Compared to the geological architecture of other European countries not exceeding a total area of 100,000km², the geology of Scotland is characterised by an unusual diversity of geological features. Due to its tectono-metamorphic complexity Scotland attracted numerous earth scientists in the eighteenth and nineteenth centuries, whose main aim was the development of theories about, on the one hand, rock formation and, on the other, metamorphic alteration of initial lithologies. Besides being the preferred target of foreign scientists, the country has also produced its own important figures in the history of geological research. In this context, James Hutton – the “father of modern geology”, after whom, for example, ‘Hutton’s Unconformity’ at Siccar Point in Berwickshire is named – has to be mentioned, but also Hugh Miller and Archibald Geikie provided valuable contributions to the enlightenment of various geological problems.
Returning to the geology of Scotland, it is possible to subdivide the country into four main geological and geographical units. The Southern Uplands, which extend south of the Southern Uplands Fault, are mainly composed of sedimentary rocks dating back to the Silurian and the Devonian. The Central Lowlands or Midland Valley, which border the Southern Uplands Fault on the north, represent a rift zone that chiefly comprises Palaeozoic rocks of both sedimentary and volcanic origin. North and west of the Highland Boundary Fault lies the Highlands and Islands, which, due to their geological diversity, can be further subdivided into two sub-units. These are the Grampian Mountains, together with the Cairngorms, which extend between the Highland Boundary Fault to the south and the Great Glen Fault to the North, and are composed of Neoproterozoic metamorphites (Dalradian metasedimentary rocks) that are intruded by Palaeozoic lavas over large areas (see the detailed geological map below). And, finally, there are the Northwest Highlands, which border the Grampians to the northwest and cover the outermost north-western margin of Scotland and which are of both Neoproterozoic (Moine metasedimentary rocks) and Archaean (Lewisian gneiss) origin. They are run through by the Moine Thrust, where the rock formations of the south were, for very long periods of time, thrust over those to the north.
The geology of the Southern Uplands
It is the Southern Uplands and, more particularly, its coastline that is the subject of this article. The area has attracted the attention of natural scientists for several reasons. From a geographic point of view, the visitor to this area is confronted by a range of hills almost 200km long and interspersed with broad valleys that were formed by glaciers covering the landscape in relatively recent times (about 10,000 years ago). From a geological point of view, about 500myrs of earth history are recorded in the rocks of the area, with lithologies ranging from soft red sandstones to hard intractable granites. Furthermore, the Southern Uplands are situated immediately to the north of one of the world’s most fundamental geological divides – the Iapetus Suture – which separates Laurentia in the north from Avalonia in the south (see sketch below). From the point of view of mineralogy, the area holds several treasures, ranging from metal sulphides, such as galena (PbS), sphalerite (ZnS) and chalcopyrite (CuFeS2), to unusual secondary minerals, such as leadhillite and susannite.
To a great extent, the geology of the Southern Uplands is the result of the destruction of the Iapetus Ocean, which took place between 500 and 420mya. During this fundamental tectonic event, an ocean measuring more than 1,000km at its widest point successively disappeared by subduction (that is, the process whereby the ocean crust sinks beneath the continental margin) due to the convergence of two continents, with Laurentia containing Scotland and Avalonia containing embryonic England as well as southeast Ireland. The closure of the Iapetus had two significant consequences:
- First, two continental masses, characterised by very different rocks and fossils were juxtaposed.
- Second, parts of the ocean crust were ridden up on top of the continental margins, a process referred to as obduction.
For the geologist, mafic (that is, rich in iron and magnesium, and low in SiO2) rocks obducted between two continents fulfil a very special function, as they exactly mark the border between the two continental masses. The borderline is called a suture or suture line.
After closure of the Iapetus Ocean and subsequent, large-scale continental collisions (250mya), all of the earth’s land masses were joined together into a single supercontinent which is known as Pangaea. The break-up of this continent due to immense tectonic forces led to the formation of the Atlantic Ocean (about 50mya), but did not follow the earlier pre-Pangaean continental boundaries, with the result that land masses with completely new shapes (for example, North America) were created. This re-structuring of the continents resulted in the odd fact that early the Palaeozoic rocks of southern Scotland have more in common with those in Newfoundland than those of northern England.
The Ballantrae Complex – impressive evidence of the obduction of the Iapetus ocean crust
The coastal between Girvan and Ballantrae was first recognised as an area of geological interest by Sir Roderick Murchison and James Nichol. In general, the Ballantrae Complex outcropping along this coastal line represents a testament to the final stage of the long-lasting process of the closure of the Iapetus. Its structure is the result of volcanic islands, which, instead of being subducted beneath the continental plates, were obducted onto the margin of the Laurentian continent. During the obduction process, various rocks originating from tens of kilometres down beneath the seafloor were stacked and mixed up with lavas that had been erupted onto the seafloor. A relic of oceanic crust exposed along a former continental margin is generally termed an ophiolite complex, even better examples (and this bit of Scottish coastline is certainly a very good example) of which can be seen in Cyprus, Oman and Newfoundland.
