Seven Stones Reef: Cornwall's Granite Ship Trap

Imagine a line of granite peaks rising out of 110 metres of water, most of them hidden, some showing only at half ebb. The sea breaks over them constantly. On a clear day you can see those breakers from 12 miles off.

That is Seven Stones, or Lethosow in Cornish. It lies nearly 15 miles west of Land's End and 7 miles east-northeast of the Isles of Scilly.

Why should a diver care? Because an estimated 200 ships have gone down here, 71 of them named, and one of them is the Torrey Canyon. There is no other piece of reef in British waters quite like it.

A reef built out of granite

The reef is two groups of rocks, roughly two miles long from north-northwest to south-southeast and about a mile across. Seven peaks, or eight depending on who is counting.

Several of them have names: Flat Ledge, Flemish Ledges, North-east Rocks, Pollard's Rock, South Rock, and a ledge called the Town. The stone is small-grained granite, part of the Cornubian batholith that formed roughly 300 to 275 million years ago.

On the vertical faces of the more exposed rocks you will find clusters of jewel anemones and hydroids, plus plumose anemones. Classic southwest wall life, growing on rock that has been sinking ships for centuries.

The wrecks

The reef sits on the western edge of a major north-south shipping lane between the Cornish coast and Scilly. That is the whole problem in one sentence.

The first recorded wreck was in early March 1656. The Primrose, a sixth rate man o'war of 22 guns, lost her main topmast near the Longships, drifted onto the Seven Stones, freed herself and then sank in 60 fathoms with sixteen men, two women and a child aboard. At the inquiry, Trinity House told the Admiralty they could not find a single chart that showed the reef.

The worst loss of life came on 27 February 1748, when the fourteen-gun sloop HMS Lizard was wrecked with over a hundred crew lost.

Torrey Canyon

On 18 March 1967 the supertanker Torrey Canyon struck Pollard's Rock. The impact tore a gash 610 feet long in her side and she spilled 860,000 barrels of oil.

Westerly winds and currents pushed the slick onto 120 miles of Cornish coast and 50 miles of Brittany. Crews sprayed enormous quantities of toxic detergent on it, and between the oil and the detergent much of the local marine life died, along with 15,000 birds. It was the costliest shipping disaster in the world at the time, and it remains the worst oil spill on the UK coast.

She now lies in 30 metres of water on the reef. French fishing boats working the reef for crab, crayfish and lobster were among the first on the scene that day.

The lightvessel

There has been a lightvessel here since 1841, and her story is almost as eventful as the wrecks she warns about. The government was petitioned in 1826 with no success, then again in 1839 with backing from Channel ports, Liverpool merchants and the Waterford Chamber of Commerce.

She was moored on 20 August 1841 and showed her first light on 1 September. She sits 2.5 miles northeast of the reef, anchored in 40 fathoms, deliberately off the rocks so that passing ships give the reef a wide berth.

Then things got strange. Her cable parted in November 1842 and she drove right over the reef at high tide, reaching New Grimsby on Tresco. She broke adrift again in January 1843 and went over the reef a second time that March.

• 15 October 1851: two crew drowned when a longboat capsized in a squall on a stores run from Scilly
• 13 November 1872: a meteor exploded overhead at 2 am, showering the deck with cinders
• Second World War: replaced by a lighted buoy after repeated bombing and strafing by German pilots
• 1987: automated and unmanned, with the accommodation filled with foam for buoyancy

She also runs as an automatic weather station these days. A nice detail to think about while you look at the forecast.

Sea temperature and swell through the year

The pattern is blunt. Wave heights run above 2.5 metres from October through April and drop to their lowest in July at 1.2 metres, which is also when the water is close to its warmest. August peaks at 19 degrees. These figures come from Open-Meteo marine data.

The land that drowned

Local legend says a country called Lyonesse once stood between Cornwall and Scilly, with towns and 140 churches, until the sea took it. One man, Trevelyan, is said to have escaped on a white horse.

In the Arthurian version, Merlin called up the flood to swallow Mordred's men as they chased Arthur's defeated army, and the survivors reached high ground in the Isles of Scilly. Fishermen still claim to hear church bells. Rising sea levels since the last ice age probably seeded the story, and there may well have been an island out here in the not too distant past. You can read the full account on Wikipedia.

If you get out to Pollard's Rock and come back with frames of the Torrey Canyon, put them in Scubaspot. Log the depth you found her at, rate the jewel anemone walls, and let your buddies see what 200 wrecks' worth of granite actually looks like. Start at scubaspot.co.