Eastern English Channel

Non-technical summary

The Eastern English Channel NSPRMF study area extends from Thanet, through the Dover Straits to the western side of the Isle of Wight and one of the key events in the environmental history of this region was breaching of a land bridge located in the Dover Straits approximately 450,000 years ago. From this point onwards, Britain became an island during periods of high sea-level changing the palaeogeography of northern Europe and influencing the presence/absence of humans in Britain.

After breaching of the Dover Straits, the Thames-Medway and Rhine-Muese rivers were diverted from their northerly courses, south through the eastern English Channel creating a large, complex network of palaeovalleys that can be seen on the seafloor today. There is evidence to suggest one to the palaeovalleys, the Northern Palaeovalley, formed through catastrophic flooding and the presence of enclosed deeps within the Dover Straits interpreted as plunge pools from large waterfalls supports this hypothesis.

While catastrophic processes have left their mark on the seabed in places, elsewhere the offshore extensions of major rivers such as the River Seine in France, and River Solent and River Arun in England can be mapped. In the central eastern English Channel, these rivers are infilled with coarse-grained sand and gravel laid down in a high energy, cold-climate environment sometime between 170,000 and 21,000 years ago according to luminescence dating. These river deposits have been dredged for aggregate resource but interestingly no archaeological material has been found to date.

Palaeovalleys associated with the submerged River Arun and River Solent are slightly different as they tend to be narrower and cut more deeply into bedrock, they are also infilled with fine-grained and occasionally organic deposits that have been dated and investigated for palaeoenvironmental remains. These channels were active and fringed by wetlands from approximately 10,500 years ago, until they flooded during post glacial sea-level rise around 9,000 years ago. These river systems would have been attractive to animals and humans and whilst there have been no archaeological finds from these channels, a small number mammoth remains have been recorded.

The only known in situ submerged prehistoric site across the NSPRMF study area is located within the Eastern English Channel at Bouldnor Cliff, off the northwest coast of the Isle of Wight. More than 1,000 Mesolithic knapped flint artefacts have been identified, along with burnt timbers from a platform structure and palaeoenvironmental assessment suggests occupation within a wooded landscape next to a river or lake around 8,000 years ago.

Stratigraphy, chronology, landscape and palaeogeography

The primary focus of research in the Eastern English Channel has been the origin and evolution of a submerged palaeovalley system (e.g. Gupta et al. 2007). Sediment cover in the English Channel is relatively sparse and bedrock is exposed at or close to seabed across large areas (James et al. 2010). As a result, there has been an emphasis on geomorphological interpretation of seabed bathymetry (Collier et al. 2015; Gupta et al. 2017; Garcia-Moreno et al. 2019). Where palaeovalleys and seabed features are infilled with sediment, seismic data collected as part of the English Channel REC and synthesis projects (James et al 2010; 2011), or to support marine aggregate licencing and prospecting (Mellett et al. 2012a; 2013), have been interrogated.

A considerable number of cores have been collected from the Eastern English Channel within marine aggregate licence areas. This data is commercially sensitive but was used by Mellett et al. (2012a; 2013) to reconstruct Late Quaternary evolution of the Eastern English Channel. As part of the Seabed Prehistory projects (Wessex Archaeology 2008d; 2008e), vibrocores were acquired in the Eastern English Channel and submerged PalaeoArun to provide material for palaeoenvironmental assessment and dating.

Table 12 Publicly available projects and published research since 2009 – Eastern English Channel

One of the most significant events in the English Channel region was breaching of the Weald-Artois anticline, a chalk ridge that connected Britain to northern France between Dover and Calais in what is now the Strait of Dover. The current prevailing theory is that, during deglaciation of the Anglian (MIS 12) ice sheet, the emptying of an ice-dammed lake ponded within the North Sea created a volume of water large enough to breach the land-bridge, incising the Lobourg Channel off the Kent coast and some of the English Channel palaeovalleys, most notably the Northern Palaeovalley, in the process (Smith 1985, Gupta et al. 2007; 2017, Collier et al. 2015; Garcia-Moreno et al. 2019). This initial catastrophic breaching of Weald-Artois ridge is thought to have been followed by further erosive events leading to the permanent breaching of the English Channel approximately 150 ka (Hijma et al. 2012; Mellett et al. 2013).

