2
The Rocks Beneath

‘Mica schist hills…as we approach their grey rocks of silky lustre, we
find that they are curved, wrinkled, contorted, so as to remind us of
pieces of ill-laid-by satin, that bear on their crushed surfaces the
creases and crumplings of a thousand careless foldings.’

Sketch-book of Popular Geology
Hugh Miller (1859)

Loch Lomondside has long held a fascination for the geologist. Although the region’s exposed rocks are representative of only a small part of Scotland’s incredibly long geological history, they still present an extremely varied cross section, not least because of massive overfolding that dominates the geological sequence in the southern Highlands. Throughout most if not all of the rocks’ formation, what is now central Scotland lay beneath or surrounded by warm seas in tropical latitudes of the world’s surface. As the result of movements of the earth’s crust and varying degrees of metamorphism, the geological structure of Loch Lomondside as seen today is very complex, with opinions as to the origins and age of the rocks under continual review.

Most of the region is covered by the British Geological Survey 1:50000 sheets 30 and 38(W). Sheet 46(W), which takes in the Glen Falloch area, has been out of print for many years, but may be consulted in major libraries.

The Cambrian period

The Dalradian rocks of the Cambrian period, which are named after the ancient Scots kingdom of Dalriada, make up the greater part of the Highland portion of Loch Lomondside. Dating back at least 570 million years, the Dalradians were formed from the top 15 km of 24 km of marine sands and muds deposited in a major subsidence in the earth’s crust known as the Highland Trough. Evidence that the lower Dalradian rocks in the most northerly part of Loch Lomondside are made up of sediments which were moved around by cross currents suggests that they were laid down on a shallow sea shelf. In contrast, the well-preserved graded bedding of the upper Dalradian rocks found elsewhere in the region’s Highland area is more typical of deep water sedimentation. The lower and upper parts of the sequence are separated by a thin, very fragmented band of crystalline limestone – the Loch Tay Limestone.

After many millions of years of deposition these sedimentary beds were compressed, heated and re-crystallised to different degrees during the Caledonian orogeny or mountain-building era, when new land was thrust up by massive folding of the earth’s surface. Three broad zones representing the degree of metamorphism to which the original sediments were subjected are characterised by their respective index mineral. The rocks of the southern zone, where the least amount of metamorphism took place, contain the green mineral chlorite, which imparts a pearly lustre to the surface. In the central zone, where the metamorphism was more intense, the rocks contain biotite, a dark coloured mica. Maximum metamorphism, restricted to the extreme northwest corner of the region, can be identified by red crystals of garnet. Occasional garnets occur further south, but only where the country rock has come into contact with the intense heat of an igneous intrusion.

Fig. 2.1 One of many new road cuttings with fresh exposures of mica schist.

The Dalradian succession in the southernmost part of the Loch Lomond Highlands – the chloritic zone – is represented by the Luss–Aberfoyle Slates and Leny Grits. A blue-black limestone that outcrops in Glen Fruin also appears to be Dalradian in age. Slate, which is derived from fine-particled muds and still clearly showing their well-marked bedding, was subjected to immense pressure but relatively little re-crystallisation. Grits contain abundant angular grains of quartz and felspar that have been carried in suspension by water currents. Both slates and grits become progressively more deformed northwards with increased metamorphism. A bright surface sheen resulting from its increased mica content indicates a change from slate to phyllite, which feels greasy to the touch. The accompanying grits become steadily more schistose. With many new rock cuttings freshly exposed with the road re-alignment and widening programme on the west side of Loch Lomond (Fig. 2.1), the traveller along the A82 has a unique opportunity to see the effects of increasing metamorphism and repeated folding of the mica schists within a relatively short distance.

