Portsoy, Banff Coast: Beautiful pebbles and tortured rocks

This lovely old fishing port was once famed for its beautiful ‘marble’, but this is just part of the story of the nearby coast that reveals a complex and violent geological past

The rocky coast west of the Old Harbour has a great variety of rock features to explore. The quartzites are noted for the rod-like structures formed when the rock was stretched tectonically; the serpentinite is famous for the wonderful variety of colours and textures; and the meta-limestones display intricate folds. Image: Peter Craig.

The constantly pounding seas at Portsoy make it a great place to study rocks because the waves ensure that the coastal outcrops are always freshly exposed to reveal detail that is all too often obscured in more weathered outcrops – and there are lots of clean pebbles too! The number of different rock types that can be found along a very short stretch of coastline is remarkable. Even more remarkable is the fact that many rock units seen side-by-side today were originally formed far apart. They have been jostled and disrupted by tectonic forces that have made them slide past each other, making it really difficult to work out relationships between the different units of rock.

In a zone only a few kilometres wide, centred on Portsoy, there are rocks that show a degree of bending, stretching, fracturing and alteration that is much more intense than those found even just to the east or west. The zone can be traced southwards towards the Cabrach and beyond as far as Deeside and Glen Muick. It is known to geologists as the Portsoy Lineament and is characterised by outcrops of significantly sheared rock along its length. It serves as a boundary between two different sets of metamorphic rock. The rocks to the south and west of the Portsoy Lineament have been subjected to deeper level and more pervasive metamorphism than the rocks of Buchan to the east.

This geological ‘lineament’ is interpreted as having been a persistent localised zone of active movement throughout a lengthy series of geological events lasting over 800 million years. The process began with large-scale sediment accumulation, followed by the crumpling and metamorphosis of these sedimentary layers and then the intrusion of two separate pulses of magma added into the mix. These rocks were then uplifted to form a mountain chain during the later stages of continental collision, before being heavily eroded to expose the ‘geological roots’ we can see at Portsoy today.

The best way to explore these rocks is to follow the coast westwards from the Old Harbour, using the network of paths to drop down to the beaches where the outcrops are freshly exposed and the rich selection of pebbles can be dipped in the water to bring out the details of their colours and textures. Look at the varied examples of masonry in older buildings too – and perhaps consider why the masons chose to use particular stones in the different contexts; in the harbour walls, doorways, and boundary walls, for example.

Text contributed by Peter Craig

Find out more
Northeast Scotland – a landscape fashioned by geology by John Merritt and Graham Leslie provides a great introduction to the geology of the wider area. Download from http://www.scottishgeology.com/find-out-more/publications/#northeast

The BGS geological map for Portsoy is full of detail for the enthusiast and can be viewed online.

More can be learned about the highly instructive outcrops along the adjacent Banffshire from this BGS Earthwise excursion guide.

Three outcrops and a boulder of Portsoy Limestone with typical interbedded dark and pale carbonate-rich mudstones. Fold structures are common and are particularly well seen at outcrops close to the Old Swimming Pool. Image: Peter Craig.

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