OR/15/053 Geological context
|Everett, P A, Gillespie, M R and Tracey, E A. 2015. Provenance of building stones in four 'galley castles' in Argyll. British Geological Survey Internal Report, OR/15/053.|
Scotland has a complex and diverse bedrock geology, which is the product of more than three billion years of Earth processes, extending from the Archaean Eon to the present day (Figure 2). The geographical area relevant to this project covers only a small part of the country — the Argyll coast and part of the Hebrides — but still encompasses a considerable amount of geological variability. This section of the report presents a brief introduction to the geology of the area to provide context for the following sections. Figures 2, 3 and 4 should be referred to while reading this section.
Some terminology and concepts
Earth’s crust consists of three main classes of rocks.
- Igneous rocks form when magma (molten rock) solidifies. Magma forms deep within the crust, and it can solidify there (forming dykes, sills, plutons and other types of intrusion) or it can erupt onto Earth’s surface (forming lava flows if it behaves like a liquid and pyroclastic deposits if it is thrown high into the air and falls back to Earth as volcanic ash and larger fragments). Common types of igneous rock include granite and gabbro (which always occur as intrusions), and basalt and andesite (which usually are erupted). Nearly all magma consists mainly of silica (SiO2), and igneous rocks can be divided according to how much silica they contain: silica-rich rocks are typically light-coloured and can be referred to generally as felsic, while silica-poor rocks are typically dark-coloured and can be referred to generally as mafic. Large bodies of igneous rock are given names to reflect their location, composition and type of intrusion, for example Arran Granite Pluton.
- Sedimentary rocks form by deposition of particulate matter (mud, sand, gravel and shell) at Earth’s surface. The particulate matter usually forms by erosion of pre-existing rocks, and is moved by water, wind, ice or gravity before being deposited as sediment. Loose sediment buried beneath accumulating layers of sediment gets compacted and is eventually converted to rock. Common types of sedimentary rock include mudstone, sandstone, conglomerate and limestone. Bodies of sedimentary rock are given names to reflect their location and composition, for example Scalpay Sandstone Formation.
- Metamorphic rocks are former igneous rocks or sedimentary rocks that have been subjected to high temperature and pressure within Earth’s crust, with the result that the original rock textures and mineral assemblages are changed significantly. Some of the character of the original igneous or sedimentary rock usually survives low to moderate degrees of metamorphism, and such rocks can be named by putting the term ‘meta’ in front of the igneous or sedimentary rock name (e.g. metabasalt, metasandstone). When rocks are subjected to a high degree of metamorphism they generally lose most of their original character and a new set of terms, including gneiss and schist, is used to name them. Bodies of metamorphic rock are given names that reflect their location and rock type, for example Beinn Bheula Schist Formation, and Lewisian Gneiss Complex.
Metamorphic rocks and most igneous rocks are crystalline (i.e. formed entirely of interlocking crystals), while most sedimentary rocks are granular (formed of adhering particulate matter, such as sand grains). Crystalline rocks lack pore spaces (voids) and therefore are essentially impermeable and relatively resistant to weathering. Granular rocks, in particular sandstone, typically have a network of connected microscopic pore spaces and are usually permeable; water can easily penetrate such stones, and they therefore tend to have relatively poor resistance to weathering.
The names and age ranges (‘date’) of all geological time divisions are given, from the earliest (Hadean Eon) to the youngest (Quaternary Period). Numbers in the ‘date’ column refer to ‘millions of years before present’; for example, the Jurassic Period lasted from 201 to 145 million years ago. This table is adapted from the BGS Geological Timechart.
The oldest rocks in the western seaboard area of Scotland belong to the Lewisian Gneiss Complex. These rocks, which date from the Archaean Eon and are thus some of the oldest on Earth, crop out widely on the Western Isles and on Tiree, Coll and Iona. Little is known about the environment under which the rocks formed, but they consist of thickly banded, strongly metamorphosed crystalline rocks (gneiss), much of which consisted originally of intrusions of granite and basalt.
The Grampian Highlands terrane is bounded by two major geological faults: the Great Glen Fault to the north and the Highland Boundary Fault to the south (Figure 3). The rocks beneath much of this area belong to the Dalradian Supergroup. This major geological unit originally was a very thick sequence of sedimentary rocks, mainly sandstone and mudstone with occasional beds of conglomerate and limestone. In some places the sedimentary strata are interbedded with lava and pyroclastic rocks, and in other places they are cut by intrusions (dykes and sills) of igneous rock (mainly of basalt composition).
The strata that now form the Dalradian Supergroup were deposited between approximately 1 000 and 500 million years ago, in a great sea-filled basin that was opening as tectonic forces pulled a continent apart. Rivers brought vast quantities of sediment to the sea, where larger grains settled quickly as layers of sand, small particles settled slowly as layers of mud, and layers of carbonate ooze (‘lime’) formed during warm periods. As the sediment was buried and compressed, the layers of sand, mud and lime became strata of sandstone, mudstone, and limestone, respectively. 600 million years ago the continent broke apart and a widening ocean (the Iapetus Ocean) separated the two sides. Now on a continental margin, the growing pile of layered sediments reached a cumulative thickness of at least 15 kilometres.
Around 500 million years ago the ocean began to narrow then closed altogether, and the land masses bordering it collided. The collision caused the flat-lying strata of the Dalradian Supergroup to buckle into giant folds; some of these are tens of kilometres in size. The massive scale of folding thickened the crust, raising the temperature and pressure within it, and causing minerals and textures in the rocks to change: the sedimentary and igneous rocks of the Dalradian Supergroup became metamorphic rocks. The intensity of folding and metamorphism peaked 470 million years ago, then diminished slowly and had ceased altogether by around 420 million years ago. These geological upheavals are known as the Caledonian Orogeny.
