Bulk mineral resources, 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).
Construction and industrial minerals form an important resource in north-east Scotland. Sand and gravel deposits are by far the most important and reserves with planning permission accounted for 25 per cent of the Scottish total in 1993 (BGS, 1998). The value of naturally occurring aggregate extracted from north-east Scotland was approximately £6 million in 1997. Of this, 431 000 tonnes were used for fill, 350 000 tonnes for concreting sand, 290 000 tonnes as coarse aggregate for concrete and 109 000 tonnes for building sand. In 1997, twenty-two major workings (Figure 8) were notified to BGS as being operational within the district (Cameron et al., 1998). Brick clay and peat also constitute significant bulk resources in north-east Scotland, although there is little current commercial peat extraction and no large-scale brick clay working from these deposits.
A brief general account of the most extensive spreads of sand and gravel in north-east Scotland is given below, but, because of their economic importance, abundance and variability, these resources are more fully described in Appendix 2.
Sand and gravel
Sands and gravels of several types occur in north-east Scotland (Chapter 6). Each type reflects its mode of deposition and the rocks from which it was derived. The extensive glaciofluvial deposits that flank the lower reaches of most of the major river valleys, and extend onto the adjacent interfluves, are the principal sources of naturally occurring coarse-grained bulk aggregates. Most were laid down at the close of the Main Late Devensian glaciation (Chapter 5). Less extensive spreads of sand and gravel accumulated around the coast as raised beach and marine deposits (Chapter 7), when sea level was higher than at present (during the Late-glacial and mid-Holocene). Sands and gravels that were deposited by postglacial streams and rivers also commonly underlie floodplains and river terraces, while extensive dunes of blown sand fringe the coast, particularly in the eastern part of the district.
The most important sand and gravel resources occur as moundy and flat topped glaciofluvial deposits. These flank the lower reaches of the valleys of the rivers Don and Dee, and extend onto the adjacent interfluves around the city of Aberdeen (the largest market for aggregate in northern Scotland). However, many of the most attractive deposits around Aberdeen have been extensively worked, and some of the resources close to the city have been sterilised by urban expansion. More extensive spreads of good quality sand and gravel, suitable for most end-uses, occur in the coastal lowlands between Forres and Elgin. They form coarsening-upward sequences that were laid down as fans at the mouths of drainage channels or in temporary ice-dammed lakes, kettled terraced spreads that were laid down on bodies of stagnant ice during deglaciation and gravelly Late-glacial raised beaches. Thick deposits of gravel and sand are locally present beneath river floodplains, but none is exploited at present, as the bulk of the resource lies beneath the water table.
Thick, easily worked resources of sand and gravel, lying above the water table, form flights of terraces flanking the North Ugie Water, South Ugie Water, and the rivers Lossie, Spey, and Ythan. The Ythan terrace gravels contain a high proportion of boulders, but have been extensively worked on both sides of the river downstream of Methlick. Gravels were also won from several pits sited on the Ugie terraces. Moundy glaciofluvial ice-contact deposits that crop out inland of Fraserbrugh, between Aberdeen and Peterhead, and between Stone-haven and Auchenblae are also major resources. However, these accumulations are commonly discontinuous and individual deposits may vary considerably in thickness and quality over relatively short distances. Typically, they contain waste partings of silt and clay and may be concealed beneath a thin overburden of till.
Most deposits, particularly those in the upland areas, contain high proportions of clasts derived from crystalline resistant rock types (Appendix 2), and after on-site screening, washing, crushing and grading, they produce high-quality aggregates that suit most end-uses (Merritt et al., 1988). However, the spreads of sand and gravel overlying Devonian bedrock, such as those in the south-eastern part of the district, tend to yield weaker aggregates, as they locally contain significant amounts of friable sandstone, mudstone and porphyritic lavas. Much of the sand and gravel in north-east Scotland is used in the production of ready-mix concrete and concrete products, as a source of mortaring and plastering sand, and as fill in civil engineering works (Plate 2). Some sand is used for coated aggregate in road building, but little gravel is used for road surfacing, as crushed-rock aggregate provides clasts with better resistance to abrasion and polishing. Poorer quality sand and gravel is used extensively in the construction of unmetalled roads and tracks.
Potentially workable deposits of coarse-grained aggregate also occur within the Buchan Gravels Formation, an unlithified gravelly deposit of Palaeogene to Neogene age, which crops out between Peterhead and the upper reaches of the Deveron and Ythan valleys (Chapter 4). In some areas, sands, and less commonly gravels, derived from decomposed igneous, metamorphic and sedimentary bedrock are also worked as fill for untarred roads.
Clays suitable for brick and tile making are widely distributed within the Quaternary deposits of north-east Scotland, and have been worked at almost 20 sites during the last two centuries. Most workings were small-scale and served local markets; no commercial brick or tile making currently takes place. The widely scattered, variable quality and remote nature of many deposits, the small size of reserves at most sites and the relatively high costs of firing superficial clays, all contributed to the decline of the industry.
