Site 7 Kirkhill and Leys quarries

The Middle and Late Pleistocene deposits preserved in and around Kirkhill [NK 011 528] and Leys [NK 005 525] quarries (Map 6), are unique and represent the most complete sequence of this age known on land in Scotland. The deposits ((Figure A1.11); (Table A1.6)) apparently record evidence of events spanning three 100 ka interglacial/glacial climatic cycles. A wide variety of sediments deposited under glacial, glaciofluvial, periglacial and fluvial regimes, are present, together with two buried soils of interglacial and interstadial aspect (Connell et al., 1982; Hall, 1984a; Connell and Hall, 1987; Hall and Connell, 1991; Hall and Jarvis, 1993a).

Kirkhill Quarry (Plate 22), alas now completely filled by landfill waste, exploited an east–west-orientated felsite dyke that cores a drumlinised ridge of similar orientation. Leys Quarry is situated in a felsitic sand and gravel deposit that lies within an ice-scoured depression to the west of the ridge, carved out of weathered basic igneous rock.

Lithostratigraphy

The lithostratigraphy described below generally follows that set up in the papers cited above. In order to adhere more closely to internationally agreed guidelines on lithostratigraphical nomenclature, new names have been proposed by Sutherland (1999). The latter have been adopted where appropriate into the present scheme presented in (Table 7) and (Table A1.6).

Unit 1 - Leys Till FormationTill Formation

Pits opened in the floor of Leys Quarry revealed up to 2.5 m of till resting on weathered bedrock. The till is a matrix-supported silty clayey sandy diamicton with pebbles of mainly quartzite and basic igneous rocks. Matrix colour is dominantly olive brown (2.5Y 4/4), but in its upper part the diamicton shows extensive yellowish brown (10YR 5/6) to strong brown (7.5YR 4/6) mottling and contains many grussified basic igneous clasts. It passes down into grussified basic igneous and kaolinised psammite bedrock and contains reworked masses of this material, demonstrating that weathering of the bedrock predates deposition of the till (Hall et al., 1989).

The Leys Till contains many locally derived clasts, but the dominance of quartzitic material indicates derivation from the west. It is the oldest known till in Buchan and has no known correlative deposits.

Unit 2 - Denend Gravel Formation (Leys Lower Gravels)

The overlying Denend Gravel Formation comprises 3 to 5 m of coarse felsite-dominated pebble to boulder gravel, with interstratified sand units. Clast shapes are dominantly subrounded to rounded. Clast lithologies comprise over 90 per cent felsite, mainly of very local origin, together with intermediate igneous rocks, gneiss, vein quartz and schist. The main primary structure in the gravels is crude parallel stratification. In contrast, large scale deltaic planar cross-stratified gravels with thick fining-upwards foresets dipping at 20° to 25° towards the west-south-west occur in the north-western corner of Leys Quarry. Massive, matrix-supported units of coarse gravel are also locally developed. Orientation of both planar cross-stratified gravels and trough cross-stratified sands indicates deposition by water moving towards the west and south-west.

Postdepositional disturbance of the gravels is widespread. Bedded sands and gravels show low amplitude flexures and higher amplitude folds, and minor faults are widespread within sand units. The folded gravel beds are further disrupted by large wedge structures with upper widths that may exceed 10 m. The wedges are infilled with sand and gravel of varied character ranging from downwarped beds of stratified gravel to unstratified, yet crudely sorted, granule to boulder gravel (Hall and Connell, 1986, fig.4). Truncation of anticlinal fold crests and the thickness of gravel beds preserved by downwarping and collapse in the wedge structure indicate that at least several metres of gravel were eroded prior to deposition of flat-bedded sands and gravels, probably part of the Pitscow Sand and Gravel Formation (see below). Locally, these gravels have more conspicuous angular clasts of felsite and appear less disturbed.

Postdepositional folding, faulting and development of wedge structures are interpreted as the result of melt-out of ice blocks originally buried within the gravels. The coarse, but variable calibre of the gravels indicates high, fluctuating palaeoflow regimes. Sedimentary structures indicate a braided channel environment, with meltwaters draining to the north-west, with subsidiary deposition within a proglacial lake. The gravels probably formed as part of a valley sandur extending from an ice margin lying relatively close to, and east of, Leys. Importantly, the easterly source of meltwater suggests that the ice mass was not that which deposited the underlying westerly derived Leys Till FormationTill Formation.

