Brief Summary

The Solent Estuarine System consists of Southampton Water, the West Solent, the East Solent and Spithead, Portsmouth, Langstone and Chichester Harbour. These are all drowned Pleistocene river systems above Tertiary (mostly Eocene) clays and sands with minor limestones. The Solent River extended from west to east and swung round the eastern end of the Isle of Wight at a time when sea-level was low and the English Channel was dry. This river channel has a series of terraces which descend to about 40 metres below sea level at the east of the Isle of Wight. The old valley is now flooded and is quite wide and relatively straight with coastal erosion at the margins.

Southampton Water is above the Pleistocene valley of the combined Test and Itchen and resembles in Solent in general geomorphology, except that the upper parts are not subject to significant coast erosion. However, from the Hamble Estuary southward coast erosion is now increasing. The Beaulieu River Estuary, the Hamble Estuary and the Itchen Estuary have incised meanders. The meanders may have developed on a Hoxnian marine surface, the Slindon Boxgrove Raised Beach (at about 30m). As sea-level fell during the Devensian, they were incised, even though the subsequent glacial phase rivers were probably of braided rather than meandering type. Meanders are only visible on a map where only the lowest gravels are flooded and the higher, wider terraces are not under water. There is also a well-developed incised meander in the former Test Valley under the West Quay Shopping Centre, Southampton (reclaimed land).

The eastern estuaries, Portsmouth Harbour, Langstone Harbour and Chichester Harbour are quite different. They represent short dendritic river systems, that are approximately limited to the north by the former cliff line of the Ipswichian Raised Beach (of the Selsey - Portsmouth area). The river system has originated in Devensian times on this beach and low level terrace. It has fairly steep slope to the outflows at the narrow harbour entrances (further narrowed by spit developments). The buried channels at these harbour entrances are at about 20m below sea-level (and may be slightly more).

The underlying geology is well-known from numerous shallow engineering boreholes (many thousands), and a few deep boreholes (e.g. Southampton No.1 and Marchwood No.1). There have been studies of sections in the Fawley Transmission Tunnel (under Southampton Water), the Fawley Outfall Tunnel (into the West Solent), the Sewage Outfall Tunnel from the Gosport area, the water pipeline excavations from Lepe southward, the study for the proposed railway tunnel from Lymington, etc. In addition there are coastal exposures in the Solent and there have also been geophysical studies. There are British Geological Survey memoirs and geological maps of the area, numerous papers and other publications. The geology is fairly simple. It is an easy area to study, assuming you have a basic knowledge of the local Eocene.

INTRODUCTION:

Topographic and Geological Maps

Palaeogene (Tertiary) Succession

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The Solent River System of the Pleistocene

A map of the hypothetical Solent River system is shown above. It has long been argued that a river valley system extending eastward from the the rivers Frome and Piddle cut a valley north of the Isle of Wight at a time of low sea level. The former existence of such a river system during some part of the Pleistocene is likely but the work of Velegrakis has shown that by the end of the Pleistocene there were direct valleys southward from the area of Poole Harbour and Christchurch Harbour. In other words the Frome, the Piddle, the Stour and Avon did not continue into a Solent River at that late stage. This, however, does not preclude the possibility that they did earlier in the Pleistocene, as shown above, in a diagram based on the work of Westaway, Bridgland and White (2006). There is little doubt that a major river coming from Southampton Water and the West Solent joined the large westward flowing Channel River. The present day bathymetry of the English Channel or La Manche, as shown above, clearly demonstrates this.

LOCATION:

