Landscape evolution, 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.
The lowlands of north-east Scotland are an excellent area for the study of long-term landscape evolution. The erosional impact of the Quaternary glaciations has been modest and a range of erosion surfaces of Palaeogene to Neogene age (see table below) are preserved (P915262). The development of these Tertiary landforms has been studied using evidence from deep weathering profiles and preglacial deposits. On land, evidence from rocks that postdate the Caledonian orogeny is limited. The geological record is confined to remnants of formerly extensive Devonian intramontane basins, Permian dykes and very small outliers of Cretaceous rocks. Much greater thicknesses of Mesozoic and Cainozoic rocks are preserved within the adjacent basins of the Moray Firth and central North Sea. These provide important evidence of environments that prevailed on the adjacent land area.
|Era||Period (System)||Epoch (Series)||Age(of base Ma)|
|NEOGENE (formerly known as the ‘Tertiary’)||Miocene||23.8|
|PALAEOGENE (formerly known as the ‘Tertiary’)||Eocene||54.8|
Knowledge of ancient landscapes in north-east Scotland is important for several reasons. Firstly, the region forms the onshore extension of a structural ‘high’ that divides the Moray Firth and central North Sea basins offshore. Buchan and its contiguous lowlands to the south and west have periodically provided sediments to both basins and, in turn, have periodically been covered with sedimentary rocks laid down beyond the basin margins. While research relating to hydrocarbon exploration has provided a large amount of new data on the tectonic development of the offshore basins, relatively little is known about events on the adjacent structural high. Secondly, the preservation of preglacial landforms is matched only in a few other formerly glaciated areas and so the region provides a rare opportunity to investigate Palaeogene and Neogene landscapes within the limits of the Quaternary ice sheets. Thirdly, the extensive presence of deeply weathered rock is not a feature normally associated with glaciated landscapes. It is more akin to crystalline terrains in parts of Europe beyond the Quaternary ice sheet limits. Deeply weathered rock is a source of aggregate and provides the parent material for Quaternary deposits and soils. Assessment of its characteristics and distribution is very important in site investigations for major engineering projects. Finally, the limited degree of glacial erosion places the lowlands of north-east Scotland within a select group of similar regions around the North Atlantic, including parts of southern Sweden (Lidmar-Bergstrom, 1982) and islands in the Canadian Arctic (England, 1986), in which it is possible to investigate the processes that have operated under dominantly cold-based ice sheets.
The evolution of the landscape of the lowlands of north-east Scotland is described below. The gross features of the relief, the main hill masses, valleys and plains, first began to emerge in the Palaeozoic and may have taken on much of their present distribution and general form by the end of the Mesozoic. The landscape continued to develop in response to uplift and differential chemical weathering under humid climates throughout the Palaeogene and Neogene, producing a wide variety of preglacial landforms. These landforms and landscapes were then variably modified by the successive ice sheets of the Quaternary period.
Palaeozoic and Mesozoic
Inherited Devonian surfaces
The oldest landscapes preserved in north-east Scotland are of Devonian age. During this period the region was desert and lay in the Southern Hemisphere. It formed the southern boundary of the Orcadian basin at the foothills of a major Grampian mountain chain (Andrews et al., 1990). These mountains were created earlier in the Caledonian orogeny and had already suffered prodigious erosion before Devonian times. At around 470 Ma, numerous large ‘Younger Basic’ igneous masses were emplaced at crustal depths of around 15 km (Droop and Charnley, 1985; Ordovician, Devonian and Carboniferous). Between 425 and 395 Ma, final phases of postorogenic magmatism culminated in the emplacement of the ‘Newer’ Granite plutons at depths of around 5 km (Gould, 1997). Many of these intrusions were already unroofed by Devonian times, as shown by the presence of sandstones and conglomerates of this age resting on exposed basement. Devonian sedimentary rocks overlie the Aberdeen granite, the Belhelvie basic intrusion (Munro, 1986), the Peterhead granite (Hall and Jarvis, 1994) and the Insch–Boganclogh and Morvern–Cabrach basic igneous intrusions (Gould, 1997).
The case of the Bennachie granite is of special interest. The granite was unroofed by about 385 Ma in the Devonian as it apparently contributed pebbles to the Turriff Old Red Sandstone basin (Mackie, 1923). Abundant aplite sheets and the occurrence of intrusive breccias along the lines of the north–south-trending faults show that rocks now exposed represent the uppermost part of the pluton (P915263). Furthermore, the pluton extends east and west of the exposed area on Bennachie under thin cover (McGregor and Wilson, 1967). Its present elevation probably reflects Tertiary uplift. An analogous situation occurs at Moss of Cruden. Here a small patch of arkosic Devonian sandstone (Appendix 1 Site 14) rests on Peterhead Granite (Hall and Jarvis, 1994). Overlying the same intrusion nearby is an outlier of Lower Cretaceous Greensand and deposits of the Buchan Gravels Formation. This association demonstrates that erosion of the Peterhead Granite since the Devonian has been minimal, like Bennachie.
