Geology of the Aberfoyle district: Introduction
|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).
This account describes the geology of Aberfoyle district of Stirlingshire and accompanies the 1:50 000 scale geological map sheet 38E (Scotland). The Aberfoyle district lies near the head of the River Forth, about 30 km north of Glasgow, and straddles the Highland Boundary Fault Zone that separates the Scottish Highlands from the lowlands of the Scottish Midland Valley, (Figure 1). This area of classical geology includes many features and localities that have been important in elucidating geological relationships between the Highlands and Lowlands, and is an area of marked topographical contrasts (P505754). The high ground occupying the north-western half of the district is characterised by rocky peaks and deeply incised valleys, some of which were overdeepened by glaciation and are now occupied by deep lochs, most notably Loch Katrine. South-east of the Highland Boundary Fault Zone and the Menteith Hills, the countryside is rolling agricultural land which falls away from about 200 m O.D. to the flat lands of Flanders Moss, at no more than about 20 m above current sea level. Flanders Moss is the western-most extension of the Carse of Stirling that was inundated by the sea in postglacial times. The land rises again to about 175 m east of Balfron [No. 548 888], before falling gently southwards towards the valley of Endrick Water.
The land rises abruptly north-west of Aberfoyle [NN 523 010] and Callander [NN 625 080], although the change in topography does not coincide precisely with the Highland Boundary Fault Zone (see Concealed geology). For example, ground in Loch Ard Forest occupying the triangular area between Aberfoyle, Loch Ard [NN 465 018] and Creagan Dubha [NO 434 954] is within the Highlands, lying north-west of the Highland Boundary Fault Zone, but is an area of relatively low, if incised, relief, only locally exceeding 200 m in elevation. To the north-east of Aberfoyle, the Menteith Hills are, geologically, part of the Midland Valley, but form a prominent topographical ridge up to 400 m high, extending from Aberfoyle almost to Callander, immediately south-east of the Highland Boundary Fault Zone (P001255). North of the Highland Boundary Fault Zone many of the mountain tops are between 500 and 700 m high. The most prominent peaks are those of Ben Venue [NN 478 062] (727 m) Ben Vane [NN 535 137] (818 m) and Ben Ledi [NN 562 098] (879 m) Landscape typical of the highlands in the district is shown in (P001253).
The district has long been popular with visitors, and tourism remains a key element of the local economy. The area between Ben Venue and the south-eastern end of Loch Katrine, known as the Trossachs, is particularly noted for its natural beauty. It has steep, wooded, craggy topography and historical connections with Rob Roy MacGregor (1671–1734), romanticised in the novels of Sir Walter Scott. Both Aberfoyle and Callander in particular rely heavily on the tourist trade for their prosperity; tourism also bolsters the local agricultural economy via numerous guest houses. Outdoor leisure activities, which are commonly linked to developing understanding of the natural environment, are making an increasing contribution to the local economy.
Much of the rest of the economy is based on traditional forms of agriculture and forestry. In the lower ground south-east of Aberfoyle, the agriculture is mixed arable and beef, with sheep and dairy herds. In the Highlands, lower ground is given over to beef cattle locally, but the upland ground supports sheep farming and field sports managed by estates. Forestry is very important and forested ground occupies many square kilometres in Loch Ard, Achray and Strathyre forests. Extensive areas of the flat-lying mosses south of Lake of Menteith are also forested. Much of this forestry is conventional conifer plantation, though more mixed woodland is now being planted as mature conifer stands are harvested for timber. Indeed, the environmental aspects of forestry and the recreational use of the forests are now being heavily promoted as part of the tourist industry. In a significant recent venture on the eastern side of Glenfinglas, north of Brig O’ Turk [NN 535 066], the charity Woodland Trust is, at the time of writing, replanting once naturally wooded areas with native Scottish woodland species dominated by birch, oak and scots pine.
The other renewable resource which is important in the district is water, a natural consequence of the region’s high rainfall. Loch Katrine and the man-made Glenfinglas Reservoir are an integral part of the water supply for Glasgow. Water is piped in a tunnel from Glenfinglas reservoir to maintain levels in Loch Katrine and, thereby, a constant water supply. In dry spells, the water in Glenfinglas reservoir, flooded by damming just above Brig O’Turk in the mid 20th century, may fall several metres.
Although a range of bulk mineral resources have been worked within the district in the past, there is no commercial activity within the district at present (2010). The largest workings are the Aberfoyle Quarries [NN 505 032], 1 km west of the Dukes Pass (A821). Forest Enterprise uses small, piecemeal quarries within their forests to provide metalling for forestry roads. These small quarries and pits have the advantage of providing outcrop in areas where exposure is commonly obscured by forestry and vegetation. Long-abandoned workings of limestones in Lower Palaeozoic and Dalradian rocks adjacent to and within the Highland Boundary Fault Zone at Leny [NN 615 098] and Dounans [NN 534 019] are now very important geological conservation sites, providing exposure of rocks crucially important in recent work elucidating the geological history of the area.
In this account, successive episodes of deformation are labeled D1, D2, etc. Folds and cleavages resulting from each of these deformation episodes are labeled F1, etc and S1, etc. Bedding is labeled S0 by convention.