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.
The original source material for these web pages has been made available by the JNCC under the Open Government Licence 3.0. Full details in the JNCC Open Data Policy
J.E Gordon
The assemblage of landforms and deposits on the south shore of Loch Lomond provides important evidence for the geomorphological changes that occurred at the end of the Late Devensian in the type area for the Loch Lomond Stadial and its associated glacier readvance. These include the formation of marine shorelines, fossiliferous marine deposits and glaciation of the Loch Lomond Readvance.
Three localities, at Portnellan
The three localities on the shore of the loch have been described by Rose (1980f). Together they show a succession of marine and glacial features. One of the most distinctive is a shore platform developed extensively along the southern shore. At Ross Priory it extends for about 0.5 km offshore to about 1 m below the loch level; it is also well displayed at Portnellan and Claddoch-side. In places it is associated with an impressive backing cliff up to about 15 m high, for example at Portnellan. At Claddochside, the cliff is cut in Old Red Sandstone bedrock. The junction of the cliff and platform is at about 12 m OD. At Portnellan, the platform and cliff are covered by till deposited during the Loch Lomond Readvance. This is the Gartocharn shelly till of the area, partly derived from reworked Clyde beds (see Croftamie) (Jack, 1875). Gartocharn Till also buries Clyde beds in foreshore sections at Claddochside, and a similar succession has been recorded on Inchlonaig
The sediments laid down in the Loch Lomond basin, immediately following retreat of the Loch Lomond Readvance, have been described from a borehole in the lower Endrick Valley
A middle Holocene marine invasion of the Loch Lomond basin is indicated by raised shorelines around the southern part of the loch at about 13 m and 12 m OD. The latter is the most conspicuous, its width in part reflecting exhumation of the Lateglacial shore platform (Dickson et al., 1978; Rose, 1980f). It is best seen at Portnellan and Ross Priory. Rose (1980f) traced these shorelines through the Vale of Leven to the Clyde estuary, thereby confirming their marine origin (see Dickson et al., 1978, figure 1). Shells in deposits related to the transgression have been recorded near Luss (Robertson, 1868; Brady et al., 1874). A further shoreline at 9 m OD was considered by Rose (1980f) to be lacustrine in origin, related to present lake level.
The middle Holocene marine transgression is also represented in sediment cores from the southern basin of Loch Lomond (Dickson et al., 1978). The marine deposits, interbedded with freshwater sediments, are distinguished by low remnant magnetic susceptibility and intensity and by the presence of marine plankton and absence of freshwater plants. Radiocarbon dates on organic material from one core place the transgression between 6900 BP and 5450 BP. More detailed results were presented by Stewart (1987). He showed that there had been two phases to the marine transgression, the earlier one lasting from approximately 7200 BP until about 6400 BP and the later one, following a brief non-marine period, terminated at approximately 5500 BP.
The three localities show a sequence of events (Rose, 1980f) beginning with a Lateglacial marine transgression and deposition of Clyde beds (see Geilston), seen at Claddochside. The existence of such marine deposits in the Loch Lomond basin has long been recognized principally on account of their fossiliferous nature (Adamson, 1823; Brady et al., 1874; Jack, 1875; Cunningham Craig, 1901). Deposition of the Clyde beds was followed by the formation of the shore platform. Rose (1980f) has correlated this platform with the Main Rock Platform of western Scotland (see Isle of Lismore) (Gray, 1974a, 1978a; Sissons, 1974d). Subsequently, during the Loch Lomond Stadial, a glacier advanced across the area depositing till over both the shore platform and the Clyde beds. Together with Gare Loch (Rose, 1980b) and Loch Spelve (Gray, 1974a), Portnellan is one of only a few localities providing important evidence that the Main Rock Platform was predominantly formed before the Loch Lomond Readvance reached its maximum extent. Also, the burial by till of the Clyde beds, which postdate c. 13,000 BP (see Geilston), provides clear stratigraphic evidence for the Loch Lomond Readvance (Cunningham Craig, 1901; Simpson, 1933).
The evidence from the borehole in the lower Endrick Valley for a marine deposit immediately overlying the Gartocharn Till deposited during the Loch Lomond Readvance, implies that the sea entered the Loch Lomond basin towards the end of the Loch Lomond Stadial (Browne and McMillan, 1989). This conclusion is dependent upon the marine fossils contained in the sediment being in situ and not derived from the underlying shelly deposits. Comparison may be made, however, with the Western Forth Valley, where Sissons (1966, 1976b) has demonstrated that a marine transgression, culminating between 10,500 BP and 10,100 BP, was approximately contemporaneous with the maximum of the readvance.
During the Holocene, the radiocarbon dates indicate that entry of the sea into Loch Lomond was some 1100 years later than the start of the Main Postglacial Transgression in the Western Forth Valley (Sissons and Brooks, 1971), a delay which reflects the time required for the sea to surmount the outwash fan barrier formed by the Loch Lomond Readvance across the Vale of Leven at Alexandria, between Loch Lomond and the Clyde estuary (Dickson et al., 1978). This early marine phase also coincides with the maximum of the transgression (6800–6650 BP) in the Forth valley (Sissons, 1983a).
The Holocene palaeomagnetic record from Loch Lomond shows similarities to those from Lake Windermere and Lough Neagh, but is particularly significant in providing much finer detail because of a higher rate of sedimentation (Dickson et a/., 1978). It provides the most precise and detailed geomagnetic record so far obtained for the last 7000 years (Turner and Thompson, 1979).
Portnellan, Ross Priory and Claddochside complement the interest at Croftamie, Gartness and Aucheneck by illustrating the sequence of Late-glacial and early and middle Holocene marine episodes and their relationships to the glacial deposits of the Loch Lomond Readvance in the type area for the readvance and the Loch Lomond Stadial. The lake-bed sediments are also of considerable importance for the detailed palaeomagnetic record they preserve and for the succession showing Holocene marine transgression deposits interbedded with freshwater lake deposits.
The landforms and sediments at this site are important for the evidence they provide for the sequence of glacial and marine events at the end of the Devensian. In particular, they show a shore platform and fossiliferous Lateglacial marine deposits that formed before the Loch Lomond Stadial (approximately 11,000–10,000 years ago). During the latter phase, a shelly till was deposited by a glacier readvance (Loch Lomond Readvance). Raised shorelines and sediments from the floor of the loch also indicate two episodes during the Holocene (the last 10,000 years) when the sea encroached into Loch Lomond. This sequence is important because of its location in the type area for the Loch Lomond Stadial.