Geomorphological features of marine 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.
Two distinct sets of raised beaches, clifflines and rock platforms are common around the coasts of north-east Scotland. The sets were formed during periods of relatively high sea level, in Late-glacial times and in the mid-Holocene (P915295). Sea level was appreciably lower than it is today between these periods. Raised glaciomarine deposits have been described in Raised marine deposits.
During the past twenty thousand years or so, sea levels in north-east Scotland have been influenced by both the isostatic depression of the land under the ice load, and by global (eustatic) sea levels. The largest amount of isostatic depression occurred in the western Highlands where the ice load had been greatest. That region has consequently been the centre of uplift since the ice melted. The rate of uplift was greatest during and immediately after deglaciation and has fallen exponentially since. The result of this pattern of isostatic recovery is twofold. Firstly, all raised marine deposits and associated erosional features in north-east Scotland are tilted eastwards and northwards away from a centre of uplift positioned in the vicinity of Rannoch Moor. Secondly, the younger, lower lying deposits and features are tilted less than the older ones. It follows that beaches of the same height in different places are not necessarily of the same age.
The older, Late Devensian set of raised features formed during Late-glacial times, that is, between the initial decay of the Main Late Devensian ice sheet in this region and the end of the Loch Lomond Stadial (Gray and Lowe, 1977). These remnants of shorelines lie up to about 30 m above OD, and are backed by discontinuous, degraded clifflines. The deposits locally merge into glaciofluvial sediments that were laid down contemporaneously. The Late-glacial set of shorelines and deposits are commonly truncated by an extensive cliffline, the Main Postglacial Cliffline. This feature formed during the mid-Holocene, at the climax of a major marine transgression, the Main Postglacial Transgression, which culminated between 6300 and 5700 BP in the Fraserburgh area (Philorth valley) and at about 4750 BP in the lower Ythan valley (Smith et al., 1999). The Main Postglacial Cliffline commonly backs the younger set of Flandrian (Postglacial) raised marine deposits, which lie up to about 10 m above OD.
Late Devensian raised beaches
Late Devensian raised beach deposits are present on the Moray Firth coast between Forres and Banff, but there is little or no record of such features between the latter locality and Fraserburgh. They are best developed between the mouths of the River Findhorn and the River Spey on Sheet 95 Elgin, particularly on the seaward side of the Covesea–Roseisle ridge, which would at one time have been an island (Peacock et al., 1968, fig. 18; P915371). They are less evident on the landward side of the ridge, where they are replaced in part by undulating, kettled glaciofluvial gravels, probably because the sea was excluded by stagnant glacier ice left behind during the westward retreat of the Moray Firth ice stream. Stagnant ice may also have been present on the southern side of the Spynie basin where Late-glacial beaches and raised marine sediments are again absent. The highest beaches occur as short stretches to the west of Burghhead (about 24 m above OD), Hopeman (about 24 and 19.8 m OD), and Easter Covesea (about 24 and 21 m OD). The hill on which Lossiemouth stands formed an island, and beach features at 22.5, 19.5 and 14.6 m OD were formerly seen in the western suburbs. These stretches of beach are formed of gravel, bouldery in places. At Greenbrae Quarry (NJ 137 692), the ‘19.8 m’ beach overlies a striated glaciated pavement. Detailed levelling has shown that the back feature of another beach descends eastwards from about 14.3 m OD south of Grange Hall (NJ 064 606) to about 13.7 m OD by Milton Brodie House (NJ 092 629) over a distance of 3 km.
There is no certain evidence for Late-glacial beaches between Lossiemouth and the mouth of the Spey, but farther east a raised beach at 14 m OD merges southwards into the Mosstodloch Terraces of the River Spey (P915371). Eastwards, there is a gravel beach up to 2.4 m thick (top surface about 15 m OD) resting on till on the low quartzite cliffs at a locality (NJ 450 675) immediately south of Craig Head, west of Findochty (P915372). A Late-glacial beach feature at about 22 m OD is seen south-east of Whitehills, and again in Banff, where it is followed by the A96 trunk road (P915373). These beaches are generally formed of a metre or so of poorly sorted gravel and boulders. Spreads of glaciofluvial sand and gravel appear to merge with raised beaches (at about 25 m OD) at the mouth of the Boyndie Burn, forming a raised delta. Other degraded Late-glacial shoreline features were identified between Cullen and Tochieneal (NJ 522 653) (P915372) by Read (1923), but they have not been confirmed by the recent revision. At Rosehearty (P915374), an area of bare rock below 9 m OD, which extends for about a kilometre west of the village, may have been scoured by the Late-glacial sea, but no deposits are known.