West Solent, including the Brambles Bank

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[ The Olympic (or perhaps the Titanic) Crashed at the Brambles Bank. This, the Brambles Bank, is at the same site that the Olympic was in collision with another ship as they tried to turn round the bank. The Olympic, sister ship of the Titanic, was claimed by Robin Gardner, in his book of 1998, to have been switched in name to the identical sister ship - Titanic. If this was really the case, then it was the Titanic which crashed at this location. Here is an extract of the detailed account, baded on a report reproduced by Gardner, of the collision in the channel next to the Brambles Bank. (Note that in the historic record, the old ship terminology relating to rudders is used; thus "hard-a-starboard" really means "hard-a-port" with regard to ship orientation.)
The captain of the Olympic at the Brambles Bank was Captain Edward John Smith (of Woodhead, Winn Road, Portswood, near what is now Southampton University), who was later in charge of the sister ship, Titanic, at its fatal disaster.
"At 12.30 pm (20th September 1911) Olympic reached Black Jack buoy which marked the end of Southampton Water. Captain Bowyer (the pilot) ordered the liner's speed reducted to 7.5 knots in preparation for the turn into Spithead ... Four buoys marked the channel around the western edge of shoal (i.e. where the cargo ship was beached). At 12.34pm Olympic passed Calshot Spit buoy and then three minutes later, she passed North Thorn buoy. By this time her speed was down to 12 knots. The next buoy, Thorn Knoll, marked the most complicated part of the vessel's turn. The order for the port engine to go "full astern" was given at 12.40 pm., then came "slow ahead" when the central turbine engine was stopped. When the ship was at two thirds of the way from the Thorn Knoll buoy to the West Bramble buoy, the helm was put hard-a-starboard [i.e. really a turn to port], the port engine was stopped and then reversed. At this point the Olympic's speed was down to 11 knots. The liner gave two blast on the whistle, a signal to other ships in the vicinity that she was going to turn to port. The turn [round the Brambles Bank bend] was completed by 12.43 pm, and one minute later, all engines (turbine included) were put "full ahead" [presumably intending to accelerate up and pass Ryde and then out to sea round the eastern Isle of Wight]. Two minutes after that the ship had accelerated up to 16 knots..."
.. The old Royal Navy cruiser, HMS Hawke had been on speed tests in the Solent, presumably the West Solent, and was returning to Portsmouth. The two ships became close, beyond (i.e. southeast of) the West Bramble bouy. The two ships were close with the cruiser appearing at first to overtake. However, the Olympic, having got round the awkward Bramble Bank turn was now accelerating as it was heading for the open sea. The naval ship turned to port (left) presumably to go behind the Olympic and head for Portsmouth. Captain Smith said "I do not believe that he will get under our stern, Bowyer!" The pilot replied "If she is going to strike let me know in time so that I can put the helm hard over to port" (i.e. presumably this means turn the Olympic hard to starboard so that the cruiser could slip by behind). Pilot Bowyer shouted "Is she going to strike, sir?"; Captain Smith shouted: Yes, she is going to strike us in the stern."
The naval ship tried to turn starboard (right) away from the liner, but the steering jammed. The cruiser's had a reinforced bow ram ripped into the liner with an explosive noise, the front ram was torn off and the cruiser twisted around by the violent impact with the Titanic's sister ship. There was a hole in the Olympic 8 feet deep into the ship, and extending down though D, E F and G decks, i.e. almost right down the side. The naval cruiser almost capsised but was able to creep away to Portsmouth.
The badly-damaged Olympic anchored in Osborne Bay, adjacent to Cowes, Isle of Wight. Passengers were disembarked by tenders. The Olympic was towed back to Southampton. There the Titanic was removed from the dry dock and the Olympic went in for assessment. Read the book by Robin Gardner for the continuation of the story and for further details. So, before the Titanic disaster, there was an accident with the sister ship, Olympic, at the Brambles Bank, under Captain Smith, the captain of the Titanic in the later disaster.
(Gardner in his book claims that the "Olympic" of this Brambles Bank misadventure, later became the Titanic, but presumbly that is disputable.)]

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The reader should consult the very useful and informative British Geological Survey Sheet, Wight, Sheet 50N 02W, Sea Bed Sediments and Quaternary. The published works of Dyer on sea bed sediments and procesess should be consulted.

The West Solent was in the past a relatively sheltered part of the Solent Estuarine System with tidal currents but only limited wave action. It once had wide mud-flats on the northern, mainland side. At about 1900 Lord Montagu introduced Spartina, the salt-marsh plant to the Beaulieu River, actually an estuary, and it subsequently spread around the West Solent and elsewhere. This converted the extensive mud-flats into salt marshes. The plants trapped sediments, perhaps roughly doubling the sedimentation rate. The shores became largely protected by the vegetated saltmarshes. This situation continued until about the last 20 years or so when Spartina started to die back. At the same times there has been a slight rise in sea level and dredging has taken place at the margins of the West Solent. Now the northern coast of the West Solent is changing quite fast from a quiet-water coast of salt-marsh and some mud-flats into a coast with small sea-cliffs. Erosion is occurring and beach sediment is being moved towards the east, almost blocking the Sowley stream and continuing on to the major, new spit developments near Needs Ore Point (near Lepe). A small amoutn of land has already been been lost on the north shore of the West Solent and this may occur on a larger scale if this continues unabated. It is true that some local sea defences give protection here and there. There is nothing to suggest this erosional prospect reducing and the storms of 2014 have made the situation worse. Loss of the Spartina saltmarshes may make this coast in due course reliant on sea-defences for almost all its length from Lymington to the Beaulieu River.

