Geology of the Aberfoyle district: Pleistocene
|This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
Glacial features include drumlins, moraines and outwash sediments, whilst raised estuarine mudflats, peat mosses and fluvial deposits record postglacial marine incursion and regression, and subsequent fluvial modification. Holocene deposits are preserved extensively in the southern part of the Aberfoyle district, particularly in the area around the Lake of Menteith [NN 580 005], and in the ground extending southwards towards Arnprior [NS 612 949] and Balfron [NS 548 892]. Part of the type area for the Loch Lomond (Younger Dryas) Stadial occurs in the south-western corner of the district, around Drymen [NS 474 885]. Here, end moraines of the type Loch Lomond Stadial glacier form ridges developed in proglacial lake sediments (Gordon 1993, and references therein).
The Quaternary glaciations of Scotland reflect the late stages of the gradual cooling of the Earth’s climate that began about 35–40 Ma ago, and continued from the Late Palaeogene through to the present day. The climate became sufficiently cold for cyclic glaciation to occur in the North Atlantic region from about 2.4 Ma (Boulton et al., 2002). However, although Scotland has experienced numerous glacial episodes in the last 2 Ma (Merritt et al., 2003), most of the deposits and features relating to glaciation in this country are less than 30 000 years old, arising from glaciation and glacier decay during the Late Devensian Dimlington and Loch Lomond stadials. These two glacial episodes were separated by a brief milder period (Windermere Interstadial). For the most part, pre-Devensian superficial deposits and their landforms have been severely modified, obscured or destroyed by glaciation during the Dimlington and Loch Lomond stadials. However, remnants of deposits arising from earlier glaciation are preserved in outcrops just to the south of the district.
Pre-Dimlington Stadial deposits
Deposits and associated landforms resulting from glaciation prior to the Dimlington Stadial (>30 000 years ago) are unknown in the district. However, the major glacial valleys may contain concealed remnants of pre-Devensian till or meltwater deposits in the valley bottoms. For example, an isolated but very important occurrence of pre-Late Devensian deposits is found just off the southern margin of the sheet at Balglass Burn [NS 581 866]. Here, an organic unit has been preserved beneath sediments (mainly diamicton) of Dimlington Stadial age, and overlies an older, weathered till (Brown, 2002). The organic unit has been dated to 35000–28000 14C BP (40000–33000 ka cal. yrs BP). This interstadial organic sand deposit accumulated during a period of regional ice recession, but under a climate still sufficiently cold for permafrost to form (Brown, 2002).
During the Dimlington Stadial (about 30 000–14 700 cal yrs B.P.), the Main Late Devensian (MLD) ice sheet flowed south-eastwards across the district, creating or deepening U-shaped valleys and producing striated rock surfaces, roches moutonnées and crag-and-tail features. The ice laid down extensive spreads of till (Wilderness Till; Rose et al., 1988), forming the oldest of the widely developed superficial deposits in the district. In the southern part of the district, around Balfron [NS 550 890], ice moulded the till into drumlins whose long axes reflect topographically influenced east-north-easterly ice flow (Figure 7).
Although glacial meltwater deposits related to the Dimlington Stadial are sparse, isolated mounds and dissected fluvioglacial terraces composed of sand and gravel occur in the valley of the Endrick Water above Endrick Bridge [NS 542 882]. These landforms were most likely created towards the end of the stadial, as the ice sheet retreated. Numerous subparallel glacial meltwater channels, on the northern and north-western flanks of Buchlyvie Muir, mark stages of this retreat. Isostatic uplift following retreat of the ice sheet, combined with more rapid global sea level rise, resulted in pronounced changes in relative local sea level. Raised beaches at 30 m OD at Arnprior and Calziemuck [NN 622 008] mark the palaeo-coastline at this time.
During the Windermere Interstadial (14700–12650 cal. yrs. BP), the Late Devensian ice sheet retreated to the north-west across the district, allowing the sea to flood the valleys of the Forth and Endrick. Thick marine silts (Paisley Formation) and laminated clays (Linwood Formation — formerly known as the Clyde Beds) were deposited, the latter containing a rich boreal fauna typified by Arctica islandica (Linné) and Modiolus modiolus (Linné). BGS boreholes at Mains of Kilmaronock [NS 4483 8829] and Killearn [NS 5100 8467] south of the district, proved 5 m of locally glaciotectonised silt and clay, and 13 m of undisturbed sediments respectively. Similar deposits almost certainly occur in the Forth valley, but they have not been investigated in detail.
Loch Lomond Stadial
The Loch Lomond (Younger Dryas) Stadial (12 650–11 550 cal. yrs. BP) was characterised by abrupt global cooling that, in Scotland, allowed the growth of a substantial ice sheet in the south-west Highlands, centred on Rannoch Moor. As the Aberfoyle district lies towards the south-eastern margin of the former ice mass, it was affected by several outlet glaciers emanating from the southern margins of the sheet.
In the north of the district, glaciers occupying Strath Gartney and the Loch Lubnaig valley, coalesced below the Pass of Leny and reached Gart [NN 641 065] east of Callander, indicated by a prominent terminal moraine (Merritt et al., 1990, Thompson, 1972). In the centre of the district, the margin of the contemporaneous Forth glacier forms a well-preserved, but broken, arcuate belt of moraines stretching from the Tom nam Broc area [NS 475 943] to Auchyle [NN 591 017], just north-east of Lake of Menteith. The extensive easternmost element of the moraine is interpreted as a thrust block moraine, as discussed below. The southern lateral moraine forms an almost continuous, single-, double- or triple-crested ridge up to 80 m wide west of the A81 at Garrauld [NS 532 920], and a persistent glacial meltwater channel commonly borders the moraine on its southern side.
