Saint Helena's geography: wet peaks above a dry and level east

Where Saint Helena sits in the South Atlantic

Saint Helena is a dependency of the United Kingdom and one of the three parts of the territory of Saint Helena, Ascension and Tristan da Cunha. Where sovereignty sits and how the arrangement works belong to the politics article; this article covers the island's ground and the sea around it.

Its distance from the mainland depends on the source. The main Wikipedia article puts the island about 1,860 kilometres west of Angola and 3,280 kilometres east of Brazil, while Britannica and the UK Overseas Territories Conservation Forum both give about 1,950 kilometres to the south-western coast of Africa. Most of its shipping comes from Cape Town.

The nearest land, in the Forum's account, is Ascension, to the north-west in more equatorial water, about 1,300 kilometres away in Wikipedia's figure, 1,100 in Britannica's and 1,125 in the Forum's. Tristan da Cunha lies to the south and outside the tropics, 2,430 kilometres away according to Wikipedia and some 2,100 according to Britannica, with the Tropic of Capricorn between them. Wikipedia measures 3,642 kilometres from the northern tip of Ascension to the southern tip of Gough Island, and the three exclusive economic zones touch neither one another nor any other country's.

Saint Helena lies on the longitude of Land's End in Cornwall and keeps Greenwich Mean Time all year, with no daylight saving time; Ascension and Tristan da Cunha do the same. Wikipedia notes that most United Nations projects, committees and papers group the island with West Africa or with Africa, and the record files it under Africa.

Why the record's summit stands on Tristan da Cunha

The record gives Saint Helena an area of 394 square kilometres and a highest point of 2,062 metres, at Queen Mary's Peak. Both figures describe the territory. Queen Mary's Peak is on Tristan da Cunha, an active volcano last reported in eruption in 1961 and high enough to carry snow in winter.

Saint Helena itself covers 122 square kilometres in Wikipedia, in Britannica and in the Forum's account. Its highest point is Diana's Peak, and the sources give it three heights: 818 metres in Wikipedia, 820 in Britannica and 823 in the Forum's description of the national park around it. Britannica gives a maximum length of 17 kilometres from south-west to north-east and a breadth of 10, and Wikipedia about 16 by 8. For the 2021 census the St Helena Statistics Office's eight districts added up to 47.6 square miles, slightly more than the 47 square miles at which Britannica and the St Helena Government put the island.

None of the sources says how the territory's 394 square kilometres were counted. On Wikipedia's figures the four biggest islands alone, Saint Helena at 122, Tristan da Cunha at 98, Gough Island at 91 and Ascension at about 88, come to 399 before Inaccessible Island and the Nightingale Islands are added.

How an extinct volcano gave Saint Helena its cliffs

Wikipedia traces Saint Helena to the St Helena hotspot, which began producing basaltic lava about 145 million years ago near the plate margin of the Mid-Atlantic Ridge. As the African Plate moved away it left a chain of seamounts, and Saint Helena sits at the south-western end of the chain, close to the plate margin. The island's last eruptions were about 7 million years ago; the St Helena Government dates its emergence from the sea to 12 million years ago.

What the volcano left, in Britannica's account, is much volcanic ash and many conspicuous rock features. Perpendicular cliffs rise 490 to 700 metres on the eastern, northern and western sides. A semicircular rim of mountains north of Sandy Bay holds Diana's Peak and Mount Actaeon, which Britannica puts at 800 metres. South of that rim, gorges cut by water deepen into valleys as they near the sea, and springs are numerous.

The harder rock has outlasted the softer. The Barn is a cap of younger lava over weaker rock, cliffed on its seaward side and overlooking Turk's Cap Valley to the south. Near Sandy Bay two solitary pillars named Lot and Lot's Wife, after the story in Genesis, top two valleys. Both are phonolite intrusions, which resisted the erosion that carried off the rock around them.

The first study of the island's geology was Charles Darwin's, made when the Beagle called in July 1836. He spent five days ashore and described the island as a desolate place, and Wikipedia records that he drew on it for the idea, published in 1844, that volcanoes rise "by slow, gradual and episodic events". Some 22 named rocks and islets lie within a kilometre of the coast. Seabirds breed on them and on the main island, in Important Bird Areas that BirdLife International has marked in the north-east and the south-west.

Why Jamestown is dry and the Peaks are wet

The climate is mild and marine, tempered by the Benguela Current and by trade winds that blow almost continuously. The Forum gives those winds as south-easterly at 16 to 32 kilometres an hour and says they make the weather highly changeable, and the St Helena Government puts the island's many microclimates down to the mountainous terrain and the trade wind together.

