North-west Hoy’s dramatic coastline, with its soaring cliffs and the famous Old Man of Hoy, has been sculpted from red sandstone that formed on desert plains 380 to 370 million years ago
The coastline of north-west Hoy has some of the highest and most spectacular sea cliffs in Britain. Up to 350 metres high at St John’s Head, they form a mighty wall of red sandstone, braced against the Atlantic Ocean. The rocks have been carved into spectacular coastal landforms, the most famous of which is the Old Man of Hoy, a sea stack which towers 137 metres high off the cliffs near Rackwick Bay. The Old Man is an iconic image of Orkney and one of Scotland’s most recognisable natural features.
The layers of red and honey-coloured sandstone that make up the cliffs are between 380 and 370 million years old and date from the Devonian Period, when Scotland lay south of the equator and was part of a huge desert continent. The rocks that formed on this ancient landmass are traditionally known as the Old Red Sandstone. In Devonian times the area including modern-day Orkney, Shetland and north-east mainland Scotland was part of a large depression known as the Orcadian Basin. The rocks that formed in this basin, on desert plains and in lakes, salt flats, dunes and rivers, are preserved today in Orkney to dramatic effect.
Hoy is one of the best places in Scotland, and the UK as a whole, to see thick sequences of Old Red Sandstone. Many of Hoy’s rock sections are inaccessible, but there are superb views from the cliff tops and also from the sea (e.g. the ferry between Scrabster and Stromness).
The Old Man of Hoy sits on a plinth of flagstones and lava. The flagstones at the base date from a time when a series of large lakes periodically filled the basin, sometimes extending from today’s Moray Firth area to Shetland and beyond. The lake teemed with fish, some of which are beautifully preserved as fossils in equivalent rocks elsewhere in Orkney, Shetland, Caithness and around the Moray Firth. During a later period of volcanism lava flowed across the landscape, covering the eroded flagstones. The lava is overlain in turn by the layers of sandstone that dominate the geology of this coastline. These rocks were deposited as sand and silt in large rivers that flowed across the Orcadian Basin in late Devonian times. Wind-blown sand also built up into small dunes that can be seen amongst the river deposits.
As well as providing insights into Devonian environments, Hoy’s coastline is also a fantastic place to appreciate active geological processes operating on timescales we can grasp. Through old maps and prints we can trace how the Old Man of Hoy has changed over the centuries. A map from around 1750 shows only a headland, but by the 1820s there was an arch supported by two legs – giving the Old Man his name. A storm then washed away part of the arch, leaving the single pillar we know today. But the upper part of the stack is weakened by large cracks, and one day in the future the Old Man will collapse into the sea – so make sure to visit him before he succumbs to the waves!
Text contributed by Elizabeth Pickett
Find out more
Orkney and Shetland – a landscape fashioned by geology provides a great introduction to the geology. Download from http://www.scottishgeology.com/find-out-more/publications/#orkney
Old Man of Hoy scientific site report in the Geological Conservation Review: Barclay, W.J., Browne, M.A.E., McMillan, A.A., Pickett, E.A., Stone, P. and Wilby, P.R. (2005) The Old Red Sandstone of Great Britain, Geological Conservation Review Series, No. 31, Joint Nature Conservation Committee, Peterborough, 393pp.
Hoy Nature Reserve: https://www.rspb.org.uk/reserves-and-events/reserves-a-z/hoy
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