Unit 3 - Kirkton Gelifluctate Bed (Kirkhill Gelifluctate 1)

In the south-east face at Kirkhill, Unit 4 is underlain by and interbedded with angular felsite rubble up to 2 m thick derived from adjacent intensively frost-shattered bedrock in channel walls. Clasts in the rubble show a well developed downslope fabric, indicating transport by avalanche or gelifluction. Silt cappings indicate incipient pedogenesis (Connell and Romans, 1984, p.76).

Unit 4 - Pitscow Sand And Gravel Formation (Kirkhill Lower Sands and Gravels)

This unit comprises up to 4 m of light olive brown sand with thin gravel beds. Most of the sands are horizontally stratified, but trough and planar cross-stratification occurs locally. In places, the sands seem to have been deposited in a series of stacked, wide and shallow channels, with thin gravel lags evident at their bases. Cross-stratification suggests transport from the east.

This unit is interpreted as a periglacial fluvial or proglacial glaciofluvial deposit. Deposition of the basal beds took place under periglacial conditions judging from the occurrence of syndepositional ice-wedge casts. The declining numbers of angular clasts in the upper beds at Kirkhill, together with the absence of ice-wedge casts and other features of cryoturbation at these levels may indicate amelioration of climatic conditions.

Unit 5 - Kirkhill Palaeosol Bed (Kirkhill Lower Buried Soil)

The Kirkhill Palaeosol Bed is developed on the eroded upper surface of the sands and gravels of Unit 4 (Connell and Romans, 1984, p.70). The upper 19 cm of the profile generally consists of light grey (10YR 7/1) sand and sparse gravel. Felsite clasts are bleached and softened and the sand fraction is depleted in less resistant heavy minerals (Connell et al., 1982). This conspicuous bleached horizon occurred throughout Kirkhill Quarry, and in the north-east face of Leys Quarry. The base of the grey horizon is marked by a sharp, undulating junction with a lower, greyish brown (10YR 5/2) mottled horizon up to 7 cm thick. This horizon is locally enriched in organic carbon and free iron as a result of translocation of these materials in the soil. Analysis of this 'Bs' horizon has indicated the presence of protoimogolite/allophane. This mineral is also found in podzols formed during the Holocene in north-east Scotland (information from M J Wilson, Macaulay Institute of Soil Research, Aberdeen, 1994) and suggests that the palaeosol was also formed under humid temperate climatic conditions. An iron pan is found towards the base of the profile (Connell et al., 1982).

At both Kirkhill and Leys, bleaching and iron/manganese staining associated with localised cementation occur extensively in the gravels and sands of Units 2 and 4. The Kirkhill Palaeosol seems therefore to be part of a weathering profile that in places extends throughout the entire thickness (about 4 m) of gravels.

Micromorphological analysis of the palaeosol has also revealed structures typical of soils of arctic environments (Connell and Romans, 1984, p.70). Superimposition of periglacial soil characteristics on an earlier interglacial soil horizon seems to have produced a composite soil horizon. The stratigraphical position of the palaeosol indicates that it developed during a pre-Ipswichian interglacial, but not necessarily the Hoxnian (OIS 11) as originally suggested (Connell et al., 1982; Hall and Connell, 1991).

Unit 6 - Swineden Sand Bed (Kirkhill Organic Muds and Sands)

At Kirkhill, the truncated Kirkhill Palaeosol Bed is overlain by 0.1 to 0.7 m of poorly stratified and weakly organic sands. These sands probably represent slope-wash deposits and are penetrated by a series of frost-cracks (Connell 1984b, p.63). Beneath the sands lies a 1 to 5 cm band of black to brown weakly laminated organic mud that drapes the truncated palaeosol in the south-west face.