The frontal terminal moraine, between Garden [NS 597 946] and Port of Menteith [NN 583 012] is up to 2 km wide in places and has a complex history, as summarised by Smith (1993). Whilst the southerly parts of this moraine assemblage, extending southwards from Dykehead [NS 598 978] to Garden, exhibit subdued relief, the northern element from Port of Menteith to Dykehead has pronounced relief with elevations up to 30 m above the surrounding, lower-lying ground. It is also markedly asymmetric, with the steeper side facing east up the valley.
The northern part of the moraine is a complex composed of thrusted masses of marine shelly clay, deposited when the sea flooded the area during the Windermere Interstadial, together with diamicton, sand and gravel. Thrusting formed multiple ridges within the moraine, each up to about 5 m in height (Smith 1993, Wilson and Evans 2003) and probably resulted from a surging glacier that excavated the unconsolidated marine sediments at its front (Thorp, 1991). The Lake of Menteith results from this excavation as the Forth Glacier surged eastwards from the upper Forth valley forming a ‘hill–hole pair’ (Wilson and Evans 2003, and references therein).
Molluscs (Mytilus edulis) sampled from masses of marine shelly clay in the moraine near Inchie Farm [NN 592 000] have been dated at 11.800 ± 170 cal. yrs BP (Sissons 1967), confirming the Loch Lomond Stadial age for the thrusting.
Sinuous meltwater channels cross the moraine from east to west, and the moraine ridge is cut by three deep glacial drainage channels floored by coarse outwash gravels. The southernmost and largest of the channels contains terraced outwash deposits that, to the east, become buried beneath an eastward-thickening succession of shallow marine shoreface, fine-grained sand, peat and ‘carse’ clay, described below. During recession of the Forth Glacier, kame terraces (not shown on Sheet 38E) developed in flights on the hillside immediately north of Port of Menteith (Smith 1993). A stillstand during the glacier’s retreat up the Forth valley is marked by large terminal moraine that almost blocks the valley immediately west of Kinlochard [NN 448 025].
The narrow, gravelly, lateral moraine of the contemporary Loch Lomond glacier can be traced intermittently from the headwaters of the Burn of Mar near Moorpark [NS 460 920], through the Garadhban Forest towards Upper Blairmore [NS 499 897] where it coalesces with a broad, sandy, deltaic, terminal moraine about two kilometres east of Drymen (Rose 1981). The marginal terminal moraines of the Loch Lomond and Forth outlet glaciers, that lie on the south-west and north-east sides of Moor Park [NS 47 93], can be traced south-eastwards at progressively lower elevations, from about 200 m OD towards the frontal terminations of each of these ice masses (Figure 8).
Well-developed, one-sided, glacial meltwater channels on the north shores of Loch Ard and Loch Venachar, at elevations up to 400 and 425 m OD respectively, indicate successive positions of glacier margins. Strath Gartney is the largest glacial trough in the district, with Loch Katrine reaching a depth greater than 150 m near Brenachoile Lodge [NN 477 100]. During the Loch Lomond Stadial, valley glaciers also occupied upper Glen Finglas and Glen Shoinne [NN 51 15], where their limits are evident from distinctive terminal morainic ridges.
Glacial meltwater deposits are extensive in the upper Teith valley, occupying the low-lying ground between Kilmahog [NN 610 084], the east end of Loch Venachar and Callander, just to the east of the district. However, the ice-contact–deltaic terminal moraine complex of the Loch Lomond outlet glacier are the most extensive glaciofluvial deposits in the Aberfoyle district. Together with their deformational history, the sedimentology of the deposits in this complex provide a comprehensive record of the behaviour of the Loch Lomond outlet towards the end of the Loch Lomond Stadial, as seen in Drumbeg Quarry [NS 480 880] (Phillips et al., 2002, and references therein). The deltaic deposits formed in the proglacial Lake Blane, impounded in the valleys of the Endrick and Blane waters due to damming by Loch Lomond glacier ice. A re-advance of the Loch Lomond glacier into Lake Blane late in the stadial led to overriding and deformation of deltaic deposits, and the deposition of subglacial Gartocharn Till Formation (Benn and Evans, 1996, Phillips et al., 2002, Rose, 1989). The elevations of former lake shorelines west of Balfron (Price 1983), which have been recorded in the course of resurvey work on Sheet 38E, occur at approximate elevations of 70, 55 and 45 m OD. These correlate well with the elevation of the Ballat spillway, now occupied by the Auchentroig Burn, the major escape route from the contemporaneous, proglacial Lake Blane (Price, 1983). Shoreline features in the upper Endrick valley, east of Balfron, occur at elevations up to 85 m OD. These correspond closely with the height of the glacial meltwater channel near Kepculloch Toll [NS 543 920], at 87 m OD, and the escape route near Strathblane at an elevation of 90 m OD north-west of Dunglass [NS 575 789]. Within the limits of the Loch Lomond Stadial ice cap, glaciofluvial deposits are confined to small eskers recorded in the Loch Ard and Achray forests, in the pass of Leny and in Gleann Dubh [NN 49 15].
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