Guernsey's geography: a high plateau and a north won from the tide

Why Guernsey stands high in the south

Geologically, Guernsey is a piece of France. It sits on the geological province of France known as the Armorican Massif, and around 6000 BC rising seas cut the Norman promontories that became the bailiwicks of Guernsey and Jersey off from the continent. How far off is a matter of which source is read: Britannica puts the island 48 km west of Normandy, and Wikipedia gives 43 km west of the Cotentin Peninsula. Neither names the two points the distance runs between.

The rock divides the island in two. The Southern Metamorphic Complex, rock that went through several phases of intrusion and deformation, stands above the geologically younger Northern Igneous Complex, and the boundary between them is also the boundary between the high island and the low one. Nine main rock types have been mapped on Guernsey. Two of them are granites and the rest are gneiss.

In the south the land rises to a plateau of about 90 metres, by Britannica's figure, and meets the sea in ragged cliffs. From there it descends northward in steps. The streams that drain it run north too, in valleys cut deep into the plateau, so the island's surface water flows away from the high coast and towards the low one.

The northern parishes are low-lying ground broken by small outcrops of harder rock. Guernsey calls these hills hougues, a word the island's Norman language took from Norse. Britannica describes the soil of this lower ground as blown sand, raised beach deposits and the fill of old lagoons, all of it ground the sea laid down or left behind.

The whole makes a rough triangle. Britannica gives the island of Guernsey 62 km² and the Bailiwick 78 km², the figure the country record carries. Wikipedia gives 71 km² for Guernsey, Herm and the smaller islands together, with about 46 km of coastline, and about 50 km of coastline for the whole Bailiwick.

The summit is where the sources part company. Britannica's 90 metres is a height for the plateau in general. Wikipedia names a point, Hautnez, at 110 metres. The record's highest point, Le Moulin at 114 metres, is on Sark, so it is the summit of Guernsey the jurisdiction and says nothing about the height of Guernsey the island.

Why the Clos du Valle was once an island

For most of its recorded history the northern tip of Guernsey was a separate piece of land twice a day. A tidal channel, Le Braye du Valle, ran across the north of the island and made the Clos du Valle a tidal island, and a bridge linked the two parts of Guernsey at high tide.

The north was church land early. In the late tenth century, Benedictine monks from Mont-Saint-Michel founded a priory dedicated to Saint Michael at St Michel du Valle, a foundation that sources variously date to about 962 to 968. Tradition holds that Robert I, Duke of Normandy, sheltered on Guernsey after a storm scattered his fleet and gave the land of the Clos du Valle to the monks. Wikipedia's article on the island dates this to about 1030; its history of the island says 1032.

The grant as the island article records it came with a condition. The abbot could extend the fief into the north-west of Guernsey "as soon as settlers could be found to clear and cultivate the land". In the eleventh century, on that account, the low north-west was still uncleared ground waiting for people to work it.

The channel was drained and reclaimed in 1806 by the British Government, as a defence measure. It was a period of military building. Fort George, south of St Peter Port, went up between 1780 and 1812 to house the troops stationed on the island in anticipation of a French invasion, and new roads were laid and the main roads metalled so the military could use them.

The reclaimed channel left marks that still organise the north. Its eastern end became the town and harbour of St Sampson's, a harbour from 1820. Its western end is now Le Grand Havre. A roadway across the end of St Sampson's harbour is still called The Bridge, after the crossing it replaced. The history of Guernsey article puts the consequence in one line: northern Guernsey was separated from the rest of the island at high tide until nineteenth-century land reclamation.

How the northern parishes were quarried

The northern ground that reclamation joined to Guernsey is igneous rock, and in the nineteenth century it was worth money. Guernsey granite was highly prized in London, where London Bridge and many important roads were repaved with it, and hundreds of quarries opened in the northern parishes to supply the trade. The island's history counts the stone industry beside maritime trade as the source of its nineteenth-century prosperity. In 1821 the island had 20,302 people, 11,173 of them in St Peter Port, and by 1901 the population had doubled.