Only the roots or stumps of the once great Dalradian range remain today, the consequence of millions of years of erosion that followed the mountains’ creation. These remnant layers of rock run approximately NE–SW, with the narrowest banding occurring in the southern zone. Examination on the ground shows progressive dipping of the rock layers from more or less horizontal in the central zone to almost vertical along the Highlands’ southern edge. This resulted from massive southeasterly overfolding hinged around Ben Lui in the northwest. Known as the Tay Nappe, evidence of this folding in the northern zone is the inversion of the Dalradian sequence, older rocks overlying those much younger. Although largely worn away in the southernmost part of the Highlands, the nose of the recumbent fold turns down sharply, with the steeply inclined slates at the nose’s core flanked on either side by grits.

Almost immediately to the south of the nose of the Nappe lies a major structural displacement – the Highland Boundary Fault. The Fault’s initial establishment is obscure, but substantial movements along the fracture line were to occur right up to the late Devonian period (see below). Even today, the southern edge of the Highland Boundary Fault zone is a prominent feature in the landscape, the glacially breached Ben Bowie–Conic Hill ridge running right through Loch Lomondside, physically dividing the gently undulating Scottish Lowlands in the south from the mountainous Highlands to the north.

The Ordovician period

Between the turned-down nose of the Tay Nappe and the Ben Bowie–Conic Hill ridge is a narrow zone of discontinuous wedges of former marine sediments, laid down initially in deep sea conditions followed by an uplift into shallow water at the beginning of the Ordovician period some 500 million years ago. Younger than their Highland neighbours, these sedimentary rocks were not subjected to the main peak of Dalradian metamorphism. Best represented are the black shales of Glen Fruin and the Highland Border Grit at Arrochymore. Closely associated with these exposures of shales and grits is a much altered band of an ultra basic igneous rock – serpentinite. At first considered to be a dyke intruded along the line of the Highland Boundary Fault, recent recognition of the serpentinite’s fragmentary nature suggests a second sedimentary phase with a history of uplift alongside the fault. As a group, these Ordovician rocks are referred to as the Highland Boundary Series or Complex.

The lack of Dalradian debris in the make-up of the Highland Boundary Complex testifies to the fact that these remnants of the floor of a long-dead ocean were far removed from the Scottish Highlands at the time of their formation. Evidence of microfossil marine creatures obtained from Aberfoyle’s Dounans limestone quarry, an exposure of the Highland Border Complex just a short distance east of Loch Lomondside, points to the limestone being similar in composition to one found in eastern North America.

The Silurian period

Further uplift, folding and metamorphism of the Dalradian rocks occurred during the Silurian period, 440–410 million years ago. Although no sedimentary deposits of this period are known to survive locally, it was from late Silurian times that intrusive masses of molten magma were emplaced below ground northwest of Arrochar, around Garabal Hill and elsewhere in Loch Lomondside’s Highland region. Of these slowly cooled, coarsely crystalline plutonic rocks, diorite at the northeast end of the Arrochar Complex is the one most readily seen in an abandoned quarry in Coiregrogan Glen near Loch Sloy. Less accessible is a huge boss of granitic rock centred on Maol Breac further north. Contemporaneous with the plutonic rocks is the Inverbeg–Rowardennan Lamprophyre Dyke, an igneous intrusion that can be traced both sides of the loch.

Fig. 2.2 Lower Old Red Sandstone conglomerates along the fault line on Conic Hill.

The Devonian period

Beginning about 410 million years ago, the Devonian period saw the waning stages of mountain building, followed by erosion of the elevated land on a considerable scale. With little vegetation to protect the land surface from the weather elements, the eroded material was washed away by fast-flowing rivers and seasonal torrents, to be deposited elsewhere in the form of cobbles, pebbles and sand. The Devonian period is represented for the most part by a broad band of red sandstone of great thickness sandwiched between the Highland Boundary Fault to the north and an overlying lava plateau to the south. The red coloration of the sandstone is due to oxidation of iron, indicative of the regular drying-out of a semi-arid plain. Scotland’s Stone of Destiny, for hundreds of years an integral part of the pomp and ceremony associated with the crowning of kings and queens, was hewn from Lower Old Red Sandstone at Scone near Perth.

