Rock Lighthouses
The Problem of the Wave-Swept Rock
Building on dry land, even difficult land, involves problems that are challenging but essentially stable. The ground does not move; workers can be on site continuously; materials can be stockpiled; the structure can be built gradually and inspected at every stage. Building on a rock that is submerged for part of every tidal cycle, exposed to open-sea conditions when uncovered, and surrounded by the kind of water that the ships being guided away from it are trying to avoid — that is a different order of problem entirely.
Rock lighthouses are the hardest structures in the history of civil engineering. The combination of demanding site, hostile weather, limited working time and the structural requirement to resist wave forces that can exceed 50 tons per square metre has produced, over three centuries of practice, a remarkably consistent answer: a tapering tower of interlocking masonry blocks, each shaped to contribute to the stability of the whole, founded on the highest point of the rock and rising to a height that places the lantern above the worst of the spray. The details vary; the principle is the same.
Eddystone: The Type Specimen
The Eddystone Rock lies 22 kilometres south-south-east of Plymouth, in the approaches to one of England's most important naval and commercial ports. It was identified as a navigation hazard from the earliest periods of post-Roman English seafaring; the rock is barely above the waterline at high water and gives almost no warning of its presence in the approach from the open sea. Winstanley's timber structure of 1698–99, the first lighthouse on the rock, was swept away in the Great Storm of November 1703. John Rudyerd's second lighthouse, also of timber, burned in 1755. John Smeaton's third lighthouse, of Portland stone and Cornish granite, stood from 1759 to 1882 and established the engineering principles that all subsequent rock towers have followed.
Smeaton's innovation was to treat the tower as an interlocked mass rather than a simple column. Each course of masonry was joggled — notched to interlock with the course below and above — and the blocks within each course were dovetailed together laterally. The mortar used was pozzolanic cement, which set under water: an essential property for construction that would be interrupted by tidal flooding at every course below the high-water mark. The tower tapered from a broad base that spread the wave impact load over a large area of rock to a relatively slender shaft above the main spray zone, in a profile that Smeaton explicitly compared to the profile of an English oak tree.
Smeaton's tower was superseded not because it had failed but because the rock beneath it had been undermined by wave erosion to the point where the foundations were at risk. Its replacement, designed by James Douglass and completed in 1882, stands 49 metres tall and remains in operation today. Smeaton's tower was dismantled to the base course and re-erected on Plymouth Hoe, where it stands as a monument to the engineer and to the principle he established.
Bell Rock: The Apotheosis
If Eddystone established the principles of rock lighthouse construction, Bell Rock proved that they could be applied in conditions even more hostile than the Plymouth approaches. The Bell Rock (also known as Inchcape) lies 18 kilometres off the Angus coast of Scotland, submerged for most of every tidal cycle. At low water it is exposed for only about two hours, and those two hours are usable for construction work only when the sea conditions are calm enough to allow safe access and working. Robert Stevenson, engineer to the Commissioners of Northern Lighthouses, overcame both the tidal constraint and the logistical challenge of building 18 kilometres offshore in the North Sea, completing the 35-metre granite tower in 1811 after four years of construction.
Stevenson's working method was ingenious in its use of the tidal cycle. A floating barracks — a converted vessel moored adjacent to the rock — housed the workforce between tidal windows. When the rock was uncovered, the workers were ferried to it and began immediately. Every minute of exposure was accounted for, and the work was choreographed to ensure that no time was wasted in the transition between the submerged preparation work and the building work above water.
The Bell Rock lighthouse has been operational continuously since 1811, making it the oldest sea-washed lighthouse in the world still in use. It was automated in 1988 and is maintained by the Northern Lighthouse Board. The light character — Fl(3) W 30s, three white flashes every thirty seconds with a range of 35 nautical miles — gives it an identity that has been printed on Admiralty charts of the eastern Scottish coast for more than two centuries.
Bishop Rock: The Smallest
Bishop Rock, at the extreme western edge of the Scilly Isles, marks the first landfall for vessels approaching the United Kingdom from the North Atlantic. The rock itself is barely large enough to build on; the first attempt, an iron screw-pile structure begun in 1847, was swept away by a storm before its lantern could be lit. The current granite tower, completed in 1858 and subsequently enlarged and encased in outer masonry between 1882 and 1887 by James Douglass, rises 49 metres above the rock. It occupies a footprint barely wider than the tower itself.
Bishop Rock has the peculiar distinction of being recognised by the Guinness Book of Records as the world's smallest island with a building on it — a characterisation that captures the character of the site accurately if not entirely elegantly. The tower is visible on clear days from the islands of St Agnes and St Mary's, and the light character — Fl(2) W 15s, a range of 24 nautical miles — marks the outer limit of the Scilly Isles approach for traffic on the great circle route from North America to northern Europe.
Wolf Rock: A Century of Difficulty
Wolf Rock, 14 kilometres west of Land's End, presented difficulties that delayed its construction for longer than almost any other British rock lighthouse. The rock has almost no flat surface at any state of the tide, and early attempts to establish a beacon on it — including a cast-iron cone designed to emit a moaning sound in the wind, which gave the rock its name — were repeatedly damaged or destroyed. James Walker's granite tower, completed in 1870, used a slightly different approach to Smeaton's dovetail system: a course of solid ashlar embedded into the rock itself provided the foundation, with the tower rising from this course.
Wolf Rock was the last Trinity House station to be de-helicoptered when helicopters replaced tenders for crew relief, and it retains the character of a genuine offshore station. The light character — Fl W 15s, a range of 23 nautical miles — serves the busy traffic lane around Land's End, where the English Channel meets the Celtic Sea.
Chickens Rock, Skerryvore and the Stevenson Tradition
The Scottish rock lighthouses built by the Stevenson family — Robert, Alan, David, Thomas and the firm they collectively constituted — form a coherent series that includes some of the most technically demanding structures ever built. Skerryvore, 18 kilometres south-west of Tiree in the Inner Hebrides, was completed by Alan Stevenson in 1844. The 48-metre gneiss tower was built on a rock that is submerged even at low water during rough weather, and the construction period required a system of preliminary operations — drilling and cutting the foundation course — entirely under the tide, with the workers crossing to the rock by boat and working in conditions of constant uncertainty about the weather.
Skerryvore's form is generally regarded as the most graceful of all the British rock lighthouses. The profile tapers more gradually than Eddystone, producing a curve in elevation rather than the more angular setback of Smeaton's design, and the finish of the ashlar is of a quality that reflects Alan Stevenson's background as both engineer and architectural scholar. The lighthouse was automated in 1994.
Common Structural Principles
Despite the variety of sites and designers, rock lighthouses share a consistent set of structural principles derived from Smeaton's original analysis. The base is as wide as the rock allows, with the tower footprint entirely occupying the available platform. The lower courses are solid masonry, with no hollow core; the thickness of the walls in the wave zone is substantial, calculated to resist impact loading rather than just the weight of the structure above. The profile tapers with increasing height, reducing the area presented to wave action as the tower rises above the spray zone. The blocks are bonded in three dimensions by joggling, dovetailing or keying rather than relying on mortar for structural integrity.
Open the map to locate these extraordinary structures on the coasts where they stand, and to trace the pattern of rock hazards that made them necessary — the shoals, ledges and submerged reefs that have claimed ships throughout the history of sail and steam and that the lights they carry continue to mark, day and night, for every vessel that passes within range.