Geomorphological features of glacial erosion, Cainozoic of north-east Scotland
|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).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul.
Features of glacial erosion
Glacial striae, roches moutonnées and glaciated rock knolls
Evidence of glacial erosion, in the form of scratched and abraded bedrock, indicates the former direction of ice movement. Striated surfaces are not common in northeast Scotland because there are few outcrops of fresh bedrock. Those that do occur are generally restricted to the coasts where glacial erosion has been concentrated. Most striae are assumed to have been produced by the last major glaciation, namely the Main Late Devensian, but this is not always the case. Some have survived from earlier glacial phases, protected beneath a mantle of till, as, for example, beneath the Craig of Boyne Till Formation at the Boyne Limestone Quarry site. Furthermore, striae probably only represent particular phases of glaciations when the ice was wet-based and able to glide over rock outcrops. Most of the striae depicted on the geological maps of the district were recorded during the primary survey and few are now visible, but those that are, together with several discovered more recently, generally confirm the earlier observations.
On Sheet 95 Elgin (P915371), there are two distinct sets of striae trending east-south-east and south-south-east respectively, with a few directed in between these points. The trend of the former set accords with the general direction of ice movement deduced from the distribution of glacial erratics in the area (P915292). The direction of travel indicated by the latter set is less clear, but they were possibly formed by ice moving inland from the Moray Firth during the final phase of glacial activity in the area, the ‘Elgin Oscillation’ (Peacock et al., 1968). Glaciated pavements are well preserved on siliceous sandstones between Carden Hill (NJ 142 622) and Quarry Wood (NJ 190 640), where plucked surfaces generally confirm the east-south-east direction indicated by the striae recorded thereabouts.
South-east- to south-south-east-trending striae are dominant on Sheets 96W Portsoy and Sheet 96E Banff, but some striae are directed towards the north-east. There is also a record of east-south-east-orientated striae crossed by northward-directed striae in the south-west corner of Sheet 96E (Read, 1923; P915373). Read (1923) linked the south-east- to south-south-east-directed striae with the glaciation that brought shelly tills and Jurassic erratics from the Moray Firth, and the north to northeast set with a major ice movement towards the coast (see also Bremner, 1934). The former agrees with the views expressed here, but the latter may be related to more limited movements of ‘inland’ ice at a late stage of the Main Late Devensian glaciation, following retreat of the Moray Firth ice stream towards the coast (Peacock and Merritt, 1997, 2000).
Striae are relatively common along the foreshore between Fraserburgh and Rattray Head (P915374). Roches moutonnées, grooves and striations are also found immediately inland where there is a gently undulating, glaciated surface developed on gneiss, granite and metabasic igneous rock. Most evidence indicates ice movement towards the east-south-east (Wilson, 1886; Milne, 1892b), but a later set, found on shore sections to the north of a line from the mouth of the Burn of Philorth (NK 028 650) to Inzie Head, suggest a later south-southeast movement (Peacock, 1997, 2000, fig. 27). Inland from these glaciated surfaces there is a transition, over a distance of a few kilometres, from rock that is little weathered at the surface to a wide area where the rocks are generally deeply decomposed with only sparse outcrops of fresh rock. Mormond Hill is one such outcrop, where striae are preserved on ice-smoothed, brecciated quartzite at the entrance to a quarry (NJ 950 568) at about 137 m above OD. Striae have also been observed on a smooth quartzite surface on the north side of the hill (NJ 982 581) at about 155 m OD. When ground as high as this was affected by east-south-eastward-moving ice, it strongly suggests that most, if not all of Buchan is likely to have been glaciated in the Late Devensian. However, large erratic boulders of Strichen granite are buried beneath thick gelifluctate on the northern and western slopes of the hill (e.g. NJ 9585 5690), suggesting that a prolonged period of periglacial conditions followed glaciation.
Striae are rare on Sheet 86E Turriff and Sheet 87W Ellon. Those that do occur are mainly orientated towards the south-east or south-south-east, and were probably associated with the deposition of overlying blue-grey tills of the Banffshire Coast Drift Group relatively early in the Main Late Devensian glaciation, if not earlier (P915251; Wilson, 1886; Read, 1923). However, some north- to north-north-east-trending striae located to the north-east of Huntly, on Sheets 86W and 96E, are associated with a later ice movement, as at two localities, they cross, and are therefore younger than, striae directed toward the east (Read, 1923, fig. 11). Several sets of striae and roches moutonnées in the south of Sheet 87W relate to a later eastward movement of East Grampian ice.
On Sheet 87E Peterhead, striae are mainly restricted to the coast where several north-north-east-orientated markings were reported by Wilson (1882) in former granite quarries around Stirling Hill and near Yoags’ Haven (NK 116 394), some 2 km to the south. These striae were possibly created by East Grampian ice that flowed north-east towards the North Sea prior to the incursion of the coastal ice responsible for laying down the Logie-Buchan Drift Group.