The basal layers of reddish-brown and purplish Lower Old Red Sandstones contain thick beds of water-rounded cobbles and pebbles set in a matrix of coarse-grained sandstone (Fig. 2.2). These are known as conglomerates and consist mainly of ubiquitous hard quartz and quartzite; but, as in the Highland Boundary Complex, there is a significant absence of soft and easily worn-away pebbles of schist or other Dalradian rocks. This, together with the abrupt northern limit of the Lower Old Red Sandstone, clearly proves that these particular conglomerates are not made up of material of local origin, but were moved into their present position by a sideways shift along the line of the fault from a great distance away, in the process displacing the original sediments washed down from the southern Highlands. Examination of the quartz and quartzite cobbles show that many were sheared during the displacement process.

Deposits of Middle Old Red Sandstone, well developed in the extreme northeast of Scotland, are unrepresented in the central belt. Renewed earth movements at this time, however, led to further uplift of the Dalradian Highland rocks, but subsidence of Lowland rocks south of the fault. The steeply inclined beds of Lower Old Red Sandstone conglomerates exposed on Conic Hill alongside the Highland Boundary Fault are directly related to this downthrow to the south.

By 390 million years ago, the southern crustal block was sliding along the fault line into its final position. Erosion of the mountains to the north continued as before, with vast quantities of sands and gravels deposited in the present day Midland Valley, a 65 km wide rift valley that was gradually sinking under the weight of the accumulating material. Some outwash was also laid down just north of the Highland Line, to rest unconformably on the near vertical Dalradian and Ordovician strata. This second deposition of sandstone – the Upper Old Red Sandstone, which is distinguished from its Lower counterpart by a brighter red coloration – can be seen to advantage in the water-worn ravine of Finnich Glen (Fig. 2.3) that passes beneath the Glasgow–Drymen road. Compared to the conglomerates of Lower Old Red Sandstone, the Upper Old Red Sandstone conglomerates are composed of more angular (less travel worn) fragments. The presence of pebbles of soft Highland schist confirm its local origin.

Fig. 2.3 Finnich Glen; a water-worn ravine in the Upper Old Red Sandstone.

Fig. 2.4 Alternating beds of cementstones and mudstones in Ballagan Glen.

With the exception of an area around Killearn, where a shallow dome of Lower Old Red Sandstone rises through the overlying Upper Old Red Sandstone, the boundary between the two occurs only along the lines of faults. The Devonian rocks on Loch Lomondside are singularly unfossiliferous, the exceptions being a few plant remains from Lower Old Red Sandstone quarries at Buchanan Castle and near Balloch. Rarer still are the fossil fish scales obtained from an Upper Old Red Sandstone quarry on Carman Muir.

Further movement along the Highland Boundary Fault followed. This time the downthrow was to the north, which was later to protect the Upper Old Red Sandstone on the Highland side of the fault from total erosion. Immediately to the south of the Highland Line the Upper Old Red Sandstone has been completely worn away, but reappears within a few kilometres where it has again been protected by a downthrow, this time on the south side of the Gartness Fault.

The close of the Devonian sandstone period beginning about 360 million years ago is characterised by the presence of cornstones – terrestrial concretionary limestones – formed by rapid evaporation of mineral-rich ground water precipitating calcium carbonate in the surface soils. As the semi-arid desert was gradually replaced by conditions where the land never completely dried out, the red sandstones gave way to green, grey and white deposits, initially alternating with the red oxide layers before replacing them entirely. Exposures of these Calciferous Sandstones skirt the northern and western lava scarps of the Campsie Hills, but are cut out on the southern side of these foothills by the Campsie Fault. The sequence can be considered as a transition zone between the late Devonian and early Carboniferous periods.