From a lithological point of view, the Ballantrae Complex includes mafic and ultrabasic rocks, which were mostly serpentinised during their uplift. Most of this rather soft serpentinite fell victim to extensive weathering processes, so local geology is partly dominated by large outcrops of dykes and enclaves of harder volcanic rocks. At the village of Ballantrae, an impressive natural phenomenon can be observed – volcanic lavas found here commonly form pillow-shaped balls of basalt. Such pillow lavas are produced during submarine eruptions, when the crystalline melt is rapidly cooled due to contact with seawater. Finally, it should be pointed out that the interpretation of basic crustal rocks and pillow lavas as components of an ophiolite complex is nowhere near as old as their discovery and geological description – the theory of crustal obduction only dates back to the 1970s.
Besides the crystalline rocks discussed in the preceding paragraph, the local lithology consists of Ordovician and younger fossiliferous sediments, which uncomformably overlie the lavas and ultramafic relicts. Therefore, they are highly useful for dating the age of the pillow lava. A last group of rocks worth mentioning in this context are basal conglomerates, shallow water limestones and deep water greywackes, representing a fore-arc succession (that is, small land masses along the continental margin) to the northwest of the complex.
Another eye-catcher – steeply dipping turbidite beds at Ardwell Bay, Ayrshire
Soon after its formation, the Ballantrae Complex (which, as explained, represents the impressive contact zone of two colliding continental masses) was subjected to an extensive geological overthrusting. This includes the subsidence of the volcanic rocks, as well as their progressive burial beneath sedimentary rocks that had been deposited from the uppermost Ordovician to the earliest Silurian. During an initial phase, deposition of sediments mainly took place in shallow water and thereby only affected the southern fringes of the ophiolite complex. Within this shallow marine environment, a rich variety of animals, such as corals, trilobites and cephalopods, were able to develop. The fauna was usually accompanied by huge amounts of algae, such that the calcareous sea-weed, Girvanella, must have been present everywhere.
The sediments accumulated in this region chiefly consist of conglomerates, pebbles and boulders originating from a landmass to the north, as well as limestones that were produced in the marine environment. With continuation of the ophiolite subsidence, a part of the sedimentary basin began to sag down to deeper levels, so that a shallow water environment in the north directly bordered upon a deep water environment to the south (see sketch below). The older strata affected by this remarkable descent were subsequently buried under thick layers of sand and mud, which had partly been deposited from underwater debris avalanches or turbidity flows.
The deposits of turbidity flows are commonly known as turbidites. They comprise hundreds of sandstone and mudstone layers, which usually measure several centimetres in thickness (see photographs). In very exceptional cases, they may be also up to two metres thick. At Ardwell Bay in Ayrshire, the turbidite layers reach a lateral extension of several kilometres, underlining their geological significance in this area.
The formation of single sediment beds from underwater debris avalanches can be characterised as a rather complex process:
- First, the slumped sediment mass continuously accelerates down the continental slope.
- Then, the turbidity flow slows abruptly, as it spreads out across the deep ocean floor.
- Deceleration of the debris avalanche causes the settling out of the sediment load, whereby the largest and heaviest particles are deposited first, and are followed by progressively smaller and lighter grains. Sedimentation according to this process results in ‘graded beds’, with coarser sand grains at the base and finer silt and mud particles at the top.
As a result of tectonic forces acting on the huge sedimentary layers that had accumulated over millions of years, the strata underwent a successive steepening, so that the original nearly horizontal structures now appear almost vertical.
From the brief report presented here, it can be concluded that southwest Scotland bears numerous geological locations that are highly worth a visit. In future articles, I will try to give some impressions of other geological milestones in this country.
Lawson, J. D. and Weedon, D. S. (1992): Geological Excursions around Glasgow & Girvan. – Geological Society of Glasgow, Glasgow.
McMillan, A. and Stone, P. (2008): Southwest Scotland. A Landscape Fashioned by Geology. – Scottish National Heritage, Porth.
Stone, P. and Smellie, J. L. (1988). The Ballantrae Area. Description of the solid geology of parts of 1:25,000 sheets NX 08, 18, and 19. – British Geological Survey, London.
Stone, P., ed. (1996): Geology of south-west Scotland – An Excursion Guide. – British Geological Survey, London.