The Quaternary history and palaeogeographic evolution of the Eastern English Channel can be broadly subdivided into four main stages;

Stage 1: Continuous terrestrial connection between southern England and northern France across the Weald-Artois ridge, regardless of sea levels, before 450,000 years ago (MIS 12);

Stage 2: A narrow Dover Strait and interfluve across the Southern Bight that linked East Anglia to the Netherlands and Belgium, after 450,000 and before 160,000 years ago (MIS 12-MIS 6);

Stage 3: Widening of the Dover Strait and progressive erosion of the interfluve after 160,000 years ago (MIS 6), with a landscape fully dissected by a major, axial Rhine-Meuse-Thames valley in the Eastern English Channel (The Channel River/Fleuve Manche) during periods of lower sea level, and;

Stage 4: Flooding of palaeovalleys during sea Holocene sea-level transgression.

The palaeogeography and landscapes of the eastern English Channel prior to the breaching event are unknown as there are no sedimentary records preserved from this time. Terraces of the River Solent in Hampshire have been assigned an Early Pleistocene age (Allen and Gibbard 1993) indicating rivers were active in the English Channel long before the Anglian glaciation. However, due to subsequent reworking over multiple glacial-interglacial cycles, the potential for preservation of these Early Pleistocene river systems is low.

The present-day seabed in the Eastern English Channel preserves a morphological record of the events responsible for breaching the Straits of Dover, including a large E-W trending palaeovalley known as the Northern Palaeovalley and a series of depressions in the Straits of Dover known as the Fosse Dangeard. Elongate streamlined bedrock islands within the Northern Palaeovalley are interpreted as evidence of a megaflood (Gupta et al. 2007; Collier et al. 2015) and support the hypothesis that the English Channel was carved by catastrophic flooding of a proglacial Lake that ponded in the North Sea during the Anglian (Smith et al. 1985). The Fosse Dangeard are interpreted as plunge pools forming at the base of a waterfalls as waters drained the North Sea (Gupta et al. 2017) and several phases of erosion and deposition within the plunge pools suggest multiple phases of fluvial and flood erosion (Garcia-Moreno et al. 2019) which is supported by estimates of palaeodischarge (Toucanne et al. 2009).

Ice sheet advance during the MIS 6 Cold Stage saw increased fluvial activity in the Channel River suggesting the opening at the Straits of Dover was a more permanent fixture (Toucanne et al. 2009) and there is evidence of fluvial activity in the eastern English Channel at 176.6 ± 20 ka (MIS 6) (Mellett et al. 2013). These river systems differ in morphology to the Northern Palaeovalley and their infill reflects deposition in braided rivers and shallow marine to littoral environments. OSL dating of the infill suggests deposition between MIS 6 and MIS 5a/5b (Wessex Archaeology 2008d; Mellett et al. 2012b) during the early part of the Devensian glacial period when sea-level oscillated driving frequent shifts between fluvial and coastal environments (Mellett et al 2013). The infilled palaeovalleys of the Eastern English Channel are mapped as undifferentiated by the British Geological Survey (Stoker et al. 2011) which may reflect the complex depositional history and issues resolving stratigraphy due to multiple erosion and incision events.

Fluvial activity was also high during the peak of the Devensian glaciation (MIS 2) (Toucanne et al. 2009; 2010) suggesting drainage of the Thames-Rhine-Meuse rivers through the English Channel during this period of lower sea level. In the Eastern English Channel, a palaeovalley investigated as part of the Seabed Prehistory projects (Wessex Archaeology 2008d) shows incision and vertical downcutting during MIS 2 creating a terrace of the fluvial and coastal deposits that formed during the MIS 6 Cold Stage and early Devensian (MIS 5a-5d). A lag deposit at the base of this channel was dated to 21.2 ± 1.5 ka suggesting incision occurred at the maximum sea-level lowstand and the channel infill records a transition from freshwater fluvial to estuarine and shallow marine environments showing the influence of post glacial rising sea levels. Final infill of the channel occurred at between 11.9 ± 0.9 ka (OSL) and 11,200-10,800 cal. BP (radiocarbon) marking final submergence of the Eastern English Channel during the early Holocene.

