Moss of Cruden - locality, 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).
Moss of Cruden (‘Buchan Ridge’)
The Moss of Cruden, 11 km south-west of Peterhead, is an area of unusual geological interest (Hall, 1993a) (P915317). Together with the Moss of Auquharney, the Corse of Balloch and the Hill of Aldie, it forms part of the so-called ‘Buchan Ridge’ (Flett and Read, 1921) (P915269, Map P915377), a gently rounded, featureless, east-north-east-trending ridge that attains an elevation of about 140 m above OD. It provides the type localities for the Smallburn Sandstone, a small outlier of Devonian sandstone, the Moreseat Sandstone, a concealed outlier of Early Cretaceous age, and the Buchan Ridge Gravel Member of the Buchan Gravels Formation, of Neogene age. It also includes the important Late Pleistocene interstadial site at Camp Fauld. These sites are described in turn.
Smallburn Sandstone Formation
The Smallburn Sandstone Formation occupies a small area on the northern flank of the Moss of Cruden (NK 023 403). Exposure is poor, but shallow pitting indicates that the sandstone rests on diorite and is partially concealed beneath Lower Cretaceous Moreseat Sandstone (P915317). The Smallburn Sandstone is a red-brown arkose, whose grains comprise dominantly quartz, with sudsidiary feldspar and lithic fragments of diorite and granite. Quartz and hematite coatings occur on grain surfaces, but development of quartz overgrowths is limited. Preserved porosity varies from 13 to 19 per cent. Clay mineralogy is dominated by interstratified mica-smectite, with some illite (information from R A Downie, Downie Geoscience Ltd, 62 Durwood Avenue, Shawlands, Glasgow). No palynomorphs appear to be preserved in the formation (information from L A Riley, Bovingdon, Herts, 1997).
Moreseat Sandstone Formation
Weathering and glacial disturbance of the Moreseat Sandstone Formation is widespread and it is only in the ground north of Moreseat that relatively fresh material was recovered from the base of trial pits (P915317). The Moreseat Sandstone here is a very dark grey, porous silty fine-grained glauconitic quartz arenite. Bioturbation is common, and burrows and moulds of mollusca are present. The rock contains well-preserved palynomorphs indicating a Late Hauterivian to Early Barremian age (information from W Braham). Thin sections reveal a spicular quartz sandstone with a mixed terrigenous/calcareous mud matrix. Grains are dominantly of quartz, but, prior to the development of voids (moulds) after the dissolution of mollusc fragments and foraminifera, the dominant grain type would have been sponge spicules. Feldspar grains and bioclastic debris have also undergone significant dissolution. Macroporosity is around 30 per cent (information from A R Duncan, Maud, Aberdeenshire, 1997).
The Lower Cretaceous rocks north of the Moss of Cruden were originally fine-grained sandy siltstones, but they are typically decomposed to clayey fine-grained sands and sandy silty clays of vivid orange, tangerine, yellow and white hues.
Buchan Ridge Gravel Member
The Moss of Cruden is the type area of the Buchan Ridge Gravel Member, part of the Buchan Gravels Formation (compare with Flett and Read, 1921; McMillan and Merritt, 1980) (P915269). Exposures also occur at Skelmuir Hill (Bridgland et al., 1997), Whitestones Hill (Kesel and Gemmell, 1981) and Den of Boddam (Bridgland et al., 1997), and other concealed, small gravel masses may well exist (Flett and Read, 1921). At Moss of Cruden, the gravel deposit forms a broad ridge, orientated north-east–south-west, some of the highest ground in this part of Buchan. The type section of the Buchan Ridge Gravel Member occurs in BGS Borehole NK04SW3 on the highest point of the ridge (McMillan and Merritt, 1980) (P915317), which proved at least 25 m of coarse flint and quartzite gravel, interbedded with pebbly sand and commonly bound by white kaolinitic clay. Reference sections have been recorded in temporary pits at Moss of Cruden (BGS records), Skelmuir Hill (NJ 987 413) (Bridgland et al., 1997), Boddam (NK 113 415) (Bridgland et al., 1997) and in BGS Borehole NK04SE6, near Berryley (McMillan and Merritt, 1980). The Buchan Ridge Gravel Member is poorly exposed. Temporary excavations and boreholes show white, clay-bound coarse gravels with minor sand and silt units (Merritt, 1981). Gravel clasts are dominantly flint with subsidiary quartzite and vein-quartz. Clasts are generally well-rounded pebbles and cobbles and bear numerous chatter marks. The deposits originally contained less resistant clasts, probably mainly granite, but these have decomposed to ghosts of white sandy clayey silt. This decomposition extends throughout the known thickness of the deposit into the underlying granite and gneiss bedrock. Both sand and gravel units are bound and, in places, supported by, white, sandy clayey silt consisting of well-ordered kaolinite with minor illite (Hall, 1982).
