Geomorphological features of glaciofluvial erosion, Cainozoic of north-east Scotland

From Earthwise
Revision as of 18:01, 8 June 2015 by BobMcIntosh (talk | contribs) (Created page with "'''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 G...")
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search

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).

Features of glaciofluvial erosion

Glacial meltwater channels

Channels cut by glacial meltwaters are very common throughout the district (Bremner, 1928, 1934; Clapper-ton and Sugden, 1977). Known locally as ‘dens’, many of them are not associated with any significant modern drainage and may be referred to as ‘misfit’ valleys (Plate 14). Three broad generic types of channel have been recognised on some maps: subglacial channels, ice-marginal channels and proglacial spillways. Although the best examples of each type are distinctive, most channel systems formed time-transgressively and distinctions between them are commonly blurred. Furthermore, many have a long and complicated history spanning more than one glaciation. For example, the Tore of Troup on Sheet 97 Fraserburgh (Map 4) and Tom’s Forest channel on Sheet 76E Inverurie (Map 8) are both deeply incised valleys that evidently predate the last glaciation because they are partly filled with till.

It should be born in mind that many of the largest and most impressive drainage channels in the district are not identified as such on the geological maps because they carry modern drainage and contain deposits mapped out within them.

Subglacial, ice-directed channels

Channels of this type, also known as ‘Nye’ channels (Benn and Evans, 1998), formed while most of the district remained buried beneath ice. Subglacial meltwaters were constrained by the regional hydraulic gradient to flow parallel to the direction of ice movement, irrespective of the local subglacial topography. The hydraulic gradient caused meltwaters to flow uphill within some subglacial channels, giving them their characteristic ‘up and down’ long profiles. They are most commonly preserved in cols cutting across topographical barriers that were orientated at an oblique angle to the former direction of ice movement. The channels commonly begin at the crests of such barriers and spurs where the hydraulic gradient and discharge was greatest (Sugden and John, 1976). Where cut into bedrock they are typically steep-sided, winding features that branch and reunite repeatedly. Subglacial channels cut into drift are generally broader, more open features, with a gently undulating long profile.

Ice-directed channels are particularly prominent to the south of Troup Head (Maps 3; 4) where they have been incised into the Middle Devonian conglomerates around the Tore of Troup (Merritt and Peacock, 2000, fig. 22). Most channels are straight, like the Den of Muck (NJ 817 607), but some are winding, like one to the south of Pennan Head (NJ 864 643).

Several subglacial channels aligned parallel to the direction of ice movement cut across the high ground linking Bennachie and the Hill of Fare (Map 8) and across relatively high ground to the west of Stuartfield (NJ 973 458) (Map 6). Particularly good examples include ‘My Lord’s Throat’, cut into fresh Bennachie Granite to depths greater than 20 m and the Bandodle channel, of similar depth, cut into the Hill of Fare Granite (both on Map 8). The latter channel extends east-north-eastwards from a col near West Bandodle for a distance of 3 km. Inland from Aberdeen, several channels cut across Tyrebagger, Elrick and Brimmond hills (Munro, 1986). Many of the major east-draining valleys such as the Don, Dee and Ythan probably also acted as major conduits for ice-directed drainage. In places they have irregular longitudinal profiles due to both ice and meltwater scour.

Ribbon channels, linking ice-scoured bedrock basins, occur north of the River Dee on Sheet 66E Banchory and numerous ice-directed channels are cut into Old Red Sandstone bedrock in Strathmore. A small, but impressive example of the latter is the Paldy Fair Den, near Glenfarquhar Lodge (NO 722 813) (Map 10); large examples include the group of north-east-trending channels around Woodburnden (NO 766 731). Similar large features, such as the Glasslaw and Barras channels occur on Sheet 67 Stonehaven. A particularly good example of a subglacially formed ‘up and down’ channel runs north-eastwards from Craig Den (NO 803 831), on the boundary between Sheet 66E Banchory and Sheet 67 Stonehaven, from where it crosses a col and links with an esker in the vicinity of Lindsayfield (NO 821 844) (Map 11). This channel, and others on the southern half of Sheet 97 are depicted on the sand and gravel resource map of Auton et al. (1990). Another impressive example is the Devil’s Kettle, north-west of Stonehaven, which is cut into resistant, gritty psammitic bedrock to depths that exceed 30 m in places.

