Glaciolacustrine deposits, Quaternary, 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).
Fine-grained sand, silt and clay laid down in standing water form part of many deposits categorised as ‘glaciofluvial’, especially those forming moundy topography. They are especially common at the base of coarsening-upward deltaic sequences, and interbedded with sands and gravels of distal outwash fans. However, only those occurring at the surface and that are extensive enough to justify mapping out separately have been identified as glaciolacustrine deposits. They are typically thinly laminated, micaceous, and commonly contain dropstones. These sediments were deposited mainly in ice-marginal, proglacial lakes that formed in the upper reaches of valleys when their lower reaches were blocked by ice. Such lakes formed extensively during deglaciation as the East Grampian ice sheet withdrew from the coasts while the coastal ice streams remained largely intact (Figure 42). Several of the more extensive glaciolacustrine deposits in the district have been worked in the past for clay for making bricks, tiles and drainpipes (Chapter 2).
Description of glaciolacustrine deposits by geological sheet
Sheet 95 Elgin
Fine-grained, glaciofluvial and glaciolacustrine micaceous sands, silts and clays underlie much of this sheet area (Map 1). Although glaciolacustrine deposits have been mapped only to the west of Elgin, similar sediments underlie much of the ground farther east, mapped as moundy glaciofluvial deposits and as raised marine and estuarine alluvium. It is commonly difficult to distinguish between the various categories of fine-grained deposit as they tend to merge one with another. The pattern of deglaciation and sedimentation was notably complicated. The deposits give rise to a particular soil association (Grant, 1960).
Most of the mapped glaciolacustrine deposits occupy kettle-holes within moundy glaciofluvial deposits. Those occurring above about 30 m OD formed while ice remained in the Moray Firth, impeding drainage. Those at lower levels may have been laid down in brackish water at the time when the sea first transgressed into the area during deglaciation. The silts and clays vary in colour, from red to brown, yellowish brown and grey. They are commonly thinly interbedded with sands, gravels and pebbly, clayey diamicton.
Evidence of an earlier period of ponding can be found in the valley of the River Spey immediately upstream of Fochabers, where varved glaciolacustrine clays underlie gravelly river terrace deposits (Peacock et al., 1968). Ponding occurred extensively farther up the Spey valley to higher levels.
Sheets 96W Portsoy, 96E Banff And 97 Fraserburgh
Glaciolacustrine deposits belonging to the Kirk Burn Silt Formation of the Banffshire Coast Drift Group occur along the coast between Portknockie and New Aberdour (Maps 2; 3). They generally underlie flat or gently undulating ground within 5 km of the coast and consist of ochreous to dark brown, thinly laminated silts and clays up to about 10 m in thickness. Bands of ferruginous nodules are common locally. The deposits were laid down after the East Grampian ice sheet had retreated from the coast, but when the Moray Firth ice stream remained offshore, forming a barrier against which meltwaters ponded to the south (Figure 42). Meltwaters entering the lakes from the south commonly formed deltas, as for example, in the valley of Burn of Boyne, near Drakemires (NJ 603 613), where sand and gravel overlies dark olive-grey silts and clays.
Most of the ponding took place during the final stages of deglaciation, but there is evidence on Sheet 96W that it also occurred during an earlier episode, before the last glacial advance across the hinterland. For example, 3.5 m of dark yellowish brown, thinly laminated silts and clays with dropstones crop out from beneath 2 m of stiff lodgement till in the valley of the Burn of Deskford near Inaltry (NJ 517 630) (Map 2). Additionally, red clay crops out beneath gravels and till in the valley of the Burn of Fordyce near Ardiecow (NJ 532 616), and laminated sands are overlain by red till in the valley of the Burn of Fishrie at the Mill of Minnonie (NJ 776 604).
Few glaciolacustrine deposits have been mapped on Sheet 97, but many of the moundy deposits there include silts and clays at the base of coarsening-upwards deltaic sequences. Terraced sands and gravels overlie extensive deposits of dark grey sandy silt and clay in the vicinity of Savoch (NK 047 588), and they probably pass beneath the alluvium surrounding the Loch of Strathbeg. The dark coloured deposits were possibly formed in a glacio-estuarine setting and are equivalent in age to the raised glaciomarine St Fergus Silts Formation occurring a few kilometres to the south-east.
Sheets 86E Turriff, 87W Ellon and 87E Peterhead
No silts and clays of glaciolacustrine origin have been mapped on Sheet 86E, but they do occur locally beneath some of the terraced glaciofluvial deposits, as for example, in the vicinity of Turriff. Glaciolacustrine deposits are most widespread on Sheet 87E and on the eastern margin of Sheet 87W, where meltwaters were ponded up against the coastal ice that occupied the North Sea and laid down the Logie-Buchan Drift Group, after the East Grampian ice sheet had receded westwards (Figure 42). Most of the ponding occurred in the valley of the River Ythan downstream of Ythanbank, where red and brown laminated silts and clays up to about 10 m in thickness normally underlie glaciofluvial terraces and the floodplain alluvium (Map 6). Outcrops are rare, however, except on the southern side of the river immediately upstream and downstream of Ellon. The glaciolacustrine deposits extend eastwards beneath glaciofluvial sand and gravel infilling the former valley of the river, which lies to the south of Kirkton of Logie-Buchan (see cross-sections on Sheet 87W Solid-and-Drift). A small patch of red clay lying within a topographical depression in the vicinity of Littlemill of Esslemont (NJ 928 285) was formerly worked for the production of tiles and pipes (Chapter 2).
