The geology of the Moray Coast

When most people think of Scotland, the images that come to mind are those of high, heather covered mountains like Ben Nevis, islands like Skye, Arran or Rum, or the endless rugged coastline of the northwest coast. However, there is another half to the country, along the east coast, which few people have explored. For example, the county of Moray offers Burghead Bay, where pill boxes sit half submerged in sand, or there are the frequently climbed sea cliffs below Cummingston and Covesea, and Findhorn Bay, the only natural harbour on the south side of the Moray Firth, where shipwrecks litter the beaches at low tide alongside remnants of an old settlement destroyed by shifting channels.

baryteFig. 1. Baryte mineralisation in Permian sandstone at Hopeman.

In terms of geology, the Moray shore provides evidence of the ancient landscape 250mya, easily found by following the coastal path, a walkable distance east from the village of Hopeman. A short detour onto the beach, behind the brightly coloured huts, reaches small outcrops of Permian sandstone, the Hopeman Sandstone Formation, which occurs continuously along the coast for several kilometres. At this particular spot, the sandstone is heavily mineralised with barytes, primarily as cement holding the medium-sized grains in place, but also as concentrations a few centimetres across that give the outcrop an overall speckled appearance and nearly obliterate the original bedding (Fig. 1). Such an outcrop can also be found near Covesea Lighthouse, as can fluorite in characteristic (but difficult to find) cubic crystals. The mineralisation most likely occurred during the Jurassic, when the Central North Sea area underwent doming, which resulted in mineral-rich fluid-flow through the Permian sandstones.

Continuing eastward, the increasingly narrow and uneven path passes Braemou Well, one of several on the Moray Coast, and Daisy rock, which is a prominent block at the waterline. Several hundred metres further on are strange, wave-beaten sandstone stacks, standing a few metres high and showing colour banding and variable resistance to erosion, This is caused by differing iron content of each bed within. Around the headland, the sandstone forms broad, sweeping bedding surfaces, dipping at an angle of no more than ten degrees towards the sea. These show cross-beds, which are smaller parallel, but inclined layers, which are all that remain of moderately-sized barchans sand dunes. That is, grains of sand were blown up a shallow ‘stoss slope’ to the top of the dune and, on becoming unstable, tumbled down the steeper ‘lee slope’. This process of erosion from the back, transport to and deposition at the front effectively moved the dune forward and, importantly, only the stoss slope is preserved. The presence of sand dunes firstly indicates that Moray in the Permian was a hot, arid desert environment, only 20°N of the Equator and, secondly, from the dip of the cross-beds approximately to the southwest, that the wind blew predominantly from the north.

clashach-coveFig. 2. Cross bedding, a fault surface and caves in Permian sandstone at Clashach Cove.

Close by, ripple marks cover several bedding surfaces suggesting standing water among the dunes, with at least a weak flow direction. Water is further indicated by bands of similarly sized, smoothed pebbles that required a more rapid flow rate and were probably deposited in occasional flooding events. Mud cracks also suggest that any remaining bodies of water dried out, with the curled up edges providing material for erosion and transport as flakes or clasts when flowing water returned. The most abundant evidence of water is the soft sediment deformation structures that appear as swirling layers, bounded by the top and base of the beds they occur in. These structures form when water becomes trapped in a layer of rapidly deposited sediment and, as more material is deposited on top, the pressure increases to the extent the water is squeezed out, hence the alternative name of ‘water escape structure’. One of the best examples occurs in a vertical face no more than a metre high, effectively showing how the layers folded through pressure exerted laterally from each side (Fig. 2). The layers also show faulting, suggesting the sediments moved when they were partly (but not entirely) consolidated, allowing the layers within to break. Sand dykes – vertical channels showing the actual course of water escaping from lower layers to the sediment surface – have also been observed, but are difficult to find. Nearby is an in situ, partly carved millstone, one of many along the coast between Hopeman and Covesea, cut using the natural bedding in the sandstone. This one was never completed or separated from the remainder of the outcrop, no doubt on the discovery of some imperfection. A few more ripple-marked surfaces can be found in the steeply dipping beds beyond – these are weatherworn and noticeably fade with each visit from the constant pounding from the sea – and also in the roofs of caves and elsewhere above sea level.

The next feature on the coastline is the highly visible Clashach Cove, also known as Cove Bay or Primrose Bay (Fig. 3). This inlet was quarried at some point, but the growth of vegetation since has made it look almost natural. The sandstone is still of Permian age, but occurs as a much more vivid orange colour in the cliffs away from the sea’s reach, with the cross-beds being much larger and more striking. A viewpoint on the west side of the cove gives the best view of a fault plane followed by one of two caves for ten or more metres. Movement on the fault occurred after the Permian rocks were deposited but, due to the absence of rocks of a younger age, the timing of movement cannot be narrowed much further. Therefore, it is described as ‘post-Liassic’ (early Jurassic).

sedimentFig. 3. Folded and faulted soft sediment deformation structures in Permian sandstone at Hopeman.

East of the cove is Clashach Quarry, where sandstone has been excavated for building stone since the mid-nineteenth century. Some time ago, it was realised that the Hopeman Sandstone was a source of fossilised reptile footprints, which show an unprecedented range of different sizes and shapes. Over 300 tracks have been recorded from the quarry. Some are similar to those in contemporaneous rocks from Germany and America, while others are unique. Examples have been laid out in an ‘amphitheatre’ near the entrance to the quarry and, although the site is now unmarked and overgrown, with the right light and a bottle of water, it is possible to pick out prints with a characteristic pit and crescent-shaped mound, or showing toes, claw marks and (relatively commonly) tail drags. Some of the latter end suddenly, providing evidence of the possible fate of their reptile trace-makers. One of the most important finds from the quarry was a block containing the ‘fossil that was not there’, a natural mould of a pig-sized dicynodont skull, found in 1997. The mould was CT and MRI scanned, producing images used to re-create a three-dimensional model of the skull now on display in the Elgin Museum. It represents one of many links between the museum’s recognised collection of fossils and the geology of the local area outside.

About the author

Dr Sue Beardmore is the Recognition Fund Curatorial assistant (geology/palaeontology) at Elgin Museum:

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