The organic mud contains pollen of Poaceae, together with a marked arboreal component of mainly Pinus and Alnus, together with charcoal. The overlying sands show a reduction in arboreal pollen and an increase in grasses and Calluna, possibly reflecting the establishment of an open, treeless environment (Connell et al., 1982). Sampling of an equivalent organic sequence in the west face at Kirkhill suggests that two components exist in the pollen spectra. Open, grassland types represent pollen contemporaneous with sedimentation of the sands. Recycled pollen, with an important arboreal component, is perhaps derived from older soil horizons at the site (information from J J Lowe, Royal Holloway University of London, 1990).

Initial radiocarbon dating gave finite ages for three samples, but contamination was suspected (Connell et al., 1982). Later dating of a much larger sample gave an age of over 47 360 radiocarbon years BP (SRR–2416) and confirmed that the sediments are beyond the range of radiocarbon dating (Hall, 1984).

Unit 7 - Camphill Gelifluctate Bed (Kirkhill Gelifluctate 2/3)

At Kirkhill Quarry, up to 2.5 m of periglacial mass movement deposits are developed above the organic Swineden Sand Bed. In Leys Quarry, a series of ice-wedge casts, spaced 6 to 9 m apart, penetrate downwards from the upper surface of this unit. The structures are filled with sand from the same unit, and not gravel from Unit 8 above. Examination of the upper surface exposed during quarrying showed that the casts form part of a fossil polygonal network. The deposit is cryoturbated with erect pebbles in its upper 0.5 to 0.9 m. The basal part of the gelifluctate is undisturbed and shows subhorizontal bedding. The evidence indicates multiple phases of periglacial climate. A luminescence date of 142 ± 19 ka BP (Duller et al., 1995) has been obtained from an exposure of this unit in the north-east face of Leys Quarry. This date, if correct, falls within OIS 6 and constrains the ages of the overlying deposits.

Unit 8 - West Leys Sand And Gravel Formation (Leys Upper Sands and Gravels)

The West Leys Sand and Gravel Formation rests in shallow channels (less than 1 m deep) scoured into Unit 7 and are only seen in the north-east corner of Leys Quarry. The sandy pebble gravel is very pale brown to pale brown (10YR 7/3-6/3) in colour and is dominated by quartzite and psammite. Crude horizontal bedding of clast-supported gravels is common, with local development of planar cross-beds indicating water flow from the north-west. These thin, probably glaciofluvial gravels and sands are separated by a sharp subhorizontal contact from the overlying unit. They represent outwash or subglacial meltwater deposits related to the emplacement of the overlying till.

Unit 9 - Rottenhill Till FormationTill Formation (Kirkhill Lower Till)

This is a yellowish brown (10YR 5/4) matrix-supported, silty sandy diamicton. Clasts include mainly pebbles and cobbles of quartzitic metasedimentary rock, pelitic schist and felsite. Clasts of red granite are common at Kirkhill and are probably derived from the White Cow Wood area to the west. Present also are red-stained quartzite cobbles, probably derived from Devonian conglomerates, and grey granite from the Strichen intrusion to the north-west. Clay mineralogy is dominated by kaolinite and illite. Normally, the till is massive, with poorly defined subhorizontal jointing, and a weak north-west–south-east fabric (Connell, 1984b, p.64). Both clast lithology and fabric indicate that the till was deposited by ice moving from the north-west or west.

Unit 10 - Fernieslack Palaeosol Bed (Kirkhill Upper Buried Soil)

Evidence of soil development on the surface of the underlying Rottenhill Till FormationTill Formation includes:

• in situ grussification of basic igneous and granite clasts
• horizon development, with formation of strong brown (7.5YR 5/6) colours in the upper 70 cm of the profile and passing down into yellowish brown(10YR 5/4) hues in less weathered material
• development of coarse yellowish red (5YR 4/6–5/8) and greyish brown to brown (10YR 5/5–5/3) mottling owing to iron oxide segregation
• clay mineralogy, with alteration of illite to interstratified mica-smectite in the preserved upper 0.3 m of the profile
• soil micromorphology, with the presence of disrupted void argillans in thin section (Connell and Romans, 1984, p.75)

The palaeosol is truncated, but resembles the B and C-horizons of gleyed brown earths developed on similar parent materials in eastern Scotland during the Holocene (Connell and Romans, 1984, p.76). Support for this interpretation is provided by the presence of Alnus, with Betula and coryloid grains from a single sample in the profile (Connell et al., 1982). The palaeosol appears to be quite widely preserved south and west of Kirkhill as a mottled zone in the upper part of the till. On the simplest interpretation and in the light of the luminescence date from Unit 7, it was probably formed during the Ipswichian Interglacial (OIS 5e).