Striae are sparse in the glaciated upland on the western side of Sheet 76E Inverurie. They have been recorded to the north of Torphins and on the northern side of Hill of Fare, where they indicate eastward flow of the East Grampian ice sheet. Glaciated streamlined rock knolls are more numerous, particularly on the lower ground and suggest slightly divergent ice-flow directions (generally towards the north-east and south-east). This divergent flow probably occurred at a relatively late stage when ice movement was influenced by the topography. Such flow directions accord well with retreat positions of the ice front indicated by moraines in cols north-east of the Hill of Fare (see below).
Striae and roches moutonnées are relatively abundant on Sheet 77 Aberdeen reflecting the cumulative amount of glacial erosion throughout the Quaternary. Inland, striae are mostly orientated towards the east, but towards the coast they swing towards the north-east, especially to the south of the Don (Bremner, 1938; Munro, 1986, fig. 31). Jamieson (1882b) noted that striae directed towards the north-north-east were superimposed on ones orientated towards the east-north-east in a former quarry south-east of Cove Bay and striae of roughly similar orientations are reported to the north of Banchory-Devenick (NJ 915 005) (Munro, 1986). Several swarms of ice-smoothed, elongated, streamlined knolls (rock drumlins) occur on the sheet, notably around Newmacher, Peterculter, to the north-west of Belhelvie, and in the vicinity of Ythan Lodge (NJ 995 270) (P915379). Most of the features on high ground to the north of the Don are orientated towards the east, but those on the flanks of the Potterton Burn drainage channel are orientated south-east. These orientations accord with that of nearby rock knolls suggesting, as on Sheet 76E, that early ice-flow was towards the east, but as the ice thinned and the higher ground became ice-free, flow directions were influenced by the underlying topography. To the south of Nigg Bay, at Doonies Hill (NJ 960 030), there are several northward-directed roches moutonnées, reinforcing the evidence from striae that ice moved in that direction along the coast.
Few striae have been recorded on Sheet 66E Banchory. To the north of the Dee and the Feugh they show a consistent north-east alignment, but north-north-east-orientated striae have been recorded south of Kirkton of Durris (P915380). Both sets indicate that the flow of the East Grampian ice sheet was directed parallel to the alignment of the Dee valley. On the watershed between the catchment of the Dee and the Bervie Water, striae trending east-south-east are present on a glaciated surface of granite exposed in a newly constructed forestry track, north-east of the Wild Mare’s Loup drainage channel. In Strathmore, a single roche moutonnée, on the northern side of the Glen of Drumtochty (P915380), indicates south-east directed ice movement. Crossing south-east- and eastsouth-east-orientated striae have been recorded on an exposure of andesite north-east of Knockbank Farm (NO 746 798). These observations suggest that the East Grampian ice sheet impinged onto the north-western flank of Strathmore, before the north-east flowing Strathmore ice stream became established (P915251).
Striae are much more numerous on Sheet 67 Stone-haven. Those occurring to the south of the Carron Water are consistent with ice moving north-eastwards along Strathmore. It is apparent that the northern margin of the Strathmore ice stream passed offshore just to the north of Muchalls, from where it ‘hugged’ the coastline closely towards Aberdeen. It encroached onshore near Portlethen, where distinctive red-brown deposits of the Mearns Drift Group were laid down.
Most of the striae and glaciated rock knolls developed on the Dalradian outcrop in the northern part of the sheet imply ice movement towards the south-east and accord with the alignment of the majority of those recorded by Bremner (1920a). However, his three records to the north of Muchalls of north-north-east-orientated striae crossed by a later south-east-orientated set, have not been confirmed during this survey. Although the trend of the north-northeast set of striae is coincident with the dominant foliation in the Dalradian rocks, the precision of his quoted measurements leave little reason to doubt Bremner’s original observations. Based on his records of crossing striae, distribution of erratics and the outcrop pattern of glacial deposits, Bremner (1920a) suggested that at some point Strathmore ice moved several kilometres inland onto the upland areas to the north and west of Stonehaven, probably reaching elevations of at least 330 m above OD (P915251).
Drumlins, drumlinoid ridges and large-scale glacial gouges
Drumlins are one of the most distinctive features of glacial moulding, yet they are relatively rare in the district. A drumlin is typically a smooth, oval-shaped hillock of glacial drift with a steeper, blunter end pointing up-glacier and a gentler sloping, pointed end in the former down-glacier direction. Most glaciologists agree that drumlins are bedforms streamlined in the direction of ice movement and produced mainly as a result of subglacial deformation of soft sediments (Benn and Evans, 1998). Apart from a few elongate till ridges of uncertain origin occurring in the Elgin district (Peacock et al., 1968; P915371), drumlins appear to be restricted in distribution to the east coast. Even there, however, the features are probably best described as ‘drumlinoid ridges’ because few are perfectly formed.
