West, Ian M. 2015. Solent Estuaries - Introduction: Geological Field Guide. Internet site: www.southampton.ac.uk/~imw/Solent-Introduction.htm. Version: 9th January 2015.
Solent Estuaries Geological Field Guide .

Ian West,
Romsey, Hampshire
and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,
Webpage hosted by iSolutions, Southampton University

Aerial photographs by courtesy of The Channel Coastal Observatory
Website archived at the British Library

|Home and List of Webpages |Field Guide Maps and Introduction |Solent Geology Bibliography - General |Solent Geology Bibliography - Topics, Alphabetically. |Beaulieu River Estuary |Calshot Spit and Stanswood Bay. |Solent - Chilling Cliff, Brownwich Cliff and Hill Head |Lymington - Keyhaven Coast, West Solent | Hurst Spit |Isle of Wight Introduction |Isle of Wight Bibliography |Solent - Lepe Beach and Stone Point |Whitecliff Bay, Isle of Wight |Alum Bay, Isle of Wight |Geology of Fawley Power Station |Geology of the New Forest

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A satellite overview of the Solent Estuaries, southern England, late 1990s

A view of the Spithead, Solent Estuaries, from the Spinnaker Tower, Portsmouth, 4th October 2012

Portsmouth Harbour, looking NNW from the Spinnaker Tower, Portsmouth, Solent Estuaries, southern England

Coast of the Solent and Isle of Wight in 1693, southern England,based with simplications and some interpretation on Collins' chart

Calshot Spit, Southampton Water, southern England, aerial photograph courtesy of the Channel Coastal Observatory, a recurved shingle spit

The end of Hurst Spit, Hampshire, with the castle, seen from the Solent to the NE

The view across the narrow entrance to the West Solent and to Hurst Castle and Hurst Spit, from Totland Bay, Isle of Wight, September 2011

Unusual channel of gleyed argillaceous sand in the Pleistocene gravel of Brownwich Cliff, northwest of Hill Head, Hampshire, Solent coast

The 1979 Marchwood Geothermal Borehole, on the eastern shore of Southampton Water, Hampshire

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.

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Topographic and Geological Maps

A generalised geological map of the central English Channel from the Isle of Wight to the Cherbourg Peninsula

An old, topographic map of parts of the New Forest and the Solent Estuaries, southern England, probably from about the 1920s

Hampshire Geology Map

Simplified geological map of the Hampshire Basin around the  Solent Estuaries, southern England

Old geological map of the Southampton Water, Hampshire, southern England

Simplified geological map of the New Forest, showing part of the Solent Estuarine System

Late Victorian geological map of the Lepe area, Hampshire

Sediment transport within the Solent Estuarine System, based on Dyer (1980)

Modified composite of old geology maps showing Brownwich Cliff, northeast of Hill Head, Hampshire, Solent coast

Location map for the coastal exposures of Chilling Cliff and Brownwich Cliff, northwest of Hill Head, Hampshire, Solent coast

Location map for the New Forest and Southampton Water

Southeastern New Forest, 1924 map

Old geological map, I.O.W.

Coast of the Solent and Isle of Wight in 1693, southern England, based, with simplications and some interpretation, on Collins' chart

A 1740 map of Beaulieu River estuary, Hampshire, and adjacent New Forest and Solent areas

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Palaeogene (Tertiary) Succession

The Palaeogene or Tertiary succession in the  Solent Estuaries, southern England


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The Solent River System of the Pleistocene

A map of the hypthetical Solent River System, present during part of the Pleistocene, when sea-level was low, Solent Estuaries, southern England, based with modifications on Westaway, Bridgland and White, 2006

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.

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West Solent


The cargo ship, Hoegh Osaka, developed a major list and was beached on the Brambles Bank, between Calshot Spit and Cowes, Isle of Wight, 4th January, 2015


The sea area of the Brambles Bank, with the West Solent beyond, seen from the Lee-on-the-Solent beach just after sunset, with the formerly beached ship, the Hoegh Osaka, in the middle distance, and part of the Isle of Wight in the left distance


The sediments of the West Solent, between the northwestern part of the Isle of Wight and the Hampshire mainland, redrawn with modifications from a BGS sea-floor map

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.

[section to be expanded]


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East Solent

Cross-sections of Southampton Water and the East Solent, with exaggerated vertical scale

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.

