Croftamie

J.E. Gordon

Highlights

The sequence of deposits at Croftamie includes plant detritus interbedded between two tills. The organic material has yielded important palaeoenvironmental and dating information, allowing the glacial history of the area during the Late Devensian to be established. The site provides important evidence for the timing of the Loch Lomond Readvance in the type area.

Introduction

The section at Croftamie [NS 473 861], which occurs in a cutting along the now disused Forth and Clyde Junction Railway, is important for Late Devensian stratigraphy in the type area for the Loch Lomond Readvance. It shows the stratigraphic relationship between the Wilderness Till (deposited by the main Late Devensian ice-sheet) and the Gartocharn Till (deposited by a piedmont glacier during the Loch Lomond Stadial). Beds of organic detritus (plant remains representative of a dwarf-shrub/grassland association) and laminated lake sediments occur between the two tills. Radiocarbon assay of the organic sediments has allowed a limiting date to be placed on the maximum of the Loch Lomond Readvance in its type area (Rose et al., 1988). This important site also has a long history of research (McFarlane, 1858; Smith, 1858; Jack, 1875; Simpson, 1933; Rose et al., 1988).

Description

At the time of their discovery, the deposits at Croftamie generated considerable interest because of their fossil content. McFarlane (1858) and Smith (1858) described 12 ft (3.7 m) of till overlying 7 ft (2.1 m) of blue clay. Near the bottom of the latter, just above sandstone bedrock, the antler of a reindeer, Rangifer tarandus (L) and a number of marine shells were found. Jack (1875) correlated the till above the clays at Croftamie with the distinctive shelly till unit that he identified in the lower Endrick Valley. He listed the macrofauna found in numerous till exposures in the area, including Arctica islandica (L.), Nicania montagui (Dillwyn), Neptunea antiqua (L), Littorina littorea (L) and Balanus sp. from Croftamie.

The most recent investigations of the Croftamie deposits are by Rose (1981) and Rose et al. (1988). The sequence comprises:

The Wilderness Till is associated with an ice-sheet moving west–east and represents glaciation during the Late Devensian glacial maximum. The Blane Valley Silts are interpreted as diatactic varves formed in a proglacial lake dammed back in the Blane Valley by the Loch Lomond Readvance. The number of varves indicates that the lake existed for at least 50 years. The overlying Gartocharn Till includes material incorporated from Clyde beds deposited in the Loch Lomond area during the Lateglacial Interstadial. The organic detritus (bed 3) contains a sparse, poorly preserved pollen assemblage (dominantly Caryo-phyllaceae, Salix, Gramineae, Cyperaceae and Cruciferae, but also with Artemisia, Valerian and Selaginella)typical of an open-habitat, dwarf-shrub/grassland environment, and gave a radiocarbon date of 10,560 ± 160 BP (Q–2673) (Rose et al., 1988). It therefore accumulated during the Loch Lomond Stadial, before the onset of the glacial lake sedimentation.

Interpretation

The significance of the Croftamie deposits in interpreting the Pleistocene sequence in Scotland was recognized in the last century. Together with other fossil remains in the drift of Scotland, the reindeer horn was considered to provide evidence for ameliorating climatic conditions and restricted ice extent at some time during the Pleistocene before a subsequent ice advance (Geikie, 1863a). Jack (1875) concluded that the marine fauna had been alive during one of the milder phases of the glacial period when Loch Lomond was an arm of the sea, although in conditions slightly colder than at present. The shelly till was therefore the product of subsequent glaciation. This explanation of the evidence was also adopted by later workers in the area (Geikie, 1894; Bell, 1895d; Cunningham Craig, 1901), following some discussion as to whether the marine event was a local extension of the sea into Loch Lomond or a great interglacial submergence (J. Geikie, 1874, 1877).

Following detailed fieldwork in the area Simpson (1928, 1929, 1933) developed the ideas of Jack, Geikie and Cunningham Craig, assembling the evidence and formulating carefully the case for a Lateglacial readvance of ice in the Loch Lomond valley following a period of marine occupation. He observed that shelly till only occurred inside a prominent end moraine, which he traced around the southern end of Loch Lomond (see Aucheneck and Gartness). He called the glacial event the Loch Lomond Readvance and noted that it was also represented in the Western Forth Valley (see below).

Charlesworth (1956) subsequently extended the limits of the readvance, incorporating the Loch Lomond and Forth valley landforms and deposits as part of his Lateglacial 'Stage M' or 'Moraine Glaciation'. The name Loch Lomond Readvance, however, was retained by Sissons (1965, 1967a), who revised the ice limits suggested by Charlesworth. More recently these limits have been further established over large areas of the Highlands and Islands and the Southern Uplands (Sissons et al., 1973; Sissons, 1979e, 1983b).

