Dr Richard J Hubbard (UK)
The Thanet Anticline is an uplifted area forming the northeast corner of Kent and is home to the four coastal towns of Birchington, Margate, Broadstairs and Ramsgate (Fig. 1). Historically, the area has been known as the Isle of Thanet and, in this article, I will look at sediment deposition and erosion around the upstanding anticlinal structure and how shorelines have shifted during the past two thousand years. I will finish with some thoughts about how shorelines might look one hundred years from now. The article is based on material drawn from three guidebooks published by GeoConservation Kent, written by Geoff Downer and myself (see below).
This article has also been written to accompany a book review that was recently published by Deposits (see Book review: The Smugglers Trail – Geology of the Thanet Coastline from Broadstairs to Cliftonville, by Richard Hubbard and Geoff Downer).
Thanet has been a high standing area for more than 300 million years and was once part of the London Brabant Massif, a raised area stretching from Wales to the Rhineland. Culminating in the times of the Alpine orogeny, a local compressional geological structure developed, which is known as the Thanet Anticline. It is an upstanding land mass measuring some 100km2 with much of its northeast coastline dominated by high chalk cliffs, which boasts the longest stretch of coastal chalk in the UK.
A wide area of northwest Europe was submerged during Cretaceous times (about 100 million years ago) largely due to global sea-level rise. Thanet lay in relatively shallow tropical water depths and the Chalk sequence is much thinner than equivalent chalk deposits in the basinal areas to the north (southern North Sea) and to the west (Wessex-Wealden Basins).
In post-Chalk times, the mountain ranges of the European Alps formed to the south, as Africa continued to move northwards buckling the rocks in Central Europe. Southern England was on the periphery of mountain building, but even so, ripples of ground movement propagated northwards creating the Wealden Anticline, the dominant structural feature south of London. Compressional forces uplifted Cretaceous sediments and erosion planed off the top of the anticline including the youngest Chalk strata.
To the northeast of the Weald, a secondary ripple uplifted the Thanet area to form a domal anticlinal structure known as the Thanet Anticline. The southern flank is marked by the Richborough Syncline, which forms a topographic low and became the routeway of the former Wantsum Channel (Figs. 1 and 3). The Channel separated the ‘mainland’ of southern England from the Thanet Anticline, which became the isolated Isle of Thanet as described below.
The Thanet Anticline
The Thanet Anticline was first mapped by a group of eminent Victorian geologists led by Dr Arthur Rowe, a medical surgeon from Margate’s Royal Sea Bathing Hospital and undoubtedly Thanet’s most distinguished geologist (Fig. 2).
Dr Rowe published some amazing work in 1900, which progressed scientific understanding of evolution by utilising the changing form of Micraster echinoderms moving up through the Chalk sequence. Prerequisite to his biostratigraphical study was the necessity to understand the structural disposition of strata within the Thanet Anticline and hence he produced structural cross sections.
The British Geological Survey (BGS) systematically mapped the Thanet Anticline during the 1980s using gravity data, the measured thicknesses in cliff sections and depths to the various geological layers in water boreholes. These data outlined the domal structure of Thanet’s ‘mini-mountain’, a compressional dome structure with a crest at 55m above sea level at Telegraph Hill, the western end of the Manston airstrip, a former WWII American airbase. The unusually long airstrip runs east-west along the spine of the Thanet Anticline and is best known today as the giant lorry park used to avoid road congestion during Brexit trade negotiations in 2019.
The northeast Thanet coast cuts right through this uplifted domal structure, creating a splendid coastal traverse, which is the site of a guided geological walk known as The Smugglers Trail. Coastal chalk cliffs attain a height of more than 20m towards the crest of the dome (Fig. 3).
The Thanet Anticline we see today developed as a result of four phases of tectonic forces as described by Ameen (1995) and others.
- Phase 1 occurred early on during Cretaceous burial and compaction of the very soft chalk ooze sediment, which had only partially lithified. This started the formation of a gentle domal drape of Cretaceous sediments over the underlying high standing basement area.
- Phase 2 occurred when extensional tectonic forces reactivated steep basement faults and fractures along the southern flank of the London Brabant Massif. This occurred in latest Cretaceous times (about 66 million years ago) and caused the area south of Thanet to subside, which became the Richborough Syncline. This steepened the southerly limb of the early compaction drape structure, bending the rocks into a ramp-like monocline (a step-like fold) diving more steeply down to the south.