Jamestown, on the north leeward shore, runs from 21 to 28 °C between January and April and from 17 to 24 °C for the rest of the year, and the central areas average 5 to 6 °C cooler. Rain divides the island far more sharply than heat. The Forum puts Jamestown's annual rainfall at 152 millimetres and Britannica gives about 200 at sea level, while the higher ground and the south coast take 750 to 1,000 millimetres. Cloud and mist collect on the Peaks, which the Forum records as overcast on about 290 days a year.

The island has no recognised seasons. The government speaks of cooler months from June to September and hotter ones from December to March, of 15 to 28 °C through the year, and of thunderstorms perhaps once a decade. Weather is recorded at stations in Longwood and Blue Hill.

The wind may also have brought the plants. A study cited in Wikipedia's history of Saint Helena reasons that on a volcanic island every plant arrived from elsewhere, most likely from southern Africa on the south-easterly winds and currents. The other parts of the territory have climates of their own, and Wikipedia describes Ascension in opposite terms in two articles, as warmer and wetter than Saint Helena in one and as warm and arid in the other.

What Saint Helena built on its level ground

Wikipedia describes the terrain as rugged and volcanic, with small, scattered plateaus and plains. The largest area of level ground is Prosperous Bay Plain, in the arid east, and Deadwood Plain in the Longwood district is another. When the island was first settled, the Great Wood ran from one to the other. In 1710 it was reported to have not a single tree left standing, as Saint Helena: history records, and the Forum notes that its site became semi-desert.

Which uses the eastern plains took first

The St Helena Turf Club held the island's first recorded sports events at Deadwood in 1818, a series of horse races. Napoleon, who landed in October 1815, was to live at Longwood House, which the Fondation Napoléon calls "a small place on an arid plateau", and he moved in that December. From 1900 Boer prisoners of war were held at Deadwood and Broad Bottom. The island's 12 wind turbines stand on Deadwood Plain, first put up in the 1990s and added to in 2009 and 2014, though as of 2023 diesel generators still made 80% of the electricity.

The plains were the wirebird's ground as well. Prosperous Bay Plain holds a desert ecosystem in its Central Basin and the ridges and valleys around it, with an estimated 14% of the island's endemic invertebrates by the Forum's reckoning, and it is an important nesting site for the plover. A review by Atkins Management Consultants in 2004 found that the survival of numerous endemic species depended on protecting it.

How Dry Gut was filled to carry a runway

The South African Air Force surveyed Prosperous Bay Plain for an airport between October 1943 and January 1944 and judged one technically feasible but not a practical proposition. When an airport was finally built, St Helena Tourism says, there was nowhere on the island large and flat enough for a runway.

Work began in 2012 at Rupert's Valley on the west coast, where a jetty was built to land supplies and construction vehicles, and a haul road of 14 kilometres was laid to the site; it was paved and opened to public traffic in 2019. Dry Gut, a gorge that was to carry part of the runway, had to be raised by almost 100 metres to make an embankment, from about 8 million cubic metres of rock fill, and the fill was complete by the summer of 2015. St Helena Tourism describes the same work in other terms, as around 3,700 tonnes of rock blown up and a valley filled in during 2015.

The runway is 1,950 metres of concrete on a raised site among bluffs, and Wikipedia attributes the turbulence on its approaches to fall winds from them. A Boeing 737-800 on a test flight in April 2016 broke off its second approach close to the ground because of wind shear. The National Audit Office records that severe wind shear and turbulence delayed commercial flights by 17 months, to October 2017. The northbound approach is generally the calmer one, but only lighter aircraft can use it, and the Embraer E190 chosen in 2017 was to fill at most 76 of its 99 seats. The St Helena Government classes it as a Category C airport, one with additional considerations for approach, landing and take-off. In February 2022 the runway ends were renumbered from 02 and 20 to 01 and 19, after a slight shift in the Earth's magnetic field.

How the vegetation changes from coast to Peaks

Britannica divides Saint Helena into three vegetation zones. A rocky, barren coastal zone with cactus runs about 1.6 kilometres inland. A middle zone, another 0.4 to 1.6 kilometres wide, carries grassy hillsides with gorse, willows, poplars and Scotch pines. The central zone, about 5 kilometres long and 3 wide, holds the endemic plants, the St Helena ebony, the St Helena redwood and the black cabbage tree among them, growing beside oak, cedar, eucalyptus, bamboo and banana.

Much of the green arrived with people. Wikipedia describes bare rock across the lower parts of the island and an interior green mainly with introduced plants, much of it New Zealand flax. The flax industry, in the Forum's account, carried the economy for over 50 years by supplying British Post Office bags, until the market for natural fibre declined in the 1960s, and large areas of what was left of the moist forest were cleared for it. Of about 420 plant species growing wild, the Forum counts 45 as endemic and 15 to 20 more as native; the other 350, or 85%, were introduced.