At St Fergus, a raised beach has been identified at about 15 m OD, but apart from this no Late-glacial raised beaches have been identified with certainty between Fraserburgh and Aberdeen. Farther south, Ritchie et al. (1978) reported a possible raised beach terrace at 10 to 12 m OD on the north-east side of the estuary of the River Ythan, above the level of Flandrian raised beach terraces at 3 to 6 m OD. However, terraces around 10 to 12 m OD are glaciofluvial (Merritt, 1981). Raised beaches may be expected adjacent to the low coastline north of Aberdeen, because raised marine clays are known at Aberdeen itself (see below) and the trend of isobases of Late-glacial beaches to the south suggests that the contemporary sea level would have been above 15 m OD (Sissons, 1983, fig. 4). The absence of Late-glacial beaches suggests that the Late-glacial sea here extended into an area of sediment-covered ice (compare with the Elgin area, above), the subsequent decay of which destroyed any evidence of beaches.
Bremner (1920a) was the first to question the extent of raised beach deposits shown as underlying much of the town of Stonehaven on early editions of Sheet 67. He recorded the presence of red till and ‘brick clay’ on the surface of the supposed ‘100 foot raised beach’ and an ‘absence of all appearance of sifting and arrangement by sea waves’ of the supposed beach gravels. These observations have been confirmed during the resurvey of Sheet 67. The gravels underlying the town that were formerly assigned to the ‘Second Raised Beach’ are shown as glaciofluvial sheet deposits on the current geological map.
Fragmentary Late Devensian raised shorelines have been recognised at six localities between Stonehaven and Inverbervie (Cullingford and Smith, 1980, table 3, fig. 2) and have been grouped with more extensive fragments that extend as far south as St Cyrus. The Stonehaven–Inverbervie shorelines occur at elevations ranging from 7.5 to 38.8 m OD between Garron Point and the mouth of the Cowie Water, at Downie Point, Bowdun Head, north of Crooked Haven, north of Upper Mill and at Kinghornie near Inverbervie (P915381). Raised beach deposits have been mapped south-westward from Garron Point, at Downie Point and at Kinghornie. The deposits appear to be heterogeneous. A resistivity sounding at Garron Point indicated that the deposit is composed mainly of clayey silt, but in a degraded bluff 350 m south-west of Mains of Cowie (NO 877 868), 3 m of well-rounded cobbly beach gravel was formerly exposed, overlying red-brown till (Auton et al., 1988).
The shoreline fragments occur at distinctly higher elevations than the majority of those found farther south, between Dundee and Montrose (Cullingford and Smith, 1980, fig. 7) and have not been directly correlated with these well established sequences. Because of their high elevation and proximity to spreads of kettled glaciofluvial outwash, Cullingford and Smith (1980) postulated that the Stonehaven–Inverbervie shorelines developed early in the deglaciation of Strathmore, while ice was still present in the vicinity. These Late-glacial shorelines are therefore probably older than any of the features in the Montrose Basin and in the Forth and Tay valleys.
Flandrian raised beaches
The highest and best developed Flandrian raised beaches in the district were formed about six thousand years ago. They generally equate with the ‘Main Post-glacial Shoreline’ of eastern Scotland, which is tilted gently north-eastwards and drops to its lowest level between Fraserburgh and Peterhead (P915296). Along the coast of the Moray Firth it falls from about 9 m OD at Inverness, to 7.5 m OD near Elgin, and to near present high-tide level at Fraserburgh (Peacock et al., 1968; Smith et al., 1982, 1983; Firth and Haggart, 1989). Along the North Sea coast, it rises southwards to about 4 m OD in the Aberdeen area, and to between 5 and 5.5 m at Milton Ness (NO 772 647), 8 km north of Montrose, and 6.4 m at Montrose (Smith and Cullingford, 1985; Cullingford et al., 1991). At Fraserburgh, however, the youngest Flandrian shoreline is about one metre higher than the Main Postglacial Shoreline, indicating that sea level is continuing to rise hereabouts (Smith et al., 1982).
