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Home , Calshot, Calshot Castle, Calshot Spit, Southampton Water

Palaeo-environmental Study Area P8 - Fawley, Solent region, south , UK

PALAEO-ENVIRONMENTAL STUDY AREA P8

CALSHOT - FAWLEY, SOLENT REGION, SOUTH COAST, UK

OBSERVATIONS ON THE CALSHOT-FAWLEY SECTION OF STUDY AREA

Plate P8 in the Solent Estuary

1 Palaeo-environmental Study Area P8 Calshot - Fawley, Solent region, south coast, UK

During the LIFE appraisal of the Solent region the Calshot-Fawley coastline was noted as area of particular interest by virtue of its geomorphology and palaeo-environmental history. Calshot Spit is a distinctive feature of the ria coastline of the Solent estuarine system (West 1980). It is set at the mouth of Water, a major arm of the drowned valley system. The spit is composed of accrued sand and shingle and it effectively constricts the mouth of by projecting outwards from its western shore. The ‘reach’ of this projection now appears to be about 0.9km which is about a third of the width of the mouth of the estuary, but the actual length of the spit is nearer 1.3km, This difference has been brought about by artificial infilling of a near-shore band of the saltmarsh during the construction of in 1962-66. This has had the effect of shortening the apparent length of the spit.

This destruction of the original shoreline had been a further step in a series of local reclamations which had been started by the construction of a neighbouring oil refinery between 1940 and 1970. Tubbs (1999) cites this and earlier reclamations as part of a serious diminution of natural habitats on the intertidal margins of Southampton Water. Here, some 690 hectares, amounting to 40% of the shoreline, had been lost to infilling and dredging since 1840. Tubbs (ibid.) also observes that cartographic evidence shows that the drying line within the Western Solent and Southampton Water has noticeably retreated in a shoreward direction since at least the 18th century. The erosion of these mudflats has involved the release of an enormous volume of sediment from the intertidal zone during the 19th and 20th centuries. Much of these estuarine clays and silts had been formerly blanketed and bonded by thriving colonies of Spartina grass but since the diminution of this species in the Solent, the loss of these deposits has been exacerbated. What happens to this material after its erosion is not currently understood.

On its northern or up-stream side, the spit is abutted by the Fawley salt marsh where a diverse intertidal habitat still survives. This is now a designated nature reserve. Tubbs (ibid.) considers that the role of shingle spits as protectors of these habitats will become increasingly important as more exposed saltmarshes are diminished by the loss of their Spartina bondings and the advance of wave attack. On its southern or seaward side the spit is abutted by a shingle-strewn intertidal zone. This thins to an ancient peaty land surface and traces of submerged woodland in the sub-tidal zone.

The relationship of Calshot Spit with the present and past geomorphology of the coastline is a factor of particular interest to this LIFE study. A little historical and archaeological evidence is available to demonstrate that the configuration of the spit has seen very little change at least since the late 16th century when a large stone gun battery, known as , was constructed by order of Henry VIII. This battery stands in a relatively vulnerable position at the outmost tip of the spit where it is set on a base of consolidatedn and and shingle. Despite this relatively vulnerable setting the castle seems to have experienced no serious threat from spit reconfiguration during of past 500 years of its life.

While the configuration of the Calshot Spit appears to be relatively stable the origin and nourishment of this feature is still poorly understood. This currently impedes any assessment of its future sustainability. The sand and shingle components of the spit are acquired by movement up its southern or seaward face but the off-shore processes responsible for this accretion are complex and uncertain. Tubbs (1999, fig 3.4) identifies the material of the spit as part of a longer shingle strand which can be traced some 4km to the southwest along the shore of Stanswood Bay and into the eastern mouth of the West Solent . He also observes that sediments appear to move into the Solent through both Hurst Narrows and Spithead and that the pattern of sediment movement can be partly deduced from the disposition of shoreline and marine features such as spits and bedforms.