Ponding also took place in the valleys of the North Ugie and South Ugie waters, where up to about 25 m of interbedded silts, clays and very fine-grained sands are concealed beneath the glaciofluvial terraces, as at Denhead (NJ 998 521), and river floodplains. The deposits are commonly colour-banded, including reds, browns, yellowish browns, greenish greys, dark greys and blacks. Few deposits are exposed, but in stream sections near Ballus Bridge (NK 001 473), south of Mintlaw, 3 m of glaciofluvial gravel with an eastward palaeocurrent overlies over 2 m of yellowish brown sand, gravel and silt thinly interbedded with reddish brown and dark grey plastic clay. The multicolouring indicates that the two coastal ice streams and the East Grampian ice sheet were close by in the area. Furthermore, in a temporary section near the bridge (NK 003 471), the laminated sequence was brown, grey and black towards the base, becoming yellowish brown and vivid reddish brown towards the top, indicating that meltwater derived from ‘Logie-Buchan’ ice entered the lake relatively late (Appendix 1 Ugie valley). All the fine-grained sediments contained good assemblages of reworked Mesozoic palynomorphs, whatever their colour.
No glaciolacustrine deposits have been mapped on Sheet 87E, but they occur extensively beneath the ground mapped as ‘undivided’ red and blue-grey tills of the Logie-Buchan and Banffshire Coast drift groups, respectively. This ground is underlain by up to 25 m or more of thinly interstratified clayey diamictons, clays, silts and fine-grained sands that were deposited in lakes (possibly brackish) at the landward margins of the coastal ice streams as they began to break up during deglaciation (Merritt, 1981). Thick deposits of red, waxy clay lie inland of the Bay of Cruden where they were worked until recently at Errollston (NK 088 370) for making bricks (Appendix 1 Site 17).
Sheets 76E Inverurie And 77 Aberdeen
During deglaciation, as the East Grampian ice sheet retreated westwards across the area covered by these two sheets, several large proglacial lakes formed on the lower lying ground. Some of the present drainage routes were blocked by sediments and residual masses of ice, causing ponding locally, but it is also probable that the regional water table was elevated as a result of the relatively high sea level during deglaciation. Sediments in the vicinity of the Mill of Dyce (Appendix 1) caused ponding in the lower stretches of the Don valley where over 15 m of dark yellowish brown, olive-brown and grey sandy silts and clays underlie the floodplain. They form low-lying terraces near Wester Fintray (NJ 811 164), where a prominent mound of glaciofluvial sand and gravel stands in the centre of the valley. The mound formed as a delta and the sandy deposits there fine downwards into over 12 m of laminated micaceous silts.
Ponding also occurred upstream of Inverurie in the valley of the River Urie and its northern tributaries. Deposits of interlaminated fine-grained sand, silt and clay were laid down in the resulting lake, which stood at about 63 m above OD. Low-lying terraces underlain by fine-grained sands, silts and clays are also present on both sides of the River Urie farther upstream, in the vicinity of Old Rayne, where they lie at elevations of 95 m OD and above. They also form extensive deposits in the valley of the Lochter Burn. The silts and clays are commonly dark yellowish brown to grey, thinly laminated and micaceous. Moundy deposits within the main valley in the vicinity of Portstown (NJ 774 231) comprise of over 13 m of interbedded silts, clayey diamictons, sands and gravels and were probably laid down at the ice margin.
Brown, laminated silts and sands of glaciolacustrine origin are common around Kemnay, where they form low-lying terraces beside the River Don and lie within several topographical depressions. Ponding took place during deglaciation when the gorge of the Don remained blocked between Burnhervie and Port Elphinstone.
Few glaciolacustrine deposits are shown on the drift edition of Sheet 77 published in 1980, but they are commonly interdigitated with the glaciofluvial sand and gravel deposits stretching to the north-east of Belhelvie, which are now placed in the Logie-Buchan Drift Group. They also occur towards the base of many coarsening-upwards deltaic sequences within the moundy glaciofluvial deposits lying between Dyce and the coast. Isolated deposits of reddish brown, stiff, waxy clay and clayey silt occur at Tipperty (NJ 970 268) and Balmedie (NJ 965 166). These sediments typically give rise to poorly drained, gleyed, brown, forest soils of the Tipperty/Carden soil association (Walker et al., 1982). Both deposits were possibly formed in lakes at the landward margin of ice retreating into the North Sea, but a glacio-estuarine origin is perhaps more likely (Simpson, 1955, p.195) (see below). The deposit at Tipperty was worked for making bricks and tiles (Chapter 2).