Of course, Hurst Spit still provides some protection in preventing large waves from travelling down the West Solent. However, this is vulnerable in part. The main seaward-facing section is partly rock armour and largely now artificial gravel. The long gravel section is periodically flattened and washed over by great storms. It may not fail completely in the near future, but if it did so, then there would be rapid erosion and cliff development in the West Solent. Of course, the future cannot be predicted and there is more than one possibility with regard to the extent of erosion on the north coast of the West Solent. However, it seems to be increasing and almost certain to further increase. Some type of human interference will probably take place.

Of course, the problems of the West Solent and other parts of the Solent Estuarine System have been observed, and are probably very well-known. See in particular:

Hooke and Riley (1991) . They have discussed some changes with time in the West Solent (and other parts of the Solent). Their statement is important and it is the best summary of the situation then and now, when it is worse. They noted that:

"A major change which has affected much of the Hampshire Coast is the narrowing of the intertidal zone by the movement landwards of the Low Water Mark. This has occurred both on shallow-gradient, salt marsh coasts such as in the Keyhaven-Pennington area and on steeper, beach-lined coasts such as Lee-on-Solent. The changes have occurred at different times in different places and are therefore difficult to ascribe to a single cause such as a rise in sea-level. It is though that dredging, construction of sea defences, growth and decline of salt marsh plants and sea level rise could all have been contributary factors. The scale of change is enormous in places, the intertidal zone now being only a fifth of its width 100 years ago."

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LOCATION:

East Solent

A geological cross-section of the East Solent or Spithead is shown above, in comparison with a cross section of Southampton Water as revealed by the Fawley Transmission Tunnel. Perhaps modern techniques of civil engineering could enable construction of a tunnel to the Isle of Wight on this line, if that was desired and financed. The Fawley Tunnel, 14 feet in diameter is relatively narrow, being used only for electricity transmission lines. Technical difficulties in the 1960s included loss of compressed air up through the Bracklesham strata. This necessitated high pressure (35lbs per square inch) and a change from concrete casing to cast iron rings. The escaping air bubbled up to Southampton Water and in the process oxidised pyrite in the Selsey Formation so as to produce sulphuric acid. This exothermic reaction produced heat that affected the tunnel. The clays are very sandy and have sand lenses that are under pressure. These burst out during mining. The large septarian nodules in the tunnel working face had to be broken up by hand.

The Fawley Tunnel was dug manually and there were at least two fatalies during its construction (both at the West Shaft and airlock area). If a tunnel was now constructed from Haslar to Ryde (or nearby) on the section shown then the problems of the Fawley Tunnel would have to be dealt with by new methods. The deeper water of the Solent means that the tunnel would have to be lower and, if dug under compressed air, the pressure would presumably be higher. The miners would have to be under longer times in the decompression chamber than about the hour needed for the Fawley Tunnel.

The Haslar to Ryde section has the particular problem of the Barton Sands (Becton Sand). These are very permeable and will let in water or loose compressed air. The Becton Sand is not entirely sand and there clays included. It is very pyritic below the water table and produces jarosite (sulphate) mineral on cliff faces. If oxidised by leaking air it will produce corrosive, acid solutions. This also applies to Bracklesham strata. The best unit for tunneling is the Barton Clay, although even this is rather sandy and not like the London Clay cut through in the London Underground system. The Chama (pronounced Kay-mer) Member is notorious for quicksands on the coast and in the New Forest. It is a blue-grey, silty,clayey sand that is very unstable. "Running sands" have been reported in the Earnly Formation of the Southampton area.