- Phase 3 occurred when this monoclinal structure was squeezed by north-south Alpine compression that buckled the step-like fold into the domal structure we see today. This occurred in Miocene times (about 15 million years ago). A line of maximum buckling developed along a zone extending southeast from Minnis Bay to Ramsgate. This zone, known as a transpressional zone, involved the horizontal movement along a steeply dipping narrow corridor of chalk as illustrated in Fig. 4.
- Phase 4 involved the release of stress following Alpine compression, which was accommodated by the steeply dipping faults and fractures (joints) as well as bedding-parallel fractures near the top of the chalk sequence.
The consequences of the polyphase tectonics outlined above can be seen around the northeast Thanet coastline as a traverse is made through the chalk cliffs of the Thanet anticlinal structure.
The routeway of the Wantsum Channel
At the time of the last glaciation, between 25,000 and 18,000 years ago, sea level was approximately 130m lower than it is today, as a result of water being locked up in massive ice sheets. At this time, Britain was joined to mainland Europe. It was only when global temperatures rose and the ice sheets melted between 12,000 and 10,000 years ago that sea level began to rise, flooding the land bridge with Europe and creating the English Channel and Straits of Dover.
During the next few thousand years, there were several fluctuations in sea level, but overall, it rose slowly as the ice sheets waned. This encroachment by the sea is an event known as the Flandrian Transgression. The relatively low-lying land between Thanet and the rest of Kent was in time flooded, creating a marine seaway known as the Wantsum Channel that resulted in Thanet becoming an island.
The strip of low-lying land that became the Wantsum Channel follows the line of the Richborough Syncline, which runs around the southern and western flanks of the uplifted Thanet Anticline. The surface expression of the structural trough is seen where North Downs Chalk strata dip down towards the east from Reculver to Fordwich, before the dip swings round to the north from Fordwich to Sandwich, thus creating an arcuate (curved) low lying channel around the southern half of the Thanet Anticline dome (Figs.1 and 3).
During the first millennium AD, the Wantsum Channel was a shallow, tidal waterway that linked the English Channel to the Thames Estuary. For ships, the benefit of the channel was partly in the shorter route, but most importantly in the safer route, avoiding the dangerous headlands of North Foreland and Foreness Point on the windward side of the Isle of Thanet.
At the time of the Roman occupation, the channel may have exceeded one kilometre in width in places, although the depth of water, especially at low tide, may not have been great. Even so, the channel could have given safe harbour to sheltering vessels, including Classis Britannicus, the Roman fleet. The Roman fort of Reculver (Regulbium) was situated adjacent to the north-western entrance, with the Roman fort of Richborough (Rutupia) adjacent to the south-eastern entrance.
Two shingle banks created by longshore drift protected the eastern entrance, which prevented the daily tides from scouring the channel floor. As a consequence, fine sediment brought into the channel by tides and rivers settled out of suspension and the waterway slowly silted up. Passage from the ‘mainland’ to the Isle of Thanet was by ferry and several hamlets, one could even call them small seaports, such as the aptly named St Nicholas at Wade, were spread out along the shores to provide this service (Figs. 3 and 14). As the channel continued to silt up, mud flats began to develop, followed by salt marsh, which in turn encouraged the further trapping of sediment.
The potential agricultural value of the coastal strips was recognised from early Medieval times and steps were soon taken to reclaim emergent land. Dykes were cut and the water slowly drained, gradually creating new pastureland. Earth-banked sea walls were erected to keep out high tides and storm surges. As sedimentation continued, the channel decreased in width and it became possible in the fifteenth century to build a bridge at Sarre. It has been reported that the last recorded ship traversing the Wantsum Channel was in 1672.
At this time, the River Stour flowed seaward on an easterly course through the once thriving medieval port of Sandwich and onto Pegwell Bay. Sandwich was eventually left stranded several kilometres upriver, while drainage of the land around the lower reaches of the River Stour continued. A northern wall between Reculver and Westgate (to the west of Margate) was finally erected in 1808 completing the closure of the Wantsum Channel. In summary, the Wantsum Channel silted up in response to both natural and manmade processes and today it remains an area of low lying agricultural land.
