Western Rocks, Isles of Scilly: Granite, Seals and 300 Years of Wrecks

Picture a low scatter of granite southwest of St Agnes, with the Bishop Rock lighthouse standing offshore beyond it. No houses. No landing stage. Just rock, seals and seabirds.

This is where Admiral Sir Cloudesley Shovell's flagship HMS Association went down on the Gilstone Reef in 1707, taking more than 1,400 lives with her. It remains one of the worst naval disasters in British history.

That is the short answer to why divers come here. The Western Rocks hold a remarkable concentration of the wrecks recorded in the Isles of Scilly, and divers both commercial and amateur still work at identifying and recording them.

Why so many ships died here

Navigation in cloudy weather used to be an informed guess based on assumed speed and direction. Add a poorly understood current in the western approaches that pushed sailing ships further north than expected, and you have a recipe for trouble.

Worse, charts made before 1750 placed these rocks ten miles further north than they actually sit. Captains thought they had clear water. They did not.

The Gilstone Reef did most of the damage. In 1784 the packet ship Nancy struck it; some crew and passengers got away in a small boat that was then dashed onto Rosevear, killing everyone aboard. Thirty-six crew, twelve passengers and one prisoner drowned, including the actress Ann Cargill, whose body was found clinging to a young child.

In 1841 the paddle steamer SS Thames mistook the St Agnes light for the Longships and turned north. She broke up off Jacky's Rock in a strong northeasterly. Sixty-two of the sixty-four aboard drowned, and the pilot gig Whale reached the wreck in heavy seas to save three women. Her figurehead is in the Valhalla Museum at Tresco Abbey Gardens.

The second heaviest loss came in 1875, when the German liner SS Schiller hit the Retarrier Ledges. Three hundred and thirty-five people died. Her captain knew he was near the Bishop Light but had no idea he was already inside the Western Rocks.

The rocks themselves

Everything here is Hercynian granite of late Carboniferous age, with thin podzolic soils on the larger lumps. The group sits southwest of St Mary's, with Annet and St Agnes to the northeast and the Bishop Rock to the west.

The larger islands run north to south: Great Crebawethan, Rosevear, Rosevean, Daisy and Pednathise Head, which is the most southerly point of the United Kingdom. Rosevear Ledge and the Gilstone Reef lie just to the west.

Smaller names you will hear on the boat include Codnors Rocks, Little Crebawethan, Jolly Rock, Jacky's Rock, Silver Carn, Gorregan and Melledgan. The Hellweathers group lies south of Annet, and there are the Rags, Inner Rags and Old Bess as well.

• Rosevear: the largest at 2.17 hectares, flat-topped to 5 metres
• Rosevean: the highest of the group at 17 metres
• Gorregan: 1.57 hectares of smooth, square, fissured rock standing 8 metres high
• Melledgan: 0.96 hectares, a grey seal pupping and haul-out site
• Daisy: 0.57 hectares, another seal haul-out
• Great Crebawethan: 0.51 hectares
• Little Crebawethan: 0.14 hectares

Rosevear is the only one with any human history worth the name. In 1709 and 1710 it served as base camp for the Herbert salvage expedition working the Association and other 1707 wrecks. In the 1840s and 1850s workmen lived there again while building the Bishop Rock lighthouse.

Seals, shags and storm petrels

The rocks were designated a Site of Special Scientific Interest in 1971 for their breeding seabirds, and the Isles of Scilly Wildlife Trust manages them largely as nature reserves, for birds and for grey seals.

Eleven species of seabird breed here. European shag and European storm petrel are the nationally important ones, alongside kittiwake, razorbill, guillemot, cormorant, fulmar, puffin, and great black-backed, lesser black-backed and herring gulls.

Scilly holds the only English breeding sites for storm petrel, with eleven colonies and an estimated 1,475 occupied sites. Rosevear now has the third largest colony in England, up from 57 occupied sites at the Seabird 2000 survey to 129 in 2006, while Melledgan went the other way, from 140 down to 69.

Grey seals pup on Gorregan and Melledgan and haul out on Daisy, Rosevean and Melledgan. On Melledgan their droppings enrich the shingle plant community, where tree mallow, spear-leaved orache and common scurvygrass grow.

For the nudibranch hunters: a rare sea slug, Babakina anadoni, was recorded in UK waters for the first time here in August 2022. In the eastern Atlantic it has only turned up a handful of times, off the west coast of Spain and further south. The Wikipedia article has the detail.

Sea temperature and swell through the year

The pattern is plain enough. Wave heights sit above 3 metres from October through April and drop to their lowest in July and August, which is also when the water is warmest at 18 and 19 degrees. September still holds 18 degrees but the swell is already climbing again.

Planning a trip

Landing on the islands is both difficult and discouraged, and there are few published records of visits by naturalists, so think of this as a boat-and-water trip rather than an island-hopping one. The rocks sit around 49.870 north, 6.401 west.

If you want the land-based history, the Thames figurehead is at Tresco Abbey Gardens, and Ann Cargill was eventually reinterred in the churchyard at Old Town on St Mary's.

If you get out to the Gilstone or the Retarrier Ledges, log it in Scubaspot with the depth and the rock you dived, and add your photos of the granite and the seals. Divers are still piecing these wrecks together, and your notes on which site is which are worth more here than almost anywhere.