Paleoenvironment

By studying the paleoenvironment or ancient landscapes we can build a picture of pre-historic landscapes and how they have evoloved to today.

Desk based Assessment of Borehole and Core Information

Dr Martin Bates undertook a desk-based assessment in 2005 to investigate existing geological borehole information and establish whether this information could be useful in reconstructing the environmental conditions and evolution of the AONB. The British Geological Survey hold an extensive collection of borehole data from the last 150 years, and there is additional data from other investigations in the harbour.

This work suggested a model for the sequence development of the region in three phases. The first phase for which evidence is available is when the region was an embayed coastline in the Middle Pleistocene. Sediments in long-buried ancient channels cut into the Manhood Peninsula are likely to be the last remaining evidence for this phase. In the second phase, the Late Middle Pleistocene, the area was open coastline, facing approximately south. Evidence for this phase is likely to consist of the oldest deposits in the AONB, perhaps preserved around Warblington.

In the third phase, the Upper Pleistocene and Holocene (up to the present), the area was a harboured coastline that underwent major changes. The climate grew warmer about 8,000 years ago melting glaciers and causing the sea level to rise dramatically, transforming the AONB. Much of the region would have remained dry until at least the middle Bronze Age, but the subsequent inundation by brackish water would have transformed the area from one containing a series of low hills separated by estuarine channels to one dominated by larger bodies of tidal water.

An important part of the project was to understand the deposits and topography in the channels. Despite being underwater now, these areas would once have been dry land. In the Mesolithic period, for example, the valleys that now form the harbour channels would have been used as access routes to the coast as well as for hunting and base camps.

The report concludes that the understanding of the evolution of the AONB is based largely on information derived from outside the AONB. It recommended:

• Modifying the mapping strategy for the area, based on the BGS (British Geological Survey) mapping, but to include information on sub-surface sediment types, ages and archaeological potential.
• Instigating a field program to recover a number of key cores to produce master sequences for the harbour region from both the wet and dry side areas.

The recommendations formed the basis for subsequent work.

Coring and Environmental Analysis

Further coring, and environmental analysis, was undertaken by MoLAS (Museum of London Archaeology Service). Seven locations (Tourner Bury Creek; Coniger Point; Thorney Island; Marker Point; Bosham Channel; Bosham Hoe and Horse Pond) were chosen and at least two boreholes drilled per location. The resulting core samples were analysed for evidence relating to environmental conditions at different times in the past. Radiocarbon dating (of seeds) was also undertaken.

Cores taken from Thorney Island preserved the best evidence for Mesolithic (12,000-4,000 BC) period environment and landscape. The sea level in c 10,000 BC lay at about 35m below today's sea level, although it was rising rapidly. The deeply-incised river valleys of the Solent drained the Chichester Harbour area. The Thorney Island cores show that spring-fed streams drained down the valley sides in an open grassy landscape. Later, the valley sides were colonised by pine forest as the freshwater tributary streams silted up with peat. High levels of charcoal in the pollen samples suggest that the pine forest was susceptible to forest fires.

As the sea level rose in the later Mesolithic period, estuarine environments are likely to have encroached up the valley systems and into the harbour area - at first constrained within the valleys. The inlets filled up rapidly - in the Thorney Island cores 3m of sediment built up in 300 years at about 4,000BC. The rate of sea level rise slowed at around this time and from this period most of the other cores obtained in this study record estuarine incursion. The Neolithic period (4,000-2,000 BC) saw estuarine conditions expand to the modern foreshore zone.

Almost all the cores show that the estuarine incursion was associated with erosion and a turbulent depositional environment. This perhaps points to storms rather than a gradual process of waterlogging of a formerly dry landscape. During the Neolithic, the encroaching water reached the shoulders of the incised valleys and by the later Neolithic was expanding across the fringes of the plateau surfaces. The spilling over of the waters would have represented a marked change in the appearance of the harbour. As the estuarine environments expanded, areas of mixed woodland of oak and lime with hazel and elm shrank. Neolithic people may also have been intermittently clearing woodland, carbon in the cores showed evidence for this at Coniger Point and Bosham Hoe. Wetland areas of sedge fen, backed by wet alder carr would have become more established by the Bronze Age (2,000-600 BC), when the landscape of the harbour would have appeared much as it does today: estuarine with extensive mudflats between islands of low ground.

Many of the borehole sequences appear to show a contraction of estuarine environments by the Iron Age (600 BC-AD 43), accompanied by a change from mudflats to a salt marsh environment. At the Bronze Age / Iron Age transition, the cores show that a woodland of oak with beech and ash developed. The extent of salt marsh may have been at its greatest from the Roman (AD 43-410) to early medieval (AD 410-1066) periods. Subsequent changes in the harbour in the medieval (AD 1066-1485) and later periods - land reclamation in particular - have obscured the impact of relative sea level fluctuations.

Available reports
Desk based assessment of borehole and core information by Martin Bates 2005
Chichester Harbour geophysical survey by Titan Environmental Surveys 2005
Chichester Harbour geophysical and auger survey by Wessex Archaeology 2005
Fishbourne meadows watching brief by Wessex Archaeology 2006
Mills, W and Corcoran, J 2006a Coring and environmental analysis - intertidal: Stage I report, MoLAS unpublished report
Mills, W and Corcoran, J 2006b Coring and environmental analysis - intertidal: Interim statement of assessment results, MoLAS unpublished report
Mills, W, Corcoran, J, Bates, M, Wittaker, J, Allen, P, Cameron, N, Roberts, K and Pipe, A 2007a Chichester Harbour AONB, Coring and palaeoenvironmental analysis - intertidal: Stage 4 Report, Volume I: data, MoLAS unpublished report
Mills, W, Corcoran, J and Bates, M 2007b Chichester Harbour AONB, Coring and palaeoenvironmental analysis - intertidal: Stage 4 Report, Volume I: data, MoLAS unpublished report

Contact Nicky Horter for more information on these reports.

Back to top