The Bass Rock, Firth of Forth

From much of the coast along the Firth of Forth in southeast Scotland, and from coastal hills such as Arthur’s Seat in Edinburgh, the impressive piece of rock called the Bass Rock forms a prominent landmark. This steep island is the neck of a Lower Carboniferous volcano, rising 107m above sea level. Scuba divers, on the north side of the island, have shown the sea bed to be around 40m in depth, so the neck would be 150m high if we could see it all.

Fig1Fig. 1 – The Bass Rock from the west. The shape of the island is due to glacial erosion.

The rock is made of phonolitic trachyte, that is, an alkali igneous rock with less silica content than a ‘normal’ trachyte, so the alkali feldspar is accompanied by one of the silica-deficient feldspathoid minerals, such as analcime. Unfortunately, this interesting mineral assemblage is too fine-grained to see easily, except in thin sections under a microscope. In winter, Bass Rock is a dark brown as might be expected, but, in summer, it turns white from both the seabirds that crowd every available surface and their accumulated guano.

The shape of the island is significant – clearly the igneous rock was more resistant to erosion than the surrounding sediments into which it was intruded. These country rocks are not visible now, having been eroded away to below water level, with an estimated one kilometre or more of overlying rock removed since the time of intrusion, along with any surface eruption products, such as lavas and pyroclastic rocks.

Fig2Fig. 2 – The steep side of the rock rises sheer from the water. Note the white guano from nesting sea birds.

The relatively gentle slope to the south southwest (to the right in Fig. 1), compared to the steeper cliffs on the other sides, is the result of glacial erosion. The rock is a ‘roche moutonnée’, reflecting ice movement seawards along the Forth of Forth (from right to left in Fig. 1). Many of the other islands in the Forth have the same shape – steep to the east, gentle slopes to the west, presumably for the same reason. The steep faces are due to glacial plucking, and the north and east faces of Bass Rock (with around 100m of almost vertical rock rising straight from the water) are impressive when seen close up (Fig. 2). (Note that the term ‘roche moutonnée’ does not refer to a sheep, as is sometimes written, but to a fleecy wig popular in the 1780s that was smeared in mutton fat.) The more accessible side of the island includes a platform a few metres above sea level (visible on the left in Fig. 3), which was presumably eroded at a time of higher relative sea level. That is, after sea levels began to rise, as a result of the glacial ice melting at the end of the last ice age but before Scotland had rebounded after the weight of the ice was removed, a process known as glacial rebound. In fact, features such as raised beaches and fossil cliffs are common around the shores of Scotland.

An interesting feature is a group of fractures that cuts through the island in an east-west direction (Fig. 3). These have been eroded out to form caves on both sides of the island and reputedly it is possible to clamber through the island from one side to the other. As the caves are often full of resting seals, who are not at all pleased to see visitors, and landing is tricky (and now requires a permit), this cannot be recommended. Otherwise, the rock is conspicuously lacking in joints or other features, which is presumably why it is so resistant to erosion. Any outer part of the volcanic plug with cooling joints must have been eroded away.

There are numerous volcanic rocks in central Scotland of Lower Carboniferous age, including Castle Rock and Arthur’s Seat in Edinburgh. An obvious question is why were there so many volcanoes at this time? There was no contemporary active mountain building in the UK – the Caledonian Mountains of the lower Palaeozoic had long since become inactive and, in many locations, had been eroded away during the Devonian. However, to the south of the UK, an ocean known as the Rheic Ocean was being subducted. Slightly counter-intuitively, subduction is often associated with tension in the over-riding plate (known as back-arc extension) allowing magma to rise through the plate. Other effects included the creation of the shallow seas in which the Lower Carboniferous Limestone of northern England was deposited.

Fig3Fig. 3 – Caves formed along fractures, with the raised wave-cut platform on the left.

Visiting Bass Rock is easy, even if you do not have a suitable kayak to hand (or the ability to paddle there and back). At least two operators offer boat tours to the rock from North Berwick, which make for a great day out for the family. In summer, the island is literally covered with seabirds – it is Europe’s largest gannet colony and the place that the scientific name for gannets is derived from (Morus bassanus). Unfortunately, the tours do not include the chance to land, which is any case is tricky in all but perfectly calm conditions – the photos show the water in conditions much calmer than normal. Unsurprisingly, I have no photographs from the days when staying upright in the kayak needed all my attention. Some of the boat tours also visit the impressive Tantallon Castle on the return, which rests upon steep cliffs of volcanic ash, but that is another story.

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