A different site might be considered for a hypothetical tunnel to the Isle of Wight. An obvious place is the Lepe Beach to Gurnard line which has been investigated for a dual water pipeline by Halcrow. This pipeline has already been drilled into place. The pipeline was entirely in the Headon Hill Formation, but it was at very little depth beneath the sea floor. On this line, it might be possible to construct a tunnel avoiding the Barton Sand, but careful study of a cross-section would be needed, and it might not be possible. Another possible tunnel route to the Isle of Wight is from Lymington to Yarmouth. This has already been investigated in the past for a proposed railway tunnel. An early 19th century plan was to take a main railway route to Alum Bay and develop the area in a quite ruinous manner so that it became another "Brighton". Fortunately, this has not as yet happened, and probably never will. Students or others might be considering as an exercise, methods of constructing a route to the Isle of Wight, and this makes a good problem. A bridge from Lepe Beach to Gurnard, on the Isle of Wight, or, perhaps, a bridge from the end of the (now artificial) Hurst Spit to Cliff End, near Totland, might be discussed. They are probably both unlikely. Another possibility is closure of the Solent for tidal power at Hurst Spit (and further east - Lepe-Gurnard or Gilkicker-Ryde) and a road across, but this also seems unlikely in present circumstances.

LOCATION:

Southampton City - Introduction

The near-surface geology of the city of Southampton can be considered in terms of two major categories of deposits, the Eocene strata beneath and the Quaternary sediments at the surface.

The Eocene strata crop out in places but are often covered by Pleistocene gravel or other younger deposits. The Eocene sediments usually occur from about two or three metres down to considerable depths, and have been deposited about 40 million years ago. They are mostly of the Middle Eocene Bracklesham Group (formerly "Bracklesham Beds"), named after Bracklesham Bay, Sussex, where they are exposed on the coast. The Bracklesham Group is Lutetian and Auversian ("Upper Lutetian") in age. They mainly consist of green, glauconitic sands (weathering yellow-brown at the surface), grey sandy clays with lignite (plant debris) and clayey sands. Marine fossils, mostly the whitened, but well-preserved, shells of gastropods and bivalves, occur in these strata, particularly in the green, glauconitic sands. Sharks teeth may occasionally be present. The shells are usually destroyed by weathering near the surface and only found in excavations. Large septarian nodules of argillaceous limestone occur at certain horizons, mostly in the glauconitic sands.

The Quaternary deposits of Southampton include a variety of poorly consolidated, or even quite soft, sediments. These have been deposited in Pleistocene and later, Holocene, times. They are mostly fairly thin and just a few metres, although alluvial muds can reach 5 metres or more in thickness.

About 3m of Pleistocene gravel cover much of the Southampton area. This is brownish or whitened, gravel of subangular flint pebbles. It occurs as a series of terraces at various levels and of various ages, within the Pleistocene Epoch. The terrace deposits are fluvial sediments, formed in the periglacial conditions to the south of the ice sheets. The higher ones are older and the lower ones are younger. The youngest (Devensian) are submerged and lie under Southampton Water. The terraces have been numbered by the British Geological Survey with No. 1 just above sea level, and No. 3 under the higher parts of Southampton City. For detailed information refer to the British Geological Survey Map, 1:50,000, Sheet 315 and to Edwards and Freshney (1987), Geology of the Country Around Southampton, Memoir of the British Geological Survey.

Over the gravel in many places is "Brickearth". This is brown silt, resembling loess. It is usually only about half a metre or a metre in thickness. It forms particularly good agricultural land, because it has a good soil-like character and usually contains no pebbles. A brickearth outcrop may have determined the location of the orchards east of the Medieval town of Southampton (Hodson, 1975). Brickearth can be described as an uncemented, structureless, fine-grained superficial deposit. It usually considered to be of aeolian origin but might, at least in part be some type of overbank fluvial silt, or in places, of solifluction origin. At least 50% of the particles of which brickearths are composed have effective size diameters of 0.05mm. and 0.07mm (i.e. coarse silt to very fine sand) (Hodson, 1975). Clay mineral content (in an example from Chilling) is of illite, kaolinite and smectite. The distribution of brickearth on the geological map above is a little uncertain because of lack of exposures in the City area.

The alluvial deposits are quite varied, but mostly consist of muds and clays with peat, particularly the Neolithic Peat. They are of Holocene or Flandrian age (younger than 10,000 BP) and occur low in the valleys, often bordering the estuaries. Some are of estuarine and some are of fluvial origin. The latter type are often calcareous because both the rivers Test and Itchen flow from a Chalk hinterland, and thus have calcareous water (unlike New Forest streams). Calcareous tufa occurs in the river valleys above the estuary. Some is also present in the sediments as far south at the Western Docks but this has probably originated before sea level rose to the present position. At the time of deposition this area was part of the valley of the River Test rather than part of the estuary. In various places the alluvial deposits have yielded artefacts of Mesolithic, Neolithic and Roman ages.

