OR/15/026 Cenozoic superficial deposits

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Gunn, A G, Mendum, J R and Thomas, C W. 2015. Geology of the Huntly and Turriff Districts. Sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland). British Geological Survey Internal Report, OR/15/026.

Palaeogene/Neogene

Palaeogene/Neogene gravels and deep weathering

Within the Huntly and Turriff districts are occurrences of unconsolidated deposits that appear to predate the Pleistocene glacial and glaciofluvial deposits. Their exact age is uncertain but they are considered to be largely of Palaeogene and Neogene age. They include the Buchan Gravels Formation and spreads of intensely and deeply weathered bedrock (saprolite) that can be divided into two distinct types: clayey gruss and sandy gruss (Hall, 1986[1]). Clayey gruss resulted from deep weathering under humid, warm climatic conditions, probably during the Miocene. Comparison of the saprolite mineralogy with that of North Sea sediments suggests that highly kaolinitic weathering mantles formed prior to the Pliocene in north-east Scotland (Hall et al., 1985[2]). The sandy gruss resulted from less intense weathering under humid temperate conditions and is considered to be Pliocene to Early Pleistocene in age. The deeply weathered rocks are best developed in the south-east part of the Turriff district on the higher metamorphic grade Strichen Formation and on parts of the Insch Pluton and related mafic intrusions.

Buchan Gravels Formation

The highly distinctive quartzite and quartz-rich gravels of the Buchan Gravels Formation are known from three localities in the Fyvie and Turriff areas. Of these, the outcrop of the Windy Hills Gravel Member at Windy Hills [NJ 795 395], 3 km north-east of Fyvie, is the largest, extending over about 2 km2. The others, at Wood of Delgaty [NJ 745 508], 2 km north-east of Turriff, and Dalgatty Forest [NJ 737 459], 4 km south-south-east of Turriff, cover a combined area of less than 0.5 km2. The deposits at Windyhills have been worked piecemeal as a resource for aggregate, generating considerable exposures, whereas at the smaller localities exposure is very poor. Hence, much of what is known about the formation comes from the Windy Hills outcrop, for which there are several detailed accounts (Flett and Read, 1921[3]; Read, 1923[4]; Kesel and Gemmell, 1981[5]; McMillan and Merritt, 1980[6]; Gordon and Sutherland, 1993[7]). A detailed account of the geology of the Windy Hills Gravel Member is given in Merritt et al. (2003)[8].

At Windyhills, some 14 m of dominantly quartzite and quartz gravel is interbedded with white quartz sand and micaceous silt overlying kaolinised pelites (Hall et al., 1989[9]; Kesel and Gemmell, 1981[5]; Koppi, 1977[10]). The quartzite and quartz pebbles are highly rounded, relatively fresh, and some show chatter marks, whereas the sparse granite and psammite clasts are generally decomposed to kaolinitic sand. A few flint pebbles are also present, some of which have been reported to contain Late Cretaceous fossils typical of the Chalk (Christie, 1831[11]; Jamieson, 1865[12]), although subsequent work has failed to confirm the presence of these fossils. Very rare chert clasts, including some of possible Early Cretaceous age, have also been reported (Flett and Read, 1921[3]; Hall, 1987[13]).

Considerable topographical change has taken place since the deposition of the gravels. Inversion of relief means that the gravels now occur on benches at elevations of around 110 to 125 m above OD, high above the current valley floors of the lower Deveron, the Idoch meltwater channel, and the middle part of the Ythan valley.

Clast imbrication, rare cross-bedding and channel forms outlined by ground penetrating radar indicate deposition of the gravels by water moving approximately north-eastwards approximately parallel to the current course of the nearby River Ythan (Kesel and Gemmell, 1981[5]; Merritt et al., 2003[8]; McMillan and Merritt, 1980[6]). Although Flett and Read (1921)[3] argued for a marine origin, the evidence indicates that the Windy Hills Gravel Member is fluviatile in origin, deposited in a pre-Quaternary river course. A glaciofluvial origin has also been proposed, consistent with the presence of quartz grains with surface textures suggestive of a phase of glacial transport and with the presence of striated metamorphic clasts (Hall, 1983[14]; Kesel and Gemmell, 1981[5]).

