Geology of the Aberfoyle district: Geological summary

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).

The oldest rocks of the district comprise strata of the Dalradian Supergroup which were deposited on the eastern margin of the ancient continent of Laurentia (Strachan et al., 2002). Deposition began sometime after about 800 Ma ago in the Neoproterozoic, and continued until at least the mid Cambrian, about 510 Ma ago. Dalradian Supergroup rocks are dominated by metamorphosed siliclastic and subordinate volcaniclastic sedimentary rocks with rare limestones, locally intruded by mafic meta-igneous rocks. Within the district, only the upper parts of the Dalradian Supergroup succession (Argyll Group and Southern Highland Group) are represented, ranging in age from about 610 Ma to about 515 Ma (Halliday et al., 1989; Dempster et al., 2002). These rocks commonly comprise coarse-grained sandstone, some of which contains abundant volcanic detritus, with subordinate semipelite and pelites and locally developed limestone, including the lithostratigraphically important Loch Tay Limestone (LLT). The rocks were deposited mainly by sediment gravity flows (turbidity currents, debris flows) (Burt, 2002). The sedimentary rocks rich in volcaniclastic detritus have been informally termed ‘green beds’ because of their distinctive, dark, bottle-green colour. Towards the top of the Dalradian, black graphitic, cherty and sulphidic rocks of the Leny Limestone and Slate Member (PL) indicate sediment-starved conditions and deposition in deep water. These rocks contain a Pagetides trilobite fauna of Laurentain affinity that indicates an early Mid Cambrian age of about 510–520 Ma.

Above the Dalradian Supergroup, the only evidence for younger Lower Palaeozoic successions comes from tectonic slivers within the Highland Boundary Fault Zone, such as the Lower Ordovician Dounans Limestone. This limestone forms part of the Highland Border Complex, a Cambrian to Ordovician igneous and sedimentary assemblage considered to represent oceanic crust. The components of the Highland Border Complex were tectonically emplaced in the HBFZ in the Ordovician, during the Grampian event of the Caledonian Orogeny.

Caledonian earth movements deformed and metamorphosed Dalradian sediments, and created most of the key structural features of the district. In common with much of the Southern Grampian Highlands, the Dalradian of the district is divided into two structural domains. These are colloquially termed the ‘Flat Belt’ and ‘Steep Belt’, and they are divided by a monoformal flexure named the ‘Highland Border Downbend (HBD). The trace of this structure trends north-east across the Aberfoyle district from just south of Loch Katrine, in the west, towards the Ben Ledi–Ben Vane massif.

The Flat Belt lies to the north-west of the HBD; it is characterised by inverted strata and cleavages that are generally shallow dipping. The Steep Belt lies to the south-east of the HBD and is characterised by generally steeply dipping strata disposed in large primary fold structures with related, steeply dipping cleavages (Shackleton, 1957). The Steep Belt results from the large-scale down-folding of strata and primary structures around the hinge of the HBD. The primary folds form the ‘nose’ of the Tay Nappe, the regionally extensive fold complex which dominates the structure within the Argyll and Southern Highland Groups (Mendum and Fettes, 1985; Stephenson and Gould, 1995).

Erosion of the newly uplifted Caledonian mountains of the Scottish Highlands is reflexted by the Devonian succession. The wide range of lithogies includes conglomerates, sandstones, siltstones and mudstones, and represent deposition in fault-bound, intermontane basins by alluvial fans and river systems under arid to semi-arid conditions. Basal Lower Devonian rocks adjacent to the HBFZ are commonly very coarsely conglomeratic and include basaltic and andesitic detritus. The volcanic clasts and the Balleich Lava Member, a 30 m thick olivine basalt lava flow that occurs within the Lower Devonian sedimentary succession, reflect the significant levels of volcanic activity that occurred throughout Scotland during early Devonian times. The Lower Devonian succession becomes generally finer grained to the south-east away from the HBFZ, with siltstone and mudstone becoming more abundant. Upper Devonian rocks were deposited unconformably on Lower Devonian strata following uplift and erosion after the Acadian Orogeny. Upper Devonian lithologies are dominated by sandstone (including some of aeolian origin) and some pebbly and conglomeratic rocks.

Lower Carboniferous rocks are conformable on Upper Devonian strata. Indeed, lowermost strata currently assigned lithostratigraphically to the basal Carboniferous Kinnesswood Sandstone Formation (KNW) may be latest Devonian in age. Rocks assigned to the Kinesswood Formation also lie unconformably on Dalradian strata where they crop out to the north of the HBFZ in the south-west of the sheet, west of Gualann [NN 459 945] and north of Breac Leac [445 933]. Carboniferous lithologies are dominated by sandstone and subordinate mudstone deposited in fluviodeltaic, lacustrine and shallow marine environments. Locally developed pedogenic horizons, nodular limestone and dolostone, gypsum and halite pseudomorphs indicate fluctuating water-table levels and subaerial exposure under hot, humid conditions.

Bedrock deposits postdating the lower Carboniferous are absent from the district. For most of the Mesozoic and Cainozoic this part of Scotland was a landmass, but a lack of preserved sediments means that little is known of its detailed geological history during this time.

Repeated glaciation occurred during the Quaternary, but only deposits resulting from the Dimlington Stadial (about 27 000–13 000 years ago) and Loch Lomond Stadial (11 000–10 000 years ago) are confirmed in the district. During the Dimlington Stadial, the U-shaped glens were created or deepened and spreads of till were deposited. In the Balfron district, the till was moulded into drumlins. During the intervening Windermere Interstadial, 13 000 to 11 000 years ago, the amelioration in climate resulted in retreat of the ice and the deposition of thick marine silt and clay when the sea flooded the valleys of the Forth and Endrick.

A sharp deterioration in the climate at about 11 000 years ago (Loch Lomond Stadial) resulted in the growth of a new, but short-lived ice-sheet centred on Rannoch Moor. This sheet reached the Aberfoyle district in the form of valley glaciers that coalesced and spread out onto the lower-lying ground. Within the district, the extent of these glaciers and stillstands during their retreat is now marked by prominent terminal moraines in the district. Glacial meltwater deposits related to retreat and decay of the Loch Lomond Stadial ice are abundant. Most notable is the deltaic terminal moraine complex of the Loch Lomond glacier, which contains the largest deposits of glaciofluvial sand and gravel within the district.

With the improvement in climate from 10 000 years ago, the sea flooded the land that had been depressed by ice in the Forth valley and Loch Lomond. This marine incursion resulted in the formation of beach deposits some 10 m above the current sea level. As isostatic rebound of the land occurred, the sea level fell and peat was deposited in the upper Forth valley. However, with the melting of the North American ice sheet about 8 000 years ago, the sea inundated the Forth again, laying down extensive deposits of silt and clay that buried most of the earlier peat deposits. Peak sea levels (transgressive maximum) occurred about 6 800 years ago. The silt and clay deposited by the flooding events of the Forth have given rise to the very flat ‘carse’ topography characteristic of the central south-eastern part of the district.