The Carboniferous and early Permian periods

A transgression of a warm shallow sea over the sunken Midland Valley of Scotland marked the beginning of the Carboniferous period in Scotland around 350 million years ago. The basement group of sedimentary deposits laid down during the early Carboniferous – the Ballagan Beds – comprises some 180 m of alternating beds of cementstones and thin-bedded silty mud-stones. Whereas the mudstones are derived from material washed down from the high ground during rainy seasons, the cementstones are impure limestones precipitated during extended periods of drought. Occasional complete drying out of the shallow lagoons is indicated by the presence of gypsum and the cavities left by dissolved crystals of salt. Few fossils have been found in these sediments, as animal life was sparse in this highly saline environment. At the type locality in Ballagan Glen (Fig. 2.4), at least 100 distinct bands of cement-stone of varying thickness are exposed. The Ballagan Beds are overlain by a thick band of hard Spout of Ballagan Sandstone, usually overhanging as the much softer mudstones and cementstones beneath erode at a more rapid rate.

Carboniferous sedimentation in the partially submerged Midland Valley was interrupted after the first 10 million years by widespread volcanic activity. From a large number of vents penetrating the marshy ground, voluminous quantities of volcanic ash were discharged, in which is preserved one of the richest assemblages of fossilised plants known for this age. The ash in turn was followed by layer after layer of molten magma – the Clyde Plateau Basaltic Lavas. At the centre of this volcanic activity it is probable that the lava flows accumulated to a depth in excess of 600 m, although thinning away towards the edge of the plateau. Well over 70 volcanic vents, plugged either by basalt or an agglomeration of debris, have been identified in the Kilpatricks and Campsies, but none in the Fintry–Gargunnock Hills. They range from relatively small multiple plugs to eye-catching landscape features such as Dumgoyne (Fig. 2.5) above Strathblane and Dumbarton Rock. Millions of years of denudation have greatly reduced the extent and thickness of the Clyde Plateau Lavas, the only remaining evidence of their former presence north of the Highland Line is to be found capping the summit of Ben Bowie on the west side of the loch. Just south of the Highland Line is the isolated volcanic plug of Duncryne near Gartocharn; so low lying that it is the only one in the district that can be seen to penetrate the Lower Old Red Sandstone. Not all of the formation can be examined at any one spot, but at least 30 lava flows amounting to a thickness of over 300 m have been identified along the steep scarps of the Kilpatrick, Campsie and Fintry–Gargunnock Hills. The well-defined terracing of these scarps is due to structural and weathering differences between individual layers of lava, with the tiers picked out here and there by a waterfall descending in a series of cascades. Although the intense volcanic activity had extinguished life over a wide area, the mineral richness of the lava bequeathed a new fertility to the land, which in turn was fully exploited by the recolonising plants and animals.

Fig. 2.5 Dumgoyne; a striking volcanic plug in the Clyde Lava Plateau.

With the gradual subsidence and irregular submergence of the Midland Valley, sedimentation become possible once more. Below the southern face of the Campsie Hills, on the downthrow side of the Campsie Fault, lies Muirhouse Muir with its Craigmaddie White Sandstones and associated quartz conglomerates. The lower Craigmaddie Sandstones comprise mainly rounded quartz pebbles. These are overlain by beds of finer sand containing some remains of fossilised plants. The bedding of the Craigmaddie Sandstones suggests the earlier material was carried down from the north by braided streams, followed in turn by flood plain deposition. The Craigmaddie Sandstones are not quite the youngest rocks to be found in southern Loch Lomondside, for cutting through them and the nearby Clyde Plateau Lavas is an intrusive igneous dyke, which can be seen to the south of Loch Ardinning. Of similar composition and presumed age to the great quartz–dolerite sill on which Stirling Castle sits, this would place the intrusion in the late Carboniferous–early Permian period, between 290 and 280 million years ago. Other quartz–dolerite dykes are present in the Highland Region.

The laying-down of sedimentary beds in the southern part of Loch Lomondside continued throughout all subsequent geological periods, but these overlying younger rocks have long since eroded away into the sands of time.

