Anguilla's geography: a limestone slab with no rivers

How the Anguilla Bank was built and lifted

Anguilla sits on the Anguilla Bank, a submarine platform whose high points also include Saint Martin and Saint Barthélemy to the south. The bank lies on the Lesser Antilles island arc, where the Caribbean and Atlantic plates meet, and it began as volcanic ground. Tuffs and volcanic breccias of Eocene age still show at the surface in a few places; the Important Bird Areas survey of 2015 names the northern end of Sandy Ground village and Crocus Bay.

Most of what stands above the sea is younger. During the Miocene the island was largely under water, and reefs built up the limestone geologists call the Anguilla Formation. The survey dates these limestones to between 20 and 30 million years ago, laid down in shallow seas and then gently folded and uplifted. Since the late Pleistocene the movement has reversed, and the island has been sinking at around 1 to 2 millimetres a year. Montserrat, farther south on the same arc, still has active volcanoes, and earthquakes are occasionally felt on Anguilla.

The result is a slab tilted one way. The limestone dips gently from the north, where the coast is a line of low cliffs mostly under 50 metres high, down to the beaches of the south coast. The main island is about 26 kilometres long and at most 5 kilometres wide, the World Factbook's coastline figure is 61 kilometres, and the whole territory covers 91 square kilometres of land with no inland water counted.

Even the high point depends on who measured it. Wikipedia and the World Factbook give Crocus Hill, on the western side of The Valley, at 73 metres; the 2015 survey instead describes a stretch of sandstone cliffs reaching 65 metres as the highest point in the islands.

Why no river runs anywhere on Anguilla

Rain that falls on Anguilla sinks. The limestone is fissured, much of it exposed as bare pavement that is hard to walk across, and water drains down through cracks, caves and caverns instead of along the surface; cave systems have formed at Katouche Canyon and along the north-east coast.

Two of those drainage features were the island's oldest water sources. The Fountain Cavern near Shoal Bay is a cave with its entrance in the roof and a floor about 30 feet below, where pools of fresh water collect; islanders drew water there even in the worst droughts. The Big Spring at Island Harbour is a sinkhole about 40 metres across and 5 metres deep whose roof collapsed long ago, with a shallow spring 30 to 50 centimetres deep under the overhang on its eastern edge.

There are no mountains to catch cloud, so the rain comes when storms bring it. The annual figure depends on the source and the period: about 900 millimetres in Wikipedia's climate summary, and 970 millimetres in the 2015 survey, which gives a range of 460 to 2,050 millimetres depending on how many storms pass over in a year. September and October are the wettest months and February and March the driest. The World Factbook notes that potable water supply sometimes cannot meet demand, largely because of a poor distribution system.

Where Anguilla's water gathers in salt ponds

What surface water there is lies still. Anguilla has a wetland system of 26 sites spread across the main island and Dog and Scrub Islands, from seasonal pools to shallow permanent ponds. The largest, Caul's Pond, Cove Pond and Road Salt Pond, each cover 35 to 45 hectares. All of them are brackish to some degree, fed by rain, run-off and springs and salted by seepage through the limestone or by storm surges.

Four are inland ponds in shallow hollows of the rock: Badcox, Caul's, East End and Mimi Bay, fed by springs and rainfall, and in winter they draw the largest flocks of waterfowl on the island, blue-winged teal above all. The others began as shallow lagoons that drifting sand closed off with beaches, dunes and bars, and in dry years they dry out. Buttonwood lines many of their edges, and small stands of red, black and white mangrove survive on the deeper ponds that hold water all year, such as Little Harbour Pond.

In the 19th century a large salt pond in the centre of the island produced around 3,000,000 bushels a year for export to the United States. Road Salt Pond is a near-circular pond immediately west of the village and port of Sandy Ground, ringed on three sides by some of the highest hills on the island, whose scrub forms its catchment. It later became the centre of the island's salt industry and was harvested until the 1970s, when salt stopped paying. The old pump-house was restored in 1995 as a bar and restaurant and is now a low-key museum of the industry.

The Key Biodiversity Area assessment of 2013 records the pond's losses: the southern catchment filled in 1996 and 1997 to widen the road, a corner filled for trailer parking in 2005, and a dune on its western side flattened and built over. Over a year it draws a wide range of wetland birds, averaging between about 500 and 750 birds, with green herons, white-cheeked pintails and common moorhens breeding at the edges. A proposal to restart salt works in the pond was at outline stage when the assessment was made.

The ponds matter to birds out of proportion to their size. Seven of them, Cove, Grey, Long Salt, Road Salt, Caul's, Rendezvous Bay and Meads Bay Ponds, are listed as Important Bird Areas for breeding least terns, and in 2007 Anguilla held around 6% of the regional population of the species. At Long Salt Pond the old salt flats and pond walls give nest sites to least terns and snowy plovers, with housing pressing in at the edges.