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Southampton City - Introduction

Southampton Walls and West Quay area in late Victorian times, Southampton, Hampshire

Geological map of the Southampton City area, Hampshire

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.

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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:

".. Henry VIII., March 22; Manoir of Grenewiche. Letters under Henry the Eighth's sign manual and signet to the Mayor and his brethren of the town of Suthampton. Trusty and Welbeloved We grete you wele, And Forasmoche as We be enfourmed that a marshe called the saltmarshe parcell of our Towne of Suthampton lyeth open uppon the see coost, and that the bankes thereof aswell as the causen nigh our Lady of Grace as in diverse and many other places of the same be worne wastyd and by stormes of the see consumed, by reason whereof the salt-water of late hathe entrid and hereafter is like to enter, Not onely to the lost and destruccion of that marshe but also by contynuance shall torne us our Towne and subgiettes thereof to grete lostes noyaunce and damages sundry wise, oneles spedy remedy in that behalf be founde, the occasion whereof is and hath bene for that the said marshe heretofore hath bene put to no good use, Whereby profyt might yerely growe for defence of itself ayenst the see, to our grete marvill and discontentacion, We specially tendring the comon welth of our said toun and to see this mater refourmed accordingly, Will therfor and commaunde you that incontinent upon the sight hereof ye not onely cause to be repeyred and amended the said bankes for the saving of that marshe but also that ye take into our handes all the said marshe putting it to suche use and order that the profyt thereof commyng may yerely defende itself agenst the see withoute any ferther charge to us or our Towne hereafter. And yf there be any persone or persones repynyng hindering or letting you or any of you for the accomplisshement of this our commaundement, that thene ye certefye us and our Counsaill there names with diligence as ye will have our favour. Ferthely We be enfourmed of dekey and povertie of that our towne whiche We and our Counsaill do studye to relyve comfort and entende to cause the same to be better inhabited and replenyshed with people, And soo We will and commaunde you to doo for your partie, And that ye preferre the co men weale of the vytelers craftymen and other inhabitauntes of that Towne befor foreyne, any acte to the contrarye notwithstanding, Whereby other shalbe encoreged to inhabite theim self among you, And also that ye see that no Ryot be used ne no idill persounes remayne amonges you, but that they be put to sundry occupaciouns or punysshed according to our lawes, Letting you wete that in the executing of the premisses or any other thing that is to the weale and comfort of that our towne ye shall not onely doo thing to the pleasur of god but in the same manner minister unto us right good and acceptable service encoragyng us by the same to helpe you fourth accordingly. Yeven under oure signet at oure Manoir of Grenewiche the xxijth day of Marche."[ unknown year]

From: 'The corporation of Southampton: Letters and loose memoranda', The Manuscripts of the Corporations of Southampton and Kings Lynn: Eleventh report, Appendix; part III (1887), pp. 97-134. URL: http://www.british-history.ac.uk/report.aspx?compid=67101&strquery=lymington

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The Marchwood Geothermal Borehole, 1979-1980

- Down to the Budleigh Salterton Pebble Bed

The 1979 Marchwood Geothermal Borehole, on the eastern shore of Southampton Water, Hampshire

The Marchwood Geothermal Borehole, seen with drilling in progress on a winter's evening, probably in December 1979

Drilling work at the  Marchwood Geothermal Borehole in 1979 continues into the night

Marchwood No.1 and Southampton No.1 borehole logs

A simplified, summary log, with notes, of the 1979, Marchwood Geothermal Borehole No. 1, at the former Marchwood Power Station, Hampshire, near Southampton Water and the New Forest

The uppermost part of the succession in the Composite Log of the 1979-1980, Marchwood Geothermal Borehole No. 1, redrawn by Ian West with some additional notes

The Middle Jurassic part of the succession in the Composite Log of the 1979-1980, Marchwood Geothermal Borehole No. 1, redrawn by Ian West with some additional notes

The Sherwood Sandstone and part of the Mercia Mudstone sequence of the 1979-1980, Marchwood Geothermal Borehole No. 1, redrawn by Ian West with mofications and additions notes

The top of the Budleigh Salterton Pebble Bed is marked by reg or hammada surface with dreikanter or ventifacts, and then the Yellow Bed, west of Budleigh Salterton, Devon, UK

The Budleigh Salterton Pebble Beds at Budleigh Salterton, Devon, with sedimentary structures pointed out by Dr. Alison Jones, 1995

The Marchwood No. 1 Geothermal Borehole was commenced on the 8th November 1979 and completed on the 27th February 1980. It preceded the other geothermal well of the area, which was Southampton No.1 Borehole. That was started on the 26th September 1981, and the well head is located in central Southampton north of the West Quay centre.