The former extent of Devonian cover rocks may be reconstructed from the present distribution and dip of these rocks and from the composition of the sandstones and conglomerates. The huge volume of clastic sediments that were deposited in the Lower Old Red Sandstone basins of Strathmore indicate that the adjacent eastern Grampian mountains were of considerable relief at this time. North of Aberdeen, the remaining Lower Old Red Sandstone strata were originally laid down on the floor of former intramontane basins. Middle Old Red Sandstone sedimentary rocks occur in the Turriff basin, where they lie with marked unconformity on older sandstones. The Upper Old Red Sandstone is found in coastal areas west of the Spey, but south of the Grampians considerable thicknesses of Upper Old Red Sandstone rest on folded and deeply denuded Lower Old Red Sandstone sequences (Mykura, 1983).
The Lower and Middle Old Red Sandstone form a continuous belt just offshore (Andrews et al., 1990) and occur as widely scattered outliers on the Dalradian basement (P915249). Apart from the main outliers of Turriff, Rhynie, Cabrach and Tomintoul, there are several small outcrops, such as those in upper Strathisla (Hinxman and Wilson, 1902). Other small, concealed outliers may exist. However, an earlier report of an outcrop of Devonian rock in the Feugh basin (Bremner, 1942) appears to have been mistaken for pale orange, sparsely feldsparphyric aplitic microgranite that crops out in the vicinity. Furthermore, the patch of Devonian conglomerate reported from Cruden Bay (Wilson, 1886) appears to be a cemented glacigenic deposit belonging to the Logie-Buchan Drift Group, and another supposed outcrop to the north of Glenlivet seems to be glaciofluvial sand and gravel.
While there is little doubt that the Middle Old Red Sandstone once covered areas well beyond its present outcrops (Sissons, 1967), it is unlikely that cover was continuous away from the current coastal belt, as some have claimed (Bremner, 1942). In the Turriff outlier, dips increase from west to east, conforming to the half-graben basin shape in section. The Turriff, Rhynie and Cabrach basins are bounded to the west by north-east-trending faults. To the east, the Lower to Middle Old Red Sandstone appears to rest with simple unconformity on basement. The basal conglomerates of these and other outliers are dominated by clasts from the local basement (Wilson and Hinxman, 1890; Hinxman and Wilson, 1902; Read, 1923; Peacock et al., 1968; Munro, 1986). These basement rocks were not buried by later deposition as shown by the continued presence of basement-derived clasts in conglomerate bands higher in the successions of the deeper basins (Read, 1923; Peacock et al., 1968). South of Elgin, conglomerates and breccias were deposited against steep hillsides marking the edge of a rugged terrain developed in underlying Moine and Dal-radian metamorphic rocks.
The original thickness of Devonian rocks in the various basins is difficult to estimate. Geophysical surveys indicate that a stratigraphical thickness of around 1 km of Devonian sedimentary rocks are preserved in the Turriff basin (Ashcroft and Wilson, 1976). The high porosity and limited development of quartz overgrowths in the Smallburn Sandstone on the Moss of Cruden suggest that postdepositional burial was limited to 2 to 3 km, although part of this burial may have taken place in the Mesozoic.
In general, sedimentation occurred in fault-bounded ‘half-grabens’ and other topographic basins with erosion of debris from intervening ridges and hills. Sediment transport was generally via a series of north-east- and north-trending valleys towards Lake Orcadie. The strata of the Deskford (Read, 1923) and Strathisla outliers (Geikie, 1878) partly infill two of these ancient valleys. Coarse sandstones and conglomerates indicate high-energy fluvial transport. Rounded and reddened quartzite clasts are abundant and they form a conspicuous component of Quaternary deposits adjacent to the Devonian outliers.
The preservation of sub-Devonian landscapes is likely to be confined to the margins of the Devonian outliers and to the ancient basins and valleys in which these rocks lie (Hall, 1991). Even where Devonian rocks no longer remain, the present subdued topography of the coastal lowlands undoubtedly results largely from post-Devonian planation.