Ice-marginal and submarginal channel systems

Complex networks of shallow (1 to 5 m deep) channels are encountered across most of the district, even where clear evidence of glacial erosion is absent, such as in central Buchan. Two distinct sets of channels occur that tend to be orientated at right angles to each other, most commonly north–south and east–west respectively, particularly on Sheet 87W Ellon (Map 6). They were created mainly during deglaciation, as the East Grampian ice sheet receded westwards, and they are particularly common where bedrock is decomposed. Most systems exhibit increasing conformity with local topography at decreasing elevation, with ice-directed channels on the cols, passing at lower levels into ones increasingly orientated downslope in the lee of topographical barriers. The higher elements would have been created first, controlled by the overall configuration of the ice sheet, whereas the lower, slope-related channels would have been cut below thinning ice in the later stages of deglaciation (compare with Sissons, 1958, 1961a, 1961b). Good examples of these types of channels with their characteristically curved or crescentic lower courses include the Pitcowdens and Mount Shade channels on Sheet 66E (Map 10) and the Burn of Muchalls and Burn of Elsick channels on Sheet 67 (Map 11). Dendritic networks of lee-side channels occur locally, as for example, to the north-north-east of New Deer and to the west of Auchnagatt (Map 6).

The origin of the ‘lee-side’, slope-directed channel networks is not absolutely clear, but they were probably formed in the submarginal zone of the retreating ice sheet, particularly where stagnant ice was stranded on the lee side of topographical barriers as they became exposed during ice-retreat. The channels were probably cut initially into the surface of the ice (compare with Price, 1983; Clapperton and Sugden, 1977), then into the frozen ground beneath. This assumes that the ice sheet was cold-based, which is likely, especially as the lee-side channels are intimately related to suites of generally north–south-orientated, ‘ice marginal’, stoss-side channels that commonly occur on the west-facing slopes of the topographical barriers. Ice marginal channels such as these are generally thought to be associated with cold-based, polar ice sheets (Benn and Evans, 1998).

North–south-orientated ice marginal channels are particularly well developed along the eastern side of the valley of the Little Water, which joins the Ythan valley at Chapelhaugh (NJ 843 393), on Sheet 87W Ellon (Map 6). At this locality, the marginal channels generally have arched longitudinal profiles, suggesting that the meltwaters that formed them eventually drained westwards beneath the ice margin into a major submarginal channel that is now occupied by the Little Water. Higher ice-marginal channels are commonly truncated by, or feed into, lower ones, indicating that they formed progressively as the ice margin retreated. Other good examples of flights of channels such as these occur in the vicinity of Berryhillocks (NJ 683 597), south of Banff, and around Cook (NJ 800 574), south of Gardenstown (Map 3).

The orientation of the two sets of channels across Buchan indicates general east-north-east to east meltwater flow within the East Grampian ice sheet, at least in its retreat phase, a direction similar to that indicated by the orientation of drumlinoid features (see above), which almost reach the east coast. The pattern thus lends support to the hypothesis that most, if not all, of northeast Scotland was glaciated during the Main Late Devensian Glaciation (Clapperton and Sugden, 1977). However, individual ice-marginal and lee-side channel systems are coupled locally, and both sets of channels formed successively as the ice front retreated westwards, the drainage being continued both through topographically controlled marginal or submarginal channels and along precursors of the present stream system (compare with Bremner, 1928, 1934). The evidence does not support the existence of an integrated, contemporaneous network across the whole region as proposed by Clapperton and Sugden (1977).

Extensive ice-marginal and interlobate channel systems

Large-scale ice-marginal channels, commonly tens of metres deep, occur along the Banffshire coast on Sheet 96W Portsoy and Sheet 96E Banff, where they arc north-east towards the Moray Firth (Maps 2; 3). They formed at the margin of the combined central Highland and Moray Firth ice stream as it retreated westwards (Bremner, 1934). Similarly orientated channels occur along the southern margin of Sheet 95 Elgin and on Sheet 85E Knockando to the south, the largest being the Blackhills channel (Peacock et al., 1968) (Map 1). The relatively large scale of these features compared with those associated with the retreating East Grampian ice sheet across Buchan, may result from later formation when the climate was comparatively warmer and there was greater meltwater production. Unlike the subglacial channels described above, they generally have regular long profiles, sloping gently towards the coast.

