Inchcoonans and Gallowflat

D.G. Sutherland

Highlights

Inchcoonans and Gallowfiat are important reference sites for the Errol beds, a sequence of fossiliferous estuarine sediments deposited largely in eastern Scotland as the Late Devensian ice-sheet melted. They provide important evidence for the high-arctic nature of the marine environment during the early part of the Lateglacial.

Introduction

The sites at Inchcoonans [NO 242 233] and Gallowflat [NO 211 202] are located, respectively, 1 km north-west and 4.5 km south-west of Errol, between Perth and Dundee. Both occur in an area of fossiliferous Late Devensian raised estuarine deposits (Errol beds). Inchcoonans comprises a small area of undisturbed deposit adjacent to a former claypit (now infilled) which provided the type sequence for these deposits; Gallowflat is a working claypit, where the sediments are exposed.

A broad twofold subdivision of the marine and estuarine sands, silts and clays that were laid down around the Scottish coasts during Late Devensian ice-sheet retreat and in the Lateglacial period has long been recognized; between those deposits containing a restricted high-arctic fauna and those containing a much more diverse boreal to arctic fauna (Jamieson, 1865; Brady et al., 1874; Robertson, 1875). This subdivision has been noted to correspond, in general, to the geographical distribution of the deposits, those on the east coast being predominantly high arctic in character and those on the west being predominantly boreal to arctic (Robertson, 1875; Anderson, 1948; Sissons, 1965). Peacock (1975c) proposed the informal terms Errol beds and Clyde beds to apply, respectively, to the high-arctic and the boreal-to-arctic deposits, this terminology reflecting the locations where the different deposits had been first described in detail. The Inchcoonans and Gallowflat area, by Errol to the north of the Tay estuary, is therefore the principal reference area of the high-arctic deposits.

At the western end of the Carse of Gowrie on the north of the Tay estuary the surface of an area of higher ground is mantled by Late Devensian estuarine clays. The clays extend to altitudes of approximately 30 m OD and their surface forms a series of terraces, the lowest of which is below 12 m OD. The western, southern and eastern sides of this higher ground are flanked by Holocene estuarine deposits. A number of claypits excavated in the upper deposits resulted in the discovery during the last century of marine fossils of species indicative of very cold conditions when the clays were deposited (Jamieson, 1865; Brown, 1867; Brady et al., 1874). The most detailed studies were carried out in the Inch-coonans claypit (now infilled) by Davidson (1932), whose work has in recent years been verified and amplified by Paterson et al. (1981) and Graham and Gregory (1981). The deposits at Gallowflat have been described by McManus (1972), MacGregor (1973) and Duck (1990). In addition, the raised shorelines contemporaneous with deposition of the marine clays have been studied in detail by Cullingford (1972, 1977). A summary of the current understanding of these deposits is given in Armstrong et al. (1985).

Description

The first systematic description of the strati-graphy of the Inchcoonans claypit was given by Davidson (1932). He identified three principal units:

Pebbles, cobbles and even boulders were scat tered throughout the deposits, the surface of which was at about 12 m OD. More recent investigations have added detail to this outline, and Paterson et al. (1981) determined the following sequence (Figure 15.2):

The main difference from Davidson's section was the recognition of an erosion surface formed during the deposition of the upper yellowish-grey sediments. In a neighbouring borehole (Figure 15.2) clayey gravels were encountered within beds 3 and 4. These were considered to be due to slumping from a mound of submerged glaciofluvial sand and gravel (Paterson et al., 1981) or deposited from an iceberg (Armstrong et al., 1985). The change in colour during the period of deposition of the clays was thought to relate to the retreat of the ice-sheet towards the west: the reddish clays at the base reflected derivation from local Old Red Sandstone, whereas the upper yellowish clays received their colour from material principally derived from Highland rock types.

The fauna described by Davidson (1932; Graham and Gregory, 1981) came principally from beds 3 and 4. The molluscan fauna consisted of the gastropods Buccinum groenlandicum (Chemnitz) and Lunatia pallida? (Broderip and Sowerby) and the bivalves Astarte borealis (Schumacher), Hiatella arctica (L.), Macoma calcarea (Chemnitz), Musculus laevigatus (Gray), Musculus niger (Gray), Palliolum groenlandicum (Sowerby) (=Arctinula greenlandica), Portlandia arctica (Gray), and Thracia cf. septentrionalis (Jeffreys). Although some of these species have wide geographical ranges, others, such as Portlandia arctica and Palliolum groenlandicum are strongly indicative of high-arctic conditions. Certain of the ostracods recovered are similarly indicative, for example, Krithe glacialis (Brady, Crosskey and Robertson), Rabimilis mirabilis (Brady) and Cytheropteron montrosiense (Brady, Crosskey and Robertson). Other macrofossils recovered include bones of the common seal, Phoca vitulina L.

The reinvestigation of the site allowed a correlation to be established between the stratigraphy and the microfaunal distribution. The lowest marine deposits (bed 2) are characterized by the above-mentioned ostracods and in the foraminiferal assemblage, Elphidium clavatum (Cushman) predominates over Elphidium bartletti (Cushman). In bed 3, Elphidium bartletti attains dominance and there is a marked reduction in the occurrence of the ostracods Rabimilis mirabilis and Cytheropteron arcuatum (Brady, Crosskey and Robertson). Krithe glacialis disappears in bed 4, and Elphidium clavatum regains dominance in the foraminiferal assemblage. Bed 5 was barren of both micro- and macrofauna. Paterson et al. (1981) have suggested that it is not part of the Errol beds but should be correlated with the Powgavie Clays, a later deposit lacking the high-arctic indicator species: these clays were intersected in boreholes in the Carse of Gowrie to the east of Errol.