Marine inundation during the Mesozoic
Reconstructions of post-Devonian palaeogeography generally show the lowlands of north-east Scotland as being above sea level for most of the time (Ziegler, 1981). Erosion of this structurally positive area has resulted in the arcuate outcrop of late Palaeozoic sedimentary strata around Buchan (P915249). Yet it is clear from the provenance of the Mesozoic rocks lying just offshore, in the inner Moray Firth and west central North Sea, that the lowlands were also inundated periodically during Mesozoic times.
In the Inner Moray Firth Basin, sequences of Jurassic rocks are up to 3.6 km thick, but thin rapidly across faults and basin highs (Andrews et al., 1990). Remnants of Lower Jurassic cover rocks are found in fault basins around Lossiemouth. Marginal marine sands were deposited across much of the Northern Highlands at this time (Hallam and Sellwood, 1976). The significant overstep of the contemporaneous margins of the Inner Moray Firth Basin in the Late Jurassic indicates that adjacent land areas had been reduced to low relief.
Tectonic activity was renewed in the Moray Firth Basin at the Jurassic/Cretaceous boundary (Anderton et al., 1979; P915264). Lower Cretaceous strata were once much more extensive both north and south of the basin, but have been removed by Tertiary erosion (Andrews et al., 1990). A small, concealed outlier of late Hauterivian–early Barremian glauconitic sandstone at Moreseat (Hall and Jarvis, 1994) is the only known remnant of this cover (Appendix 1 Moss of Cruden). Greensand clasts are known from the Neogene Buchan Gravel Formation at Windy Hills (Flett and Read, 1921) and Moss of Cruden (Kesel and Gemmell, 1981). Large blocks of Lower Cretaceous sandstone of late Hauterivian–early Barremian and Aptian age occur also as erratics in glaciofluvial outwash around Cardno, near Fraserburgh (Cumming and Bate, 1933; P915374). These marine sands of marginal facies must originally have covered most, if not all of Buchan, from the Hauterivian onwards.
The basement rocks in the lowlands of north-east Scotland may not have emerged extensively prior to the Late Cretaceous transgression. That a cover of chalk once existed across Buchan is indicated by the large volume of flint contained within the Buchan Gravels Formation. Furthermore, small remanié lags of nodular flint occur at the base of the Buchan Gravels at Skelmuir Hill (Bridgland, et al., 1997, 2000) and Moss of Cruden (Hall, 1993). These rest on kaolinised crystalline rocks and place the sub-Cenomanian surface close to current summit levels in the area. As the chalk accumulated at water depths of 100 to 600 m (Hancock, 1975) the former chalk sea must have extended across all low ground in north-east Scotland. The lack of terrigenous debris in the chalk offshore (Andrews et al., 1990) also implies that any areas of high ground were remote from the contemporaneous shoreline.
Palaeogene and Neogene
The onset of igneous activity in western Scotland in the Palaeogene at about 63 Ma was accompanied by widespread tectonic activity (Pearson et al., 1996). The Grampian Highlands were uplifted by at least 1 km and tilted eastwards towards the North Sea. Thick sand sequences derived from the raised block accumulated in the Moray Firth basin (P915249). Magmatism was at an end by about 52 Ma and both uplift and erosion slowed. Sedimentation rates dropped in the North Sea, with deposition of mud replacing that of sand. Uplift was renewed in the late Oligocene and it probably continued through the Neogene. The scale of Neogene uplift in Scotland may have been underestimated. Estimates of burial depths of Mesozoic rocks indicate that up to 1.5 km of basin fill was removed from the Inner Moray Firth during the ‘Tertiary’ (Thomson and Hillis, 1995), possibly reflecting Neogene uplift of the western North Sea and adjacent areas (Japsen, 1997).
In north-east Scotland, major uplift of the eastern Grampians occurred in the Palaeogene. A proto-Dee and Don river system during the Eocene fed sediment to the Gannet Fan in the western North Sea (P915265). Uplift of the Cairngorms and accelerated erosion led to the exhumation of sub-Devonian valley systems. Yet the position of a major Paleocene depocentre in the inner Moray Firth (Andrews et al., 1990) suggests that the coastal fringe of north-east Scotland may not have been greatly uplifted at this time. Cretaceous cover rocks may have survived in eastern areas into the Neogene, finally being removed by erosion as a result of later regional uplift.