South-eastwardly orientated channels north of Aberdeen, such as those drained by the Potterton and Blackdog burns, were probably initiated as ice-directed subglacial features (Map 9). However, most of the melt-water drainage along them took place when they emerged from the margin of retreating East Grampian ice sheet. The Culter Burn, Silver Burn and Den of Murtle channels near Peterculter carried drainage around the margin of the East Grampian ice sheet into the Dee valley as the ice-front retreated inland.

Several channels were carved out at the former margins of abutting ice masses. For example, on Sheet 87E Peter-head, a prominent channel system links the upper reaches of the Water of Cruden, west of Hatton, northwards to the valley of the River Ugie, to the east of Longside. The route follows the ‘misfit’ valleys of the Laeca Burn, Den of Aldie, Dens and West Den (Map 7). The system was probably formed close to the landward margin of ice that pushed in from the coast on more than one occasion, when meltwaters were diverted across the high ground linking the ‘Buchan Ridge’ and the Hill of Longhaven. The Den of Boddam (NK 114 415) is partly infilled with gravels of the Buchan Gravels Formation and certainly has a long history. Meltwaters also flowed eastwards at the margin of the Moray Firth ice stream. For example, several major channels ‘loop’ inland and back again along the coast between Buckie and New Aberdour (Maps 2 to 4). To the east of New Aberdour, on Sheet 97, meltwaters flowed farther inland via several channels, towards Rathven and the Loch of Strathbeg.

The Strathfinella–Glen of Drumtochty channel on Sheet 66E Banchory (Map 10) and the south-west–northeast-trending Ury Home Farm–Limpet Burn channel, on Sheet 67 Stonehaven (Map 11), were eroded between the abutting margins of the East Grampian ice sheet and the Strathmore ice stream (Simpson, 1955). A group of secondary interlobate channels is present on the northern flank of the former channel. These channels drained eastwards into the main channel in the vicinity of Drumtochty Castle (NO 699 801). The White Hill, Blackhills and Den of Cowie channels on Sheet 67 were formed by drainage at the margin of the Strathmore ice as it retreated (Plate 14a). This meltwater probably included drainage from ice-dammed lakes that developed between the two adjacent ice masses.

Proglacial spillways

In contrast to the subglacial and ice-marginal types of drainage channels described above, proglacial spillways generally have regular longitudinal profiles. When cut into bedrock, they also, typically, have a pronounced V-shaped cross-section, and interlocking spurs. Many formed as ‘overflow’ channels where ice-dammed lakes drained across cols (compare with Charlesworth 1926, 1956; Synge, 1956). Others formed when major drainage routes became blocked by glacier ice. Some would have been cut catastrophically as a result of glacier outburst floods or ‘jökulhlaups’ (Maizels and Aitken, 1991; Brown, 1994).

A good example of a spillway, the ‘Towie Spillway’, occurs on Sheet 86E Turriff between Darra (NJ 744 474) and Towie Castle (NJ 744 439) (Map 5). It formed when the valley of the River Deveron was blocked by ice downstream of Turriff, causing all drainage in the upper catchment of that valley to be diverted southwards into the Ythan valley (Figure 42). The Blackhills channel on Sheet 95 Elgin probably also functioned as an overflow channel, as did the valley of the Black Burn, which joins the River Lossie at Elgin (Map 1). Two prominent spillways draining former water-filled basins have been identified on Sheet 77 Aberdeen, near Blackburn and Kingswells (Munro, 1986).

The Westhills channel, west of Belhelvie, may have acted as a spillway for meltwater draining from the East Grampian ice sheet, before ice-marginal drainage became established in the nearby Potterton Burn channel (Map 9). Although no discrete channel-feature is preserved, it is clear that the extensive spreads of sand and gravel around Corby Loch were laid down by eastward flowing meltwater draining along the Don valley and across the interfluve towards the sea. This drainage route existed before flow was re-established in the lower reaches of the valley of the present river (Aitken, 1998).