Rapid coastal erosion
To the west of the Wantsum Channel, the north coast of Kent comprises Paleogene soft sands and clays, which have suffered coastal erosion since the end of the last Ice Age. A conservative estimate for the distance between the Roman fort at Reculver and the shore at the time was between one and three kilometres, an implied erosion rate of between 0.5m and 1.5m each year. What is left to see of the Roman fort today is little more than half of the perimeter wall (Fig. 5). Indeed, it is possible that the offshore sand bank on which the Kentish Flats wind farm is built, may have been dry land during Roman times.
The Reculver fort is one of a series of Roman coastal fortifications in the southeast of England, known today as the Saxon Shore forts. Together, they were constructed over a period of about 100 years, and it is now believed that early development of these defensive structures had little to do with the Saxon people. They were originally built by the Romans as places of refuge and as commercial centres for trading ships and military bases during the time of the Roman occupation.
To the east of the Wantsum Channel, winter storms have long caused coastal erosion of the soft white chalk cliffs in the Thanet Anticline. The Thanet Chalk comprises two chalk members of Santonian age (about 85 million years old), with a total thickness of 42m. The lower is known as the Broadstairs Chalk (formal name, Seaford Formation), which contains numerous flint bands. The upper is known as the Margate Chalk (formal name, Margate Member of the Newhaven Formation), which is very soft indeed and contains far fewer flints.
The 1793 Thomas Pouncy engraving of chalk cliffs at Kingsgate Bay, just north of Broadstairs, captures a time lapse view of the full forces of coastal erosion. It shows that Holland House, the 1780’s retirement home of Lord Holland, was about 15m further back from the cliff top than today (Fig. 6).
Lord Holland built several flint conceits,or follies, around Holland House and the one at Whiteness Point called Arx Rouchim (Neptune’s Tower) was built in the style of the Henry VIII coastal fort in Walmer. Lord Holland added to the myth by claiming that the folly was built on the site of a tower built by King Vortigern in about 458 AD. As with Holland House, Arx Rouchim was also a lot further back from the cliff top at the time. As seen today, the central chalk tower has completely eroded away. Margate Chalk erodes very quickly and is far too soft to be used as building stone.
Whiteness Point, just north of Kingsgate Bay, was a remote place during the eighteenth century and famous for smuggling. A natural line of geological weakness (a prominent north-trending joint plane) cutting the chalk cliff was dug out to create a cut down to Whiteness Point known as Kemp’s Stairs. Dr Rowe’s geological cliff section of 1900 shows this as a lifeboat slip, but it seems to be the work of smugglers. The Noble Captain Digby (as the inn in Kingsgate Bay was originally known) itself became a victim of coastal erosion and much of the building fell over the cliff in 1861. Some of the building material was salvaged and incorporated into the new Captain Digby we see today.
Chalk cliffs, caves and sea stacks
Geologists and engineers have long studied the mechanisms of coastal erosion controlled by natural jointing within the chalk of the Thanet Anticline. The high coastal cliffs around the northeast corner of Thanet displaytwo closely spaced joint sets,intersecting at right angles and cutting the softchalk layers into small blocks. The joints are sub-vertical (more than 70°) and trend in both easterly and northerly directions. The easterly set is dominant, and the northerly set abuts the easterly set, propagating from top to bottom to form northerly trending caves (Figs. 7 and 8).
The Keyhole Cave below the Captain Digby Inn in Kingsgate Bay eroded into the very soft Margate Chalk and is a great example of a sea cave in this area (Fig. 7). The dominant easterly joint set cuts the cliff face at right angles forming frequent polished joint plane surfaces, which trend out toward the offshore wind farm. The cave itself cuts through along the cliff-parallel northerly joint set. Erosion of the Keyhole Cave has been accelerated by smugglers, who widened the upper part of the cave to allow vessels to be hidden away with their protruding square rigged cross trees beyond the width of the hull.
This pattern of coastal erosion continues to Whiteness Point, where a cave cutting through the promontory on the northerly joint trend grew to become the splendid arch we see today.At the moment, Whiteness Arch looks its best, but not for much longer (Fig. 8). On closer inspection, it becomes clear that the roof of the cave will soon fall in to create an isolated sea stack. Over enthusiastic cave excavation by brandy smugglers in times past has accelerating coastal erosion on this headland.
