Gordon, J.E. and Sutherland, D.G. GCR Editor: W.A. Wimbledon. 1993. Quaternary of Scotland. Geological Conservation Review Series No. 6. JNCC, Peterborough, ISBN 0 412 48840 X.
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D.G. Sutherland
The deposits exposed in stream sections at Geilston include a sequence of fossiliferous marine sediments, the Clyde beds, which provide important evidence for marine palaeoenvironmental conditions during the latter part of the Late Devensian. Geilston is one of the few sites with good exposures which have been studied in detail.
The site
It has long been known that certain of the silts, sands and clays exposed at low altitudes (generally below 35 m OD) and in many foreshore areas around the head of the Firth of Clyde contain marine fossil faunal assemblages that are indicative of a climate colder than that of the present (Smith, 1838; Jamieson, 1865; Crosskey and Robertson, 1867–1875; Brady et al., 1874). Furthermore, it was also realized by these researchers that the changes in the fossil faunas reflected changes in the level of the sea (Robertson, 1883) as well as changing climates. In recent years, radiocarbon dating and quantitative analysis of the faunal assemblages have clarified the age of the sediments and the changes in climate and sea level that accompanied their deposition (Peacock et al., 1977, 1978; Peacock, 1981b, 1983a, 1989b). The informal term 'Clyde beds' was proposed by Peacock (1975c) for those marine sediments deposited around the Scottish coast (but known principally in the area of the Firth of Clyde) subsequent to the deposition of the arctic 'Errol beds' (see Inchcoonans and Gallowflat), but prior to the establishment of a marine climate similar to that of today in the early Holocene. The Clyde beds thus cover the period from approximately 13,000 BP to around 10,000 BP.
Rose (1980a) identified the following stratigraphic succession in the Geilston deposits
5. Beach sands and gravels unconformably overlying bed 4
4. Clayey silt, Clyde beds
3. Laminated silts, Clyde beds
2. Glaciomarine till
1. Lodgement till
At the base the stiff, reddish-brown lodgement till (bed 1) has a relatively high silt–clay content and a dominant NW–SE fabric. The overlying glaciomarine till (bed 2) is similar in colour and lithology to the lodgement till but has a much lower silt–clay content. It contains a marine microfauna. Resting on the glaciomarine till is a sequence of colour laminated (pale olive-brown to brown) silts (bed 3) of which 26 couplets were reported by Rose (1980a). The individual laminae fine upwards and the colour change relates not to differences in particle size, but to the relative proportions of materials derived from Dalradian (pale laminae) or Old Red Sandstone bedrock. Foraminiferal remains increase in abundance through the lower half of the bed, remaining constant in frequency in the sediments above this level. Mollusc shells (Yoldiella lenticula (Müller) occur in the upper part of the laminated silts. The upper laminae merge into an overlying homogeneous clayey silt (bed 4) in which shells are abundant, in particular Arctica islandica (L). The clayey silts are truncated by a marked unconformity, above which are horizontally bedded sands and gravels. The clasts are generally subrounded and become progressively more disc-like or blade-shaped upwards.
Three samples of Arctica islandica from close to the base of the homogeneous clayey silt have been radiocarbon dated and have given ages (Sutherland, 1986) of 11,390 ± 130 BP (GU–12) (Baxter et al., 1969); 11,900 ± 210 BP (N–475) (Yamasaki et al., 1969); and 12,310 ± 250 BP (SRR–833) (Harkness and Wilson, 1979).
The sediments were interpreted by Rose (1980a) in the following manner. The basal lodgement till was deposited by the Late Devensian ice-sheet, which flowed from the south-west Highlands towards the east along the Clyde valley. The long axes of drumlins in the area accord with this direction of ice movement. During deglaciation, the tidewater ice front retreated along the Clyde estuary, allowing deposition of the glaciomarine till beneath the floating ice margin. With further ice retreat the laminated sediments were deposited, the upward thinning of the laminae reflecting the increasing distance of the ice margin from the site. Each lamina is considered to represent the suspension fraction of subaqueous sediment plumes discharged from the ice margin. The homogeneous clayey silt with abundant macrofauna was deposited after the area was completely deglaciated, and when clearer water allowed Arctica islandica to become established (Peacock, 1981b).
The radiocarbon dates confirm the silts to have been deposited during the Lateglacial Interstadial but the considerable range in age is either due to mixing of the sediments by bottom current activity or very low sedimentation rates (Peacock et al., 1978; Peacock, 1981b; Sutherland, 1986). The oldest radiocarbon date also places a minimum age on deglaciation of this area although consideration of dates from a variety of sites in the Clyde estuary suggests deglaciation had occurred by 12,600 BP (Sutherland, 1986).
The unconformity which truncates the Clyde beds is shown by Rose (1980a) to correlate with the period of erosion responsible for the cutting of the Main Rock Platform during the Loch Lomond Stadial. Both in accord with that age for the unconformity and their altitude, the sands and gravels capping the section are considered to be part of the Holocene raised beach deposits which occur widely around the coasts of the head of the Firth of Clyde.
Despite the wide distribution of Clyde beds sediments around the coast of Scotland and, in particular, around the Firth of Clyde, there are very few localities where they can be examined in section. Geilston is one such locality and one of the few where detailed sedimentological work has been carried out and the stratigraphic sequence and origin of the sediments is well established. Furthermore, radiocarbon dating here has placed the deposits in a clear geochronometric framework. The Clyde beds are a very important sedimentary unit, not only for historical reasons related to the evolution of ideas on climatic change and the ice age, but also for the evidence they contain of climatic and sea-level changes during the Lateglacial. Such evidence is an important counterpoint to the terrestrially-derived models of climatic change during the same time interval (cf. Peacock, 1989b; Peacock and Harkness, 1990). As a representative of the Clyde beds, Geilston is hence a particularly important site.
Geilston is a key reference site for the Clyde beds, a sequence of fossiliferous glaciomarine and marine sediments that formed in the period during and following the wastage of the last ice-sheet (approximately 13,000–11,000 years ago). These deposits provide important evidence for changing conditions in the marine environment during the phase at the end of Devensian times, the Lateglacial. Geilston is one of the few sites where there are both good sections available and the sediments have been studied and dated.