drift Group.

References

Full reference list