In the West Solent the bed of the main channel is largely composed of waves of gravel and coarse sand. These are predominently shifting in an easterly direction while being interrupted by various recirculating eddies. This channel of the Solent estuarine system experiences the highest current velocities and these appear to be responsible for the active resorting and movement of the bedload. This material offers a suitable source of shingle for the sustaining of

2 Palaeo-environmental Study Area P8 Calshot - Fawley, Solent region, south coast, UK

Calshot Spit but east of Newtown Harbour and the Solent Bank its easterly movement is poorly understood. The Solent Bank is a location where commercial gravel dredging has formerly taken place.

Where the Western Solent meets the Eastern Solent off Cowes, the bedload is still heavily dominated by shingle. It is considered that some of this material shifts in a south easterly direction coming to rest in the Prince Consort Shoal which lies off the mouth of the Medina river at Cowes. A second lobe reaches to the north east, passing along the sub-tidal and the intertidal margins of Stanswood Bay where it seemingly provides the raw material for the sustaining of Calshot Spit.

In contrast, the present bed of the Eastern Solent is dominated by sands and silts which are inappropriate for the nurturing of a substantial shingle spit such as Calshot. Nevertheless, both Southampton Water and the Eastern Solent are lined with extensive deposits of earlier gravels which lie on the floor of the old Pleistocene river valley. Modern gravel beaches are to be found on the eastern shore of Southampton Water and these traverse the coastline which is immediately opposed to the projection of Calshot Spit. Extensive shingle beaches are also to be found on the northern shore of the Eastern Solent. These amount to an extensive tract of more than 30km on the open coast of and West , extending from to West Selsey.

While long tracts of beach head gravel have been noted on the eastern shore of Southampton Water and on the northern shore of the Eastern Solent, the present evidence favours the Western Solent as the source of beach material for Calshot Spit. This assumption is based upon the physical continuity of the spit with the beach shingle in Stanswood Bay and the sub- tidal continuity with the gravel bedload of this channel. This concords with the view of Hodson and West (1972) who drew similar conclusions based upon the easterly drift of longshore sediment.

The historical evidence offered by cartographic sources and the construction of the Tudor castle suggest that spit accretion at Calshot has been extremely slow. This means that the past and present behaviour of the spit might be used as an index of past environmental conditions in the Solent estuarine system. This opportunity was readily grasped in 1972 when major engineering works at Fawley power station required the drilling of a series of boreholes through the Holocene sediment archive which lay in the lee or cusp of the spit (Hodson & West, ibid).

The Fawley boreholes traced the sediment Water down to a depth of some 25m where the floor of the old Pleistocene river valley was lined with a train of old terrace gravel. Beneath Calshot Spit, at a depth of -21m OD, the Pleistocene gravel was found to overlay a thin deposit of Holocene of sand and shell (bed A). This deposit offered the first evidence of a marine incursion into the estuarine system. At this particular location the event seems to have been short-lived for after this deposit had accrued to a height of about 0.6m the environment changed to more terrestrial conditions in which clay and peat began to form (bed B). By analogy with a further series of boreholes set 800m to the north this deposit seems to represent sheltered marsh conditions.

At a level of -16m OD the marsh conditions at Calshot had been overwhelmed by the deposition of spit sand (bed C). The two most southerly boreholes were able demonstrate that this deposit had continued to accrue at this location in an unbroken sequence until the present day. Where boreholes were sunk in an 800m line running north of the spit some significant variations could be detected in the stratigraphy. In the more northerly locations the estuarine intertidal deposits had continued to accrue in tandem with sea-level rise while the spit had persisted in its growth. These deposits showed that for most of this time mudflats and saltmarsh had continued to develop in the sheltered lee of the spit but on two occasions the spit had either shifted or expanded to the north depositing a broadened spread of gravel.