Unit 11 - Corsend Gelifluctate Bed (Kirkhill Gelifluctate 4)

A range of periglacial deposits and features formed in the interval prior to deposition of Unit 12 and after formation of the weathering profile within Unit 10. In stratigraphical order from oldest to youngest these are:

1. Truncation and cryoturbation of the Fernieslack Palaeosol Bed, with erection of pebbles in the uppermost 0.6 m.
2. Deposition of a thin (0.6–0.8 m) brownish yellow (10YR 5/3) silty gelifluctate unit with angular felsite clasts (Corsend Gelifluctate Bed).
3. Incipient pedogenesis under periglacial conditions, with development of silt droplet fabrics in the gelifluctate.
4. Ice-wedge growth. The presence of an ice-wedge cast extending downwards from the surface of the gelifluctate and filled with matrix material from Unit 13 indicates that glaciation of the site began while ground ice remained in the wedge (Connell, 1984c, p.68).

Unit 12 - Corse Diamicton Formation

This unit has been observed to overlie both sediments of Unit 11 and Unit 9. In turn, it is known at both Kirkhill and Leys Quarries to be either overlain by the Hythie Till FormationTill Formation (Unit 13) or to be reworked into the base of that till. The Corse Diamicton was formerly regarded as a component of the East Leys Till, but these units, while of broadly similar lithology, appear to lie below and above the Hythie Till respectively.

The Corse Diamicton Formation was first exposed at Kirkhill as a large mass of dark grey clay consisting largely of reworked Mesozoic mudstone. This black (10YR 3/1) to dark grey (5Y 4/1) silty clay contained dinoflagellate cyst assemblages of Late Jurassic/Early Cretaceous age (Connell, 1984c, p.69) and had a smectite-dominated clay mineralogy. The unit contained sparse small, rounded pebbles of quartzite or psammite. At Leys, the unit included deformed bodies of pale pinkish yellow fine-grained sand, very similar to the glacial rafts at Oldmill Quarry (see below).

Unit 13 - Hythie Till FormationTill Formation (Kirkhill Upper Till)

This unit is a mainly brown (10YR 4/3) massive, sandy silty matrix-supported diamicton, up to 3 m thick. Clast lithology is dominated by psammite and pelitic schist, with basic igneous rocks and felsite. A single clast of rhomb porphyry typical of those found in Norway was recovered from this unit. A characteristic feature of the till at Kirkhill is the presence of boulders of basic igneous rock that were probably originally formed as corestones. Clay mineralogy there is dominated by an interstratified smectite/chlorite mineral. At Leys, the till contains rare clasts and thin lenses of red clay (Hall and Connell, 1986, p.23).

The dominance of quartzitic and basic igneous rocks in the Hythie Till FormationTill Formation indicates a western provenance. The unit can be correlated on the basis of stratigraphy and lithology with tills along the valleys of the North and South Ugie Water, which also have strong west–east fabrics (Hall and Connell, 1991).

Unit 14 - East Leys Till FormationTill Formation

This till is a very dark grey (2.5YR N/3) massive clayey silty diamicton with scattered matrix-supported pebbles and cobbles. Clast lithologies are dominated by dark grey-stained gneissose quartzite and psammite, with chalk, pelites and reddish brown (possibly Devonian) sandstone. The till is weathered in its upper 2.5 m, with grussified basic igneous, softened chalk clasts, the absence of shell fragments and development of reddish brown (2.5YR 4/4) to dark greyish brown (10YR 4/2) hues. The clay mineralogy of fresh diamicton comprises more or less equal proportions of interstratified mica smectite, illite and kaolinite.

Striated shell fragments appear below a depth of 2.5 m and include specimens of Macoma sp. and Astarte sp. The matrix contains reworked Late Jurassic/Early Cretaceous dinoflagellate cysts, Mesozoic and Tertiary pollen, and Quaternary foraminifera (information from R Harland, BGS, 1980 and L A Riley, Bovingdon, Herts, 1997). Together these organic remains indicate derivation of the matrix from Mesozoic mudstone, early Tertiary sediments and Quaternary glaciomarine and marine muds in the Moray Firth basin. Southward transport by ice from the Moray Firth is supported by the presence of chalk clasts and by the transport of dark gneissose rocks comparable to the Inzie Head Gneiss of the Fraserburgh area.