The detailed work of Rose (1981) and Rose et al. (1988) has allowed a clearer interpretation of the full sequence of events at Croftamie: (1) deposition of till by the Late Devensian ice-sheet (Wilderness Till); (2) deposition of marine clayey silts (Clyde beds) (elsewhere in the area); (3) formation of a proglacial lake by the advancing Loch Lomond Readvance (Blane Valley Silts); (4) deposition of the shelly till (Gartocharn Till), in part derived from Clyde beds sediments; and (5) maintenance of the proglacial lake which drained into the Forth valley (Blane Valley Silts). The radiocarbon date from the organic sediments implies that the Loch Lomond glacier reached its maximum extent after 10,560 ± 160 BP, in agreement with the evidence from the Vale of Leven that glaciomarine sedimentation continued until after 10,350 ± 125 BP (SRR–1529) (Browne and Graham, 1981). This also accords with the inference that the Loch Lomond Stadial glacier in the Western Forth valley reached its maximum extent between 10,500 BP and 10,100 BP (Sissons, 1983a; Sutherland, 1984a), and that the Creran glacier reached its maximum after 10,500 BP and perhaps as late as 10,000 BP (see South Shian and Balure of Shian) (Peacock et al., 1989). However, these dates are significantly at variance with dates previously obtained from elsewhere that have been used to infer the time of the maximum glacier extent (see Sutherland, 1986; Rose et al., 1988): they are at least 400 years younger than the other dates from sediments overridden by Loch Lomond Stadial glaciers, and they also overlap in age ranges with dates from lacustrine sediments that were deposited immediately on retreat of the stadial glaciers (Rose et al., 1988) (see also Kingshouse, Loch an t-Suidhe, Mollands, Tynaspirit, Rhu Point and Loch Cleat). Rose et al. (1988) argued that this variance could relate to three factors. First, many of the dates are from marine shells, but environmental and sedimentation factors tend to favour a higher probability of sampling shells from the middle part of the interstadial, rather than the later part (see also Sutherland, 1986). Second, there may be errors in the dates on basal organic lake sediments caused by mineral carbon and hard-water contamination (see Sutherland, 1980). Third, because of variations in individual glacier dynamics and glacier response to climate, the fluctuations of the glaciers are likely to have been diachronous. Since the material dated at Croftamie is plant detritus, the date obtained should be less susceptible to mineral carbon and hard water errors than dates obtained from organic lake sediments (gyttja); Rose et al. (1988) suggested that dates on the latter material may be slightly too old. The evidence from Croftamie and Inverleven therefore indicates relatively later deglaciation of Loch Lomond Stadial glaciers than suggested elsewhere (Lowe, 1978; Lowe and Walker, 1980, 1984; Walker and Lowe, 1980, 1982; Dawson et al., 1987a) (but see Kingshouse). A further complication in interpreting the different radiocarbon dates arises from the variations in atmospheric radiocarbon production known to have occurred during the Lateglacial (Amman and Lotter, 1989; Zbinden et al., 1989; Bard et al., 1990), but this may have affected all the dates.

Croftamie is a site of highest importance in several respects. It provides a key sequence of Lateglacial deposits in the type area for the Loch Lomond Readvance. In stratigraphic terms, it unequivocably demonstrates the superposition of till deposited during the Loch Lomond Stadial on till deposited by the main Late Devensian ice-sheet. It is the only site at which plant material has been used to date the maximum of the Loch Lomond Readvance. Moreover, the results of this dating, corroborated by additional dates from Inverleven and South Shian and Balure of Shian, provide significant evidence that the maximum extent of the Loch Lomond Readvance glaciers may have occurred after 10,500 BP. The interest at Croftamie complements that at Aucheneck, Gartness and South Loch Lomond (Portnellan, Ross Priory and Claddochside), and together these sites provide a comprehensive demonstration of the Lateglacial stratigraphy and landforms, both glacial and marine, in the area where the Loch Lomond Readvance was first recognized.

Conclusion

Croftamie is important for interpreting the glacial history in the type area for the Loch Lomond Readvance. The site has a long history of research and is particularly significant in showing organic deposits interbedded between tills deposited by the Late Devensian ice-sheet (approximately 18,000 years ago) and a Loch Lomond Stadial glacier (approximately 10,500 years ago). The organic deposits represent a warmer phase between the two glacial episodes, and the pollen they contain provides details of environmental conditions during that period. The organic deposits also provide an important means of dating the readvance. Croftamie is therefore a key reference site for establishing particular details of the nature and timing of the Loch Lomond Readvance in its type area.

References