This may be apply to the quartzite gravels at the south-west end of the outcrop, which have been modified during the Pleistocene by glacial erosion, local deposition of overlying till, and by periglacial disturbance.

Paleogene/Neogene deep weathering deposits

Deeply weathered bedrock and other deposits generated by weathering are widespread but not abundant in the Huntly and Turriff districts, except locally in the south-east part of the Turriff district. The deposits appear to predate the Pleistocene, and show features compatible with their formation under humid tropical and then temperate conditions during the Neogene. This period of intense weathering coincided with the uplift in north-east Scotland, which began in the early Palaeogene and continued into the Early Pleistocene.

Deep weathering affects almost all rock types to varying depths (Fitzpatrick, 1963[15]). The distribution of weathered bedrock is governed mainly by rock type, structure, topography and the variable intensity of glacial erosion. The mafic-ultramafic igneous rocks of the Insch Pluton and the Aberchirder biotite granite are widely and deeply weathered. In contrast, Southern Highland Group rocks and Devonian sandstones and conglomerates are less significantly affected (Hall, 1986[16]), although geophysical surveys suggest that in parts of the Turriff outlier alteration extends down to 10 m (Ashcroft, personal communication to A Hall, 1983). Beds of quartzo-feldspathic psammite within the Dalradian quartzites are frequently kaolinised.

Rocks in shear zones in the low relief depression at the eastern end of the Insch Pluton are weathered to depths of about 50 m (Leslie, 1984[17]). Similar depths of weathering may also occur along the northern margin of the Insch depression and beneath the floor of the Drumblade depression (Ashworth, 1975[18]). Boreholes in the Knock Pluton east of Ruthven also reveal 20 to 30 m of weathered basic igneous rock, although weathering depths are highly variable (Hall, 1983[14], fig. 12.6.iv).

‘Scarp-foot’ zones of enhanced weathering occur along the northern margin of the Insch depression at the foot of the Slate Hills (Hall, 1986[19], fig. 5). Glacial erosion was variably effective but did result in the stripping of weathered rock in some areas. Ice-moulding generally increases eastward along the Insch depression and the distribution of weathered rocks becomes more restricted, although numerous deep pockets and zones still remain. Similarly, weathered rock profiles appear to be less commonly preserved north of Huntly in the area affected by the relatively vigorous eastward flowing Moray Firth ice stream. Outcrops of fresh rock are also more abundant in this area in addition to the fewer occurrences of weathered rock.

Very intense weathering during the Miocene produced clayey gruss that is dominated by kaolinite and illite and small amounts of hematite. Examples of this degree of weathering are known only from the pelitic rocks beneath the Buchan Gravels and silicate clasts within the gravels themselves (Hall et al., 1989[9]; Koppi, 1977[10]). Koppi (1977) also described a highly weathered biotite-bearing feldspathic psammite from Clashindarroch Forest at [435 305], just within the Glenfiddich district.

Within the Huntly and Turriff districts, most saprolites are sandy grusses with low fines, considered to have formed under the humid temperate conditions in the Pliocene and during warmer periods of the Pleistocene (Hall et al., 1985[2]). The clay mineralogy of grusses is closely controlled by bedrock composition. Granitic saprolite clay mineral assemblages are dominated by kaolinite and white mica (Hall et al., 1989[9]), whilst basic igneous saprolites contain a wide range of clay minerals. Plagioclase feldspar and hornblende in the Insch Pluton are largely unaffected by weathering. Pyroxene alters first to iron oxides and then vermiculite, whilst biotite weathers to hydrobiotite and vermiculite and, locally, kaolinite and gibbsite (Basham, 1974[20]). Kaolinite and mica clays predominate in weathered metamorphic quartzo-feldspathic rocks, but the presence of ferro-magnesian minerals results in an increased smectite content (Hall et al., 1989[9]).