The base of the Buchan Ridge Gravel Member in places contains large nodules of flint and boulders of quartzite (P104121, P104122). Quartzite blocks up to 30 cm in diameter were recovered from a trial pit close to the western margin of the gravels at Moss of Auquharney (NK 019 400). In pits at an elevation of approximately 130 m (NK 028 403) the base of the gravels revealed a lag of unworn, nodular flint with blocks up to 25 cm in diameter. In places the gravel is partially cemented by iron concretions containing goethite. Similar deeply weathered boulder gravel containing large nodular flints occurs on Skelmuir Hill at an elevation of about 148 m OD (Bridgland et al., 2000). The Buchan Ridge Gravel Member rests on kaolinised granite and metasedimentary rock (McMillan and Aitken, 1981; Hall et al., 1989) and on highly weathered Lower Cretaceous Moreseat Sandstone (Hall and Jarvis, 1994).
A ground probing radar survey (Greenwood et al., 1995) on the northern slope of the Moss of Cruden has confirmed that the feather edge of the Buchan Ridge Gravel Member rests on weathered Lower Cretaceous sandstone (see Appendix 3). Within undisturbed parts of the Buchan Ridge Gravel Member, low-angle cross-stratification dips towards the axis of the ridge (P915339 line 1). Resistivity depth probes and ground probing radar indicate that around 15 m of Buchan Ridge Gravel Member lies beneath the crest of the ridge, resting on weathered granite. The gravels here fill a channel running north-east–south-west and parallel to the line of the ridge.
The Buchan Ridge Gravel Member is overlain by a variable thickness of till, and disturbance attributed to overriding ice is common. At Corse of Balloch, BGS Borehole NK04SW4 (McMillan and Merritt, 1980) indicates that in situ Buchan Ridge Gravel Member is overlain by almost 10 m of till, derived in large part from the underlying gravel. Temporary excavations east of Camp Fauld revealed rafts of flint and quartzite gravel within till (Whittington et al., 1993). To the north of the ridge, a sheet of soliflucted flint gravel, up to 2 m thick, extends well downslope on to weathered granite (Hall, 1993a). This gravel forms part of the ‘Flint-quartzite Head’ deposits described in Chapter 6 that are widespread around the Buchan Ridge on sheets 87W Ellon and Peterhead 87E Peterhead (P915376, P915377).
This site (NK 049 410) lies on the southern slope of the Moss of Cruden, about 900 m north-west of Moreseat (P915318, P915377). It is important for the presence of a buried peat of probable Early Devensian age that separates two till units (Whittington et al., 1993). The lithostratigraphy described below generally follows that set up by Whittington et al. (1993), but in order to adhere more closely to internationally agreed guidelines on lithostratigraphical nomenclature, new names have been adopted here (P915347, P915359). Those proposed recently by Sutherland (1999) are not appropriate.
The stratigraphy is known only from trial pits. The oldest deposit recognised is the Camp Fauld Till Formation. This is at least 2 m thick and is a grey (2.5Y N5) to very dark grey (7.5YR N3) massive to crudely horizontally bedded diamicton with sparse pebbles. Clast lithologies comprise mainly rounded quartzite and flint, with lesser amounts of subangular granite, basic igneous, red sandstone and soft grey siltstone clasts. The matrix is a micaceous sandy silt and clay mineralogy is largely kaolinite, illite-smectite and minor glauconite. The diamicton rests on, and locally passes down into, and incorporates rafts of Moreseat Sandstone. The direction of ice flow during deposition of the Camp Fauld Till is difficult to establish owing to the dominance of very locally derived material. The presence of clasts of grey granite and red sandstone suggest transport from local outcrops to the west of the site.