There is a marked difference between the palaeovalleys of the Channel River/Fleuve Manche and palaeovalleys preserved in shallower water nearshore areas fringing the eastern English Channel that trend broadly N-S forming tributaries of the Channel River/Fleuve Manche. These palaeovalleys include offshore extensions of the River Solent and River Arun amongst other unnamed channels related to antecedent drainage networks.

The submerged PalaeoArun was investigated as part of the Seabed Prehistory project (Wessex Archaeology 2008e) revealing a relatively deep (~20 m) channel that was likely incised during the Devensian and subsequently infilled by a transgressive sequence during the early Holocene. Peat preserved along the channel margin was dated to 10,490-10,200 cal. BP and a late boreal pollen assemblage confirms this channel was active during the early Holocene.

The submerged forest and associated peat deposits preserved at Bouldnor Cliff, located off the coast of the Isle of Wight provides further environmental evidence of the early Holocene landscape in the Solent region (Momber et al. 2014; 2017). Understanding the paleogeographic evolution of the Solent region in response to rising Holocene sea levels provides crucial contextual information for Mesolithic human activity at Bouldnor Cliff. Palaeoenvironmental reconstructions show a transition from a fenland to a brackish saltmarsh and later mudflat environment during the Early Holocene (Momber et al. 2021). The Western Solent at this time was a sheltered embayment/estuarine environment that did not become fully connected with marine waters in the eastern Solent until ~ 2.5 ka. The Western Solent was therefore relatively well protected and experienced lower energy leading to greater preservation of archaeological material (e.g. Bouldnor Cliff and Cowes).

Records of Holocene sea-level transgression are not restricted to palaeovalleys and an Early Holocene submerged barrier coastal system is preserved ~15 km off the coast of Hastings in the eastern English Channel (Mellett et al. 2012a). Preservation of such systems is extremely rare as coastal processes tend to rework and transport sediment landward during sea-level rise. However, under certain scenarios, it is possible for a coastline to become stranded and abandoned offshore as sea-level continues to rise (Mellett and Plater 2018). At Hastings Bank, there is evidence of a gravel barrier beach and back-barrier tidal lagoon that formed between 8.4 ± 0.2 ka and 5.3 ± 0.5 ka (Mellett et al. 2012b).

Palaeoenvironmental records

Limited paleoenvironmental data from direct sampling of sediments through vibrocores is currently available for the Eastern English Channel (Table 13).

Table 13  Quaternary palaeoenvironmental studies of vibrocore data carried out in the Eastern English Channel

Evidence obtained from 30 km offshore south-west of Beachy Head, West Sussex, between marine aggregate licence Areas 464 West and 464 East, has identified Quaternary deposits in-filling a palaeovalley (Wessex Archaeology 2008c). Although these sediments span the late Middle Pleistocene to early Holocene, identified paleoenvironmental material is principally associate with late Pleistocene and early Holocene units. These datasets record estuarine, freshwater, wet herb fen with local pine and birch woodland and riverine settings thought from reflect the Windermere Interstadial, Loch Lomond Stadial and the earliest Holocene; these are periods associated with the late Upper and final Upper Palaeolithic, and earliest Mesolithic.

Investigations in the submerged catchment of PalaeoArun River and the western Solent have also identified Quaternary sediments that preserved early Holocene paleoenvironmental evidence (Wessex Archaeology 2008d; Scaife 2011).

As part of the capital dredge project to support the operation of Queen Elizabeth Class aircraft carriers in Portsmouth, a series of vibrocores were acquired from Holocene deposits that were truncated at the margins of the approach channel. These cores were subject to a suite of palaeoenvironmental proxies and radiocarbon dating which document vegetation and landscape response to rising sea level during the early Holocene between 8,990-8,600 cal. BP and 8,520-8,320 cal. BP (Wessex Archaeology 2019c).

Investigations of deposits within the submerged former course of the River Arun, in an area 18 km south of Littlehampton, Sussex, have studied the upper units of Quaternary sequences that are in total up to 20 m thick (Wessex Archaeology 2008d). These identified gravels, sands, silts, clays and peats, with the peats dated to between 9,333±45 BP to 9,155±50 BP. The palaeoenvironmental data is indicative of early Holocene landscapes and environments. The pollen data is characteristic of developing Pre-boreal (10 ka–9 ka) and Boreal (9 ka–8 ka) woodland, whilst the foraminifera attest to frequent variations in salinity associated with periods of saltmarsh and estuarine conditions, with an increasing marine influence towards the top of the sequences.

