The rocks of Ballantrae Bay offer a rare glimpse into the layers of our Earth, giving us chance to journey through a slice of ancient ocean, past underwater volcanoes and down to the deep rocks of the mantle
Many puzzled geologists have scrambled over the rocks to the north of Ballantrae Bay. Their story tells an important chapter in the formation of Scotland, and raises questions that continue to challenge geologists even today.
The rocks of Ballantrae are a slice of ancient oceanic crust. The rocks date from the Ordovician, and so formed some 500 million years ago, not long before they were tectonically spliced onto an adjacent continent. Such occurrences, where a fragment of oceanic crust is emplaced onto continental crust, are called ‘ophiolites’, and they are globally rare. You can see a further Scottish example at Unst in Shetland, another Best Place to see Scotland’s Geology. The process of ophiolite emplacement usually jumbles up the rocks, and at Ballantrae they are smeared out like a squashed slice of cream cake laid messily on its side. Nonetheless, ophiolites give geologists (and you!) a fabulous ‘window’ into the Earth’s make-up. Here, you can walk over rocks of the upper mantle, onwards to igneous rocks of the oceanic crust, and then on to the sedimentary rocks of the ancient seafloor. What a treat!
The rocks of the upper mantle are exposed at Pinbain Beach and Balcreuchan Port. These rare rocks, which were once composed almost completely of beautiful green olivine and lay over 5 kilometres beneath the seabed, are now altered to green and red (or ‘piebald’) serpentinite, a soft and majestic rock with a soapy feel. At both locations, the serpentinites of the ancient upper mantle and the igneous rocks of the crust are separated by a major fault. At Balcreuchan Port, the fault cuts in front of a cave formed in basalts which, legend has it, was the home of the famous murderer and cannibal Sawney Bean’s tribe. Further down the beach towards the foreshore at Balcreuchan Port you can see a prominent igneous dyke which intruded the softer serpentinites in the Tertiary period. At Pinbain Beach a fault separates the serpentinites from a sequence of sediments and volcanic rocks which would have once formed the shallow seabed. Graptolite fossils in these sediments date these rocks to the middle Ordovician. The fossils are notoriously difficult to spot.
You might notice that some of the sediments are slumped and folded on a centimetre scale, and in other places these layers are completely chaotic. This records the instability of the sediments once laid down on a tectonically-active continental margin. Even more fabulous folding and slumps can be seen on the foreshore at Bennane Lea. Some of these sediments are layers of volcanic tuff, and so the oceanic crust was not too far from some explosive volcanism. In fact, the shallow ocean that you can see the remains of here at Ballantrae was a hive of magmatic activity. Lavas oozed on to the sea floor, forming ‘pillow’ structures as the molten rock rapidly cooled and cracked upon contact with seawater. More lava extruded through these cracks, enlarging the pillows. You can see excellent exposures of numerous pillow lavas at Downan Point, sometimes with pockets of seabed sand preserved amongst them. The pillows here are very large, which evidences their formation in shallow water, since pillows that form in deep ocean are smaller and more compact.
Ophiolites are emplaced in rare tectonic environments close to subduction zones. These rocks therefore demark an ancient subduction zone, thought to have been active for at least 60 million years before the ophiolite was emplaced. The Ballantrae rocks are a relic of the Iapetus Ocean which has long disappeared, subducted into the Earth’s mantle during the Caledonian Orogeny. The Ballantrae ophiolite is actually one of several from this tectonic event, with others located on Unst in Shetland , north-east Norway, and Newfoundland in Canada.
As you ponder this magnificent concept, look out to the sea from Ballantrae and see if you can spot Ailsa Craig, which is some 15km away in the Firth of Clyde. Then, look around your feet on the nearby beaches and you might find pebbles of the very pale speckled microgranite from which Ailsa Craig is formed, and from which curling stones are fashioned. The coastline here, particularly at Bennane Lea, have been shaped into ledges by sea level changes leaving raised beaches. Thus, the coastline of Ballantrae takes its visitors through a journey that documents geological processes from 500 million years ago to the present day.
Text contributed by Jennifer Roberts
Find out more
Stone, P., (2014), A review of geological origins and relationships in the Ballantrae Complex, SW Scotland: Scottish Journal of Geology, v. 50, no. 1, p. 1.
Stone, P (1996). Girvan and Ballantrae – an excursion. Excursion 8. From: Stone, P (editor). Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey. http://earthwise.bgs.ac.uk/index.php/Girvan_and_Ballantrae_-_an_excursion
Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.
Geological Excursions around Glasgow and Girvan by J.D. Lawson (Editor), D.S. Weedon (Editor) Geological Society of Glasgow (1992) ISBN-10: 0902892096.
This web page is published by the Scottish Geodiversity Forum under a Creative Commons ‘Attribution Non-commercial’ (CC BY-NC) licence, which permits non-commercial reuse provided the original work is properly cited.