Geological history of the area
Chichester Harbour lies within the Solent River system that is dominated by Cretaceous Chalk and overlying muds, sands and gravels of the early Tertiary deposits. The river system developed during the Pleistocene, and has been significantly altered by repeated glacial action. The Solent and surrounding estuaries and inlets have cut into the Mesozoic and early Tertiary rocks of the Hampshire basin that formed part of the Anglo-Paris-Belgian basin. The Cretaceous in this area is dominated by Chalk, with outcrops at the head of the Chichester inlets amongst others. Overlying the chalk are Palaeocene–Lower Eocene silty and sandy clays, clayey silts and sandy silts. The Pleistocene geology is dominated by the development of the west to east flowing Solent River System. Gravel deposits throughout the area originate from material carried and then deposited by the river system.
Human beings first appear in the archaeological and geological record about half a million years ago. Since then, the coastline of Sussex and the winding creeks of Chichester harbour have undergone enormous changes. During this time there have been vast changes of climate and in particular a series of alternating cold glacial and warmer interglacial periods. Although the ice never reached as far south as Sussex , its proximity had a profound effect. At the height of a glaciation the great volume of water locked up in ice sheets and glaciers around the world would have dramatically reduced sea-levels, at times by over 100 metres, whilst the weight of the ice would have caused the land mass to be depressed. The conditions produced thick layers of gravels. These were laid down as a result of the mass-movement of semi-frozen rock and underlying deposits down the slopes of the Downs on the coastal plains, into the river valleys and onto the floor of what is now the English Channel .
Peter Adeline has contributed the following
Traditional geology refers to the glaciation of the British Isles by ice which moved southwards then receded northwards, leaving behind the characteristic topography of upland northern Britain and masses of clays, gravels and silts in lowland southern Britain. In about 1985 we started walking the shores of Chichester Harbour, and found two very puzzling large lumps of rock, about 2 feet in diameter. One was on the foreshore at the southern end of Thorney Island, the other near the southern tip of the Chidham peninsular. They were coarse granite boulders, so had either been dumped by someone (most unlikely) or had been transported by ice, having been picked up by the ice from their natural area of outcrop. The nearest outcropping granite is in Brittany, otherwise it would be Devon / Cornwall or the north of England. By their nature the ice sheets which moved south over the British Isles couldn’t have dumped such erratics on the south coast – but conventional (ie geology student!) wisdom didn’t allow for anything else. I spent some time in Chichester Library at the time trying to puzzle this out, and eventually found an obscure reference to ice sheets which might have headed our way from N France, which would of course explain these boulders. More recently, with the benefit of Google, I was able to learn a bit more about the Saalian ice, which seems to have moved in a broad sweep across N Eurasia, with a limb down the English Channel which – literally – ground to a halt on the W edge of the Continental Shelf. It probably formed a damn for Lake Solent, and may have been responsible for the so-called ‘100 foot raised beach’ of Sussex and also some recently recognised submarine features of the English Channel.
These processes were a key determining factor to human activity in the area. Prior to the Anglian glaciation about 480,000 years ago, the eastern part of Sussex was joined to the continent by a substantial land mass that was eventually breached by water overflowing from an ice-dammed lake in the southern North Sea. The rivers that cut through the Sussex Downland today represent remnants of much larger river systems that originally flowed into an embayment south-west of this early land mass and later into the Channel river, flowing south of the present Sussex coastline.
Some 13,000 years ago during the last major glaciation an ice sheet over 1km thick covered north-west England and Scotland . As climatic conditions improved, hunter-gatherers were able to cross to Britain on the bed of the North Sea , left largely dry because of the volume of water frozen in the glaciers. As the ice melted and the sea level rose, the river valleys began to flood while the land rose under the decreasing weight of the ice mass. Sea levels rose rapidly during the Mesolithic and into the Neolithic, leading to a continued alteration of the coastal environment. By about 8,000 years ago the land mass had disappeared and Britain was separated from the continent by the Channel. Coastal sites from this period will lie far off-shore – attested by finds of flint axes and barbed harpoon points dredged from the floor of the North Sea . Rising sea levels would have flooded the lower reaches of Chichester Harbour, while wetter freshwater conditions would have been encouraged further up the river valleys, leading to peat growth.
Peat is important because it not only provides a picture of vegetational history for archaeologists, but also indicates the nature and chronology of sea-level rise and coastal change, especially when the peats are interleaved with inorganic marine silts and clays. The shape of the early coastline would have been strongly influenced by the topography of the landscape prior to the sea level rise, the geology of the coastal environment, the availability of gravels and sediments on the floor of the Channel that may have been redeposited in the coastal zone, the effects of tidal flow and of periods of storms, and sediments redeposited from the land, as well as human intervention.