Unit 15 - Kirkhill Church Sand Formation (Kirkhill Upper Sands)

These sparsely developed sands and subordinate gravels overlay the Corse Diamicton Formation in the north face of Kirkhill Quarry, where they appear to fill a shallow channel. Cross-bedding indicates transport towards the west. The heavy mineralogy of the sand fraction resembles that of the Corse Diamicton in that it contains significant amounts of garnet together with apatite.

As no diamicton similar to the Hythie Till FormationTill Formation was identified in exposures along the north face of Kirkhill Quarry, it is possible that a hiatus occurs at this point in the sequence. Alternatively, the channel within which the sands sit may have been locally eroded into the Hythie Till as well as the Corse Diamicton (as accepted here). If this was the case, the sands relate to the subsequent advance of ice derived from the Moray Firth, which deposited the East Leys Till. It is likely that outwash delivered through the channel contributed the sediment that was formerly visible at Sandhole [NJ 998 521], which formed as a delta built in Lake Ugie during the Main Late Devensian Glaciation (Connell, 1984d)(see below).

Unit 16 - Manse Gelifluctate Bed (Kirkhill Gelifluctate 5)

This unit overlies the Hythie Till and includes felsite rubble, crudely bedded and gently dipping sand, and pebbly diamicton. At Leys, the unit displays involutions with stone pillars, near-surface concentrations of frost-heaved clasts, erected and frost-cracked pebbles and associated development of ice-wedge casts (Hall and Connell, 1986, p.24). Ice-wedge casts have also been noted within sands of the fan delta at Sandhole (Connell, 1984d, p.85).

Discussion

The Pleistocene sequence in the Kirkhill area comprises a record of three complex stadial–interstadial–interglacial cycles. In the oldest cycle, the till of Unit 1 is succeeded by the outwash gravels of Unit 2, then by the periglacial deposits of Unit 3 and at the base of Unit 4 and, lastly, by a period of interglacial conditions during which the Kirkhill Palaeosol developed. A second cycle begins with the overlying organic sands and muds of Unit 6, which record a climatic deterioration, which then deepens to provide the periglacial slope deposits of Unit 7 and culminates in glaciation of the site, with deposition of Unit 9, the Rottenhill Till. Subsequently, the Fernieslack Palaeosol Bed developed on the till and its pedogenic characteristics imply a return to interglacial conditions.

The youngest cycle commences with the varied periglacial deposits of Unit 11. This phase of climatic decline terminates with the deposition of the Hythie Till FormationTill Formation on to permafrost. The flanks of the drumlinised ridge at Kirkhill are underlain by this till (Unit 13), suggesting that it was laid down during the final shaping of the feature. An important distinction is made here between the Corse Diamicton and the East Leys Till, both dark grey, mud-rich diamictons derived from the Moray Firth, but separated by the Hythie Till. This situation resembles that at Oldmill (see below) where a raft of glaciomarine mud derived from the Moray Firth is overlain by a thin till of inland derivation (Hall, 1984a). Subsequent deglaciation involved minor glaciofluvial deposition and was accompanied, or succeeded by, a phase, or phases of periglacial activity until soil formation began at the start of the present interglacial.

Chronostratigraphical information is provided by the luminescence date of 142 ± 19 ka BP (Duller et al., 1995) from sandy gelifluctate in Unit 7, which together with the overlying Rottenhill Till date to OIS 6. If this is correct, then the Fernieslack Palaeosol Bed is likely to be of OIS 5e age. The Corse Diamicton probably correlates with the Whitehills Glacigenic Formation of late Middle or, more likely, early Late Devensian age. The age of the Kirkhill Palaeosol Bed and underlying glacial and periglacial deposits is uncertain. The simplest model would have them of OIS 7 and OIS 8 ages respectively, but they may be older.

(Table A1.6) Lithostratigraphical units at Kirkhill and Leys Quarries.

References