Deep weathering profiles are highly variable (Hall, 1983[14]). Weathering of igneous rocks normally leaves a residual coarse grit, with or without core stones. Weathering of metamorphosed pelitic and psammitic rocks results initially in deep blocky disintegration with the progressive development of fines as weathering becomes more intense. Borehole intersections of the weathering profile and the bedrock show that typically there is a gradational contact. However, in rare cases a sharp contact is observed (Hall, 1986[19]).

Quaternary

Figure 3 Distribution of the glacigenic drift groups in north-east Scotland showing their relationships. Arrows show the directions of movement of the different ice streams (after Merritt et al., 2003[8]).

Quaternary superficial deposits are dominated by those formed as a result of main Late Devensian glaciation between about 26 000 and 13 000 years BP (before present) (Merritt et al., 2003[8]). Note that most of the ages quoted in the Quaternary section relate to calibrated years BP (taken as 1950). Uncalibrated radiocarbon (14C) ages are also quoted here, but they have not been adjusted and recalibrated. Radiocarbon years can be converted graphically to calibrated years (BP), but the adjustments can be complex and may result in a spurious accuracy. Where ages lie at or above the limit of 14C dating (c. 50 000 years BP), as in the two instances cited below, no advantage is obtained by their conversion. Regional setting and timing

The Huntly and Turriff districts lie in a region which underwent complex glaciation and deglaciation during the Quaternary, dominated by ice sheet build-up, movements and subsequent decay in the Late Devensian between about 26 000 and 10 000 years BP (Sutherland and Gordon, 1993[21]; Merritt et al., 2003[8]). Interaction between separate Late Devensian ice sheets, one cold-based and sourced in the Cairngorms, the other wet-based and occupying the Moray Firth, occurred to the north and east of Turriff, with the Moray Firth ice stream transporting and depositing large rafts of Pleistocene marine sediments and Jurassic clays on the mainland (Figure 3). A cold, dry interstadial possibly occurred between about 21 000 and 18 000 years BP as has been documented in the Fennoscandian ice sheet (Sejrup et al., 2000[22]). Amelioration of the climate during the Windermere Interstadial (13 000 to 11 000 years BP) was interrupted by the Loch Lomond Stadial about 11 000 years BP. This period of intense cold lasted about 1000 years until the start of the Holocene (10 000 years BP). Permafrost conditions were re-established, leading to the formation of ice wedges, periglacial solifluction deposits and frost shattering.

Pre-late Devensian deposits

Evidence of interstadial deposits that pre-date the Late Devensian glaciation is rare in Scotland, but in 1980 a drainage contractor discovered organic deposits beneath the till in a drainage ditch by Crossbrae Farm at [NJ 753 512]. The site was first described in Hall (1984)[23] but further excavations in 1992 (10 pits in total) resulted in more extensive studies (Whittington et al., 1998[24]). The pits reveal topsoil over a brown and red diamicton up to 2.5 m thick, which in the more north-easterly pits is underlain 0.4 to 1.2 m of pale brown-grey, crudely bedded sandy quartz and quartzite gravel. In two of these pits the gravel overlies a unit up to a metre thick, consisting of pale grey, disturbed, laminated medium-grained sand lying above organic sand material and sandy peat. A thin, lenticular, sandy peat bed forms the base of the unit and lies on weathered sandstone bedrock. This lower organic unit has a maximum thickness of 55 cm and is termed the Crossbrae Farm Peat Bed (Merritt et al., 2003[8]). Pollen analysis has revealed that dwarf shrub tundra vegetation with Betula nana and Salix herbacea was formerly abundant. The spike heath, Bruckenthalia spiculifolia (Ericaceae), found presently in the Balkans, has also been identified. Forty coleoptera taxa including Olophnum boreale, Acidota quadrata and Boreaplius hinningianus have been reported from the bed (Whittington et al., 1998)[24]. At the present day none of the beetles are found in the British Isles but all are found in northern Fennoscandia. Based on the overlap of the climatic envelopes of 23 coleoptera species the mean temperatures of the warmest and coldest months are estimated at 10°C ± 1°C and -9°C ± 3°C respectively. Two samples of the peat were subject to radiocarbon age determination and gave uncalibrated ‘humic carbon’ ages of 44 030 +910/-820 14C yr BP and 47 180 +1390/-1190 14C yr BP, and ‘humin carbon’ ages of >53 630 14C yr BP and >61 900 14C yr BP, respectively (Whittington et al., 1998)[24]. These ages are interpreted as minima for the Crossbrae Farm Peat Bed with possible contamination by younger carbon in groundwater. Correlating the Crossbrae site with other interstadial peat sites at Camp Fauld, Burn of Benholm, Sel Ayre, Fugla Ness and Allt Odhar suggest that the peat formation may date from the Early Devensian Brørup or Chelford Interstadial, equivalent to OIS 5c (95 to 104 ka). The overlying glaciofluvial gravel is interpreted as of probable Late Devensian age, but may be older, as glacial deposits farther east at the Howe of Byth, and farther west in the vicinity of Tiendland, have been ascribed to cold stages in Oxygen Isotope Stages 4 and 3 (Merritt et al., 2003)[8].

