Geology of the Aberfoyle district: 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 Aberfoyle district lies about 30 km north of Glasgow, straddling the Highland Boundary Fault Zone (HBFZ) that separates the Scottish Highlands from the Midland Valley, near the head of the River Forth. This area of classical geology is important in advancing our understanding of the geological relationships between the Scottish Highlands and Lowlands and is an area of marked topographical contrasts and considerable scenic beauty.
To the south of the HBFZ, rolling arable and pastoral country is underlain by Devonian and Carboniferous sedimentary rocks with variable cover of Quaternary superficial deposits. Extending from Aberfoyle to Callander, the Hills of Menteith form the highest ground within this domain, locally reaching elevations of 400 m above sea level. Lower ground around the Lake of Menteith forms part of the Carse of Stirling — land which was inundated by the sea following the end of the last ice age.
North of the HBFZ, upland countryside underlain by metasedimentary and meta-igneous rocks of the Dalradian Supergroup is deeply dissected by glens overdeepened by Quaternary glaciation. Marked relief in the north-west of the district endows the area known as the Trossachs with its particular scenic appeal. Although hill farming, forestry and estate activities are important elements of the economy of the upland area, tourism and outdoor leisure in the forests and mountains is increasingly contributing to the local economy.
Dalradian Supergroup rocks are of Neoproterozoic to mid Cambrian age and are dominated by metamorphosed siliciclastic 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 are represented. The Dalradian succession is generally inverted in the north, the oldest rocks cropping out on the highest ground around the mountain massifs of Ben Ledi and Ben Vane. The southern Dalradian outcrop is dominated by metamorphosed coarse-grained sandstone and mudstone. Distinctive, dark green-coloured metasedimentary rocks in the lower parts of the Southern Highland Group, informally termed ‘green beds’ and now assigned to the Loch Katrine Volcaniclastic Formation, are rich in volcanic detritus. The thin and only locally developed, Leny Limestone Member, found near the top of the Dalradian outcrop, provides key fossiliferous evidence for deposition of the Dalradian extending into the lowermost part of the mid Cambrian, about 515–520 Ma ago.
Within the HBFZ, slivers of igneous and meta-igneous rocks, the fossiliferous Lower Ordovician Dounans Limestone and conglomeratic rocks constitute crust of oceanic affinity. These units are included in the Highland Border Complex, representatives of which occur sporadically along the HBFZ from Stonehaven on the east coast of Scotland, to the Isle of Arran in the west. The units within the Highland Border Complex were tectonically juxtaposed with Dalradian rocks during Caledonian orogenic events in the Early Palaeozoic.
Devonian and Carboniferous rocks include conglomerate and sandstone with subordinate mudstone and concretionary calcareous rocks underlie the ground to the south of the HBFZ. Carboniferous strata also occur in a narrow outcrop immediately north of the HBFZ in the south-west of the district.
The ground immediately south-east of the HBFZ is underlain by conglomerate and sandstone of Early Devonian age. Andesitic and basaltic conglomerate clasts and a 30 m thick basaltic lava flow testify to contemporaneous volcanic activity in the lower part of the Lower Devonian within the district. Upper Devonian sandstone and conglomerate are unconformable on those of the Lower Devonian. The Devonian rocks were deposited by fluvial and aeolian processes within terrestrial basins under generally warm to hot, arid to semi-arid conditions. The sandstone beds of the succeeding Carboniferous successions are exclusively fluvial and reflect a change to warm, humid conditions. Strong evaporation is indicated by pedogenic concretionary dolostone and limestone in the sandstone, and by veins of fibrous gypsum and rare halite pseudomorphs.
Quaternary superficial deposits chiefly result from the last two glacial episodes and are between about 10 000 and 30 000 years old. They include spreads of till and hummocky deposits of sand and gravel, together with spreads of glaciofluvial sand and gravel deposited by glacial meltwaters from valley glaciers.
Igneous intrusions in the district comprise dykes and sheets. Pre-metamorphic mafic dykes and sheets intrude Dalradian strata in the Ben Vane area and are now amphibolites with relict igneous fabrics. Late Caledonian lamprophyric to microdioritic dykes and felsite sheets of Silurian age are quite common in Dalradian rocks. East–west orientated late Carboniferous quartz-dolerite dykes are also present and are commonly in excess of 10 m in thickness. North-west to south-east orientated Tertiary basaltic dykes are present but rare.
The geological structure is an important element of the geology of the Aberfoyle district. To the north of the HBFZ, rocks were subjected to complex deformation during the Caledonian Orogeny, and Dalradian rocks were metamorphosed to upper greenschist facies. To the south, Devonian and Carboniferous rocks in the Midland Valley have been tilted and folded by Acadian and Variscan earth movements. The Devonian and Carboniferous rocks within the district lie in the northern limb of the Strathmore Syncline, a major, open fold structure that occupies the northern margin of the Midland Valley of Scotland. The rocks dip to the south-east; dips are very steep adjacent to the HBFZ but become more shallow to the south-east. Faulting is widespread and is particularly complex along the HBFZ. Fault movements during the Palaeozoic played a key role in the development of the structural architecture of the Aberfoyle district, and in the development of the basins into which Devonian and Carboniferous sediments were deposited.
The Dalradian rocks north of the HBFZ were metamorphosed during Caledonian orogenic events. Metamorphic grade increases from sub-greenschist facies adjacent to the HBFZ to upper greenschist facies in the north.
At present, there is no commercial quarrying or mining within the district, but the area is important for public water supply, maintained by a high annual rainfall. Loch Katrine has supplied Glasgow since Victorian times, supplemented by water from Glenfinglas since the mid 1950s.