A natural right angle bend occurs in the coastline at Whiteness Point. It forms where the layer of very soft Margate Chalkcomes down to sea level for the first time. The dominant easterly jointing now starts to control coastline orientation (Fig. 9).
The full force of North Sea winter storms batters this corner of Kent and, combined with the extremely soft nature of Margate Chalk, the speed of coastal erosion is fairly rapid. Winter storms regularly bring down sheets of soft chalk along lines of easterly joint sets known as shatter belts. Within a week or so, the rock fall is redistributed along the beach, leaving little or no visible evidence other than a bright white cliff face where fresh chalk is exposed. This process lays bare the dominant easterly joint plane surfaces, as particularly well seen in Whiteness Bay and in the Botany Bay sea stacks. They are the sub-vertical smoothly polished surfaces caused by the friction of chalk moving against chalk either side of the joint plane.
By courtesy of the BGS photo archive, we are fortunate to be able to include a ‘time lapse photographic sequence’ showing a century of coastal erosion. Back in 1927, the Botany Bay headland was cut by a small natural arch, little more than head height, as captured in the colour enhanced image of Fig. 10. BGS commented on the 1927 arch saying:
nearly vertical, closely spaced jointing in the Margate Chalk, Uintacrinus Zone has weakened it, leading to the formation of the natural arch. Eventually the arch will collapse leaving the seaward part of the promontory as a sea stack on the wave-cut platform”.
By 1963, the Botany Bay Arch had grown to almost full cliff height, as shown on the cover of the 1965 BGS The Wealden Guide (Fig. 11), which says:
vertical jointing in the Margate Chalk has been picked out by marine erosion to form rectilinear inlets and the natural arch. Frost action has broken up the upper few feet of chalk at the cliff tops”.
Only three years later in 1966, the year Bobby Moore lifted the World Cup for England, the Botany Bay Arch collapsed. Two sea stacks remain today, much reduced in footprint size, but far from diminished in interest: they are the most painted and photographed features along The Smugglers Trail and often used as a film location (Fig. 12).
Migrating Margate Sands
While walking along the sandy bays of the northeast Thanet coastline,it would not be surprising if you ask yourself, exactly where have these beautiful golden yellow beach sands come from? They do not look like the chalk cliffs, nor do they look like Thames Estuary mud, nor the pebble shingle we see elsewhere in Kent. Indeed, they are perfect bucket and spade sands, ideal for making sandcastles as shown on the road sign when entering Thanet. They are famous for their appeal, as some of the best beaches around and the main draw for Thanet’s five million annual visitors.
The answer to the question is that Margate Sands do not come from Margate. As you can see from Fig. 13, it is a wonderful sand made up almost entirely of small, rounded quartz grains, with just a few other grains of darker minerals. Thanet has 16 sandy bays stretching all the way around the exposed northeast corner of Kent, from Pegwell Bay to Minnis Bay. The guided tour of The Smugglers Trail walks along some of the most scenic bays at Stone, Joss, Kingsgate, Whiteness and Botany Bays.
Having accepted that Margate Sand does not come from Margate, the answer to the provenance is hinted at by looking seawards to the Straits of Dover. During the last Ice Age, ancient river systems carried large quantities of quartz-rich sediment from highland areas in mainland Europe and the British Isles to the lower-lying area between France and England.
At this time, the English Channel was land occupied by a river system. Quartz is an extremely hard and durable mineral and, while many other minerals break down into clays, quartz is gradually abraded into progressively smaller grains. As the Ice Age came to an end and sea level rose once more, renewed tidal and current action sorted the sediment by size and weight. Many marine sandbanks formed, including the well-known Goodwin Sands and Margate Sands, which break water at low tide and are dangerous shipping hazards.
As offshore sand has been moved around by tidal action over the past few thousand years, some has drifted onto the northeast corner of Kent. A bit like the migratory birds seen touching down for a rest at Foreness Point, we can enjoy our beautiful golden yellow quartz sands as they pass by along the coast.
The next 100 years. Will Thanet become an Isle once more?
As we face the realities of global warming and sea level rise over the century ahead, the winter storm of 1953 might act as a harbinger of what lies ahead. On 31 January 1953, a combination of an extreme low pressure weather system over the southern North Sea, strong north-westerly winds, a high river discharge and high spring tide combined to create a storm surge with waves over 12m high along the north Kent coast from Herne Bay to Birchingtonnear Margate.