The general absence of drumlins probably results from the relatively sluggish flow of ice across most of the district, rather than to any lack of deformable substrate, although drift deposits are relatively thin. The East Grampian ice sheet may also have been ‘cold-based’ during most of its existence, in which case little subglacial deformation is likely to have occurred (Benn and Evans, 1998). However, there are local exceptions, as for example to the south-east of Lurg Hill (NJ 506 575) on Sheet 86W Huntly, where there is a swarm of drumlinoid ridges and peat-filled gouges glacial sculpting of Old Red Sandstone bedrock is apparent o the south-western flank of the Hill of Finden (NJ 803 637) (P010096), south of Gardenstown (P915373).
A swarm of drumlinoid ridges and intervening, anastomosing gouges extends from a kilometre or so west of a line from Strichen to Maud, on Sheet 87W Ellon, eastwards towards the coast (P915376; P915377). The ridges typically have a relief of 10 to 25 m, and are formed mostly of deeply decomposed bedrock and till. The trend of the moulding suggests a direction of ice-movement between east and east-north-east, which is slightly at variance with east-south-east- directed striae recorded nearby on Mormond Hill (see above) and two sets of north-east-orientated striae on Sheet 87E Peterhead. The moulding affects, and therefore postdates, most of the deposits at the important glacial/interglacial succession at Kirkhill, including the blue-grey Corse Diamicton Formation. The patchy uppermost till at Kirkhill, the Hythie Till Formation, has a fabric indicative of ice flow towards the east-north-east, suggesting that it was laid down during the streamlining event. The blue-grey till and rafts of the Banffshire Coastal Drift Group occurring at Oldmill has been streamlined similarly. There is little or no evidence of glacial streamlining on the ‘Buchan Ridge’ and across central Buchan, but there are distinct east-south-east to south-east-directed gouges in the vicinity of Ellon, depicted on Sheet 87W (P915376). These features might have been produced by the ice that pushed inland from the North Sea basin, laying down the deposits of the Logie-Buchan Drift Group, but it seems unlikely that the margins of this ice mass would have had the power to create such features.
Glacial streamlining is evident across large parts of Sheet 76E Inverurie and Sheet 77 Aberdeen, and it generally becomes more pronounced eastwards, especially to the east of a line between Oldmeldrum and Inverurie. Many of the ridges have rock cores, particularly at their stoss (up-glacier) ends and only a thin covering of till. Around Newmachar (P915379), for example, there is a gradation between small south-east-trending rock-cored drumlinoid till ridges and ice-moulded rock knolls, which is difficult to delineate by surface mapping alone. Limited drilling and trial pitting in the area (Auton and Crofts, 1986) has shown that till on the crests of ridges can vary in thickness between 0.6 and 2.6 m. The orientation of the drumlinoid features, particularly those on higher ground, generally indicates an eastward flow of ice towards the North Sea. West–east-orientated drumlins are well developed between Belhelvie and Udny Station where several examples of crescent-shaped depressions filled with peat occur at the stoss end of the features (P915379). In the southern part of Sheet 77, the orientation of the features swings around to the eastnorth-east, but again indicates seawards flow of the East Grampian ice sheet. The south-east trend of ridges on the lower ground around Hatton of Fintray, Newmachar and south-west of the Potterton Burn may be preserving evidence of an early south-east directed ice movement (Bremner, 1928). Alternatively, the features may have formed late in the glaciation when flow directions of the thinning East Grampian ice sheet became more influenced by the local topography.
On Sheet 66E and Sheet 67E, drumlinoid ridges are well developed only within the outcrop of the East Grampian Drift Group. Their orientation is compatible with the ice flow directions that can be inferred from nearby striae and glaciated rock knolls (see above).
These typically oval-shaped hollows scoured out of bedrock have a broadly similar distribution to the drumlinoid ridges described above and tend to occur in the areas where there has been most cumulative glacial erosion. They are commonly peat filled because of poor drainage, and are especially common in the area underlain by the Strichen Granite on the margin of Sheet 87W Ellon and Sheet 97 Fraserburgh. Smaller, more irregularly shaped features are common to the west of Maud (P915376) where highly weathered gabbro and norite crop out. Harestone Moss occupies an ice-scoured hollow on fresher ultrabasic rocks of the Belhelvie Pluton on Sheet 77 Aberdeen. Numerous alluvium-filled depressions lying to the east of Bennachie and the Hill of Fare on Sheet 76E were scoured out by eastward flowing ice, the largest one being occupied by the Loch of Skene (P915378).
Large elongate elliptical hollows have also been eroded by eastward flowing ice north of the River Don on Sheet 76E. They are linked by west–east-trending drainage channels, and are filled with fine-grained alluvial sediments and peat. Marshes occupy the largest of the hollows, where former lochs, such as Loch of Park and Loch of Leys (NO 702 978) have been drained. South-east-flowing ice has also scoured large irregular hollows in resistant Dalradian bedrock on Sheet 67, north of Stonehaven. Many hollows are occupied by peat mosses, the largest of which is Red Moss (NO 860 940).