The map above includes some modification resulting from borehole information (Hodson, 1975). Some notes on this are given below:

Boreholes at Chapel Pumping Station (S.U. 1131) and at Ryde Terrace (S.U. 1107) showed alluvium below made-ground (i.e. infill - reclamation) down to about 2.74m. below O.D. That is to say down to the present level of low spring tides at Southampton. Both these localities lie in an area that was formerly a harbour behind a curved spit (i.e. mirror image of old Portsmouth).

Borehole S.U. 1032 at the Terminus Railway Station revealed alluvium and shingle down to minus 1.52m. below O.D. and occupies a site shown by Crawford (1949) as being formerly salt marsh. A little to the south, just south of Canute Road, S.U. 980 an adjacent borehole pentrated the flint shingle of Chapel Spit shown a maximum thickness of 3.05m. of shingle down to low spring tide level. In Medieval times, before the urban development of this area, this base would have been round about 1m. or a little more above low spring tide level (because sea-level has risen at about 1 to 2mm. per annum). A shingle spit with a thickness of 3m. is not huge, but it was a significant feature for the estuary. In lateral dimensions it was almost of the size of Calshot Spit, and certainly larger than Hamble Spit. Development, rising sea-level and compaction, means that the former spit has disappeared under Platform Road and Canute Road. The contrast in level between Platform Road (shingle spit) and Queens Park (back spit marshland) gives some visual indication of the former feature.

Other boreholes provide some further detail. The borehole S.U. 1030 has indicated that the higher Pleistocene gravel extended a little further south than recognised on the older Geological Survey map but correctly shown on the new one.

LOCATION:

Southampton City - History - Flooding

There is an interesting record of coastal flooding problems at Southampton Saltmarsh in the reign of Henry VIII (King - 1509-1547). I thank Jeremy Greenwood for drawing my attention to this. It seems that the banks (and causeway?) of the Saltmarsh were eroded and broken through by storms. Sea water seems to have flooded the marsh, which presumably had already been at this early date closed off from the estuary for grazing purposes. The banks were probably artificial embankments (like those at Pennington and Keyhaven) on the eastern, Itchen side. What was the "causen" near our Lady of Grace? Presumably this was a causeway.

The letter follows:

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LOCATION:

Browndown (near Gosport)

Browndown and Gilkicker Point where the southeast end of Southampton Water meets the Solent. The relatively bare ground of Browndown consists of accumulations of gravel transported by longshore drift from the low cliffs of Pleistocene gravel around this area. Gilkicker Point further southeast (bottom) is underlain by a thick Quaternary gravel deposit on Bracklesham Group. There is a line of boreholes from here across to the Isle of Wight at Ryde, providing a good cross-section of the eastern Solent.

LOCATION:

Langstone Harbour, Eastern Solent

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LOCATION:

Cowes, Isle of Wight

Part of the Solent seen from above the Isle of Wight, looking northwestward. East and west Cowes are separated by the estuary of the river Medina which is on the line of a fault. The promontory south (to the left) of the ship consists of Solent Group with a capping of Pleistocene gravel. Queen Victoria's residence, Osborne House, is situated on the hill here. The low cliffs around this part of the Solent contain fresh and brackish water gastropods and bivalves in the Solent Group clays and sands and also charophyte oogonia (freshwater alga). Wootton Creek is the small estuary in the lower part of the image.

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LOCATION:

West Solent - General

The western Solent is the valley of the Solent river, which flowed from west to east, flooded in the Flandrian Transgression. This view is towards the west. Near the left of the aerial photograph is the coast of Christchurch Bay, ending to the southeast at Hurst Spit. The conspicuous dark feature near the bottom of the aerial photograph is Parkhurst Forest. Appearing rather murky in the middle distance is the Newtown estuary where Pleistocene vertebrate remains occur. Bison bones are particularly numerous near the mouth of the estuary and mammoth remains occur too.

For more on Hurst Spit and Hurst Castle see:
Hurst Spit webpage.

For more on Lepe Beach see:
Lepe Beach webpage.

LOCATION:

West Solent - Shingles Bank

The photographs above seem to show the Shingles Bank, a well-known bank of subangular flint shingle, that lies between Hurst Spit and the Needles. It is near low tide level and may be exposed from time to time.

