Geology of the Aberfoyle district: Applied geology: Quarry products
|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).
Apart from small quarries used by Forest Enterprise and exploited piecemeal to pave forestry roads, there are no active quarries for road metal in the district. Tertiary dolerite dykes were formerly quarried at Fir Hill [NS 5395 9675] and near Kepculloch [NS 5495 9120].
The winning of slate from the Aberfoyle Slate Quarries [NN 505 032] was a major industry from about the 1820s until the 1950s. Prior to this Cunningham-Craig (2000) recorded roofing slates being worked from 1742, and there is documentary evidence of slate working here dating from 1625 (Richey and Anderson 1944, Walsh 2000). Cunningham-Craig, writing originally in 1901, stated that the quarrying company then active had been working for 19 years, employed between 80 and 100 men and produced about 6000 tones of slate per annum. Slate quarrying appears to have survived at Aberfoyle well after it had ceased in many other Scottish slate-producing areas, probably because of proximity to the industrial heartland of Scotland. Indeed, the quarries at Aberfoyle were some of the few that re-opened in 1947 after the war (Walsh 2000). However, the revival was short lived, and the quarries finally closed in 1954. By the time quarrying ceased, the workings covered some 40 ha (Walsh 2000).
The quarrying was sufficiently important to warrant the building of a branch railway connecting Aberfoyle to the Glasgow–Stirling line in 1882. This also opened up Aberfoyle and The Trossachs to the burgeoning Victorian tourist trade. The slate from the quarries was carried to the railhead by a mineral tram line, laid out on the north side of Craigmore [NN 510 021], just North-West of Aberfoyle. At the end of the tramway, the slate was lowered 225 m to the road via a about 0.65 km-long inclined plane. The weight of full trucks was used to raise empty ones to the quarry. Cunningham-Craig (2000) recorded that ‘by this means a thousand slates can be delivered to the rail head at Aberfoyle station at a cost of two shillings and fourpence for carriage’. The tram line route is shown on Ordnance Survey maps, and can still be traced through the forest to the quarries. Some of the winding gear and abutments for the inclined plane are still in place on the forested slopes between Craigmore and the course of the tramway.
In their account, Richey and Anderson (op. cit.) described the slate produced here as being blue, grey, green and purple in colour, with a purple and green mottled variety that was termed ‘tartan slate’. Cunningham-Craig (op. cit.) noted that, because of fashion and comparative rarity, the green slates commanded higher prices.
Layers of different coloured slate are noted as running parallel to the cleavage. The slates are described as being slightly rough, with a fine to medium texture, but coarser than the best Welsh slate. Many thin quartz veins occur, parallel to the dominant cleavage. The slates were worked in seven main seams in the Low Quarries, varying in thickness from 2 to 6 m, with thinner seams of workable slate in between. The higher West Quarry, to the west and south of the Low Quarries, contained a seam some 10 m thick. There are still considerable reserves of slate here.
Apart from frequent small quarries, where sandstone was won locally to build ‘dry stane’ dykes, sandstone was quarried on a larger scale at the following quarries:
Balfron [NS 547 889]: red sandstone, Stockiemuir Sandstone Formation
Edinbellie [NS 577 894]: red sandstone, Stockiemuir Sandstone Formation
Ballikinrain [NS 558 869]: buff sandstone, Kinnesswood Formation
Ballikinrain [NS 559 871]: buff sandstone, Kinnesswood Formation
Tombrake [NS 5465 9080]: pale purple sandstone, Teith Formation
Buchanan Castle [NS 459 890; NS 468 888]: pale green sandstone, Teith Formation
Limestone and carbonated serpentinite
Accessible resources of limestone and other carbonate rocks that have been used historically for lime, such as carbonated serpentinite (principally dolomite) and cornstones, are limited in occurrence in the Aberfoyle district.
The most extensive outcrop of limestone is that of the Loch Tay Limestone, on the north-east side of Ben Vane [NN 535 137]. Cunningham-Craig (2000) recorded that this limestone had been worked for many years, although not in the 80 years prior to his work in the area in the 1890s. The limestone was carted down to Laggan [NN 560 146], on the west side of Loch Lubnaig where it was burnt for lime. The lime was then loaded on to boats and transported down Loch Lubnaig.
The main limestone quarry within the district is Leny Quarry [NN 615 098], discussed earlier with reference to the Cambrian trilobite fauna hosted by the limestone. The limestone is now worked out and little remains except the excavations. Cunningham-Craig (2000) described this and another smaller quarry in some detail in his draft of the original, but unpublished memoir for Sheet 38. Because of the historical importance of the quarry, Cunningham-Craig’s description is included here and it is notable that fossils and other organic remains were considered to be absent, following petrographical inspection.
‘The best known limestone, the Callander (now Leny) limestone, was formerly extensively worked in two quarries, one, the larger of the two, about 1000 yards, and the other about three-quarters of a mile slightly east of north of Leny House. A little above each quarry there are the sites of old ponds, the waters of which have been used to wash off drift etc. and bare the limestone. Below the quarry there are many loose pieces of limestone which are clean from dirt and give a better idea of the lithological character of the rock than the present exposure in the quarry.
Most of the limestone, when fresh, is of a dark grey colour, and some parts blacken the fingers. The colour of weathered surfaces is usually rusty. A fine banding into subparallel laminae of slightly different colours and characters and often about one inch thick, is frequently observed. Cleavage planes frequently cross the bedding at a considerable angle, and these planes are somewhat lustrous. Iron pyrites is abundant within the limestone, and occurs sometimes in lumps the size of a hen’s egg, and sometimes in thin layers about one eighth of an inch thick, which can be traced a foot or two, and which are occasionally disposed in sharp folds. In the large quarry, in sheltered places under some pyritous beds, there were in 1896 considerable encrustrations composed of sulphates which had been formed during the oxidation of the pyrites and resulting chemical actions.