The variation in the microfauna appears mainly to reflect variations in salinity, bed 3 with the dominance of Elphidium bartletti being indicative of more fully marine conditions than either beds 2 or 4. The reduced salinity of bed 2 may be due to meltwater influx from the retreating ice-sheet, whereas bed 4 may have been deposited in shallower water as sea level fell consequent upon isostatic uplift.

At the Gallowflat claypit (surface altitude about 25 m OD), deposits similar to those at Inchcoonans are revealed. McManus (1972) described the sedimentary characteristics of the deposits, noting that they comprised laminated silty clay with thin sand layers. The lower part of the succession shows rhythmic bedding in silty clays or clayey silts, whereas the upper part comprises fine and medium sands. Pebbles and boulders up to 1.3 m in size occur as drop stones in the succession, and calcareous concretions are also present (Duck, 1990). The erratic material includes dolerite, metamorphic rocks and Old Red Sandstone sediments, sometimes striated (MacGregor, 1973; Duck, 1990). The only macrofossils recovered were Portlandia spp., and the micro-fauna consisted of the ostracod Cythere montrense (Brady, Crosskey and Robertson) and the Foraminifera Elphidium clavatum and Cassidulina obtusa (Williamson), E. clavatum being dominant (Paterson et al., 1981).

Interpretation

By analogy with modern polar environments, McManus (1972) considered that the Errol beds were deposited in association with seasonal pack-ice and icebergs in water depths of up to 100 m. Analysis of the calcareous concretions led Duck (1990) to support McManus's suggestion that the clays were deposited from flocculated suspensions in a strongly stratified water body and in the absence of significant currents. As the water shallowed and became more mixed, sedimentation of coarser particles occurred.

The earliest of the Errol beds were deposited when relative sea level was at least 28 m OD in this area. During the subsequent fall in sea level particularly pronounced terraces were formed at 24–25 m OD, and these have been correlated with the Main Perth Shoreline (Cullingford, 1972, 1977). This shoreline was formed when ice lay some distance to the west of the present area (see Almondbank) and, it has been argued (Paterson et al., 1981), at approximately the time of cessation of deposition of the Errol beds and the start of deposition of the Powgavie Clays, with their fauna indicative of a milder climate. The shoreline has been traced widely along the coasts of east-central Scotland (Sissons et al., 1966; Smith et al., 1969) and has a marked tilt to the south-east of 0.43 m km⁻¹ resulting from isostatic uplift subsequent to the formation of the shoreline. The lower terraces in the Inchcoonans and Gallowflat area have been correlated with shorelines formed as sea level continued to fall. These terraces have successively lower tilts, reflecting the decrease in isostatic uplift during the period of their formation (Cullingford, 1972, 1977).

The Errol beds have not been dated directly. Their base is clearly diachronous as they were laid down in front of a retreating ice-sheet. On the basis of shoreline gradient calculations (Andrews and Dugdale, 1970) the start of deposition of the Errol beds can be placed at as early as 17,000 BP (Sutherland, 1984a), and if deposition ceased at the time of change from arctic to more boreal conditions as the oceanic polar front retreated north of the Scottish coast (Ruddiman and McIntyre, 1973, 1981b; Peacock, 1981b, 1989b), then this may be placed at approximately 13,500 to 13,000 BP.

The Errol beds are an important element in the Late Devensian stratigraphy of Scotland. They are typified by a high-arctic fauna and their wide distribution along the east coast of Scotland indicates that the majority of the last ice-sheet had melted prior to the climate amelioration at the opening of the Lateglacial Interstadial at around 13,000 BP. They are the equivalents, now on land, of the St Abbs Formation of the North Sea Basin (Stoker et al., 1985). There are apparently few deposits on the west coast that may be correlated with the Errol beds, with two exceptions possibly at Stranraer (Brady et al., 1874) and in the North Minch (Gregory, 1980; Graham et al., 1990), and their absence from much of the west coast has been attributed to these areas being covered by ice during the period of their deposition (for example, Sissons, 1965; Peacock, 1975c). This hypothesis, however, awaits full substantiation (Sutherland, 1984a). Inchcoonans and Gallowflat constitute the principal reference area for the Errol beds, where the most abundant macro- and microfauna has been recovered and where the deposits have been examined in most detail in recent years.

The former pit at Inchcoonans, which yielded the most abundant faunas is now infilled, but the deposits can still be examined in sections at Gallowflat claypit.

Conclusion

Inchcoonans and Gallowflat are reference localities for a sequence of fossiliferous estuarine deposits (Errol beds) restricted almost entirely to eastern Scotland and formed during the melting of the last ice-sheet, about 17,000–13,000 years ago. These sediments and the fossil fauna (marine mollusc shells) they contain, provide important evidence for marine environmental conditions at this time. In particular, they indicate that the estuarine waters were high-arctic in character.

References