A much older name for the strait is the "Sea of Orcs", in reference to the Pictish tribe who inhabited Orkney. One version of Nennius's 9th century Historia Brittonum states that "the Britons originally filled the whole island with their peoples from the British Sea to the Sea of Orcs". "Britons" in this case means the British-speaking peoples in contradistinction to the Gaels and the English.
The southern shore of the strait is Caithness and this shore extends from Dunnet Head in the west to Duncansby Head in the east, while the northern shore, with Orkney, is from Tor Ness on Hoy in the west to Old Head on South Ronaldsay in the east.
The most northerly point of the headland of Dunnet Head, Easter Head, is also the most northerly point of mainland Great Britain. The famous John o' Groats, Mey (site of the Castle of Mey), and many smaller villages are also to be found on the Caithness side as are the town of Thurso and Scrabster Harbour in Thurso Bay, on the western fringe of the Firth.
Crossing the Firth
Ferry crossings are:
- Scrabster to Stromness (the oldest continuous ferry service across the firth, started in 1856 as a continuation of the railhead at Thurso)
- Gills Bay to St Margaret's Hope on South Ronaldsay. Historically this route was by way of the islands of Stroma and Swona and thence to South Ronaldsay, and is known as The Short Sea Crossing.
- John o' Groats to Burwick on South Ronaldsay (foot passengers only, summer months)
The Firth is well known for the strength of its tides, being among some of the fastest in the world, a speed of 16 knots is reported close west of Pentland Skerries. The force of the tides gives rise to overfalls and tidal races which can occur at different stages of the tide.
Such is the reputation of the Pentland Firth that shippers have for centuries preferred to sail to the north of Orkney rather than enter the firth.
Some of the principal tidal races are:
- 'The Merry Men of Mey'. Forms off St John's point in the west-going stream and extends as the tide increases NNW across the firth to Tor Ness. The worst part is over a sand wave field about 3½ miles west of Stroma. The waves formed by this race form a natural breakwater with relatively calm water to the east of it, particularly noticeable when a westerly swell is running. Tides in this area can exceed 10 kts.
- 'The Swelkie'. The race at the north end of Stroma, off Swelkie Point is known as 'The Swelkie'. It extends from the point in an easterly or westerly direction depending on the tide and can be particularly violent. The whirlpool of the same name was, according to a Viking legend, caused by a sea-witch turning the mill wheels which ground the salt to keep the seas salty. The name derives from an Old Norse term, Svalga meaning "the Swallower".
- The 'Duncansby Race' forms off Ness of Duncansby at the start of the SE-going tidal stream (flood). Initially extending ENE but wheeling gradually anti-clockwise until it extends about 1 mile NW some 2½ hrs later at which point it is known as 'The Boars of Duncansby'. During the time of the SE stream there is additional turbulence off Duncansby Head, particularly to the East. The race temporarily ceases at the turn of the tide before forming in an ENE direction in the NW-going tidal stream (Ebb) before ceasing again at the next turn of the tide. The race is particularly violent and dangerous when the tidal stream is opposed by gales in the opposite direction. During the east-going stream a race forms off Ness of Huna. This race can be particularly violent in an easterly or southeasterly gale.
- The 'Liddel Eddy' forms between South Ronaldsay and Muckle Skerry in the East-going stream (flood). A race also forms for part of the time off Old Head at the SE part of South Ronaldsay.
In addition to 'The Swelkie', races form at both the north and south ends of Stroma and Swona. Between the races there is a calm eddy which extends down tide as the tide strengthens. The races are highly visible with overfalls and whirlpools. Large swell waves can also be present, especially in bad weather conditions. When entering or leaving the eddies, crossing the races, even large powerful vessels can be pushed off course, such is the demarcation between the relatively calm eddy and the fast-moving tide in the races. There are other races in the firth particularly off Brough Head.
The potential economic significance of the tidal currents in the Pentland Firth is great. It may be capable of accommodating 10 GW of installed generating capacity. This is up to 25% of the estimated total capacity for the European Union.
In October 2008 tidal power developer Atlantis Resources Corporation announced it was considering a site near the Castle of Mey for a computer data centre that would be powered by a tidal scheme in the Firth.
In October 2010 MeyGen, a consortium of Morgan Stanley, Atlantis Resources Corporation and International Power, received operational lease from the Crown Estate to a 400MW project for 25 years.
Seals can be seen at all times of the year in all parts of the firth. They usually bask on the rocks on the ebb (falling) tide since it saves them having to move as the water rises. Large groups of seals can be seen at the north of Stroma, from the lighthouse round to the north-west part of the island and at the south end in the area of the beacon and to the east between the old jetty and the lighthouse. On Swona they can be seen around the mid point of the west coast.
The common seals come in various colours with spots and have their pups in May, June and July. They are of a similar colour to the adults and can swim from birth. Grey seals are more common in this area having one of the largest British populations, also comes in a number of colours and live in colonies giving birth to white pups from October through to January. These pups do not swim at first until they shed their white coat. When giving birth seals can often be seen inland, sometimes quite a surprising distance.
Porpoises can be seen all year round in all parts of the firth although they tend to favour shallower water. Usually they are seen singly or in small numbers although in the autumn many groups may be seen in Gills Bay. Dolphins are not very common but seen from time to time. Many different types visit the firth and may be seen anywhere at any time.
Killer whales are mostly seen around May to July although may be sighted at any time of the year. Usually swim in a pod of a maximum of from six to twelve individuals led by a male who will be about 25% larger than the females and juveniles who make up the remainder of the pod. Minke whales are quite often seen in June and occasionally through to October but also at other times of the year.
Basking sharks are rare but being seen more often around May to August. A fish rather than a marine mammal spends most of its time cruising on the surface filter feeding. The dorsal fin can be seen moving steadily through the water with the tip of the tail moving from side to side behind it. Usually seen feeding inshore at high tide around here in the Gills Bay, Stroma area though may be seen in deeper water.
Turtles are very rarely seen, however they are occasionally found caught in fishing nets in this area. The Leatherback is the most frequently recorded species in Britain's waters and probably the only one in this area.
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- Anderson, Joseph (Ed.) (1893) Orkneyinga Saga. Translated by Jón A. Hjaltalin & Gilbert Goudie. Edinburgh. James Thin and Mercat Press (1990 reprint). ISBN 0-901824-25-9
- Watson (1994) pp. 62-63
- "Marine Briefing" (December 2006) Scottish Renewables Forum. Glasgow.
- RSPB Scotland, WWF Scotland and FOE Scotland (February 2006) The Power of Scotland: Cutting Carbon with Scotland's Renewable Energy. RSPB et al.
- "Major tidal power plan revealed" BBC News. Retrieved 26 October 2008.
- Major Scottish tidal project unveiled New Civil Engineer, 28 October 2010. Retrieved: 4 November 2010.
- Watson, W J (1994) The Celtic Place-Names of Scotland. Edinburgh; Birlinn. ISBN 1-84158-323-5. First published in Edinburgh; The Royal Celtic Society, 1926.