Late Devensian glacial deposits

Superficial deposits of Pleistocene age within the Huntly and Turriff districts are dominated by tills with subordinate glaciofluvial deposits.

The glacial, glaciofluvial and glaciolacustrine deposits can be divided into two groups: the East Grampian Drift Group and the Banffshire Coast Drift Group (Merritt et al., 2003))[8] (Figure 3). These were formerly referred to as the ‘Inland Series’ and the ‘Blue-grey Series’, respectively (Sutherland, 1984[25]; Hall and Connell, 1991[26]) and also roughly equate with the ‘Upper or Northerly’ and the ‘Lower or Southeasterly’ drifts of Read (1923)[4].

Deposits of the Banffshire Coast Drift Group generally underlie deposits of the East Grampians Drift Group and were probably laid down early in the Late Devensian before the East Grampians ice sheet had expanded towards the Buchan coastline (Figure 3).

Although tills are widely distributed, they only attain significant thicknesses in the northern and northeastern parts of the Huntly and Turriff districts. Over the remainder of the area, the till cover is patchy and thin and much of the superficial material is weathered bedrock. The tills are diamictons characterised by poor sorting of clasts in a dense and cohesive sandy, silty and clayey matrix.

Glaciofluvial deposits are concentrated particularly in the valleys of the rivers Ythan, Deveron and Idoch Water and are generally terraced (see also Applied geology section).

East Grampian Drift Group

These deposits occur over the whole of Sheet 86 and generally were sourced locally from the East Grampian ice sheet. In the Huntly and Turriff districts they consist generally of thin to very thin (< 2 m), typically sandy diamictons, derived mostly from disturbed, weathered bedrock. The uppermost metre has commonly been severely disturbed by periglacial activity. The diamictons strongly reflect their local bedrock albeit with a small eastward or southeastward overlap. They range from typically yellow-brown to grey, sandy tills in the northern part of the Huntly district to dark brown, more clayey tills, full of arenite and slaty pelite fragments, in the areas underlain by the Macduff Formation. The Old Red Sandstone gives rise to a red-brown sandy till. Read (1923)[4] recorded that in Strath Isla west of Meikle Balloch the till is a dark-grey clayey diamicton, derived from the local graphitic pelite unit.