Earthquakes and tremors

It is fortunate indeed that the Highland Boundary Fault – one of the great fractures in the earth’s crust – seems to have remained dormant within historical time. Periodic earthquake swarms near to the line of this massive fault have been recorded in the Comrie area of Perthshire, but apparently attributable to a neighbouring fault.

Most of the earth tremors that have occurred in the region since records were kept would seem to have originated from the still settling down Campsie and Gartness Faults, the former running west along the south face of the Campsie Hills and the northern edge of the Kilpatrick Hills, the latter through Balfron, Croftamie and Balloch. The two faults almost converge in the vicinity of Dumbarton. From several documented accounts of earth tremors in Dumbarton, one of the strongest took place during the hours of darkness on 8 November 1608, the townsfolk scurrying from their homes to the kirk, where with their minister they prayed for deliverance from imminent destruction. Situated almost directly on the Campsie Fault, Strathblane too is no stranger to seismic activity, the most recent incident severe enough to attract newspaper coverage in January 1990. What must be one of the strongest tremors recorded in Strathblane occurred mid-morning on 6 January 1787, the terrified inhabitants running for safety to the open fields as their houses shook and locked doors flew open. The sounds accompanying these sudden movements in the earth’s crust have been variously described as a loud rumbling or rushing noise to a deep-seated cavernous growl.

A famous incident involving Loch Lomond itself began suddenly at 9.30 a.m. on 1 November 1775, at the same time as the great Lisbon earthquake. Without warning, the surface of the loch rose rapidly about 2.3 ft (0.7 m) in height, then dropped to a level only seen during the driest summers. The sudden rise in water level was almost immediately repeated, the oscillations continuing at five minute intervals for an hour and a half before gradually subsiding, leaving behind a ‘tide-line’ of debris and small boats up to 40 yards (37 m) from the shore.

Rocks in the service of man

Man’s ingenuity in utilising the most readily available materials is well reflected in the region’s rocks. Serving both local and distant needs, almost every type of rock found on Loch Lomondside has been put to practical use at some time. As timber dwellings gave way to something more permanent, the earliest buildings and defensive walls were simple constructions made from gathered loose stones. Only with the development of rock-cutting implements came the use of quarried stone hewn to the desired shape and size. First choice was the easily extracted and dressed red sandstones, as many of the older houses show. Not all was used locally; for many years, flags of Old Red Sandstone used for flooring and pavements were transported by boat from the Port of Aber on the loch’s southern shore. For more durable roofing, the early turf and thatch coverings were replaced by readily split slate, from quarries opened up on workable seams both sides of the loch (Fig. 2.6). Like the sandstone flags, slate was also exported by boat. The coming of the railway and the increasing use of building stone from further afield was to lead to the closure of the Loch Lomondside sandstone and slate quarries one by one. Gone too are the clay pits, which had been opened up to meet the need for field drainage tiles at the height of nineteenth-century agricultural improvement. Still continuing in demand by the building industry, glacial gravels and sands have been worked in a number of localities around the southern end of the loch. Crushed, washed and screened, the Craigmaddie Sandstone, until recently quarried on Muirhouse Muir, produced an aggregate of the highest grade. In the more northern part of Loch Lomondside, well over 300,000 tonnes of coarse-grained diorite was used as a crushed hard-rock aggregate in the construction of the hydro-electric reservoir dam at Loch Sloy.

Like the earliest stone houses, the hundreds of kilometres of dry-stone dykes (unmortared field and boundary walls) were invariably built from the nearest source to hand. In contrast, the choice of suitable road stones in pre-Tarmacadam days was far more selective. Lowland igneous rocks (whinstone) and Highland grits were frequently used as road ‘metal’, the softer sandstones and schists proving totally unresilient to wheeled traffic.

Agricultural mineral fertilizers were at one time obtained from burning serpentinite (some of the Lomondside outcrops having high concentrations of calcium), cornstones and cementstones in specially built kilns. In the absence of any coal deposits in the immediate vicinity, large amounts of peat were used as fuel in the firing process. At the other end of the cereal production cycle, millstones were quarried in several places, but the local material was not of a high enough quality for the industry to be sustained.