The Berryley Peat Bed is up to 0.5 m thick in pit A and rests on the mottled top of the Camp Fauld Till (P915318). The peat is truncated by, and both reworked and sheared into, the lower 30 cm of the overlying Aldie Till. In Pit B, the peat bed rests on bedrock and is overlain by sands and gravels containing stringers of peat. This unit is separated from the overlying Aldie Till by a gravel-rich diamicton with a strong downslope clast fabric (P915318). This intervening deposit, named here as the Hardslacks Gelifluctate Bed, appears to mark a phase, or phases, of periglacial conditions and slope instability.
The Aldie Till at Camp Fauld is a greenish grey (5GY 6/1) sandy diamicton that is massive or shows a crude horizontal bedding and is up to 2.5 m thick. Clast lithology is dominated by broken rounded pebbles and cobbles of flint and quartzite reworked from the Buchan Gravels Formation, with sparse mica-schist, granite, basic igneous and red sandstone clasts. The matrix is kaolinitic quartz sand. Away from the type-site, the character of the Aldie Till varies greatly in response to marked lateral changes in bedrock and even drift geology. On the Buchan Ridge Gravels at Hill of Aldie, the Aldie Till closely resembles that at Camp Fauld. However, in the area underlain by the Moreseat Sandstone Formation, it is indistinguishable from the underlying Camp Fauld Till. To the north and east of Camp Fauld, the Aldie Till partly incorporates large masses of weathered brown sand with sparse quartzite clasts of unknown derivation and origin. This sand, named here as the Moreseat Farm Sand Bed, was also probably formerly exposed at an infilled gravel pit (NK 0529 4108), from where a cobble of brown, porous arkose of Lower Cretaceous appearance was recovered in 1979. BGS Borehole NK04SE2 shows that this unit of sand is up to 7 m thick, and rests on a dark grey till that may be equivalent to the Camp Fauld Till.
South and east of Moreseat, the Aldie Till passes beneath red clayey diamictons of the Logie-Buchan Drift Group. It thickens on the southern and eastern slopes of the Moss of Cruden in accord with the apparent eastward carry of flint and red granite from the Moss of Cruden, and its weak north-west–south-east clast fabric. These features indicate that the unusual till thickness is probably a product of only minor subsequent glacial erosion as it lies in the lee of the Buchan Ridge. The presence of red granite erratics on the Moss of Cruden has been used to infer ice movement from the north-east prior to the last glaciation (Bremner, 1934a, 1939; Synge, 1956). However, the situation is complicated by the presence of coarse-grained red granites, similar to the Peterhead Granite, in the ground 1 km north and west of Moreseat.
The Berryley Peat Bed has yielded radiocarbon ages (SRR–3724) of 39 590 ± 550 BP for the humic fraction and 34 600 ± 910 BP for the humin fraction (Whittington et al., 1993) and luminescence ages of 160 ± 16 and 251 ± 24 ka BP (Duller et al., 1995). The significance of these contrasting ages is uncertain. Pollen analysis of peat layers from the adjacent pits suggests that the peat formed during an interstadial in which birch–pine woodland first developed, probably followed by tundra vegetation in which Bruckenthalia spiculifolia was present. This heath is found today in the Balkan Mountains, but is extinct in Scotland (Whittington, 1994). The vegetation assemblage of the Berryley Peat, including the presence of Bruckenthalia, is akin in certain aspects to that found at other Early Devensian sites in Scotland at Allt Odhar and Sel Ayre. Hence, the Berryley Peat may correlate with Oxygen Isotope Stage 5c or 5a, when pine–birch woodlands developed widely in north-west Europe during the Brørup and Odderade Interstadials (Behre, 1989) (P915254).