LOCATION:

Lepe Beach, West Solent

Go to webpage on: Solent - Lepe Beach and Stone Point ?

Lepe Beach and Stone Points are localities of special interest for periglacial flint gravels of the Pleistocene, with an interglacial peat and mud deposit. Elephant remains have been found in this. There are small exposures of the Headon Hill Formation in this area. A dinosaur footprint in Purbeck stone from Dorset has been found here, and there are blocks of Purbeck Marble which have transported from the Isle of Purbeck and wrecked on the beach here.

LOCATION:

Beaulieu River Estuary

Go to main webpage on Beaulieu Estuary.

Go to:

The Geology of the Beaulieu River Estuary

Quaternary Deposits and Saltmarshes of Lymington - East (Lymington-Keyhaven Nature Reserve)

Please go to separate webpage:

Lymington, Keyhaven and the northern shores of the West Solent

LOCATION:

Hythe Saltmarshes, Southwest Bank of Southampton Water

- formerly the "Hythe Spartina Marshes" now (1917) with Spartina replaced by Halimione

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For introductory information go to:
Hythe to Calshot Marshes SSI [Site of Special Scientific Interest]

[example extract] "The upper shore levels within the SSSI comprise saltmarshes, which grade from monospecific cord grass Spartina anglica swards upshore to mixed marsh in which the dominant species are generally sea purslane Halimione portulacoides, saltmarsh grass Puccinellia maritime and Spartina. The marshes near Hythe are the location in which hybrids between the indigenous cord-grass Spartina maritima and the introduced north American species Spartina alterniflora were found in the 1860s."

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Go direct to Google Earth and find Hythe, Hampshire (southward, across Southampton Water from Southampton). At Shore Road, southeast of the built-up, small town, the undisturbed saltmarshes commence. Only a short stretch can be seen from Shore Road before it swings inland as Frost Lane. The saltmarshes continue on southeast to the oil refinery, as a nature reserve, without access for the public.

The 2017 image shows that neither the saltmarsh or the shell beach or chenier has changed much. There has only been some minor displacement of part of the shell beach within the Shore Road coastal stetch. This particular saltmarsh area has not been modified very much by wave action. This is because there is only a small fetch in this rather restricted area of Southampton Water. However, there may have been some, limited, landward movement of the shell beach, but that has not been studied in detail here.

(These were written earlier, about 2009, before a later phase of Spartina die-back in the British region).

The saltmarshes southeast of Hythe on the southwestern bank of Southampton Water are special conservation features. They have been listed as forming the Hythe Spartina Marshes Nature Reserve. This is (or was) a notable place for the historic development of Spartina anglica which has vegetated former mudflats and accumulated and bound silty sediment so as to produce supratidal saltmarshes.

As the old Hythe notice board, shown above, as it was, indicates, the original cordgrass of southern England was Spartina maritima. However, Dr. Bromfield discovered Spartina alterniflora, an American relative, here in 1829. It had probably been accidently introduced by a ship. Later, in 1870 an infertile hybrid, Spartina x towsendii, was discovered. Further evolution led to Spartina anglica, a distinct and fertile species. From the 1890s it colonised every estuary from Chichester to Poole. Because it was so successful as a means of turning mudflats into saltmarshes, it was exported around the world, including to Holland and China. It has had a significant role in reclamation, because Spartina have in many cases been later turned into industrial or grazing land.

The Hythe Spartina Marshes, seen here, are owned by the New Forest District Council and managed under lease by Hampshire Wildlife Trust. Apart from Spartina anglica these marshes also contain Sea Purslane (Atriplex portulacoides), which tends to grow at creek margins. Sea Aster is another common plant plant.

After a long expansive phase of growth, some decline of Spartina saltmarshes commenced in about the 1950s. Die-back was considered a major problem and a threat to existance of the saltmarshes. Examination both in the field and from aerial photographs seems to indicate that erosion and creek-enlargement may be more serious. When this reaches an advanced stage there may also be considerable die-back.

Aerial photographs, above, show how the saltmarshes at Hythe are being lost, mainly by frontal cliff erosion and partly by the widening of marsh creeks. As the low front cliffs (only a metre high) retreat, there is the accumulation of shell beaches. These are white accumulations of aragonitic, Cerastoderma (i.e. cockle shells - "Cardium") shells which, rest, on the seaward part of the saltmarsh surface. These were fairly patchy in 1968, but by 2007 have become better developed and, in the Shore Road stretch, have joined to form an almost continuous white ridge or shell chenier (the name is from the Mississipi area where there are similar features). The shells have been washed out of the tidal flats in front of the marshes.