The subsequent evolution of the drainage network is of interest here (P915266). Despite the proximity of the Moray Firth, it is clear that the major drainage routes in north-east Scotland ran west to east. The headwaters of the Dee and Don, prior to river capture, lay in the Cairngorms and the topographic trench that extends from the Cabrach to Insch marks another west–east drainage line. Significantly, the Windy Hills Gravels of Neogene to Early Pleistocene age were also transported by a proto-Deveron–Ythan river system flowing eastwards towards the North Sea. River capture and glacial diversion disrupted this pattern of drainage to the advantage of rivers following the south-west to north-east Caledonian structural trend, some re-occupying Devonian valleys. Nevertheless, the ancient drainage pattern is still clear, and it clearly indicates tilting towards the North Sea and Neogene uplift of Buchan across the Banff Fault.
There is evidence of Neogene differential tectonic uplift and subsidence in the region. The existence of prominent scarps that appear to be unrelated to lithological boundaries, as around Bennachie and the Hill of Fare (P915378), suggest relatively recent fault movement. Continued uplift along the Highland Boundary Fault also seems to have been necessary to raise the Mounth erosion surface 500 m or so above the floor of the Mearns. Block faulting seems widespread in the Elgin area (Hall, 1991) and such faulting may account for the marked elevation of Devonian sandstones and conglomerate of Hill of Fishrie (P915373) and Windyheads Hill (P915374). More general uplift of central Buchan is suggested by the elevation of the Buchan Ridge Gravels, which have been raised to around 150 m OD since deposition possibly at, or close to, contemporaneous sea level at some time in the Tertiary.
Climate and weathering
Climates during the Paleocene and Eocene were warm and humid throughout north-west Europe and deep kaolinitic weathering profiles developed widely (Hall, 1991). Humid to subtropical conditions prevailed during the formation of organic deposits offshore to the north of Scotland in the late Oligocene (Evans et al., 1997). A sharp drop in temperature in the late Miocene (Buchardt, 1978), combined with an influx of less mature terrigenous debris derived from uplift of Fennoscandia and Scotland, led to increasing amounts of feldspar, illite and chlorite and decreasing quantities of kaolinite in North Sea sediments (Andrews et al., 1990). The mineralogical changes reflect a fundamental shift in weathering styles at this time from predominantly kaolinitic weathering profiles to less mature sandy weathering products.
The relief of the area covered by this memoir is mainly lowland, dominated by a single, complex erosion surface, the ‘Buchan Surface’ (Hall, 1987; P915262), whose relative relief seldom exceeds 60 m OD. In detail, this subdued terrain resolves into a tiered landscape showing pervasive litho-structural control. An upper tier of isolated, low hills and broad interfluves developed on pelites and quartzites (with pockets of kaolinitic weathering) passes downslope into an extensive middle tier that includes open, saucer-like basins developed on deep sandy weathering covers (P915269). Set into this middle tier are negative landforms developed on rocks of low resistance. These include the large, shallow basins of Maud and New Pitsligo, developed on norite and biotite granite respectively, and broad valleys such as that of the South Ugie Water, which follows a septum of biotite granite through the quartzite belt of central Buchan. The general correspondence between rock resistance to chemical weathering and topographic position reflects the dominant style of landscape evolution in this region during the Palaeogene and Neogene. In other words, deep weathering paved the way for subsequent erosion and heavily influenced the distribution of the main components of the preglacial topography.
The hilly terrain at the inland margin of the Buchan Surface reflects the interplay of differential weathering and tectonics. Uplift of blocks and erosion surfaces has created ridges and plateaux at different levels. The summit of Bennachie displays a fine series of tors. The tors have been only slightly modified by the passage of ice, with removal of blocks and the beginning of streamlining and lee-side plucking. They must predate at least the last ice sheet. Uplift has caused drainage incision. That this incision began prior to Quaternary glaciation is indicated by prominent benches that mark the preglacial valley floor along the middle reaches of many valleys in the eastern Grampians (Hall, 1991). Such benches occur along the Dee downstream of Banchory and above the Ythan gorge. Representatives of the intramontane basins of the eastern Grampians (Linton, 1951; Hall, 1991) occur along the inland margin of the district and include the basins of Feugh on the Dee, Alford on the Don, the Insch depression and the Knock basin on the Deveron. These basins have a long history of development, perhaps extending in some cases back to the Devonian, and generally reflect the presence of rocks with low resistance to chemical weathering.