Although the tills are of local derivation, reflecting the largely cold-based nature of the ice sheet, the distribution of erratics implies a more complex pattern of ice movement. Read (1923)[4] documented the nature and extent of erratic blocks across north-east Scotland and drew attention to the distribution of the olivine-gabbro and troctolite that form the western part of the Huntly Pluton around The Bin and Dunbennan Hill. A prominent train of these mafic boulders can be followed northwards as far as Portsoy. Blocks of gabbro and troctolite 2 m to 3 m across are found on the summit of Knock Hill (430 m above OD), and about 735 m north-east of the summit at 249 m above OD, an erratic block known as the Cloven Stone measures 4 m x 3 m x 1.2 m. Similar metre-sized boulders occur on the south-west flanks of Knock Hill. On Sillyearn Hill (quartzite) erratic gabbro and troctolite boulders are also prominent. A prominent ovoid boulder called the Gillymule Stone sits some 275 m south-south-east of Edingight Wood. Erratic mafic boulders are also common in the low ground around Knock, but these may be derived more locally from the gabbros and troctolite of the underlying Knock Pluton. In addition to the obvious northward carry of the Huntly mafic rocks, Read (1923)[4] also documented their northwestward carry to near Cairnie and the Glen of Coachford. The Huntly olivine-gabbros are also found as erratic blocks southeast from The Bin in Strathbogie. Southeastward transport in the southern part of the Huntly district is also shown by spotted slaty pelitic rocks, derived from the aureole of the Insch Pluton, which Read (1923)[4] recorded as occurring as striated blocks in the till overlying the igneous rocks of the pluton. Read (1923)[4] also pointed out that erratics of the prominent garnetiferous cordierite-sillimanite-bearing pelitic hornfels that outcrops immediately southeast of Knock Hill are found farther to the north-north-east.

The erratic blocks were probably incorporated into the basal or lower parts of the ice sheet and hence reflect the ice sheet flow patterns over time. Some may have been derived during earlier glaciations and redistributed in the Late Devensian. Many may have originally been corestones, particularly where derived from the lower relief areas. The distribution of erratics agrees moderately well with information on ice movement directions derived from striae (Read, 1923[4], Fig.11).

Glaciofluvial deposits associated with the Late Devensian ice sheets only rarely occur in the Huntly and Turriff districts. Eskers are sparse but an excellent example occurs immediately west of Gartly church [NJ 527 350] where two narrow bedded sand and gravel ridges trend approximately 010°, The more easterly known as The Riggin is some 1.3 km long and up to 10 m high. The more westerly ridge, known as Little Riggin, is about 750 m long. Read (1923)[4] recorded that eskers are also found in the parish of Alvah, where disconnected elongate mounds of very coarse gravel trend northwards. A near-continuous feature runs from approximately 375 m west of Rosyburn [NJ 667 560] north for about a kilometre. The northward continuation runs from Fattahead [NJ 661 577] to Mallyrust [NJ 662 596] but lies within the Banff district.

North-east of Huntly lying on both sides of the Knightland Burn is an area some 4 km long by 1 to 1.5 km wide where clays have been worked for brick and tile manufacture at the former Kinnoir Brickworks in the 18th and early 19th centuries. Around Longmoor Wood the clays pass into sands that underlie the low ground of the Corse of Kinnoir [NJ 550 434] and can be traced westwards to the River Deveron where gravels are present. This low-lying area is interpreted as the site of a former late glacial lake (Read, 1923)[4].

The apparently weathered nature of the tills and their overall thinness, together with the dearth of glaciofluvial deposits, has been taken by some workers to indicate that central areas of Buchan are ‘moraine-less’, and thus were unglaciated during the Late Devensian (e.g. Synge, 1956[27]; Hall and Connell,1991[26]).

Banffshire Coast Drift Group

Deposits of this drift group lies at depth in the northern and eastern parts of the Turriff district and have all been assigned to the Whitehills Glacigenic Formation (Merritt et al., 2003[8]). They are dominated by brown weathering, bluish grey, clayey tills that locally are up to 9 m thick but in this district normally only attain few metres in thicknesses. The tills commonly include abundant fragmentary and occasionally striated shells that are the remains of deeper water mollusca, whose living relatives are found in the colder boreal waters of the North Atlantic. Good sections occur in several places beside the Burn of King Edward [NJ 722 561] and were originally described in detail by Jamieson (1858[28], 1865[29], 1906[30]) and summarised by Read (1923)[4]. A good section was formerly exposed about 100 m south-west of the old bridge on the Banff-Turriff road. Here Jamieson reported up to 8 m of glaciofluvial sand and gravel (with ice-wedge casts) overlying 9 m of dark grey pebbly mud with striated shells in its basal part. This diamicton, termed the ‘Shelly Boulder-clay’ by Read (1923)[4], following Jamieson (1865)[12], is now known as the Castleton Member (Merritt et al., 2003)[8]. It overlies a 60 cm-thick layer of brown shelly sand, interstratified with over 3 m of stone-free dark grey silt that contains crushed and decayed arctic shells with a few whole specimens apparently in situ. Jamieson viewed this lower shelly silt and sand as an in situ deposit thus requiring sea level to be at over 45 m above current OD at some stage during the Quaternary. Read (1923)[4] favoured an interpretation that viewed the whole sequence as erratic and hence transported from the Moray Firth. This model fitted well with the presence of large and small rafts of Jurassic mudstones and Quaternary marine clays in the till, also derived from the floor of the Moray Firth.