Thus the Marchwood well was the first geothermal experiment in the region and a drilling rig was put in place so as to be capable of drilling to the Total Depth (T.D.) of 8580 feet (2615m.). The Marchwood and Southampton area is part of a region of high heat flow through the underlying strata. At Southampton the rate is 59 milliwatts per square metre compared to 40mW/m 2 for more northerly parts of Hampshire (Edwards and Freshney (1987). This is almost a 50% increase, and seemed worth exploiting. I was at the Geology Department of Southampton University at the time and followed, to some extent, the progress of both wells. The initial hope was that the Sherwood Sandstone reservoir, a porous red, Triassic sandstone exposed in the Sidmouth to Budleigh Salterton area, would contain hot water at about 100 degrees Centigrade. The usual geothermal gradient is about 30 degrees per kilometre, so that without considering the details, a temperature like this would be expected at round about 3 kilometres. Thus, if the Sherwood Sandstone had been near the total depth of the well, about 2600m, water with a temperature near that of boiling water might have been obtained, and this could have been useful. It was intended to use the heat produced to supplement the energy supply for the Marchwood, coal-fired Power Station, to which the well was adjacent.

However, the Marchwood Borehole, did not produce hot water at this temperature. We shall consider, below, why the temperature was lower than expected. A further complication was that a year or so after the drilling, and for other reasons, the Marchwood Power Station was demolished. Thus, there was no real use nearby for the warm water. However, in spite of the lack of commercial success with regard to geothermal energy, the borehole was certainly a successful research venture. It provided much information on potential oil reservoirs in the area, and particularly helped the understanding of the Bridport Sands, the Upper Reservoir of the Wytch Farm Oil Field.

The Sherwood Sandstone of the Trias was the selected target for hot water. This is because it was the deepest formation in the area that was expected to be sufficiently porous and permeable. This was proved to be the case. An older sandstone, the Devonian Old Red Sandstone lies lower in the borehole sequence, but it is tight, that is, non-porous and non-permeable from a practical point of view. The Sherwood Sandstone can transmit water from a considerable distance and is recharged up-dip in Devon. It should be appreciated, though, that the water is saturated brine. This is because the Trias contains substantial salt deposits in places and some evaporites were proven in the Marchwood Borehole. Even if hot water is pumped from a well it cannot be used directly but has to be directed through stainless steel heat exchangers with the aid of downhole, stainless steel pumps. The brine may also contain heavy metals in solution, and has to be discharged somewhere. The ideal is to have two adjacent boreholes, a production borehole for producing hot water and an injection borehole for disposing of used brine. There was no second borehole at Marchwood. At Southampton Geothermal Well there is only one borehole but the used brine and any associated trace element content is discharged into Southampton Water (already a polluted estuary). Thus the matter is not necessarily simple.

The British Government department, BERR, have commented that the geothermal aquifer resource within the Wessex Basin, under the Bournemouth area, appears to be the most attractive for possible future exploitation, but the commercial risks of speculative drilling remain high. At 3.5p/kWh or more, the cost of heat from the aquifer resource is still significantly higher than that from conventional industrial boilers (approximately 1.44p/kWh). See also Rollin, for more information on low temperature geothermal energy in the UK, including the Wessex Basin.

