Stonehaven: The power of colliding continents

The spectacular rock outcrops along the Stonehaven coast have exciting stories to tell of the sheer force of colliding continents, uplift and rapidly eroding mountains

Crumbling cliffs of vertical sandstones and mudstones fronted by a well-developed wave-cut platform where the sandstones stand out as jagged layers separated by water-filled hollows where mudstone has been eroded. Image: Peter Craig.

During the continental collision that formed the Caledonian mountains, the main plates collided at an angle, forming a major tear in the Earth’s crust – the Highland Boundary Fault – where two sets of rocks began to slide past each other in response to the extreme compressional forces. The fault marks the boundary between lowland and highland Scotland all the way from Bute, through another of the 51 Best Places at Balmaha, to cut through Craigeven Bay, north of Stonehaven, and head off under the North Sea at Garron Point. On the north side of the fault are metamorphic rocks of the Dalradian supergroup, shot through with many large intrusions of granite and other igneous rocks. To the south lies younger sedimentary rocks, the Old Red Sandstone, here thick sequences of sandstones, mudstones and conglomerates.

The Dalradian rocks form the landward side of Craigeven Bay, and the rugged coastline north between Garron Point to Newtonhill. Close-up, many of these outcrops have a gritty texture and show coarse foliation, locally crinkled into small folds. The original sedimentary rocks here are some of the youngest in the Dalradian and formed when turbidity currents transported sediment down the continental slope into deep sea water.

The Highland Boundary Fault itself is seen as a narrow zone of intensely altered rock, up to 20 metres wide, cutting across the tidal outcrops in Craigeven Bay and forming the spine of Garron Point. Being a zone of thoroughly shattered rock, many hot fluids and even magma have risen up through it. A type of carbonate rock that weathers a distinctive orange-brown colour is the most obvious local manifestation of the fault at these locations.

Immediately to the south of the Fault at Craigeven Bay and Garron Point, there is a narrow section of altered black, dense basaltic rocks whose relationships with the adjacent rocks has puzzled geologists for many years. They are now recognised the Highland Border Complex, one of several outcrops found along the line of the Highland Boundary Fault. This is a small sliver of an ancient ocean floor that got caught up in the continental collision. Some outcrops exposed at low tide near Garron Point reveal dark mudstones, red chert and rather deformed pillow structures in the basalt, characteristic of deep ocean floor environments.

Nearby to the south, further evidence of the forces generated by these same events can be seen in the outcrops Cowie and Garron Point. Here sedimentary rocks have literally been upended as they were caught in the vice-like jaws of the colliding continents. At Cowie Harbour, there is a display describing how one of the oldest known air-breathing fossils (Pneumodesmus newmani) was discovered there in 2003. It is the first myriapod, and the oldest known creature to have lived on land.

South of Stonehaven, the cliffs are formed by a great thickness of boulder conglomerates. More than 400 million years ago, as the great Caledonian mountains were thrust upwards to heights of over 10,000 metres by continental collision, the high ground began to erode rapidly. The conglomerates represent the debris swept off these mountains by repeated flash floods, before being dumped on arid plains to the southeast of the mountain range.
This section of coast is also notable for much more recent geological events. The steep cliffs are fronted by a wave-cut platform that dissipates much of the sea’s energy before it reaches the base of the cliffs. Even so, coastal erosion is very active here as can be seen in the freshly crumbling outcrops and the many new and old landslips that scar the steep coastline.

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 https://www.scottishgeology.com/find-out-more/publications/#northeast

Much more detailed information can be found in the following online publications. Although these are written for teachers and their pupils, they are excellent sources of information for anyone wishing to explore the rocks of the Stonehaven and Crawton areas:
http://www.geowalks.co.uk/eso/CowieTeachersGuide-v4.pdf
http://www.geowalks.co.uk/eso/CrawtonTeachersGuide-v4.pdf

Three Geological Conservation Review accounts of the same areas provide highly detailed information about the rocks that outcrop there and their particular relevance in deciphering the geology of this part of Scotland:
http://jncc.defra.gov.uk/pdf/gcrdb/GCRsiteaccount2563.pdf
http://jncc.defra.gov.uk/pdf/gcrdb/GCRsiteaccount356.pdf
http://jncc.defra.gov.uk/pdf/gcrdb/GCRsiteaccount2493.pdf

Serpentinite and carbonate rock occupying the Highland Boundary fault zone near Garron Point. Image: Peter Craig.

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