Merritt et al (2003)[8] reported a more recent excavation of the river bank 200 m south-east of the original locality at [NJ 7236 5604]. This confirmed the original succession of terrace gravel over dark grey muddy diamicton, resting in turn on over 6 m of intercalated brown sand, grey silt and mud and dark-grey muddy diamicton. Shell fragments occur in varying concentrations and states of preservation throughout the sequence. Whole shells, including specimens of Lunatia pallida, and valves of Arctica islandica and Mathoma balthica were recoverd from the sand layer at 12 m depth. The basal contact of the sequence was not found, but in a pit beneath the adjacent floodplain of the burn at [NJ 7234 5602] the lower muddy diamicton rests on coarse glaciofluvial gravel and on Devonian bedrock. In addition, the sequence is distorted and steeply dipping in parts, suggestive of glacial disturbance. Merritt et al. (2003)[8] provide a full list of the mollusca recovered from both recent and older excavations at the King Edward sites, and at Gardenstown and Gamrie. They noted that the taxa represented were very similar but pointed out that only two species, Tachyrhynchus reticulata and Serripes greenlandicus, can be classed as truly arctic to sub-arctic. One species (Yoldiella lucida) is a deep water taxon and two others, Polinices nanus and Turritella communis, are boreal taxa. However, most of the listed mollusca have also been found in the Windermere Interstadial Clyde Beds of western Scotland. Hence the listed mollusca are typical of a non-arctic, interstadial offshore fauna. This data supports Read’s hypothesis that the shelly tills, muds and sands were transported by the Moray Firth ice stream onshore from the bed of the Moray Firth (Read, 1923)[4].

At King Edward five Arctica shells were collected for determination of amino-acid ratios from a till exposure 200 m north-east of Jamieson’s section. Ratios ranging from 0.073 to 0.095 (mean value 0.078 ± 0.010) were obtained (Miller et al., 1987)[31]. Uncalibrated AMS radiocarbon ages of > 44 200 and >41 500 14C years BP were obtained from two of the analysed shells. On the basis of this data Merritt et al. (2003)[8] assigned the Castleton Member of the Whitehills Glacigenic Formation to the interval between 40 and 80 ka BP. They pointed out that this age is consistent with the faunal evidence of interstadial conditions and that the marine muds and sands were thus originally deposited on the floor of the Moray Firth during the late stages of the Early Devensian or in the Mid Devensian (OIS stage 4 or 3).

Sequence of Late Devensian glaciation

Although there has been considerable debate about the extent of glaciation over the of Buchan region, which includes the Huntly and Turriff districts, the current view is that the area was overwhelmed by ice during the Late Devensian (Peacock and Merritt, 1997[32]; Whittington, et al., 1998[24]).

During an early phase of Late Devensian glaciation, ice belonging to the Moray Firth ice stream, derived largely from the Northwest and Central Highlands, flowed south-east across the north-eastern part of the Turriff district (Figure 3). This general south-eastward-directed flow was caused by the presence of Scandinavian ice in the North Sea. This large ice mass deflected the Moray Firth ice stream onshore, where it deposited dark grey, clay-rich tills and associated large rafts of Jurassic mudstones and Quaternary sea-floor sediments derived from the floor of the Moray Firth. Erratics derived from inland sources include granitic rocks derived from plutons to the west of the Buchan region, such as the Auldearn Granite Pluton near Nairn and the Inchbae Augen Gneiss (Read, 1923)[4].