In both quarries the limestone is sharply folded, and also crossed by many fault lines, and it is now impossible to ascertain the original thickness, or to what extent shale was at first interbedded with it.
On the north-west side of the upper end of the big quarry there is a band of quartz felsite, the south-east side of which is crushed by a north-east to south-west fault. The black shale and limestone occur south-east of this fault. The best exposure of limestone is near the middle of the quarry; here it is in two bands separated by four or five feet of black shale. The upper band diminishes in thickness from five feet to one foot in a distance of eight or nine yards. The lower band is about two feet thick. Both bands appear to end rather suddenly before they reach the top of the rock on the south-east side of the quarry. The higher band is probably folded out, but it is also crossed by a crush line.
Twenty yards north-east of the lower end of the big quarry, on the north-west side, there is a little sharply folded black shale and limestone. This exposure is perhaps separated from a larger exposure of limestone on the same side by a fault hading north-west. The limestone in the larger exposure is also repeatedly folded, and lies just under a rather pale grey shale, which in turn lies under alternating black and grey shales. It is doubtful if the limestone exists at the lower end of the quarry. Perhaps it is cut out by a north-east to south-west fault nearly parallel to the others in the quarry.
The limestone exposures in the small quarry are clearer than those in the other. On the north-west side of the quarry we see the quartz-felsite band already spoken of, an again in a crushed state, and south-east of this band come the black shale and limestone. Limestone occurs in small exposures every here and there for a breadth of 12 or 14 yards. Near the south-east end of the quarry there is a fault hading north-west and with prominent slickensides. Most of the limestone lies north-west of this fault, and the various folded bands of limestone are thrown out of sight by it. Some of the limestone is in more or less lenticular courses from two to four inches thick and apparently surrounded by shale. In one place we also observed within the shale several courses of soft calcareous sandstone, two or three inches thick.
About 100 yards slightly south of east of the small quarry there is an exposure of limestone close to a dolerite dyke, but separated from it by ‘crush breccia’ altered by the dyke. No traces of organisms have been discovered in the limestone. A specimen (S.6971) from the big quarry was sliced and examined by Mr Teall and Dr Flett and disclosed some curious dark fragments of uncertain character and some small grains of quartz and feldspar, mostly plagioclase, which are present as impurities in the limestone.’
Very little limestone now remains in the quarry, and it is clear that much of it had been worked out over a century ago at the time Cunningham-Craig was working in the area.
The other important limestone quarry within the district is at Dounans [NN 535 020], about 2 km north-east of Aberfoyle. The degraded remains of the old track way used to transport the limestone down the steep hill are still visible, and now used as one of the recreational footpaths in the Achray Forest. Very little of the original limestone remains visible in outcrop, although the outcrop of dolomitised serpentinite is still extensive.
Cunningham-Craig (2000) recorded between 7.5 and 15 m of limestone in his account, but, if any limestone is still present, it must be heavily covered by the cobbles and boulders that form talus derived from the cliffs in Craig of Monievreckie Conglomerate.
Limestone, or more likely dolomitised serpentinite, is recorded to have been worked from Lime Hill [NS 473 963]; apparently, the lime from this resource was ‘greatly esteemed on account of its purity‘ (Cunningham-Craig, 2000).
In addition to quarries in carbonated serpentinite, cornstones were also worked quite extensively along their outcrops. The limestone (thought to be mostly dolomitic) was burnt nearby using local peat (kilns are present on Balgair Muir and at the Burn of Mar). Working was abandoned many years ago. There are quarries at Balgair Muir [NS 602 857 to NS 614 861] and [NS 604 863 to NS 607 868], and at Burn of Mar [NS 437 935]. There is a kiln near the site of Bofrishlie Farm at [NS 5075 9965].
Baryte veins associated with north-west–south-east faults, were formerly quarried between Arndrum and Clashmore, at [NS 4980 9839 and NS 4985 9842].
Sand and gravel
Commercial sand and gravel working in the Aberfoyle district is confined to one pit at Drumbeg [NS 48 88], which operated for many years, but is now dormant (1999). Although glacial meltwater deposits associated with the Loch Lomond Stadial are most extensively developed east of Drymen, other noteworthy spreads occur near Buchlyvie and around the Lake of Menteith. Some of these deposits have been worked in a small way in the past. Forest Enterprise has extracted sand and gravel from several eskers in the Loch Ard Forest.
Glaciolacustrine silty clays, deposited in an ice dammed lake in the valleys of the Blane and Endrick waters towards the end of the last ice age, were formerly worked for brick clay near Little Drumquharn [NS 507 878].
Extensive spreads of peat covered the carse of Forth after the last glaciation, for example Flanders, Collymoon and Gartrenich mosses. During the 19th century large quantities of peat were removed for agricultural purposes, especially adjacent to the River Forth, for example at Pendicles of Collymoon [NS 58 96]. The irregular but linear margins to many of these mosses attest to the past activities of man. Large peat mosses also exist on low, till covered ground such as Cardross Moss [NS 57 99]. Hill peat is extensive on Moor Park [NS 47 93], Buchlyvie [NS 58 91], Balgair and Kippen [NS 61 92] muirs.
- Cunningham-Craig, E H. 2000. Explanation of Sheet 38, Loch Lomond. . British Geological Survey Technical Report, WO/00/05.
- Richey, J E, and Anderson, J G C. 1944. Scottish Slates. Wartime Pamphlet. No. 40.
- Walsh, J A. 2000. Scottish Slate Quarries. Technical Advice Note, No. 21. (Historic Scotland.)