Deep Time on the West Coast: Lewisian Gneiss, the Ardnamurchan Volcano and the Parallel Roads of Glen Roy

Each major claim carries one of three labels. established means supported by primary records, official bodies or several independent reliable sources. shared means the core fact is sound but sources disagree on details, dates or interpretation. traditional means widely repeated but not traceable to a primary or independent source.

The weak points here are numbers that circulate in popular accounts. Ages for the Lewisian are a range, not a single figure, because the complex records events across about 1,400 million years. The Ardnamurchan complex is dated by official bodies to "around 60 million years ago"; more precise single figures appear in some accounts but vary. The interpretation of individual Ardnamurchan intrusions is an active research question.

The oldest rock in the United Kingdom

established Lewisian gneiss is the foundation of the far north-west. The British Geological Survey describes it as a mainland coastal strip from Cape Wrath to the Sound of Sleat that continues westward across the Outer Hebrides. Most Lewisian rocks formed more than 2.8 billion years ago. The oldest are felsic gneisses in the Outer Hebrides, whose source rocks date to between 3.2 and 2.8 billion years. The oldest mainland gneisses are about 3 billion years old. The Natural History Museum describes Lewisian gneiss as the oldest rock in the UK.

established The gneiss is not one event but a long record. The complex preserves at least two major episodes of heating and deformation. The earlier, Archaean, episodes are called Scourian and Inverian. A swarm of dykes, the Scourie dykes, was mainly intruded about 2.4 billion years ago. The later Laxfordian events ran from about 1.9 to 1.6 billion years ago on the mainland, and peaked around 1.8 billion years ago in the Outer Hebrides. In all, the complex records some 1,400 million years of Earth history.

established A common popular line says that touching Lewisian gneiss means touching rock "close to a quarter of the age of the planet". That understates it. The Earth is about 4.5 billion years old, so a rock 3 billion years old is roughly two-thirds of the planet's age.

Ardnamurchan: inside a volcano

established The Ardnamurchan peninsula, which reaches the most westerly point of the British mainland, preserves the eroded core of a volcanic complex. NatureScot's citation for the Ardnamurchan Site of Special Scientific Interest dates it to the Palaeogene, around 60 million years ago. The Lochaber Geopark describes the ring complex as having formed beneath a great volcano at the time Europe and Greenland began to drift apart, the start of the modern North Atlantic.

established Geologists divide the complex into three volcanic centres, numbered 1, 2 and 3, which record the shifting focus of activity over time. Two kinds of curved intrusion dominate. Cone sheets are thin, curved dykes that occur in swarms. Ring dykes are larger and thicker curved bodies with vertical or outward-dipping sides. NatureScot states that the amphitheatre of Centre 3 is accepted internationally as the most perfect example of a nested series of ring dykes.

established The classic description is the Geological Survey memoir by J. E. Richey and H. H. Thomas, The Geology of Ardnamurchan, North-west Mull and Coll, published in 1930. E. M. Anderson's 1936 work on the mechanics of these intrusions, published by the Royal Society of Edinburgh, made Ardnamurchan a reference point for the study of what happens beneath volcanoes.

shared The textbook story is not settled. The gabbro known as the Great Eucrite has long been described as the type example of a ring dyke, a body that let a block of crust sink as a magma chamber filled and emptied beneath it. In 2006, Brian O'Driscoll and colleagues argued in the journal Geology that the Great Eucrite is instead a dish-shaped intrusion, and proposed a new model for the centre. The Geological Society treats the question as open.

The Parallel Roads of Glen Roy

established On the steep sides of Glen Roy, north-east of Spean Bridge in Lochaber, three dead-level terraces run for miles along both sides of the valley. They look like roads, and before the Ice Age was understood they were explained as artificial roads, even as works of art, or as the hunting roads of the ancient kings of Scotland or of the legendary hero Fingal. They are in fact the shorelines of a lake. NatureScot gives their heights as 260, 325 and 350 metres above sea level.

established The lake belongs to the Loch Lomond Stadial, a late cold spell that began about 12,900 years ago. Glaciers growing in the Highlands advanced north and east into Glen Spean and then north into Glen Roy, blocking the glens' normal drainage. Water ponded behind the ice, and the lake's surface stood at the height of the lowest outlet still open. Wave action cut a narrow shelf at that level. As the ice front advanced, successive overflow channels were blocked, the lake rose to the next outlet, and a new, higher shelf was cut.

shared When the climate warmed, the ice dams melted and the lake drained, suddenly and catastrophically according to the Scottish Geology Trust. The British Geological Survey dates the landscape to more than 11,700 years ago; the Scottish Geology Trust puts the melting of the dams at about 11,500 years ago.

established The roads are protected in the Glen Roy National Nature Reserve. On 27 August 2024, at the International Geological Congress in Busan, the International Union of Geological Sciences named them in its second list of 100 geological heritage sites, citing their part in establishing the glacial theory in the nineteenth century.

Darwin's great failure

established Charles Darwin went to Glen Roy in 1838, two years after returning from the voyage of the Beagle, and described the expedition in a letter to Charles Lyell dated 9 August 1838. In a paper read to the Royal Society in 1839, he rejected the idea that the roads were lake shores, because he could not see what could have dammed the glen. He argued instead that they were sea beaches, cut as the land rose in stages, with the country slowly elevated and the movement "interrupted by as many periods of rest as there are shelves".

established The missing dam was ice. Louis Agassiz, who argued for a former age of great cold, toured Britain in 1840 and proposed that a wall of glacier ice across the foot of the glen had held back a lake. Darwin accepted glaciation elsewhere, and in 1842 wrote of Cwm Idwal in Wales that the signs of ice could not have been clearer if a glacier still filled it. He resisted the explanation for Glen Roy far longer, and was still defending the marine theory, half-heartedly, in letters as late as 1861, the year the geologist Thomas Jamieson wrote to him on the subject.

established In his autobiographical recollections, Darwin called the Glen Roy paper "a great failure" and said he was ashamed of it. The episode is now a standard lesson in how a strong theory, in Darwin's case the slow rise of land, can lead a brilliant observer to the wrong reading of a landscape.

• Lewisian gneiss takes its name from the Isle of Lewis.
• The oldest Lewisian rocks are around two-thirds of the age of the Earth.
• Ardnamurchan Point is the most westerly point on the British mainland.
• Ardnamurchan's volcanic rocks formed as the North Atlantic began to open.
• The Parallel Roads of Glen Roy were once said to be the hunting roads of ancient kings, or of Fingal.
• Darwin spent years defending a wrong explanation of Glen Roy.
• Glen Roy shares its 2024 international listing with two other Scottish sites, the Rum igneous complex and the Barrow Zones.

• Myth: Lewisian gneiss is "close to a quarter of the age of the planet". At up to about 3 billion years old it is roughly two-thirds of the Earth's age.
• Lewisian gneiss has no single age. It is a complex spanning about 1,400 million years.
• Disputed: "the oldest rock in Europe". The supported claim is that Lewisian gneiss is the oldest rock in the UK.
• Disputed: the age of Ardnamurchan. A precise single date such as "58 million years" is not definitive; official sources say about 60 million years.
• Disputed: whether the Great Eucrite is a ring dyke. Its interpretation has been disputed since 2006.
• Myth: the Parallel Roads are roads, or beaches of the sea. They are shorelines of an ice-dammed lake.