A major spillway, Slack Den breaches the watershed between Glen Dye and Strathmore (the ‘Slack of Birnie’ of Synge, 1956) on Sheet 66E (Map 10). The spillway is cut, locally to a depth of more than 100 m, into resistant hornfelsed interbanded psammites and semipelites. The intake of the spillway occurs at about 350 m above OD, about 1.6 km north of the present watershed. The spillway was cut by drainage from a glacial lake that was ponded behind East Grampian ice blocking the lower reaches of Glen Dye. The south-west-directed drainage also dissected ice-contact sands and gravels laid down at the margin of the Strathmore ice stream during its retreat, depositing a fan of sand and gravel (containing clasts predominantly of psammite and semipelite) in the vicinity of Clatterin Brig (NO 664 782). The surface of the fan stands some 30 m above the level of the present valley floor and has been dissected by the Slack Burn. The south-western end of the spillway also bisects the group of secondary interlobate drainage channels on the northern flank of the Strathfinella–Glen of Drumtochty channel (see above), indicating that meltwater drainage along the spillway commenced after the initial parting of the East Grampian and Strathmore ice streams.

Buried valleys

There are several examples of deep, buried valleys around the coasts of north-east Scotland. These channels were probably formed by subaerial drainage when sea level was relatively low during glacial stages, but when the land was not greatly depressed isostatically. Most are unlikely to date from the early Late-glacial period because sea levels were relatively high during deglaciation. They are commonly capped by glaciofluvial deposits, thus ruling out formation during the low sea-level stand of the early Holocene (Chapter 5). Moreover, many are partly filled with till and clearly predate the Main Late Devensian glaciation. Some may be very old features indeed, like one underlying the lowest reaches of the valley of the River Spey (Map 1), which is partly filled with unusual pale greenish grey sand (Aitken et al., 1979).

Deep channels have been located at the mouths of both the Don and Dee at Aberdeen (Figure 46). Both are largely filled with till and predate the Main Late Devensian glaciation. The River Don flows through a gorge cut in rock immediately upstream of the Bridge of Don (NJ 940 097), yet boreholes in the vicinity of Seaton Park, some 600 m to the south of the gorge, show that rockhead is well below sea level (Lumsden, 1958; Munro, 1986). A buried channel bottoming at about 15 m below OD lies beneath the floodplain of the River Dee between the Bridge of Dee (NJ 928 036) and the Wellington Suspension Bridge (NJ 943 050) (Peacock et al., 1977; Munro, 1986). Downstream of the latter bridge, a drift-filled channel diverges from the present valley and extends eastwards beneath the valley between Torry and Tullos Hill (Simpson, 1948). It reaches the coast at Nigg Bay (Map 9), where it bottoms at a depth of at least 40 m below OD, and extends for more than a kilometre beneath the bay (Munro, 1986). Geophysical data suggests that the feature is cut in bedrock and is narrow and gorge-like (Law, 1962). Another buried channel has been located beneath the centre of Aberdeen (Figure 46).

A further buried valley has been located beneath the valley of the River Ythan at Ellon (Merritt, 1981) (Map 6). It diverges from the present valley downstream of the town, passing south of the Hill of Logie (NJ 978 297) before reuniting with the present course at the estuary (see cross-sections on the Solid and Drift edition of Sheet 87W). Like those at Aberdeen, the buried channel of the Ythan is gorge-like and contains till. Seismic evidence suggests that it descends to some 40 m below OD at the Snub (NK 002 282) (Quaternary Research Association, 1975).

It is likely that many buried channels remain to be discovered, particularly along the coast. For example, the River Ugie flows through a gorge cut in bedrock near Inverugie (NK 102 482) (Map 7), yet boreholes drilled upstream failed to reach bedrock at 2 m above OD (McMillan and Aitken, 1981). This suggests that a buried channel occurs somewhere between Inverugie and St Fergus village (NK 098 520). Inland, segments of old channels cut in rock and locally up to 10 m deep are known, for example, at Kirkhill Quarry, where the protection from glacial erosion provided by the features has helped preserve the complex Middle to Late Devensian succession there (Connell et al., 1982; Connell, 1984a; Appendix 1 Site 7).

References

Full reference list