The only comprehensive evaluation of clays in northeast Scotland suitable for brick, tile and pipe-making was undertaken by Eyles et al. (1946). This work, which forms the basis of the following account, provided geological descriptions of all of the worked and potentially workable clay deposits that had been identified in the district prior to the end of the Second World War. It also reported the results of physical and chemical tests on deposits at a selection of clay pits, which were primarily undertaken to establish the suitability of each clay deposit for brick making. The results showed the material to be of variable, but generally rather poor quality, but suitable for the production of drainage pipes, tiles and common bricks.
The data provided by Eyles et al. (1946), together with that gleaned during more recent geological surveys have been used to compile a map of the main sources of brick clay in the district (Figure 9). A total of 29 brick clay sites are recognised, of which 21 have been worked commercially. Eyles et al. (1946) established that common bricks had been made from a wide variety of clayey Quaternary sediments, including, ‘boulder clay’ (till), ‘fluvioglacial clay’ (clayey glaciolacustrine deposits), clayey raised marine deposits and alluvium. Workings for brick clay were also recorded in large glacially transported rafts (‘erratics’) of Jurassic mudstone. Data on the type of clay deposit and its maximum thickness at each site are summarised in Table 4. This shows that clays of marine, glaciomarine and glaciolacustrine origin (Chapter 6) have been the most widely worked for brick making, possibly because they are the least heterogeneous, widespread clayey deposits in the district.
Silty clays of marine origin (Spynie Clay Formation) were formerly worked for brick and tile making in two pits close to the town of Elgin (Figure 9; Map 1). The clay deposits occur close to sea level, beneath shelly sands and peat near Loch Spynie. Considerable resources are thought to be present north of Milltown Airfield (NJ 266 658) and beneath Lossiemouth Airfield (NJ 210 695) (Peacock et al., 1968).
Dark blue-grey clay with wisps of sand was worked for pipes, bricks and tiles in a clay pit at Tochieneal (NJ 521 652) (Map 2), south of Cullen, though test results indicate that mixing sand with the clay would improve its suitability for brick making. Previous accounts are undecided as to whether the worked deposits are part of a large erratic of Jurassic mudstone or a Quaternary marine deposit. It is probable that they are rafts within the Whitehills Glacigenic Formation (Chapter 8). Similar rafted material was also worked in pits at Blackpots (NJ 659 657) near Whitehills (Map 3), and at Plaidy (NJ 730 550), north of Turriff (Map 5).
Greenish grey laminated clays and silts, containing dropstones and marine shells of Late Devensian age, have been worked for brick making in a pit near Annachie (NK 105 529), north of Peterhead (Appendix 1 St Fergus). Much of the resource is concealed beneath alluvium, but 16 m of laminated silt and clay were recorded in a nearby BGS borehole (NK15SW1).
Most recent brick production has been concentrated on Quaternary clay resources between Peterhead and Aberdeen, which are reviewed by Ridgeway (1982). Until the middle part of the 1980s, reddish brown laminated silty clays and associated clayey tills were worked for brick making at the Cruden Bay Brick and Tile Works at Errollston (NK 088 368), south of Peterhead (Appendix 1 Errollston; Map 7). Laminated silts, sands and clays, interbedded with silty clayey diamicton, were recorded to a depth of 6.2 m, in a section at the working pit examined during 1974 by Peacock (1984). A comparable section was recorded in 1944, by Eyles et al. (1946), although the workable deposits were reported to extend to more than 24 m depth in places. The test results indicate that the Errolston deposit is suitable for the manufacture of facing bricks as well as common bricks.
Reddish brown clay and silt was also worked by the Cruden Bay Brick and Tile Company in a pit in the valley of the Tarty Burn at Tipperty (NJ 971 268) (Map 9) until the middle part of the 1980s. Eyles et al. (1946) record 5.3 m of laminated clay resting on sand and gravel in the working face exposed in 1944. The form and extent of the deposit, which was investigated by mapping and trial pitting during the 1970s, is illustrated in Munro (1986, fig. 37). At the time, 2 m of unstratified ‘red’ clay passing downward into 2 m of laminated silt and clay was recorded from the working pit. The laminated unit was seen to overlie sand and gravel resting on red-brown till. In 1944, the clay was only worked for the manufacture of agricultural drain tiles, but facing bricks were produced during the 1980s.
In and around the city of Aberdeen several clay pits were worked in the past; for example Seaton, Blackdog, Ferryhill, and Torry (Map 9). The sites of all but the first mentioned have long been abandoned and the exact extent of each working is unclear. About 1 m of crudely laminated ‘bluishyellow’ clay was exposed in the pit at Seaton Brick Works in 1944, but 4.9 m of similar material, beneath 0.9 m of gravel was recorded from the site by Jamieson (1858). Bricks were produced at Seaton during the 19th century. By 1944, however, the deposit was worked solely for the manufacture of earthenware pottery and horticultural ware, such as flower pots. The worked deposits are assigned to the Tullos Clay Member of the Logie-Buchan Drift Group.