The oldest relief in the region, apart from localised sub-Devonian surfaces, is probably found in central Buchan (P915267). Here the juxtaposition of kaolinitic weathering profiles and the flint gravels of the Buchan Ridge defines an area of Neogene terrain. The kaolinitic weathering appears to predate the cooling of climate that occurred in the late Miocene (Hall et al., 1989) and the constituents of the flint gravel indicate stripping of siliceous residues from pre-existing highly weathered landsurfaces. On the Buchan Ridge, the hilltops are close to the level of the sub-Cretaceous surface, as shown by the presence of Greensand and remanié deposits of flint. The flint gravels occur close to the highest tops in central Buchan. If it is accepted that the flint gravels are fluvial in origin then the headwaters of the rivers that deposited them have been lost to erosion. The marked drop in elevation of the base of the Buchan Ridge Gravels, from 145 m OD at Whitestone Hill and 140 m OD at Skelmuir Hill to 70 m OD at Den of Boddam, 14 to 15 km to the east, suggests tilting towards the east. The age of the gravels is uncertain, but postdepositional uplift and tilting is likely to be late Neogene in age. Kaolinitic saprolites also occur to the north, within the outcrop of the Mormond Hill Quartzite. The inselberg of Mormond Hill is a very ancient feature and may have emerged from beneath Cretaceous cover rocks in the Neogene (P915269).
Elsewhere in the region, the widespread development of deep sandy weathering covers of late Miocene to early Pleistocene age indicates that all but the largest relief elements are of late Neogene age. Stripping of older kaolinitic regoliths and replacement by deep, but geochemically immature, weathering covers indicates regional erosion in response to continued uplift. Further etching out of lithological variations in the bedrock is demonstrated by the influence of geology on the distribution of sandy weathering patterns and by the landforms of differential weathering and erosion that comprise the meso-scale relief of the Buchan Surface.
Cumulative glacial erosion during the Quaternary
Compared with other areas of Scotland, the geomorphological impact of Quaternary glaciations on the lowlands of north-east Scotland has been modest. However, although most of the region is a zone of limited glacial erosion (Linton, 1963; Clayton, 1974) there is significant variation in the intensity of glacial erosion (Hall, 1986). Regional weathering zones indicate that deep weathering profiles are rare in areas affected by the relatively vigorous ice streams that occurred along the coastal fringe of the Moray Firth and North Sea. The area formerly covered by ice from the East Grampians (P915251) shows fewer signs of active glacial erosion and deep weathering is preserved widely. In the area between the Don and the Ythan valleys, there is clear inverse relationship between the distribution of landforms of glacial erosion and of preglacial landscape remnants (Hall and Sugden, 1987; Sugden, 1989; P915268). By comparing morphology and sediment volumes along a transect from the Cairngorms to the central North Sea, it has been estimated that 16 to 42 m of weathered and fresh rock will have been removed from an area of relatively restricted glacial erosion (such as the lower Dee valley) during the Quaternary (Glasser and Hall, 1997). In central Buchan, the preservation of the Neogene Buchan Gravels Formation implies even more modest levels of glacial erosion.
The morphological contrasts described above reflect differences in glacier basal thermal regimes and rates of ice flow at different scales. The Moray Firth and North Sea ice streams were sourced well to the west of the district in areas with high snowfall and relatively elevated temperatures. These ice streams were warm-based and capable of scouring bedrock. In inland areas the survival of sandy weathering indicates a general lack of erosive capacity and it is clear that successive ice streams were generally cold-based and frozen to their beds. The local presence of ice-smoothed rock surfaces on hills, cols and in major valleys, however, shows that at these sites ice was at some time at its pressure-melting point and starting to slide and erode (Hall and Sugden, 1987). The contrast between the tor-studded plateau of Bennachie, where ice modification has been limited to removal of detached blocks, and the adjacent summit of Cairn William (NJ 656 168), with its striated pavements (Gould, 1997), is of interest here, as it shows that ice at this elevation (about 500 m OD) was only sliding in zones of convergent flow and on slopes that faced up-glacier.
The former glaciers played an important role in removing and reworking weathered materials. Weathering profiles are generally truncated and covered by a variable thickness of glacial deposits. In glacigenic deposits of the East Grampian Drift Group there is commonly a large proportion of material reworked from saprolites (FitzPatrick, 1963; Basham, 1974) and rafts of saprolite occur (Sugden, 1986). In soils developed on till, clay mineralogy is often found to be independent of drainage status and the clays are regarded as relict, being inherited from the underlying till (Wilson and Tait, 1977). In turn, the clay mineralogy of tills mirrors that of subjacent saprolites and it is clear that soil clays were also largely derived directly from former saprolites (Glentworth and Muir, 1963). Inherited material may also dominate the sand and gravel fraction of tills.
Abundant partially altered primary minerals occur in till below the depth of Holocene soil formation (Basham, 1968). Corestones are a conspicuous component of tills in areas down-ice from certain acid and basic igneous rocks (Wilson and Hinxman, 1890). In contrast, the amount of far-travelled material is substantially greater in glacial deposits of the Logie-Buchan and Banffshire Coast drift groups.