Sheets 66E Banchory and 67 Stonehaven
Glaciolacustrine deposits are quite widely distributed on these two sheets (Maps 10; 11). Those that were laid down in proglacial lakes at the margin of the East Grampian ice sheet, as it retreated north-westwards across the area, have been assigned to the Glen Dye Silts Formation of the East Grampian Drift Group (Chapter 8). Those formed at the margin of the Strathmore ice stream are assigned to the Ury Silts Formation of the Mearns Drift Group. A considerable thickness of waterlogged, thinly laminated sandy silt and clay is commonly present beneath the alluvium and peat in the elongate ice-scoured basins north of the River Dee (Chapter 7). These deposits, which are commonly grey or greyish brown in colour, have been proved to exceed 7.1 m in thickness in BGS Borehole NO79NW16, sited on the lacustrine alluvium within the Loch of Park basin. They have been interpreted as ranging between 7.9 and 9.2 m in thickness at nearby resistivity sites.
Many of the moundy and flat-topped spreads of glaciofluvial sand in the Water of Feugh catchment fine downwards into laminated sandy silt and clay. Notable spreads crop out on the western side of the Water of Dye near Bogarn and on the flanks of a small tributary stream at Miller’s Bog (NO 637 861). Up to 3.0 m of orange-brown and grey, laminated, stiff waxy clay was formerly exposed in the latter area. Similar deposits are present beneath flat-lying ground in the vicinity of Blairydryne (NO 749 926), in the valley of the Burn of Sheeoch, where they underlie deltaic glaciofluvial sands and gravels (Appendix 2). The deltaic sediments, which were formerly well exposed in Lochton Pit (Auton et al., 1988; Brown, 1994) prograded northwards into a glacial lake at an elevation of about 120 m OD (Brown, 1994). Glacial lakes may also have been present farther up the valley, at elevations of about 133 and 126 m OD, though fine-grained sediment has been recognised in association with them only in BGS boreholes and trial pits (NO79SW6, 8 and 11).
The only notable outcrop of the Glen Dye Silts Formation on Sheet 67 occurs within a shallow ice-scoured hollow north of Rickarton (NO 816 891). However, silty lacustrine deposits are probably concealed beneath peat and alluvium in several other ice-scoured basins in the northern part of the sheet. Fine sand with partings of laminated clay underlies glaciofluvial gravel at Bossholes (NO 812 884) indicating the presence of a small ephemeral proglacial lake.
Thinly laminated reddish brown fine sand, silt and clay of the Ury Silts Formation crop out extensively on the low-lying ground of Strathmore, where they also constitute a major part of the concealed succession recorded in boreholes and trial pits. Notable flat-lying spreads crop out east of Fettercairn, west of Fordoun and on the western side of Stonehaven (where they were formerly worked for making bricks and tiles). Smaller outcrops occur on the flanks of sand and gravel mounds north of Ury Home Farm and on the sides of the valley of the Carron Water. Red-brown silts and clays also occur between the ridges of sand and gravel that formed in contact with the southwestward retreating Strathmore ice stream, east of Auchenblae.
Within the valley of the Cowie Water concealed patches of red brown clay and silt extend up to at least 4.5 km inland from the coast. Moderate red, waxy clay, with partings of yellowish brown sand and silt occurs between 6.7 and 12.8 m depth in BGS Borehole NO88NW12 at Nether Findlayston (Auton et al., 1988). The clay occurs between two units of sand and gravel. The underlying silty gravel, which contains pebbles of quartz and fine-grained, dark igneous (possibly andesitic) rock as well as granitic, psammitic and semipelitic clasts, rests on semipelitic bedrock. The composition of the gravel indicates that it was deposited by meltwater sourced from the Strathmore ice stream. The moderate red colour and waxy nature of the clay are typical of lacustrine sediments within the Mearns Drift Group. It is suggested that a proglacial lake formed, fed principally by meltwaters draining north-westwards (up valley) from Strathmore ice that was retreating towards the coast. The 6.7 m of sand and gravel overlying the red clay fines downwards and becomes more silty and cohesive with depth suggesting an origin as a delta or fan-delta. Pebbles within the sand and gravel are predominantly granitic (with minor amounts of semipelite) indicating that the deltaic sediments were laid down by meltwaters from the East Grampian ice sheet draining south-eastwards down the valley into the lake.
Interbeds of red-brown silt and silty sand, 1 to 2 m thick, with sparse dropstone pebbles, are present within deposits of the Drumlithie Sand and Gravel Formation between Auchenblae and Glenbervie. In these sequences, however, all of the sediments were laid down by meltwater issuing from the Strathmore ice stream as it decayed.