Returning to specific matters regarding the Marchwood No. 1 borehole, there was, as mentioned above, a problem regarding depth of the geothermal aquifer. The part of interest is the succession from the Lower Greensand downward. It is well-known that there is usually, in this region, a Pre-Albian unconformity at the base of the Gault, and the Basal Gault Pebble Bed is an indication of this. At the base of the Gault in the Marchwood Borehole (depth - 2240ft. - 682.8m.) , the pebble bed does not seem to have been identified (there was coring in the Gault but without recovery). Under the Gault is a very thin sequence of Lower Greensand. It is from 2240ft. to 2267ft.,only 27ft. or 8.2m (compare to the southwestern Isle of Wight where it is, at Atherfield, about 800ft. or 243m.). There is an unconformity, as is common, at the base of the Lower Greensand. It is lying directly on top of Kimmeridge Clay. The Wealden, which can reach more than 600m in the subsurface, is missing. So too is the Purbeck Formation which reaches about 120m. in Durlston Bay. The Portland Stone and Portland Sand is missing. These units can reach about 70m in total thickness. Thus if about the greatest thicknesses could have present, then the Sherwood Sandstone would have been about 790m. lower, although this would have been the extreme. If you add this to the base of the Sherwood Sandstone in Marchwood No. 1 then you obtain 2515m, just a little less than the total depth of the well. Of course the unconformity present means that some of the Kimmeridge Clay is probably missing, and in any case, the Lower Greensand is abnormally thin. A large drilling rig, shown above, was chosen and the plan for drilling the Marchwood Geothermal Borehole was appropriate to deal, if necessary, with a thick Jurassic-Cretaceous sequence including Wealden, Purbeck and Portland. This was a sensible plan, and a reasonably thick sequence was expected so that the Sherwood Sandstone would prove to be deep and hot. It was therefore a disappointment when the unconformity at the base of the LGS was discovered. It was thus possible to use the spare capacity of the drilling rig to penetrate the Devonian and to gain an understanding of the strata below the economic basement (i.e. below the base of the Permo-Trias). So that is why so much Devonian has been drilled. A follow-up to this matter was a similar problem at Southampton No.1 Geothermal Borehole. A fault between the two sites might have led to a deeper succession and higher temperatures, but, in fact, the situation proved to be similar to that at Marchwood. However, at Southampton the geothermal energy could, and is, being put to use (for central heating at Southampton Civic Centre and the South Hants Hospital).

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Southampton No.1 (Western Esplanade) Geothermal Well, 1981.

In addition to the Marchwood Borehole, there is another deep borehole nearby which provides useful information on the geological succession and lithologies. It has provided information relevant to petroleum exploration in addition to its primary purpose in providing geothermal energy and data on geothermal prospects. This well is the Southampton No.1 Geothermal Borehole. It was drilled the year after the Marchwood No.1 Geothermal Well so that this was the second borehole in the area for geothermal energy. The location is Western Esplanade and the well-head is easily seen by parking in the car park of Toys-R-Us and Babies-R-Us, northwest of the West Quay Centre, Southampton.

Southampton No.1 (Western Esplanade) Geothermal Borehole, during the drilling in 1981

Publication on the Southampton No. 1 Geothermal Well: Geological Well Completion Report, 1982 - available from British Geological Survey

Summary Well Data (after Thomas and Holliday, 1982)):

Well Name: Southampton No.1 (Western Esplanade)
Location: Western Esplanade, Southampton
National Grid Reference: SU 41559 12018
Latitude: 50° 54' 24''
Longitude: 1° 24' 33''
Hydrocarbon Exploration Licence: Amoco/Ultramar XL 138
Client and Operator: Department of Energy and Institute of Geological Sciences (now British Geological Survey, BGS)
Drilling Contractor: Kenting Drilling Services Ltd.
Rig: Kenting No. 12.
Spud Date: 26 September 1981
TD Reached: 20 November 1981
KB Elevation: 24.92 ft. AOD
Ground Level: 10.96 ft. AOD
Terminal Depth (TD): 5994 ft. below Kelly Bushing
Hole Diameter (depth below KB)
26 inch to 165 ft.
17.5 inch to 3005 ft.
12.5 inch to 5994 ft.
Casing and completion depths below KB:
20 inch casing to 158 ft.
13 and 3/8 inch casing to 2995 ft.
9 and 5/8 inch liner was hung from 2826 ft. to 5635 ft.
Cores - six cores were taken with total core length of 226 ft. See Thomas and Holliday (1982) for further details of coring.
Wireline logging: Schlumberger Inland Services Inc., Tessel Well Services Ltd.
Mud Logging: Exploration Logging (UK) Ltd.
Drilling Superintendent: Mr. D. McIntyre, Drilcon Ltd.
See Thomas and Holliday, (1982) for further information on the well.

