Historic River Avon

Case Study – Historic River Avon (from the book ‘Post-Glacial Flooding Hypothesis’)

The Quaternary deposits within the Avon Valley encompass a range of geological constituents, including clay-with-flints, head, gravelly head, river terrace deposits, brick earth, alluvium, and sporadic peat. Clay-with-flint deposits, being of residual nature, have resulted from the alteration of Palaeogene sediments and the dissolution of the underlying chalk, with their formation predating that of river terrace deposits. While the precise age remains uncertain, it is likely that clay-with-flint deposits in the southwest of England date back to the Pleistocene epoch (Gallois, 2009).

Head deposits, often found on the upper slopes of the valley, are closely associated with clay-with-flint sediments and are believed to have formed through processes such as solifluction and the dissolution of underlying bedrock. Moving downstream within the valley, deposits like ‘head gravel,’ ‘gravelly head,’ and ‘head’ are deposited through various mechanisms, including fluvial transport, hill wash, hill creep, and solifluction (Barton et al., 2003; Hopson et al., 2007).

Fluvial deposits in the region comprise a series of 14 river terraces, with the highest terraces situated approximately 100 meters above the Avon Valley floor, extending up to 12 kilometres in width from the present-day river axis. In contrast, the lower terraces within the Avon catchment have widths ranging from 3 to 6 kilometres and are located alongside and below the current river. The extensive coverage and limited altitudinal variation of the highest terraces suggest their deposition as draped layers over the landscape (Clarke and Green, 1987).

These terraces maintain relatively consistent thicknesses along the valley, a characteristic often observed in systems where sediment overloading from upstream and input from tributaries to the central valley play a role. Lateral erosion and the redeposition of fluvial sediments are likely contributing factors (Brown et al., 2009a, b). This mechanism could account for the progressive reduction in floodplain width observed between successive erosion-aggradation phases (Brown et al., 2010).

The presence of a substantial number of terraces in the Avon Valley raises questions regarding their direct linkage to Marine Isotope Stage (MIS) cycles, as suggested by some terrace formation models (Bridgland, 2000).

In discussing the Avon terraces and pre-Quaternary geology, a publication titled ‘Crustal uplift in Southern England: evidence from a river terrace records’ has highlighted several terraces from the River Avon still visible in the Hampshire basin, designated as T5 to T10.

‘Little has been done to determine the age of either the terrace sequence or the older River Gravels in the Avon Valley’

However, the age of these terrace sequences and the older River Gravels in the Avon Valley remains relatively understudied. There have been some indications of interglacial sediments within the two lowest terraces of the Solent, assigned to OIS 7 and OIS5e, with organic remains of probable Ipswichian (OIS 5e) and probable early Devensian age described from sediments beneath low-level terraces in the Avon Valley. However, no organic remains have been reported at higher elevations in the valleys of the Avon and its tributaries (Allen et al., 1996; Barber and Brown, 1987; Green et al., 1983).

Recent research, specifically ‘Pleistocene landscape evolution in the Avon Valley, southern Britain: Optical dating of terrace formation and Palaeolithic Archaeology’ by Egberts et al. (2019), has generated results challenging previous geological dating methodologies for river terraces. Optical Stimulated Luminescence (OSL) dating results for T10-7 suggest deposition during or prior to MIS10/9, which includes the Last Glacial Maximum (LGM). These findings, while broadly aligned with prior relative chronologies, potentially offer refinements for dating the archaeological record of T7 and for calculating regional uplift and incision rates (Egberts et al., 2019).

In summary, the Avon Valley’s geological formations, including river terraces, exhibit complexity and have been the subject of ongoing research, especially regarding their formation processes and chronological interpretation.The Quaternary deposits in the Avon Valley encompass a variety of geological formations, including clay-with-flints, head, gravelly head, river terrace deposits, brick earth, alluvium, and occasional peat (Gallego-Sala et al., 2016). Clay-with-flint deposits are residual and have formed through the alteration of Palaeogene sediments and the dissolution of the underlying chalk, with similar deposits in southwest England likely attributed to the Pleistocene epoch (Gallois, 2009).