The cloud forest is what remains of the original cover. The Forum confines its fragments to High Peak and the Diana's Peak range, both degraded by invasive plants, where black cabbage trees act as a keystone species that lets the endemic St Helena dogwood and whitewood germinate. The St Helena Government calls it "the last natural cloud forest on British soil". Diana's Peak became the island's first national park in 1996. It is one of 23 National Conservation Areas designated under the Land Development Control Plan of 2012, which comprise three national parks, six nature reserves, five Important Wirebird Areas, Man and Horse among them, and nine Historic Conservation Areas.

Which species live on Saint Helena alone

The St Helena Government's World Heritage scoping report of July 2019 counted 612 endemic species on the island and in its marine protected area: 45 vascular plants, 26 bryophytes, 10 lichens, 455 terrestrial invertebrates, more than 50 marine invertebrates and 25 marine fish. In 2012 the panel that reviewed the island's case for the United Kingdom's tentative list found potential value on natural grounds alone, for its endemic species and genera and for habitats ranging "from cloud forest to desert".

Several endemics are gone. The last wild St Helena olive died in 1994 and the last cultivated one in December 2003, and the St Helena earwig was last seen in the wild in 1967. Kew, in an undated account from its Tropical Nursery, puts the wild false gumwood at six plants on a single rocky outcrop and the wild bastard gumwood at one, growing from a crack in a cliff above a drop of more than 50 metres. Both have been raised at Kew from seed stored in the Millennium Seed Bank.

There are no native land mammals; cattle, cats, dogs, donkeys, goats, mice, rabbits, rats and sheep were all brought in. The wirebird, the St Helena plover named for its thin legs, is the only surviving endemic vertebrate, and surveys found 559 mature birds in 2016, a stable or rising population. Spiders described in 2023 and 2024 include a mole spider that builds its own small mounds and the Mount Vesey wolf spider, whose range is restricted to a single waterfall.

Replanting has started on the dry east. The Millennium Forest, launched in 2000 by the Forum's dating and established in 2002 by Wikipedia's, set aside more than 250 hectares of the former Great Wood, and over 10,000 native gumwoods have been planted there since 2002 under the St Helena National Trust.

Which districts hold Saint Helena's people

The census of 7 February 2021 counted 4,439 residents, against 4,534 in 2016, in eight districts that serve for elections and statistics. Jamestown is the only town, with about a sixth of the population in Britannica's account. The only practicable landing for ships is James Bay on the north-west side, and the town lies in a narrow valley running 2.4 kilometres inland from it. In 1871 the Royal Engineers built Jacob's Ladder, 700 steps up the valley's steep side, and in 1890 a fall of rock killed nine people in the town. Freight now lands at the island's only wharf, at Rupert's Bay, built for the airport works.

Half Tree Hollow had 1,034 residents on 0.6 square miles in 2021, 1,723 to the square mile, while Blue Hill had 174 on 14.2 square miles, 12 to the square mile. No household in Blue Hill, Sandy Bay or Levelwood was on a public sewer, and in Blue Hill 64.2% of households had a garden for growing vegetables, against 15.6% in Jamestown. Longwood lost 128 residents between the two censuses, which the Statistics Office puts down to foreign construction workers leaving the camp at Bradley's once the airport was finished. The roads between the districts are mostly paved and mostly single lane, with steep and narrow stretches and hairpin bends, under a speed limit of 30 miles an hour.

What the sea around Saint Helena holds

Saint Helena has a territorial sea of 12 nautical miles and an exclusive fishing zone of 200. In late 2016 that whole zone became a marine protected area in IUCN Category VI, for sustainable use, in which fishing is allowed but managed; the St Helena Government describes it as nearly the size of France. The Marine Management Plan for 2023 to 2027 governs it, and in 2023 it was recognised as a Hope Spot.

Wikipedia describes the shoreline as deep, with abundant red crab. Tuna is the main catch, yellowfin, bigeye and skipjack, and a Darwin Plus project begun in 2015 showed yellowfin staying in local waters for many months. Two lobsters are fished inshore, the brown spiny lobster, called crayfish, down to 35 metres, and the endemic red slipper lobster, called stump, at 9 to 75 metres.

Whale sharks, called bone sharks on the island, gather offshore in the December to March season of best weather, according to the government's tourism strategy for 2026 to 2031. St Helena Tourism reports that they arrive in equal numbers of males and females, which it takes to mean they come to mate. Humpback whales, turtles and resident dolphins use the same water.

The other two parts of the territory protect their seas more strictly. Ascension declared its whole marine zone of 445,000 square kilometres a marine protected area in 2019, closed to large-scale commercial fishing and seabed mining, and on 13 November 2020 Tristan da Cunha announced a Marine Protection Zone of 687,247 square kilometres closed to all fishing and extraction.