The most extensive Flandrian raised beach deposits in the district occur on Sheet 95 Elgin (P915371), where they form storm beaches and shingle ridges that stretch from Kinloss to Burghead and from Lossiemouth to Portgordon (Ogilvie, 1923; Steers, 1937; Peacock et al., 1968). The western belt ranges from 180 m to 1.6 km in width and comprises shingle ridges that are more or less parallel to the beach and that are separated from one another by strips of peat. The ridges are locally buried beneath blown sand. The eastern belt ranges from 700 m to 2 km in width and contains similar features, except that the ridges bend southwards away from the shore at the mouth of the River Lossie (P239922). The ridges are formed of openwork, well-sorted, well-rounded, pebble to cobble grade gravel with sparse beds of sand. Although generally spherical, wave action has created local accumulations of tabular or bladed clasts. Shell fragments are common locally in the lower ridges. The gravel is generally very durable and has been exploited widely for aggregate (Bulk mineral resources). The highest ridges occur at the back of the two belts, where they locally reach about 9 m OD. These features probably formed during the creation of the Main Postglacial Shoreline, whereas those at lower levels towards the sea formed subsequently, while sea level was falling.
There is a belt of peaty and sandy flats that stretches between Kinloss and Loch Spynie, behind the beach ridges (P915371). The flats have been variously mapped as raised marine deposits, alluvium and peat, depending on what sediment occurs at the surface. They mark the sites of former brackish lagoons and tidal flats that existed when Roseisle and Branderburgh were islands in the mid-Holocene. The marine deposits generally consist of olive-grey or brown silty clay and yellowish brown micaceous sand. The River Lossie seems at various times to have found outlets northwards near its present mouth at Lossiemouth, and westwards south of Burghead. In historical times, there was a seaport beside the Palace of Spynie (NJ 231 658) until the late 15th century.
Flandrian raised beaches, formed mainly of shingle, occur discontinuously between the mouth of the River Spey and Troup Head, mainly at the heads of inlets and coves along this rugged, indented stretch of coast. The back feature of the raised beaches locally coincides with the base of the cliffline backing the pre-Late Devensian rock platform.
Raised Flandrian estuarine deposits are widespread in the Fraserburgh area, where they consist mainly of brown silty clay. The clay locally overlies peat, within which two laterally persistent layers of minerogenic material occur (Philorth valley). The lower layer consists of grey sand and lies at about 1 m below OD. It has been dated to about 7 ka BP, and is attributed to a regionally significant tsunami (‘tidal wave’). The upper layer consists of micaceous sandy silt that was laid down between 6.3 and 5.7 ka BP during the Main Postglacial Transgression. The brown clay at the surface began to accumulate after about 4.8 ka BP during a subsequent transgression that probably continues today.
Shingle ridges and sand dunes, some of which were formed in historical times, extend along the coast between St Combs and Rattray Head, on Sheet 97 Fraserburgh. They separate the Loch of Strathbeg (formerly a tidal lagoon) and the historical port of Old Rattray from the sea (Peacock, 1983). Between Rattray Head and Peterhead (P915377) the highest Flandrian beach is only a little above high tide level, and is backed by a low cliff cut in glacial deposits. A borehole drilled on the raised beach 350 m south-east of Annachie (NK 1085 5280) proved 2.6 m of sandy gravel resting on till at an elevation of 1.1 m below OD (McMillan and Aitken, 1981). The gravel is well rounded with a matrix of medium- to coarse-grained shelly sand. However, this raised beach, and others facing Sandford Bay and the Bay of Cruden, is largely concealed by blown sand. A borehole positioned amongst sand dunes behind the beach at the Bay of Cruden (NK 0858 3555) proved 8.1 m of grey sand and sandy gravel with shell fragments resting on till at 9.5 m below OD.
Several small deposits of grey silty clay flank the estuary of the River Ythan downstream of Kirkton of Logie-Buchan on Sheet 87W Ellon. These deposits resemble estuarine carse clays of the Forth estuary and are associated with the highest Flandrian shoreline at about 4.5 m OD (Smith et al., 1999). At Waterside (NK 003 273), a layer of grey sand attributed to a major tsunami (see above) has been found lying at about 0.5 m OD (Long et al., 1989). The main raised Flandrian shoreline is extensive between the Ythan estuary and Aberdeen, but it is generally buried beneath blown sand, like the low cliffline backing the feature. However, information from boreholes and trial pits in the vicinity of Menie Links indicates that the shoreline is mainly underlain by over 2 m of dark grey, micaceous fine-grained sand and sandy silt (Auton and Crofts, 1986). The deposit is peaty at the top and becomes gravelly towards the base. The peaty top is probably equivalent to several beds of peat that have been recorded below present-day beach deposits in the vicinity (Jamieson, 1858; Bremner, 1943). Extensive building development has taken place on the raised beach deposits between the mouths of the Don and the Dee, which are mainly composed of sand and gravel (Munro, 1986).