A summary log of Southampton No.1 Geothermal Well (borehole), Southampton, Hampshire, 1981

Formation tops data for Southampton No.1 Geothermal Well, 1981, based on Thomas and Holliday, 1982

The well head of Southampton No.1 Geothermal Borehole, Southampton, Hampshire, July 2008

Simplified diagram showing the Southampton Geothermal Well in use and supplying heat energy to the Southampton City Geothermal and CHP Scheme

The power station at West Quay, Southampton, Hampshire, which processes hot brine from the geothermal well in the Sherwood Sandstone of the Triassic

Marchwood No.1 and Southampton No.1 borehole logs

In many respects Southampton No.1 proved to be similar to Marchwood No.1 and most of the formations are similar and at similar depths. A slight difference is the occurrence of Portland Group beneath the sub-Lower Greensand unconformity. At the time it was hoped that, a sub-Albian fault occurring between the two sites might have led to a significantly deeper level for the Sherwood Sandstone at Southampton, and, consequently, higher temperatures. However, in fact, the situation proved to be similar to that at Marchwood and the downhole brine temperature was at about 76 degrees C. A better figure near 100 degrees might have been obtained had the Cretaceous sequence been without major unconformity and was as thick as on the southern Isle of Wight and at Swanage Bay, Dorset (i.e. if, therefore, the Sherwood Sandstone was about another half kilometre or one kilometre lower). The hot rather than boiling temperature was accepted as satisfactory, and this time, unlike Marchwood, the wellbore was not continued deep into the Devonian (Old Red Sandstone) and merely went into the top part of it before TD'ing.

In spite of the moderate, rather than high temperature, the hot brine could be used at Southampton to provide geothermal energery for central heating. The heat has been used to some extent for local buildings, such as Southampton Civic Centre, West Quay Centre and the Royal South Hants Hospital. The brine is brought to the surface by stainless steel, downhole electric pumps. It is not only saturated for sodium chloride and therefore corrosive, but it also contains very small quantities of undesirable trace elements, heavy metals. The brine is thus not usable directly for central heating but is processed through stainless-steel heat-exchangers before being discharged into Southampton Water (ideally there should be another borehole into which the used brine is discharged downwards, but there is not another borehole at Southampton). Thus, the hot brine can usefully provide geothermal energy, but it is a complex matter and is not cheap or easily. In practice the geothermal energy is used in combination with heat from fossil fuel sources at the power station near the West Quay Centre (note the conspicuously red building).

Pumping is from a downhole turbo pump at about 600m., and at about the level of the Gault and the Lower Greensand. The geothermal aquifer is the Sherwood Sandstone which is much deeper ranging from 1729m. to 1767m. Thus the brine is rising under hydrostatic pressure for about two/thirds of the height, and is pumped up for the last part.

The pumping scheme is explained in basic form in a large colour poster, kindly provided by Utilicom and Southampton Corporation. At the well head, shown above, there is an electrical pump which forces a certain proportion of the brine back down a central pipe so as to provide motive power for the downhole turbo pump. This long, narrow, fluid-driven turbine is fairly deep in the borehole and it pumps the hot brine up an outer concentric pipe to the surface (together with the recycled brine used for the power to drive it). The equipment has to be made of stainless steel to avoid corrosion from the brine.

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Coast Northwest of Hill Head, including Chilling Cliff and Brownwich Cliff.

Recent rapid increase in erosion at Chilling Cliff near the Brownwich valley, northwest of Hill Head, Hampshire, Solent coast

A general view of Brownwich Cliff, northeast of Hill Head, Hampshire, Solent coast, looking northwestward, up Southampton Water

Please go to the webpage: Solent - Hill Head to Hook

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Hill Head

Accretion of beach shingle at Salterns Park, Hill Head, Solent, Hampshire, 27th July 2009

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Browndown (near Gosport)

Browndown and Gilkicker Point from the air

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.

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Langstone Harbour, Eastern Solent

Langstone Harbour at dusk, looking northward from the entrance, Solent Estuarine System, 11th September 2008

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Cowes, Isle of Wight

Cowes and the river Medina from the air

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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West Solent - General

The end of Hurst Spit, Hampshire, with the castle, seen from the Solent to the NE

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.

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West Solent - Shingles Bank

A cruise ship passes the Shingles Bank, at the entrance to the West Solent, southern England

A small island formed by part of the Shingles Bank, near the Needles, Isle of Wight, seen from Barton-on-Sea, January 2011

 The exposed Shingles Bank at the entrance to the West Solent, seen from Milford-on-Sea, 28th January, 2011

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.

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Lepe Beach, West Solent

View from just east of Lepe Beach, Hampshire

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.