The diagram in Fig.62 presents the OSL results per terrace and in relationship to the MIS stages. The schematic valley cross-section shown in Fig. 63 is also based on the 3D model of the superficial geology of the Avon Valley, built-in Rock works based on BGS borehole data. The OSL ages for T10-7 suggest deposit during or before MIS10/9 (Fig. 5 – including the LGM). They broadly agree with, but potentially offer a refinement of, previously proposed relative chronologies used for dating the archaeological record of T7 and the calculation of regional uplift and incision rates.

Figure 62 – OSL Results Avon River

The initial analysis of these results reveals a somewhat perplexing pattern characterised by inconsistencies and, in some cases, what appears to be a near-random sequence. These findings have surprised our research team as they deviate from the expected chronological order. For instance, the terrace labelled T10, positioned at the highest elevation 102 meters above Ordnance Datum (OD), was determined through Optical Stimulated Luminescence (OSL) dating to have formed over more than 200,000 years. This is a significant departure from the anticipated sequence.

Further complicating is the revelation that T7, situated at a lower elevation of 58 meters OD, has been dated as older than T10. Such findings challenge the previously held hypothesis regarding the age sequence of these terraces. However, perhaps the most compelling evidence highlighting the inadequacies of the older dated terrace hypothesis is the Loess Terrace, positioned at an elevation of 77 meters OD and classified as ‘Undifferentiated T.’ According to OSL dating, this terrace was formed during the Last Glacial Maximum (LGM), a period associated with extreme cold conditions. In a similar vein, T4, another terrace, also yields the youngest dates among the samples.

These findings underscore the complexity and intricacies involved in deciphering the history of these terraces. The results defy conventional expectations and challenge assumptions about their formation and chronology.

Figure 63- Avon River Terrace Levels

The authors attempted to account for the intriguing results by proposing possible explanations. They suggested, “Therefore, more plausible explanations for this discrepancy between the age estimate of T4 at Fisherton and that at Bickton are either that T4 at Bickton includes sediments ‘reworked’ during more recent fluvial processes or that T4 is a ‘compound’ terrace exhibiting differing depositional behaviour in the upper and lower catchments.”

However, it’s important to note that the reliability of the Optical Stimulated Luminescence (OSL) dating method used in this study raises some questions. Notably, there are instances of questionable dates, particularly within the same soil level. For example, sediment sample GL 14039 was dated as 70ka +/- 8, while sediment sample GL14041 at the same level was dated 58ka +/- 4. Similarly, sample GL14038 was dated 86ka +/- 6, and sample GL14040 was dated 70ka +/- 4, both at the same soil level. Such inconsistencies raise doubts about the accuracy of OSL dating.

These uncertainties in dating are reminiscent of the historical challenges faced by radiocarbon dating, which has seen improvements over time. It is possible that, as with radiocarbon dating, OSL dating methods may become more precise and reliable over the next few decades.

Additionally, a 14C vs. OSL dating paper (Gaigalas, 2000) revealed discrepancies between OSL and carbon dating, with OSL dates being significantly older, suggesting that OSL may overestimate ages by 30% to 40%. This observation is consistent with the dating range presented in Fig.50, indicating potential inaccuracies in OSL dating.

Furthermore, it is essential to consider the impact of the Holocene river floods on the deposition of sediments and terraces. The study identified over one hundred such floods, some lasting hundreds of years, which would have contributed to the erosion and sedimentation dynamics in the region. These floods, driven by intense precipitation and the flow of rivers, played a significant role in reshaping the landscape.

 This may explain the apparent absence of sufficient alluvium or colluvium at Stonehenge Bottom during the Mesolithic and Neolithic periods.

In conclusion, while the study’s findings present challenges and inconsistencies, they underscore the need for a more comprehensive approach that combines multiple dating methods, accurate modelling of deposit thickness, and a nuanced understanding of landscape evolution to unravel the system’s and diachronic evolution complexities.

Further Reading

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