The area around Loch Awe and Knapdale is characterised by numerous intrusions of basalt and related rocks that have been metamorphosed. These dark, fine-grained metamorphosed igneous rocks are difficult to describe and classify in the field, so they are referred to broadly as metamafic rock, and sometimes as metamafite (Figure 4).
Towards the end of the Caledonian Orogeny (between roughly 430 and 408 million years ago) vast volumes of magma rose through the crust to form intrusions within the Dalradian Supergroup. Several large intrusions of granite (including the Cruachan Granite Pluton) formed close to what is now the Argyll coast. Numerous smaller, sheet-like intrusions known as dykes and sills were also emplaced at this time (the great majority are too small to show on Figure 4).
Some of the magma erupted, and in the Lorne area of Argyll a pile of lava flows overlying the Dalradian rocks (the Lorne Lavas) is preserved across an area of approximately 300 km2 and has a maximum thickness of c.800 metres (Figure 4). A smaller area of lavas formed at about the same time is preserved in Glen Coe.
During the Caledonian Orogeny the crust thickened and rose quickly (in geological terms), forming a large mountain chain (possibly akin in scale and character to the Himalaya) that eroded rapidly as it rose. Huge quantities of sediment created by erosion were deposited in topographically depressed areas around the mountains and now form the Old Red Sandstone Supergroup. These sedimentary rocks are of Devonian age and consist mainly of beds of sandstone, conglomerate and siltstone. At this time the land that is now Scotland lay to the south of the equator, roughly at the latitude that the Namib Desert in southern Africa occupies today. The Devonian sediments therefore were deposited in a desert environment, usually as a result of flash floods which drained into lakes (hence they include beds of coarse conglomerate and very fine siltstone). Sandstones formed in desert environments typically are brightly coloured, because the small quantities of iron they contain oxidises in the atmosphere (i.e. it effectively rusts, producing sandstones that are brown, purple, ochreous, orange or pink); the Devonian sandstones typically are brown or purplish.
Over most of the Scottish Highlands the rocks that were metamorphosed during the Caledonian Orogeny are exposed at the surface today (i.e. they are not covered by younger rocks). There is therefore little evidence in these areas of what has happened in the last 350 million years of geological history. However, along the western seaboard of Scotland large volumes of lava erupted onto the land relatively recently (in geological terms) and where these occur today (mainly on Skye, Mull and Morvern) they occasionally reveal sedimentary rocks beneath them. These sedimentary rocks presumably existed across most of the Highlands at one time but have been removed completely by erosion where they were not protected by a cap of lava.
The sedimentary rocks revealed beneath the lavas include strata formed during the Carboniferous, Permian, Triassic, Jurassic and Cretaceous periods.
The Carboniferous strata typically are buff to pinkish brown, coarse or gritty sandstones. At the time they were deposited Scotland lay more or less on the equator, and the Carboniferous sediments therefore were deposited in tropical rivers and swamps. The sediments that were deposited at this time were interspersed with layers containing huge quantities of dead plant matter. Over geological time these have transformed into coal. Large parts of the Midland Valley of Scotland are underlain by thick sequences of Carboniferous strata, including numerous seams of coal, but on the western seaboard the strata are relatively thin and there is little or no coal; however, the Carboniferous sandstone beds do contain occasional scattered fragments of black fossil plant matter, and where these occur they usually are a good indication that the strata date from the Carboniferous Period.
Permian strata typically are pink or reddish sandstone and Triassic strata are typically buff to orange sandstone. At the time they were deposited Scotland lay to the north of the equator, roughly at the latitude that the Sahara Desert occupies today. The Permian and Triassic sediments were deposited in a desert environment, and in many areas they represent fossilised sand dunes.
Jurassic and Cretaceous strata typically consist of white to light grey, and occasionally greenish, sandstone and brown shales. By the time they were deposited Scotland was roughly at the latitude of the Mediterranean Sea, and the earlier (Permian and Triassic) had been submerged beneath a (pre-Mediterranean) warm shallow sea.
The Atlantic Ocean began to open in the Jurassic Period. Around 60 million years ago, in the Palaeogene Period, the crust became so stretched as this process continued that large volumes of magma were able to rise through the crust where they formed intrusions (which now underlie the Cuillin Hills on Skye, the Cuillin hills on Rum, and the mountains of Mull and Arran) and extensive lava flows (now exposed in north Skye, Eigg, Mull and Morvern). These are the lava flows beneath which the older (Carboniferous to Cretaceous) sandstone strata are preserved.
The last significant part of the geological history of Argyll and adjacent areas is the Ice Age, which began 2.6 million years ago and continues today. During that time Scotland has experienced many cold (glacial) intervals interspersed by warmer (interglacial) intervals. Vast ice sheets from Scandinavia and interior parts of the Highlands scoured the entire land surface from time to time, and narrow ice streams gouged out valleys and corries. Melting ice deposited vast quantities of glacial sediments (including till, sand and gravel), which now overlie and conceal much of the bedrock and in places form distinctive landforms such as drumlins, eskers and terraces. The Ice Age therefore is responsible for most of the topographic features that characterise the Highlands (and the rest of the country) today.
- TREWIN, N H and ROLLIN, K E. 2002. Geological history and structure of Scotland. 81–148 in The Geology of Scotland. TREWIN, N H (editor). (London: The Geological Society.) ISBN 1-86239-126-2