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Beaulieu River Estuary

Dieback of Spartina saltmarsh near the main channel, west bank of the Beaulieu River estuary, Hampshire, opposite Exbury, 22nd July 2008

Go to:

The Geology of the Beaulieu River Estuary

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

Shell Island, a relict area of Spartina saltmarsh, south of Lymington, West Solent, Hampshire, seen at high tide from a boat on a breezy day, August 2008

Please go to separate webpage:

Lymington, Keyhaven and the northern shores of the West Solent

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Hythe Spartina Saltmarshes, Southwest Bank of Southampton Water

Part of a topographic map showing the location of the Hythe Spartina Marshes, Hythe, Southampton Water, Solent Estuaries, southern England

Hythe Spartina Marshes, Hythe, Southampton Water, southern England

Hythe Spartina Saltmarshes, seen from the northwestern end at Shore Road, Hythe, Hampshire, in October 2006, and progressively narrowing over recent years by frontal erosion

Spartina saltmarshes at Hythe, Southampton Water, seen in a modified version of an old aerial photograph of 1968 Spartina saltmarshes at Hythe, Southampton Water, shown in a modified version of an aerial photograph of the Channel Coastal Observatory, 2nd June 2007 - observe saltmarsh erosion since 1968 photo

Comparison of aerial photographs of Hythe Spartina Marshes from 1968 to 2007, Solent Estuaries, southern England

The cordgrass saltmarshes southeast of Hythe on the southwestern bank of Southampton Water are special conservation features. They form the Hythe Spartina Marshes Nature Reserve. This is 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 Hythe notice board above 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

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Hythe Spartina Marshes - the Shell-Beach Cheniers

Landward-moving cheniers, or shell beach ridges, at Hythe Spartina Marshes, Solent Estuaries, southern England, January 2009

The northwestern end of the shell-beach cheniers at Hythe Spartina Marshes, Hythe, Solent Estuaries, southern England

Unbroken aragonitic, shells of a chenier or shell-beach ridge on saltmarshes, Hythe Spartina Marshes, Solent Estuaries, southern England, January 2009

Broken Cerastoderma and Littorina shells of an aragonite shell ridge, or chenier, Hythe Spartina Marshes, Solent Estuaries, southern England, 30th January 2009

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.

Deterioration and erosion of saltmarsh seaward of a shell-beach chenier, Hythe Spartina Marshes, Hythe, Southampton Water, Solent, southern England

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.

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Fawley Oil Refinery

Fawley Oil Refinery seen across Southampton Water from Chilling, Solent Estuaries, southern England, 2011

An old aerial photograph of the southwestern side of Southampton Water, with Fawley Oil Refinery, probably from about the 1970s</A>

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Fawley Power Station, Southampton Water

An old topographic map, 1936 with roads updated to 1947, of the Fawley and Calshot area, Solent Estuaries, southern England

Fawley Power Station, Southampton Water, Solent, 21 January 2006

Washover Fans of Cerastoderma shell beach debris on the degrading Spartina saltmarshes, in front of Fawley Power Station, Southampton Water, southern England, 1st August 2008

Geological cross-section of the Fawley Tunnel, Southampton Water, southern England


Fawley Power Station webpage.



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Calshot Spit

Calshot Spit, Southampton Water, southern England, aerial photograph courtesy of the Channel Coastal Observatory, a recurved shingle spit

A location map for Calshot Spit, Fawley Power Station, Quaternary geology and the locations of cross-sections shown, Solent Estuaries, southern England

FOR CALSHOT - see also the webpage on:

Calshot Spit and Stanswood Bay



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Bibliography of the Geology of the Solent - General Section - Undivided This is a list of Solent Estuarine System references, not separated under topics. It is not comprehensive and the user is referred to the continuation - the alphabetical section below for more complete coverage .

Bibliography of Geology of the Solent - Topics Alphabetically

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).

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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.

Borehole Data - Solent Estuarine System

Records of more than 1,400 boreholes

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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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| Home and Contents |Field Guide Maps and Introduction |Solent - Hill Head to Hook | Hurst Spit, Barrier Beach of the West Solent |Isle of Wight Introduction |Isle of Wight Bibliography |Solent - Lepe Beach and Stone Point |Whitecliff Bay |Alum Bay |Solent Bibliography, General - General |Solent Geology Bibliography - Topics, Alphabetically |Fawley Power Station - geology

Copyright © 2015, Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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Dr Ian West, author of these webpages

Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.


at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.