At the time of the investigations at Fawley, in 1970, the opportunities for absolute dating seem to have been somewhat restricted and despite the presence or organic remains in beds B & C no radiocarbon dates were obtained from these deeper horizons. This was particularly

3 Palaeo-environmental Study Area P8 Calshot - Fawley, Solent region, south coast, UK unfortunate because levels are equated with the early stages of marine activity. In another borehole further onshore, at the main power station site, absolute dates were obtained from a stratigraphic sequence which was equated with events at the spit. Here a date of 6366 +/-124 BP [5561-5047 cal. BC] (Q834) was obtained from a saltmarsh environment where human coastal activity was attested by charcoal particles scattered at a level of -7.33m OD. This seems to have been contemporary with a time when the spit was in its second episode of shift or expansion. Some time after this event a wood fen environment was established at the power station site at a height of -1.19m OD (bed E). This is dated at 3563+/-90 BP [ BC 2174-1674] (Q832). After this date the stratigraphy shows oscillating deposits in which transgressive estuarine clay is intercalated with wood fragments and these seem to represent localised shifts in a natural boundary between saltmarsh and coastal fen.

For the purpose of the present LIFE project, the Calshot-Fawley study is of particular interest because of the notable antiquity of the spit. The evidence suggests that the spit was first formed some time after the mid-Holocene sea had reached a height of at least -16m OD. Caution must be applied in using this level as a chronological indicator because, given a tidal range of approximately 5m for this location in the Solent, the shingle body of the spit would require an initial standing height of at least 6m to become an effective barrier. This is confirmed by the height of the present spit which maintains by a generous margin for a human habitation level which is well placed above highest astronomical . This analogy suggests that although the base level of the spit deposits at Calshot are set at -16m an effective shingle barrier may not have been operative until mean high water had reached a height of at least -10m OD. Such conditions were probably not reached until after the 7th millennium BC.

A shoreline management plan has been prepared for Calshot Spit and this proposes to hold the line. Given that the spit is a dynamic geomorphological feature dependent upon natural accretive processes, it is important that the sources of its shingle supply are adequately understood. This LIFE study has shown that the sediment sources for the sustaining of this spit are not fully understood although the movement of gravel bedloads in the Western Solent and the easterly movement of longshore drift are suspected to be the major mechanisms. The investigation of the sediment archives beneath and behind the spit have been highly revealing and have indicated a surprisingly long and stable Holocene pedigree. The deep cored stratigraphy of the spit has alsol provided critical evidence to support more shallow investigation which have since been conducted in other parts of Southampton Water (Long & Tooley, 1995; Long & Scaife, 1996; Long, et al., 2000). For this LIFE study a particularly important aspect of the spit is its relationship to the geomorphological history of its region. Coupled with the other notable spit of the Western Solent, at , Calshot Spit provides vital evidence for calibrating the coastal-change chronology which has governed the behaviour of all other coastal process units in the Western Solent (Tomalin, 2000).

4 Palaeo-environmental Study Area P8 Calshot - Fawley, Solent region, south coast, UK

BIBLIOGRAPHY FOR CALSHOT-FAWLEY

Hodson, F. & West, I. M., 1972. ‘Holocene deposits at Fawley, Hampshire and the development of Southampton Water’. Proceeding of the Geologists Association 83, 421- 444. Long, A. J. & Tooley, M. J., 1995. ‘Holocene sea-level and crustal movements in Hampshire and southeast , ’, Journal of Coastal Research, special issue 17, Holocene cycles, climate, sea-levels and sedimentation, 199-310. Long, A. J. & Scaife, R. G., 1996. Pleistocene and Holocene evolution of Southampton Water and its tributaries. Unpub. report. Long A. J., Scaife R. G. & Edwards R. J., 2000. ‘Stratigraphic architecture, relative sea level and models of estuary development in southern England; new data from Southampton Water’. Forthcoming. Tomalin, D. .J. 2000. ‘Geomorphological evolution of the Solent seaway and the severance of Wight; a review’, in K. Ansell & M. C. Collins (eds), Solent Science Review. Elsevier. Amsterdam. Tubbs C. R., 1999. The ecology , conservation and history of the Solent, Packard. Chichester. West, I. M., 1980. ‘Geology of the Solent estuarine system’, in The Solent estuarine system. NERC Publications series C, 22, 6019.

Plate P8a Calshot Castle, on the tip of Calshot Spit, built AD1539-1540, Solent Estuary. This shore fort demonstrates that no significant changes have interrupted the stability of this gravel deposit during the last 450 years

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