The dark tills occur at depth north of a line between Turriff, Blackhills and Deskford, indicating the minimum extent of the incursion of Moray Firth ice. A raft of Lias clay enveloped in till was found when the railway cutting was made at Plaidy [NJ 730 550]. This mass was large enough for a brick pit to operate in the late 19th century (Jamieson, 1859[33]; Jamieson, 1906[30]).

Dark grey clay-rich till and beds of sand and gravel occur in the King Edward Burn (Read, 1923)[4]. These sediments contain cold water to arctic marine shells. These are commonly fragmented and striated, but locally appear undisturbed. Sutherland (1981)[34] suggested that certain of the shelly sediments are in situ marine deposits of Mid Devensian age. However, temporary excavations near Castleton Bridge [NJ 722 561] have shown, extensive shearing and disturbance within similar sediments and it is likely that these materials represent glacitectonic rafts (Peacock and Merritt, 1997[32], 2000[35]).

Later in the Late Devensian, local East Grampian ice covered the Huntly and Turriff districts, flowing to the north and northeast. This is indicated by striae orientated north and north-east that locally are superimposed on older easterly striae, e.g. at Rothiemay [NJ 541 498], and at Alvah, in the Banff district (Read, 1923)[4]. The train of basic igneous erratics that has been documented north and north-east of the Huntly Pluton, apparently extending as far as the coast, testifies to the extent of this ice sheet (Read, 1923)[4]. The presence of basic igneous erratics at >400 m above OD on Knock Hill indicates an ice sheet of considerable thickness. The only significantly thick tills that have been attributed to this ice movement occur mainly in the coastal zone outwith Sheet 86 (Peacock and Merritt, 2000[35]). Read (1923)[4] regarded this north to north-easterly ice flow as the final ice movement within the Huntly and Turriff districts, but complex movements of the margin of the Moray Firth ice lobe have been deduced from localities along the Moray Firth coast (Peacock and Merritt, 1997[32], 2000[35]).

The system of large meltwater channels which extends from Gardenstown to Turriff and the associated high level terraces around King Edward and Turriff show that Moray Firth ice extended to the present coastline, even at a relatively late stage in the deglaciation of the Huntly and Turriff districts. This ice dammed the lower Deveron system, resulting in the deposition of silts (Kirkburn Silts Formation) in the resulting lakes. Meltwaters from the Deveron catchment were diverted into the Ythan via the Towie Spillway, south-south-east of Turriff, where they deposited the only extensive spreads of glaciofluvial sand and gravel within the Huntly and Turriff districts.

This model of glaciation in the north-east Grampians is almost certainly over-simplified and relates only to the Late Devensian. Thus, no account is taken of pre-Devensian glacial events and their possible role in the distribution of erratics. Read (1923)[4] provided a thorough account of the distribution of erratics, but it is possible that certain blocks were not attributed to their true sources, due to the poor exposure. Drift thicknesses are limited over much of the district due to nondeposition and few multiple till sequences are known. The relative age of the two periods of main ice movement rests on evidence of crossing striae at just two sites and on the stratigraphical relationships seen in the till sections exposed at Boyne Bay and at Gardenstown in the Portsoy (Sheet 96W) and Banff (Sheet 96E) districts, respectively (Peacock and Merritt, 1997[32], 2000[35]). Deglaciation occurred under cold, dry ‘periglacial’ conditions over a long period, during which time there was considerable cryoturbation and gelifluction (Sutherland, 1984[25]).

Windermere Interstadial and Loch Lomond Stadial deposits

The Windermere Interstadial was a period of milder climate which began around 13 000 years ago, but was terminated by climatic cooling at around 11 000 years ago. It was succeeded by a further cold period, the Loch Lomond Stadial, which persisted for only about 1000 years. Organic and inorganic deposits from these two periods are known for a number of sites in the Huntly and Turriff districts and pollen analysis indicates the presence of a tundra vegetation cover of mainly grasses and sedges, with some dwarf birch and willow. Ice cover was not re-established in these districts but corrie glaciers were present in the Cairngorms and a significant ice sheet formed farther west in the Grampian Highlands.