Horticulture spread across the same low ground. Glasshouses built for grapes were turned over to tomatoes, and tomatoes became an important industry from the 1860s. Britannica's account of the island's farming still divides it by height: dairy farming with the Guernsey breed of cattle is largely confined to the high land in the south, and market gardening is concentrated in the north, where greenhouses grow tomatoes, flowers and grapes, mostly for England.

That split follows the rock. The plateau, cut by its northward valleys, carried the cattle. The northern lowland, built on laid-down sand and lagoon fill and full of workable stone, carried the quarries and the glass. Cattle still appear on the north's open ground, where Guernsey steers can occasionally be seen grazing on L'Ancresse Common, though the number of islanders raising them for private supply has fallen since the 1960s.

Both industries are small in money terms now. The States of Guernsey's Economic Overview of December 2020 put agriculture, horticulture, fishing and quarrying together at 0.2% of GDP in 2019. Fishing and quarrying were among the traditional businesses that the history of the island records as failing to return after the German occupation of 1940 to 1945.

Quarrying left the north scattered with deep holes cut into hard rock, and the water supply later put them to use.

Where Guernsey stores its drinking water

Britannica calls Guernsey's water supplies somewhat scanty. A century of engineering has gone into the problem, and the storage follows the rock: dams in the valleys of the southern plateau, flooded quarries in the northern lowland.

How the valleys were dammed

The Guernsey Water Company, formed in the late nineteenth century, supplied 1,850 domestic customers by 1920 from seven wells in the St Martins and Forest areas, and those customers used an average of 550 megalitres of untreated water a year. In 1920 the States took the supply over and formed the States Water Board. Guernsey Water records that 1921 brought "the lowest rainfall ever recorded", which put supplies under great strain in the Board's first full year.

Over the next fifteen years the Board built treatment works at Kings Mills and St Saviours and put pumping stations on the water flowing through the island's valleys. By 1930 consumption had doubled to 1,100 megalitres a year. In 1936 the Board proposed an impounding reservoir in St Saviour's valley, holding 1,090 megalitres. Work began in 1938, was halted during the occupation and resumed in 1946. The dam was sealed in January 1947, and by April 1947 the reservoir was full to overflowing.

How the quarries became reservoirs

The north had few valleys to dam and a great many quarries. Around 1930 the Board began buying disused quarries in the north of the island to hold water, and it bought more as supply rose past 3,400 megalitres a year by 1958.

It tried the sea as well. In 1960 a seawater distillation plant opened at La Hure Mare in the Vale, described by Guernsey Water as "the first of its kind in the British Isles". It turned 80,000 gallons of seawater into 20,000 gallons of raw water each hour and helped the island through the dry summers of 1964 and 1965. It proved uneconomical to maintain and was decommissioned shortly after 1970, in favour of more above-ground storage in mined-out quarries.

Demand reached 5,000 megalitres a year in the hot summer of 1970, and the Board bought Longue Hougue quarry, which backs onto St Sampson's church. Guernsey Water's history page gives its capacity as 1,160 megalitres and calls that more than a third greater than St Saviour's, a comparison its own figures do not bear out: 1,160 against 1,090 is about 6% more. Figures from Guernsey Water published by the Vale parish in December 2017 gave Longue Hougue 1,159 million litres, St Saviour's 1,091 million litres and Juas 586 million litres, and put Longue Hougue's supply to customers at around 5.2 million litres a day. The utility's statement that year called Longue Hougue "the island's largest and most important reservoir".

What the tides do around Guernsey

The sea around Guernsey rises and falls through a tidal range of 10 metres, and currents run at up to 12 knots among the islets, rocks and reefs, a combination that makes sailing in local waters dangerous. Wikipedia's article on the island attributes the size of the tidal flows to the flatness of the sea floor for nearly 32 km to the west.

What the tide uncovers is a wide intertidal zone, and some sites in it hold Ramsar Convention designation. The foreshore is used as well as protected. The ormer, a variety of abalone, is harvested under strict laws from beaches at low spring tides, and the island's language has its own word, vraic, for a type of seaweed.