Widespread commercial exploitation of peat in north-east Scotland has been limited by transportation costs and environmental concerns. Production for horticultural use and as fuel has been concentrated around New Pitsligo and Strichen in Buchan, but resources there are almost exhausted. Other patchy deposits of peat are widespread, but there are few major resources. Most are restricted to relatively inaccessible spreads of hill peat, many of which are covered by forestry (Chapter 6). Areas of basin peat occur on lower ground, but the bulk of these have been worked to the water table, or are preserved as nature reserves and Sites of Special Scientific Interest (SSSIs). The use of peat as fuel by farmers and crofters has not been important in the recent past, but persists on a small scale. In western parts of the district it has a specialised application in whisky distilling.
The first systematic assessment of the peat deposits in Scotland began during the Second World War (Fraser, 1943), with the aim of evaluating their potential uses as fuel and in agriculture. The results from the first of a planned series of surveys, dealt with the deposits of Aberdeenshire, Banffshire and Morayshire (Fraser, 1948) and are summarised in Table 5; subsequent systematic surveys were abandoned in the immediate postwar period. Assessments recommenced in 1949, but concentrated only on major deposits with a view to their potential use as fuel for peat-fired power stations (Department of Agriculture and Fisheries for Scotland, 1962). These surveys evaluated resources in south-west Scotland, the Western Highlands and Islands, Central Scotland, and Caithness, Shetland and Orkney (Department of Agriculture and Fisheries for Scotland, 1964, 1965a, b, 1968). None of the deposits in north-east Scotland was included in these surveys, but the overall potential of Scottish peat for electricity generation was reviewed by Dryburgh (1978).
Fraser (1948) identified 46 areas of peat in north-east Scotland that were considered potentially workable (Table 5), but none of the deposits on 1:50 000 sheets 66E and 67 was investigated. In general terms, the hill peats are extensive, but thin, with much of the resource forming hags above the water table. The basin peats are thicker, but less extensive and predominantly water-saturated; many are relict and now eroding.
By the mid 1940s, most of the 4845 hectares (ha) of peat identified by Fraser, between Aberdeen and Elgin had been worked in a piecemeal fashion for fuel. In the 1960s, eight of the major peat mosses between Aberdeen and Fraserburgh (Table 5) were examined in detail by the Soil Survey (Glentworth and Muir, 1963). In general, the extent of the peat and amount of cutting that had occurred at each site, recorded in the later survey, were comparable to the figures given in Fraser (1943). The detailed later survey showed that maximum thickness of peat at each site was generally greater than that recorded by Fraser. The average thickness of peat that remained in cut areas, ranged between 25 per cent of maximum thickness recorded at Moss of Air, to 60 per cent of the maximum at St Fergus Moss. Recent geological mapping indicates that piecemeal working has continued, particularly up to the late 1970s, and many of formerly identified resources are now too thin and waterlogged to be attractive for commercial extraction.
Commercial extraction of peat around New Pitsligo and Strichen reached its peak in the early 1990s. Seven to eight thousand tonnes per year is currently extracted from Lambhill Moss, by the Northern Peat and Moss Company, although remaining reserves here are small. Some 15 to 20 per cent of production is sold throughout Scotland for domestic fuel; the remainder is exported, via Fraserburgh, to southern Sweden, for use as fuel in a district heating plant. Commercial extraction of peat from St Fergus Moss commenced during 1998, with 7–8000 tonnes being produced in the first year of working; most of the material is also exported to Sweden for use as fuel, but some peat for horticultural use is also produced.
More than 3000 ha of peat has been mapped on Sheet 66E Banchory, most of which occurs as extensive hilltop spreads on the watershed between the catchments of the River Dee and Water of Feugh and the rivers and burns that drain south-eastwards into Strathmore. The hill peat is generally less than 2.5 m thick and much of it rests either on thin till or directly on the dominantly granite bedrock. The most notable spreads occur on the uplands, north and south of Glen Dye (1500 ha), between Little Kerloch (NO 674 874) and Leachie Hill (NO 739 853) (900 ha) and around Little Sheil Hill (NO 796 917) (350 ha). The hill peat has been worked on a piecemeal basis in several places, but much of the resource is currently sterilised by extensive forestry plantations. Minor deposits of thin, waterlogged basin peat are present around Loch of Park, on the northern edge of Sheet 66E.
About 400 ha of basin peat occurs in the northern part of Sheet 67 Stonehaven within rock basins and in the floors of glacial drainage channels. Significant deposits (160 ha) occur in basins, linked by drainage channels, between Broomhill (NO 844 901) and Forester’s Croft (NO 873 883), and at Red Moss (120 ha), near Netherley (NO 855 934). Mapping indicates that, apart from the deposit at Red Moss, the basin peat is generally thinner (1–2 m) than that commonly found north of Aberdeen, and it has not been worked as extensively.