Fig. 2.6 Camstradden slate quarry when actively worked in the nineteenth century (G.W. Wilson/Author’s collection).

Not all past quarrying or mining operations in the region have surviving records. There appears, for example, to be no documentation on graphite extraction from the blue-black limestone in Glen Fruin, or on the barytes mine beside Kilmannan Reservoir. A reputed venture to extract gold from Lomondside quartz ca.1880 and a worked-out vein of silver near Dungoil in the Campsies must equally have tales to tell. The only deposit of precious metals to be worked in recent years is just outside the Loch Lomond area at the abandoned Cononish lead mine, immediately northeast of Ben Lui. Test results show that the Cononish mine can produce on average 11.3 grams of gold and 60.1 grams of silver for each tonne of processed rock.

Finally there is the influence of the underlying rocks on the locally produced ‘water of life’. According to whisky connoisseurs, it is the water draining from the mineral-rich lavas and sandstones of the Campsie Hills to a well-known distillery at the foot of Dumgoyne that helps impart to the finished product its distinctive flavour.

Mineralogy

The collections of local minerals and semi-precious stones housed in the museums of Glasgow and Edinburgh bear witness to the dedicated work of the amateur collector. In Scotland, agates are always the first to come to mind, but Loch Lomondside’s Carboniferous Lavas cannot compare in either numbers or variety of these ‘scotch pebbles’ which occur in the earlier Devonian Lavas of the Ochils east of Stirling or the Carrick Hills and coastal cliffs to the south of Ayr. But what the Lomondside foothills lack in agates is more than made up for by a richness in zeolites and associated minerals, with the Kilpatrick Hills a hunting ground of world renown. Zeolite crystals are found where mineral-laden water has permeated into fissures and cavities within the cooling lava. Amongst the best known types are:

• stilbite: sheaf-like aggregations of crystals coloured orange-red due to the presence of iron oxide, the finest specimens up to 38 mm long;

• heulandite: locally taken specimens are also orange, but with ‘coffin-lid’-shaped (wider in the middle) crystals;

• thomsonite: white, occasionally flushed or lined with orange; and

• prehnite: very similar in composition to a zeolite; crystals usually pale green. In the mid-nineteenth century, a mini-bonanza of zeolites was made available to the keen-eyed opportunist from rock spoil heaps as tunnelling work for the Loch Katrine aqueduct progressed through the Strathblane–Mugdock area en route to the Glasgow water supply reservoirs at Milngavie.

Amongst other minerals described for Loch Lomondside, gypsum and garnet have already been mentioned. Gypsum as a sedimentary evaporite is well known from the Ballagan Beds, and both red and yellow varieties are easily collected. Garnets are common in the highly metamorphosed schists in the northwest, but the stones are generally small and lack gem quality. Elsewhere in the Highland region, crystals of tourmaline have been obtained from schists on An Caisteal and titanite in the plutonic rocks of Garabal Hill. Not to be forgotten are the local veins of both banded and interwoven red and yellow jasper, an opaque form of quartz which occurs in the Clyde Plateau Lavas. Jasper takes a fine polish, and in the past was made into seals and buttons. In modern times, a specially chosen specimen from the Campsie Hills was used to decorate the ceremonial baton carrying the Queen’s message to the 1986 Commonwealth Games.

The overlying soils

The wide variety of rock types found in the region has given rise to a complex pattern of derived soils. At one extreme are the Highland sub-alpine soils, characteristically shallow and containing parent material that is little altered. In complete contrast to these immature soils at high levels are the Lowland forest brown earths and river flood plain silts that have both been extensively utilised for arable farming. Such soils have long since lost their natural profiles through centuries of ploughing; their chemical composition changed with repeated applications of mineral fertilizers.

Loch Lomondside is covered by 1:250000 sheets 4 and 6 from the Macaulay Institute’s soil survey of Scotland.