The shoreline has also been built on and argued over for as long as the north has been reclaimed. In the 1930s depression, unemployed labourers were given work building sea defences. In December 2017 the Vale Douzaine debated proposals to reclaim land from the sea off Bulwer Avenue, where the States had identified Longue Hougue South as the preferred site for the island's residual inert waste. Douzeniers raised fears that the project signalled longer-term intentions to reclaim more of Belle Greve Bay, and asked why the Longue Hougue quarry could not take the waste instead.

The States' Trading Assets answered that the quarry had been evaluated and ruled out because it is a major water resource for the island. The exchange set a quarry cut out of the lowland and filled with rainwater against a bay on the east coast that could be filled with rubble and turned into land, as the Braye du Valle was in 1806.

Why frost is rare on Guernsey

The Met Office's averages for its station at Guernsey Airport over 1991 to 2020 describe a climate held steady by the sea. The station recorded 7.1 days of air frost a year over that period, against 53.4 for the United Kingdom as a whole. February, the coldest month, averaged a daily maximum of 7.8 °C and a minimum of 3.9 °C; August averaged 19.0 °C and 13.5 °C. Britannica puts the annual temperature range at only about 9 °C.

The rain falls unevenly, and that is the part of the record the reservoirs answer. Over 1991 to 2020 December brought 104.7 mm on average and July 37.3 mm, while sunshine ran from 51 hours in December to 251 hours in July. Summer on Guernsey is bright and dry, the reservoirs refill in the wet months, and the 1960 distillation plant was built to augment supply in the dry ones.

Wikipedia's figures, which state no period, run close to the Met Office's without matching: February at 6.9 °C, August at 17.1 °C, December at 119 mm of rain and 58 hours of sunshine, July at 253 hours. Britannica gives annual rainfall of 750 to 900 mm. Snow rarely falls and is unlikely to settle. Sea temperatures have been rising and, by Wikipedia's account, now vary from 8 °C in February to 20 °C in August.

Wind is the other constant. The Met Office's monthly mean wind speed at 10 metres ran from 10.2 knots in August to 15.3 knots in December over 1991 to 2020, against 7.9 and 10.0 knots for the same months across the United Kingdom.

How Alderney and Sark differ from Guernsey

The Bailiwick's two other jurisdictions are each a single plateau with no lowland half.

Alderney is separated from Cap de la Hague on the Normandy coast by the 16 km Race of Alderney, which Britannica calls dangerously swift. The Swinge, on its west side, separates it from the uninhabited Burhou, Ortac and smaller islets, and beyond them the Casquets, a group of jagged rocks, carry a lighthouse. The nearest English coast is 88 km to the north. The island is a tableland mostly above 60 metres and little cut by valleys. Steep cliffs rise to 83 metres on the south and west, while old cliffs stand behind raised beaches on the north and east. Braye Harbour on the north is protected by a breakwater of 914 metres, and Longy Bay in the south-east is backed by a nearly filled lagoon, a smaller version of the process that shaped northern Guernsey.

Sark lies 11 km east of Guernsey and about 40 km west of the Cherbourg Peninsula. It is 5 km long and 2.4 km wide at its broadest, a plateau rising to 114 metres and ringed by precipitous cliffs. It comes in two parts, Great Sark and Little Sark, joined by La Coupée, an isthmus about 90 metres long and only about 10 metres wide. The privately owned island of Brecqhou sits across the narrow Le Gouliot Channel. Three small harbours break the cliffs, at Creux, La Maseline and Havre Gosselin. Britannica gives Sark and Brecqhou together 5.5 km² and put Sark's population at 591 in 2001; farmland covers about a third of the island. Sark bans cars, and farm tractors are the only motor vehicles allowed on its roads.

Britannica's figures for 2001 gave the island of Guernsey 59,710 people and the Bailiwick 62,692, which left about three thousand for everywhere else. The country record gives 67,787 as of 1 January 2024. Herm, Jethou and Lihou fall under the States of Guernsey, and Herm belongs to one of Guernsey's ten parishes. On the 2001 figures, about 95% of the Bailiwick's people lived on Guernsey itself.