How the dry scrub covers the limestone

From a distance the main island looks green, because a dry evergreen scrub covers most of the limestone. The plants are adapted to low, intermittent rain, long sunshine and a constant drying wind; in sheltered spots bushes and trees reach 5 metres, and in the exposed east the wind keeps them low. Tree cactus rises out of the scrub, and at Windward Point, the eastern tip, the ground is thick with Pope's Head cactus. Taller dry forest survives only in folds of the ground. The largest stand is 32 hectares in Katouche Canyon, with smaller patches at West End and along Crocus Bay. Charcoal burning had almost entirely cleared the island's trees by the First World War, and where goats still browse the cover is a low thorn scrub.

The land-use figures do not agree with each other.

The World Factbook, reproduced in Wikipedia, lists arable land at 0% and forest and woodland at 61.1%, and puts forest cover at 5,500 hectares in 2020, unchanged since 1990. The 2015 survey found about 1,335 hectares of the main island in agricultural use, mostly vegetables and fruit, with goats, some cattle and sheep, and small hydroponic schemes growing salad vegetables and herbs for the tourism market.

Several animals common on the island today arrived from elsewhere. The Cuban tree frog and the red-footed tortoise are introductions, and green iguanas reached Anguilla across the sea after the hurricanes of the mid-1990s. Five species of bat are recorded.

What the outer islands and cays hold

The territory counts 22 islands in the 2015 survey's reckoning, the main island and a scatter of outer islands and cays totalling around 660 hectares, none of them inhabited. Except for Sombrero, which stands on a separate bank to the north-west, all are visible from the main island. The survey describes them as essentially seabird islands: 16 seabird species have bred since 2000, and 16 Important Bird and Biodiversity Areas were identified, 12 on the main island, all but one of them ponds, and four offshore.

Why Dog Island matters to seabirds

Dog Island is low and rocky, with two large ponds behind its beaches and a centre of impenetrable thorn scrub and cactus. Its sooty tern colony was estimated at over 113,000 pairs in 2007 and over 135,000 pairs in 2013, and it is one of two breeding sites in the Lesser Antilles for the magnificent frigatebird, with 518 pairs in 2013. Rats were eradicated in an operation in 2012 and the island has been declared rat-free since. A herd of feral goats remains from earlier livestock farming.

How mining reshaped Sombrero

Sombrero is named for the hat it once resembled from the sea. Guano miners cut it down to sheer sides and a flat top about 12 metres above the water, pitted with craters up to 10 metres deep; phosphate output reached some 3,000 tons a year by 1870 and was exhausted by 1890. The island is about 1.67 kilometres long and 0.38 kilometres wide, and there is no beach, so landing means climbing a ladder up the cliff. A lighthouse has marked the Anegada Passage between the Atlantic and the Caribbean since 1 January 1868. The present tower dates from 1962, and the light was automated in 2001 by one account and 2002 by others, after which the island had no inhabitants. A proposal to build a rocket launch site there prompted the first systematic seabird counts in 1998 to 2000 and was withdrawn. Sombrero became Anguilla's first Ramsar site in 2018, and its endemic ground lizard is found nowhere else.

When the hurricanes reach Anguilla

The climate is tropical, with daily mean temperatures between 26.4 and 29.8 °C and trade winds blowing from the east at about 15 to 28 kilometres an hour. Hurricanes can come from June to November, with the peak from August to mid-October.

The 2015 survey named Luis in 1995, Lenny in 1999 and Omar in 2008 as the most damaging hurricanes of recent years, and Irma followed two years later. Hurricane Luis in 1995 left water over the whole of Sombrero and set back the vegetation there. Hurricane Lenny in 1999 brought flooding of 1.5 to 6 metres. Hurricane Irma passed on 6 September 2017 as a category five storm, its eye over Saint Martin and its northern eyewall over Anguilla, with maximum sustained winds of 185 miles an hour. About 90% of roads were left impassable, East End was hit hardest, and all but three of more than 40 mobile towers went offline. The damage assessment counted about 1,759 dwellings affected, 3% of them destroyed.

The coastline is mostly low, fissured limestone broken by long stretches of white sand beach. The highest cliffs line the north coast, sandstone at Crocus Bay and limestone at Little Bay, where red-billed and white-tailed tropicbirds nest.

The main island is separated from Saint Martin to the south by the Anguilla Channel, and for administration it is divided into fourteen districts. The sea around the territory is claimed to 3 nautical miles as territorial sea, with an exclusive fishing zone of 200 nautical miles. Coral reefs lie close to shore in places such as Shoal Bay East, beds of turtle grass at Forest Bay, Little Bay and Island Harbour feed green turtles, and hawksbill turtles forage on the northern reefs.