Erosion along the Sussex coast is well documented – for example Selsey extended considerably seaward of the present coast. The history of the Sussex coastline is dominated by the formation and migration of coastal and off-shore barriers. These sand and gravel deposits can migrate landward under the influence of rising sea levels. In the early Neolithic period, the coastline would have been deeply indented, perhaps with sandy beaches and barriers close inshore. Most river estuaries would have been broad and tidal. More substantial coastal shingle barriers would have formed as the sea level began to stabilise. West of Selsey peninsula, the direction of longshore drift is westward (the opposite direction to the rest of the Sussex coast, in fact). This, and the sheltering effect of the Isle of Wight , meant that Chichester Harbour has managed to escape the most profound effects of barrier formation and consequent silting.
Sea level appears to have stabilised by the Early to Middle Bronze Age, but by the end of the Late Bronze Age marine influence extends up the river valleys, indicating a period of dramatic change. The depletion of the barriers, perhaps due to climatic deterioration and increased storminess, allowed the sea to flood right up the lower reaches of the rivers and their estuaries. This turned former marsh, meadow and fen into tidal salt-marsh to produce and environment that is similar to Chichester Harbour today.
The highly indented coastline remained a constant feature through the Iron Age and Roman periods. Sea level during these times was some 1 to 2 metres lower than the present day. As longshore drift continued, this may have restarted the process of barrier building encouraging silting that may have allowed land reclamation to take place. Shingle bars would have continued to form into the early medieval period across the broader embayments of former marshland. The silting of the former salt marsh would have been ideal for the extraction of salt and reclamation as silting continued. Other changes in the coastal morphology occurred more recently. Historical accounts suggest a period of increased storminess at the end of the 13th century. There may also have been a slight rise in sea level. Severe storms and associated flooding continued into the 14th century which contributed to the abandonment of some arable fields and their reversion to pasture. Some of the most dramatic changes in coastline in historic times have been at the mouth of the Harbour. East Head has shifted dramatically in the past 250 years, retreating progressively east since the late 18th century by over 500m. This process may have been accelerated by the building of groynes along the beaches to the east in Bracklesham Bay as far as Selsey Bill that have starved East Head of the material required to sustain it.
A study that used historic maps to plot changes in the coastline of Hampshire found that there was a considerable landward movement in the high water mark of the south-eastern coastline of Hayling Island . The shift between the 1st and 4 th editions OS (mid to late 19th century to mid 20th century) was over 80m north. The study also found a dramatic growth in salt marsh area between the early 20th century OS mapping and the modern period. In Chichester Harbour along the coast of Hayling Island during this period the inter-tidal zone becomes categorised as ‘marsh’ rather than ‘mud,’ suggesting salt marsh accretion is taking place. The natural changes, in the form of coastal erosion and accretion, the movement of spits and inter-tidal / offshore changes have a greater effect along the more exposed areas that are subject to powerful marine forces, like Hayling Island.
Coastal change has had an inevitable effect on the use of the area as a harbour. The establishment of a naval base was rejected by the Navy Board in 1698 because the harbour was ‘too dangerous to enter.’ At this time the entrance to the harbour was barely a quarter of a mile wide. An Admiralty chart of 1845 shows a sounding of just two feet in places at Mean Low Winter Springs. Entry to the harbour seems only to have been possible from half-tide onwards, with a pilot steering many non-local vessels. The spur of land to the south of Chidham Point has migrated slightly north. An extensive sandbank at Ferry Barn to the west of Furzefield Creek shown on the 1st edition OS map has reduced in size, although a path on the top of the bank running south to West Itchenor – marked ‘Hard’ on the OS map is still in use today. Channels have also altered over time. A channel shown on the 1st edition OS branching from Sweare Deep heading for Emsworth and then curving to the west and meandering to Conigar Point has disappeared from the modern mapping.
Attempts to control coastal change and to restrict the flow of the sea have met with varying success in the harbour. The most successful reclamation of land was in the 19th century when the north part of Thorney was enclosed. Sea defences were largely in place on the vulnerable west coast of Thorney by the time of the 1st edition OS map. At Conigar Point, an embankment was also in place, although this did not prevent the loss of irregular area of marsh and a sand bank. An embankment around Chidham Point was later extended north by a sea wall up the northeast coast of Chidham . A gap in the sea wall on the east coast of Chidham at Cobnor House may have been a landing place. There were the remains of a possible quay at Cobnor Hard, on the east coast of the Chidham peninsula. These consisted of ‘stumps … still visible in the mud in front of the boathouse’.
A sea wall running from the west of Cobnor Point on Chidham to Pilsey Island shown on the 1st edition OS represented an ambitious but failed attempt to exclude the sea from Thorney Channel. A further attempt to enclose the bay south of Prinsted with a sea wall also failed. These walls – or ‘cuts’ – can still be seen, for example on aerial photographs. There are great differences between the smooth coastline of the present day and the deeply indented inlets of the Neolithic, the fen covered peat valleys of the Bronze Age, the inundated coastline of the Iron Age and Roman periods, and the reclaimed coast of the medieval period.