The classic saltmarshes here Hythe may have lost on average about a metre per annum between 1968 and 2007, as shown in the comparison illustration above. They are, nevertheless, less affected by erosion and creek-widening here than to the south in the area of the oil refinery, Fawley Power Station and Calshot Spit (a place of major wave erosion), where the loss is quite severe. Eventually the saltmarshes on the southeastern bank of Southampton Water will probably return to mudflats. Spartina marshes will be very limited in area in Southampton Water. In the long term this southwestern shore will probably become somewhat similar to the northeastern shore and possess only eroding low cliffs and mudflats.

See also webpages showing saltmarsh erosion at:
the Beaulieu River Estuary, and
the Lymington - Keyhaven Coast

LOCATION:

Hythe Spartina Marshes - the Shell-Beach Cheniers

(see also later notes re Google Earth, above)

The seaward margin of the Hythe Spartina Marshes is characterised by whitish beach ridges of shells. Similar features occur in various parts of the world, including the Mississippi Delta where they are known as "cheniers". The cheniers at Hythe are moving landwards and joining to form more continuous ridges. This can be seen in comparative aerial photographs from 1968 and 2007 above. The 2007 aerial photographs are courtesy of the The Channel Coastal Observatory , National Oceanography Centre, Southampton.

The cheniers or shell beach ridges at the Hythe Spartina Marshes have been discussed by Quaresma in a thesis (2008) and in a paper by Quaresma et al. (2007). The shells are of Cerastoderma glaucum (or C. edule?), a common, shallow-burrowing, edible bivalve, known in Britain as the "Cockle". They live in the intertidal mudflats but the dead shells are eroded out and transported shorewards as bedload during the early flood tide. Shell transport rates can reach up to 0.7m. per tide over the upper mudflat, according to Quaresma et al. Shell-induced erosion can become an important parameter on morphological changes of a coastal area. Shells are transported over the mudflat, deposited at the base of the cliff, and eventually transported over the salt marsh where they form the extensive chenier deposits, shown above. The Cerastoderma shell cheniers move over the saltmarshes only during storms, but at such times they can move up to 3 metres.

Shell transport and deposition over the salt marsh surface at Hythe can contribute to vegetation deterioration, as shell deposits migrate over the marsh surface. As a result, the seaward limit of the salt marsh shows deteriorated vegetation, as shown in the photograph above, which leads to scouring and consequent surface lowering. Quaresma et al concluded that the morphological characteristics of Hythe intertidal area appear to be controlled by a combination of prevailing hydrodynamic conditions and shell transport over the salt marsh.

With regard to cheniers, see also the next section on Fawley Power Station.

LOCATION:

Fawley Oil Refinery

LOCATION:

Fawley Power Station, Southampton Water

FOR MORE ON FAWLEY POWER STATION GEOLOGY - please go to:

Fawley Power Station webpage.

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LOCATION:

Calshot Spit

This is divided into sections such as Bracklesham Group, Radionuclides, Saltmarshes etc. and is more complete. Some topics such as material relating to the Isle of Wight are only covered briefly and full lists are in other bibliographies (although not necessarily available here).

SUPPLEMENTARY NOTES

Map of the Solent River Buried Valley

This is just a small scale map showing the general features of the Pleistocene valley under the Solent Estuarine System based on the borehole data referred to below and geophysical work. The original Pleistocene map for Southampton Water is much more detailed with contours at 10 feet (3m.) intervals and is on the scale of 1:10,000. A structure contour map of the Bracklesham strata , based on the variolarius Bed, has also been produced for Southampton Water.

Records of more than 1,400 boreholes

ACKNOWLEDGEMENTS

I very much appreciate discussion in the field with Toru Tsuzaki and on many occasions with Professor Yining Chen, who has been particularly helpful. I am grateful to Professor Dorrik Stow and his students for study and discussion on the coastal sections at Hill Head, Chilling and Brownwich Cliff. Keith Talbot has provided invaluable and enthusiastic help at Lepe Beach. I very much appreciate the receipt of helpful historic information from Jeremy Greenwood. I am particularly obliged to the staff of the The Channel Coastal Observatory , Southampton, for kind permission to use their excellent aerial photographs.

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