There were breaches of the Reculver-Birchington sea wall and the Wantsum Channel was flooded to a depth of 2.5m, with much damage to agricultural land and loss of livestock. The railway service from Faversham to Margate was severely disrupted by flooding, which needed very significant and urgent repair.
A week after the storm struck, giant landslips occurred in the soft London Clay cliffs between Reculver and Herne Bay. A strip of land some 14m wide belonging to the cliff top Miramar Hotel dropped down on an inclined slip plane,lubricated by excessive ground water and wave scouring at beach level. The land slips ruptured gas mains and sections of gas pipe were carried downhill within the landslide. The Herne Bay Press described the event as “an extraordinary geological phenomenon”.
Recent assessments from the Intergovernmental Panel on Climate Change (IPCC) imply that global mean sea level is likely to rise by 1.1 m within this century and will continue rising beyond. A recent study by Grinsted and Christensen (2021) suggests even higher rates of sea level rise. Analysis of climate change is beyond the scope of this article but, for reference, we assume a notional 2m rise in sea level and look at the impact that might have on the position of the shorelines around the Thanet Anticline (Fig. 14). In short, Thanet could well become an Isle once more in the next 100 years.
The high chalk cliffs around the northeast of the anticline would remain as a barrier to encroachment by the sea, but you will notice a return of the Wantsum Channel waterway running around the Richborough Syncline on the southern flank of the Thanet Anticline. All low lying land would be at risk of serious flooding and train lines to London and Dover would be severed. No doubt, public roads and services would be put at risk along with all buildings in low lying areas.
The Wantsum Channel is noteworthy in that it is one of those low-lying areas such as the Somerset Levels which will be among the first to go underwater if and when the time comes and will change the geography for all around.In summary, the Isle of Thanet would shrink in size by some 20% and be surrounded by water on all four sides.
If you would like to know more, GeoConservation Kent has published three local guidebooks covering the NE Thanet and North Kent areas.
The Smugglers Trail. Geology of the Thanet Coastline from Broadstairs to Cliftonville,by Richard Hubbard & Geoff Downer (2021) GeoConservation Kent
The guidebook is written for the whole family to explain and illustrate some marvellous geology, while spending a great day out on the beach. Located on the northeast corner of Kent, The Smugglers Trail visits some wonderful sandy bays along what is the longest coastal exposure of Chalk anywhere in the country. The Smugglers Trail is a four hour guided walk along the foreshore taking in some very interesting social history, a description of the natural environment and an engaging explanation of the Cretaceous Chalk geology. For those limited by time, there is a 1½ hour Highlights Tour to visit and enjoy the key sights. Newly published by GeoConservation Kent with support from the Geologists’ Association. The guidebook is available for online purchase through the Geologists’ Association at https://geologistsassociation.org.uk/shop/, as well as local outlets in Thanet and elsewhere. ISBN 978-0-9561690-3-7.
The Geology of Reculver Country Park, by Geoff Downer (2011) GeoConservation Kent
The short stretch of coast from Reculver to Belting has much to interest those who want to find out more about the physical features of the land and man’s interaction with the landscape over the past two thousand years. It is published by GeoConservation Kent with support from Natural England. The guidebook is available through www.kentrigs.org by contacting Geoff Downer at email@example.com. Up to date information about the Reculver Country Park and its facilities can be accessed through the Canterbury City Council or from Kent Wildlife Trust. ISBN 978-0-9561690-1-3.
The Stones of the Reculver Country Park, by Geoff Downer (2011) GeoConservation Kent
A companion guidebook to The Geology of Reculver Country Park, this describes two ancient scheduled monuments at Reculver, and the different building materials used in their construction, the Roman fort and the Saxon St Mary’s Church. The “Stones” guidebook also describes the rocks introduced to Reculver to protect the coastline from further marine erosion. It is published by GeoConservation Kent with support from Natural England. The guidebook is available through www.kentrigs.org by contacting Geoff Downer at firstname.lastname@example.org. ISBN978-0-9561690-2-0
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Mortimore, R.N. 2021. The Chalk of the South Downs of Sussex and Hampshire and the North Downs of Kent, Geologists’ Association Guide No. 74, The Geologists’ Association, London.
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