A broad raised beach, at about 5 m OD occurs immediately inland of the present high-water mark around Stone-haven Bay where it forms the flat-lying ground beneath Stonehaven sea front and shopping centre. The raised beach is generally about 200 m wide and is backed by a degraded cliffline, trending north–south, cut in bedrock capped by thin till and head (Barclay Street follows the break of slope at the base of the cliff). The raised beach reaches some 350 m in width near the mouth of the Cowie Water, where the flat ground has been developed as holiday caravan parks. Records from site investigations near Stonehaven harbour indicate that the raised beach deposits comprise pebbly sand and gravel, containing small amounts of shell debris locally. The deposits generally range in thickness from 2 to 5 m and overlie either red-brown laminated clay or bedrock. The nature of the stratification within the raised beach is unknown, but surface topography suggests that Allardice Street follows a north–south-trending beach ridge (Carroll, 1995a).
Flat-lying raised beach deposits have been mapped, inland of the high water mark between Bervie Bay (Inverbervie) and the southern margin of Sheet 67. They extend to between 5 and 10 m OD. Some of the highest deposits may be of Late-glacial age, but most of the lower spreads are products of marine transgression during the Holocene. The deposits consist of coarse-grained pebbly sand and gravel, with minor amounts of shell debris (Carroll, 1995b). The raised beaches are backed by the steep Main Postglacial Cliffline, which is cut into till and bedrock. Widespread landslides are developed along the cliff face and in many places, the raised beach deposits are concealed beneath slipped material at the foot of the cliff.
Raised wave-cut platforms and associated features
Wave-cut rock platforms with back-features a few metres above present high water mark are common along the rocky northern coast of the district on sheets 95, 96W and 96E. Between Burghead and Lossiemouth, at Cullen and at Gardenstown, a prominent rock platform and cliff is cut into the relatively weak Permo–Triassic and Devonian sandstones. There are ‘fossil’ sea-stacks on the platform above high tide level to the east of Covesea (P915371), and also at Cullen. These features are probably partly post-glacial in age, but between Portsoy and Troup Head, the platforms probably occur at more than one level and some of them are partially overlain by glacial deposits of the Whitehills Glacigenic Formation. The main platform reaches some 300 m in width between Whyntie Head and Whitehills (P104117), but where the cliffs are particularly high, such as at Troup Head, it is either missing or is represented only by rock reefs with concordant levels. There is little evidence of correlative platforms along the eastern coast of the district. One does occur at the ‘Bridge of One Hair’, south of the Bay of Nigg, on Sheet 77, where a sea-stack is joined to the mainland by a plug of red till some 30 m thick (Synge, 1956).
A raised rock platform standing at an elevation of about 23 m OD extends between Ruthery Head (NO 888 872) and the Glen Ury distillery (NO 871 869), north of Stonehaven. Bremner (1920) suggested that the platform is a ‘preglacial’ feature because it is capped by till. It is more likely to have been formed following a major pre-Late Devensian glaciation(s) when sea level was relatively high as a result of glacio-isostatic depression of the land. The till-covered feature is, after all, capped by Late Devensian raised beach deposits. Other fragmentary rock platforms occur to the south of Stonehaven at Downie Point, Bowdun Head, near Crooked Haven by Kinneff, and at Kinghornie, just north of Inverbervie.
Submerged wave-cut platforms
Along the eastern coast, the sea cliffs locally descend to a topographical break, possibly a wave-cut platform, at 55 to 60 m below OD (Crofts, 1975). A series of till-covered platforms have been detected from seismic records offshore between Stonehaven and Inverbervie (Stoker and Graham, 1985). These occur at about 30, 45 to 50 and 60 to 70 m below sea level. Wave-cut platforms such as these have been attributed to severe marine erosion during periods of lowered sea level coincident with periods of glaciation (Sutherland, 1984a). Submerged geos and stacks partially enclosed by till occur to the north of Newburgh and south of Aberdeen (Synge, 1956; Walton, 1959).
‘Pre-glacial’ raised beaches
Deposits of angular and rounded boulders, gravel, and sand lie locally between bedrock and till along the Banffshire coast. They were ascribed by Read (1923) to a ‘pre-Glacial raised beach’. However, re-examination of these sediments at Portessie (NJ 4510 6722), in a gully (NJ 4710 6815) between Findochty and Portknockie, and at Portnockie harbour (NJ 4882 6858) (P915372), suggests that most, if not all, are of glacial or glaciofluvial origin (Peacock and Merritt, 2000). The deposit at the first locality is interpreted as ‘immature’ till whereas the gravel at the other two localities is almost certainly glaciofluvial, laid down or reworked immediately prior to the advance of the Moray Firth ice stream of the Main Late Devensian ice sheet across the area.