Almost continuous vegetation cover was established during the Windermere Interstadial, following the final retreat of the Late Devensian ice sheets from north-east Scotland. Peats and organic muds from this period and the later Loch Lomond Stadial are known from excavations at Woodhead, Fyvie [NJ 788 384] (Connell and Hall, 1987[36]), Fisherie Green [NJ 791 589] and North Gorrachie [NJ 7385 5857] (Whittington et al., 1998[24]).

The Loch Lomond Stadial was a period of intense cold that resulted in permafrost conditions in north-east Scotland. Ice wedge casts are commonly present in sand and gravel quarries and at least some these structures formed during this period (Gemmell and Ralston 1984[37]). Examples have been noted in gravel pits south of Turriff [NJ 736 493] and elsewhere (Connell and Hall, 1987[36]; Galloway, 1961[38]). Involutions and vertically orientated clasts, rotated as a result of freeze-thaw action, are also widespread in gravel pits in the Turriff area (Connell and Hall, 1987[36]). However, the finest example of rotated clasts in the districts occurs in the quartzite gravels at Windy Hills (see above). Here, the entire upper metre of the deposit locally consists of sub-vertically orientated clasts.

Frost shattering of rock is also widespread. Some shattering predates the last glaciation, as with the shattered quartzite as Newbigging [NJ 527 591], which is overlain by till (Galloway 1958[39]). Shattering is particularly well developed on the Durn Hill Quartzite, for example, at Gallowhill [NJ 484 525] and it reaches a reported depth of 8 m on Sillyearn Hill [NJ 507 514] (Galloway 1958[39]). The phyllitic semipelitic rocks around the headwaters of the Ythan and in the Glens of Foudland also show shattering to depths of 3 to 5 m (Galloway 1958[39]).

Periglacial mass movement deposits are also widespread. In an old quarry at Cadgers Road [NJ 662 344], up to 3.5 m of head is exposed, developed on a surface slope of 12°. The upper 2 m consists of tabular pelite clasts, in parts up to 30 cm in diameter, but mainly less 5 cm, in a matrix of fine sand and silt. The basal metre contains larger blocks and forms an open-work scree deposit. Numerous examples of solifluction deposits occur in the district, for example, at Bruck-hills [NJ 692 379], near the headwaters of the Ythan [NJ 630 379], at Woodside [NJ 612 407], and at Gallows Hill [NJ 685 437] (Galloway, 1958[39]). These deposits are typically 1 to 2 m thick, with a well-developed clast fabric orientated near parallel to the slope. However, they lack morphological expression and generally form a smooth blanket of reworked till and weathered and shattered rock.

Evidence that significant solifluction occurred during the Loch Lomond Stadial is provided by the soliflucted tills that overlie Windermere Interstadial peats. Examples are seen at Woodhead, Fisherie Green and North Gorrachie, and similar relationships occur at several localities outside the districts (Connell and Hall, 1987[36]).

Flandrian

Flandrian deposits formed during the last 10 000 years in the Huntly and Turriff districts consist mainly of peat and recent river alluvium, with some fine-grained deposits developed in minor enclosed basins. Spreads of alluvial materials are most extensively developed beneath the floodplains of the main river systems of the Deveron, Idoch Water and the Ythan.

Peat was extensively developed over much of the Huntly and Turriff districts and was notably abundant on ground to the north-east of Fyvie and north of the Deveron in the Huntly district. However, most of the mosses have been locally and commercially exploited and the peat areas reclaimed for agriculture, resulting in significant depletion of the former peat deposits (see also Applied geology section). Read (1923)[4] recorded that the peat moss lying immediately north-east of Windyhills around [NJ 804 405] was stated to be 6 to 7 m deep in its centre. He also noted that a moss once stretched along the southern margin of the Slate Hills from Kennethmont to Old Meldrum but now only small relic patches remain unreclaimed. Wartle Moss [NJ 723 325] constitutes the largest remaining moss but even this has been worked down to the water table.

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