Paleoenvironmental data from the sites at Bouldnor Cliff has provided evidence of late Mesolithic (c. 8,000 cal. BP), Boreal environments in the western Solent (Scaife 2011). This data indicates that the associated landscape and environment witnessed a transition from freshwater fenland to brackish water fen fringing estuarine lagoons, with salinity associated with development of salt marsh and estuarine mudflats levels increasing through time.

Similar late Mesolithic landscapes and environments are indicated by paleoenvironmental data from the current mouth of the River Medina in the western Solent (Wessex Archaeology 2006). Here, sequences of sandy clays, overlain by peat and silty estuarine alluvium were sampled. The top of the peat is dated to between 7,970-7,790 cal. BP and 7,930-7,700 cal. BP, whilst associated palaeoenvironmental data was indicative of brackish water and the development of extensive saltmarsh. Boreal pollen indicative of oak and hazel woodland in dry areas, with alder along wet margin was also identified. Both the evidence from Bouldnor Cliffs and the Medina is indicative of marine inundation at the end of the Boreal period (9 ka–8 ka).

Faunal assemblages

There are no historic records of vertebrate faunal material from submerged context in the English Channel region. The known terrestrial vertebrate faunal record from the Eastern English Channel is dominated by individual finds reported through the BMAPA protocol. All Pleistocene discoveries identifiable to species are mammoth. Multiple finds of mammoth remains have been recovered from Area 127, located south of Christchurch Bay, Dorset and from Area 122/1A, which is situated south of Littlehampton and Worthing, Sussex (Figure 6).

Additionally, consistent finds of horse, cattle and deer have been reported through the BMAPA protocol from Area 122/3, located in the eastern Solent (Figure 6). These are undated, but their condition and the range species present suggest that these are most likely Holocene. One bone exhibits possible human modification (butchery marks). This material may indicate the presence of early Holocene terrestrial sediments. Small amounts of vertbrate fauna have been recovered from Holocene deposits dated to the late Mesolithic in western Solent at Bouldnor Cliffs (Momber et. al. 2011)

Archaeological assemblages

Although Palaeolithic archaeology is well known from the intertidal zone in the Eastern English Channel region (White et al. 2006, Bates et al. 2009), and Palaeolithic archaeology is known from below MLWS in the western English Channel (Cliquet et al. 2011), no Palaeolithic archaeology is known from below MLWS in Western Channel. Later archaeology is documented. This consists of late Mesolithic archaeology from Bouldnor Cliffs, located in the Western Solent, offshore from the Isle of Wight (Momber et al. 2011, Momber et al. 2021).

The Quaternary deposits at Bouldnor Cliffs consist of a series of submerged alluvial clays, peats and fluvial sediments, associated with a cluster of archaeological sites. These sites have been subject to various phases of investigation since 1999. Bouldnor Cliff II (BC-II), identified in 1999, has produced more than 1,000 Mesolithic knapped flint artefacts and burnt flint clasts (Momber 2014). The Mesolithic lithics include typo-technologically late Mesolithic pieces (Momber et al. 2011). Palaeoenvironmental investigations, including pollen analysis and sedaDNA, has established that the Mesolithic activity at BC-II occurred within a wooded landscape in proximity to a riverine or lacustrine environment (Momber et al. 2011; Smith et al. 2015). Radiocarbon dates on organic material from BC-II have provided a terminus ante quem of 7,970-7,790 cal. BP, indicating a late Mesolithic date.

Subsequent to the discovery of BC-II, further Mesolithic findspots have been identified. One of these, Bouldnor Cliff V (BC-V), has produced worked timbers forming a platform structure, some of which were burnt (Momber et al. 2021). The same geological unit associated with the platform has produced wood chippings, fragments of string, scatters of charcoal and two pits containing burnt stones. Radiocarbon dates on woody material from BC-V have provided a terminus post quem of 8,170-7,860 cal. BP (Momber et al. 2011) suggesting a late Mesolithic date for the archaeology.