Case Study – River Avon

Extract from the book: The Post-Glacial Flooding Hypothesis

The Quaternary deposits in the Avon valley include clay-with-flints, head, gravelly head, river terrace deposits, brickearth, alluvium and occasional peat. Clay-with-flint is a residual deposit created by modification of Palaeogene sediments and the solution of the underlying chalk, deposited considerably earlier than the river terrace deposits.

The exact age is uncertain, but clay-with-flint deposits in south-west England are likely of Pleistocene age (Gallois 2009). Clay-with-flint is mainly found on the flats of hilltops. Older head deposits, associated with clay-with-flint sediments, formed through solifluction and solution of the latter and the underlying bedrock. It is often found on the upper valley slopes. Further down the valley ‘head gravel’, ‘gravelly head’ and ‘head’ are found, deposited by fluvial transport, hill wash, hill creep and solifluction (Barton et al. 2003; Hopson et al. 2007).

The fluvial deposits in the area form a flight of 14 river terraces. The highest terraces, up to 100m above the Avon valley floor, spread up to 12km wide from the present-day river axis. The lower terraces in the Avon catchment are 3km to 6km wide and are found alongside and below the present-day river (BGS 1991, 2004, 2005). The massive extent and limited altitudinal separation between the highest terraces point to the draped deposition of these terraces over the landscape (Clarke and Green 1987).

The terraces are of relatively constant thickness along the valley. This is typical for systems where sediment overloading from upstream and input from tributaries to the main valley occurs (Blum and Tӧrnqvist 2000). This is probably combined with lateral erosion and redeposition of fluvial sediments (Brown et al. 2009a, b).

This mechanism could explain the progressive restriction of floodplain width between each erosion-aggradation phase (Brown et al. 2010). The high number of terraces may indicate that terrace formation in the Avon valley cannot directly be linked to MIS cycles, as suggested in the model of terrace formation developed by Bridgland (2000).

The Avon terraces, the pre-Quaternary geology is briefly discussed In a publication ‘Crustal uplift in Southern England: evidence from a river terrace records’ Illustrated that there were several terraces from the river Avon still visible in the Hampshire basin which they called T5 – T10.

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

‘However, interglacial sediments within the two lowest terraces of the Solent have been assigned to OIS 7, and OIS5e (Allen et al., 1996) Organic Remains of probable Ipswichian(OIS 5e) age (Barber and Brown, 1987), and probable early Devensian age (Green et al.,1983) have been described from sediments underlying low-level terraces within the Avon valley, but at higher elevations in the valleys of the Avon and its tributaries, no organic remains have ever been described’.

This dating of River terraces is more confused when a recent publication called: ‘Pleistocene landscape evolution in the Avon Valley, southern Britain: Optical dating of terrace formation and Palaeolithic archaeology’ by Egberts et al., 2019.  Produced a set of results that questioned the way previous geologists have dated these river terraces.

Figure 5 – OSL Results Avon River

The diagram in Fig.5 presents the OSL results per terrace and in relationship to the MIS stages. The schematic valley cross-section is shown in Fig. 6 is also based on the 3Dmodel 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 are broadly in agreement 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.

The first observation you can make from these results is that they are ‘inconsistent’ at best and almost random at worst?  It is clearly not what was expected by the team as the supposed oldest terrace at the top T10 (at the height of 102m OD. Fig 6) was laid according to the OSL dating method) over a 200,000-year period.  This is compounded with T7 terrace (58m OD) being dated BEFORE T10?? But the most compelling evidence of the problems (with this old dated terrace hypothesis of yesteryear) is the Loess Terrace at (77m OD – Undif. T) which was laid during the LGM, as was T4 which has the youngest dates.

Figure 6– Avon River Terrace Levels

The authors attempted to explain this remarkable result by suggesting “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.”

When we look at this new OSL method of dating we are not fully assured of its accuracy – as seen by the dating of sediment sample GL 14039 which was dated as 70ka +/- 8, and sediment sample GL14041 at the same level was dated 58ka +/- 4 and also  sample GL14038 dated 86ka +/- 6 and sample GL14040 dated 70ka +/- 4, again at the same soil level.

Moreover, there are not only questionable dates at the same level (almost within the error limits) but the amount of sediment laid down below the topsoil – the first two samples 62ka for the 83cm depth (1.33 cm per 1000 years) and then just 16ka for another 70cm (0.23 cm per1000 years).

This would therefore question the accuracy of the OSL dating method. If history is repeating itself, as with radiocarbon dating, then over the next 50 years, these results will be more accurately defined.  But what these samples do show is that dating these levels by visual evidence alone is not accurate as the report also confirms.

This is verified in a 14C v OSL dating paper (Gaigalas, 2000) when they concluded on their results “The observation of an anomaly high OSL age (from 39+/- 4 to 40 +/- 2 ka) for sands which covered the peat layer with 14C dates 24,430 +/- 210 of organic detritus, 27,800 +/- 340 of carbonate tuff…. Strongly suggests that these sands were exposed only for a short time.”

This showed that the OSL was as much as 30% to 40% older than carbon dating.  This is confirmed in the dating range in Fig.60 and illustrates that the Undifferented Terrace could easily be dated to the LGM.

What also was missed by the report was the evidence as previously shown in our case studies in this section was the number of river flooding that occurred in the Holocene and consequently must have flooded some of the older river terraces. We suspect that is why the terraces between T7 and T10 are of river silt and dates that are out of sequence to the above and below terraces.

“There are obvious problems with uplift modelling based on relative altitudes of terrace deposits and the use of the Palaeolithic record as a chronological marker, and indeed the age proposed by Maddy et al. (2000) and Westawayet al. (2006) for T7 (Woodgreen Palaeolithic site) is not in agreement with our chronometric date of 389-243 ka”.

This apparent problem is compounded by the fact we have also shown in this section is that during the period 389 –  243ka the sea level analysis shows that the ice during this period was of a LESSER extent that the last LGM and therefore would affect the river terraces less rather than more!

Moreover, what we agree is the sentiment that “The reinvestigation of archaeological sites in the Avon valley shows that terrace deposits offer an opportunity to provide a valuable relative chronological framework but that in conjunction with chronometric age control and accurate height and deposit thickness modelling, a far better appreciation of the complexities of the system and diachronic evolution can be achieved. The more detailed understandings of landscape evolution which can be realised have direct implications for our interpretations of hominin landscape use, behaviour and predictive modelling of Palaeolithic sites”.

Finally, archaeologists and geologists resist the fact that the river Avon was in Stonehenge Bottom during the Mesolithic and Neolithic period.  They insist that there is no evidence in the form of Alluvium or Colluvium in sufficient quantities to support my hypothesis.  This objection has a simple solution as Julian Richard’s suggested in his book ‘The Stonehenge Environs Project’: “colluvium sediments may have been removed or thinned by the action of seasonal streams or higher water tables in the past”. 

Macklin, as we have now seen in this section has identified over one hundred Holocene river floods, twelve of which lasted hundreds of years, that would have contributed to this lack of alluvium or colluvium at Stonehenge Bottom.  Moreover, the sources of the rivers that lay this sediment over the centuries of water flow, rely on massive precipitation entering the rivers, cutting through rocks and valleys making them flow at extreme levels which create this erosion and consequential sediment. However, the source of Palaeochannel water are natural springs found locally underground and therefore would not contain the same alluvium levels as active flowing rivers – resolving this dilemma.

More information on the River Avon can be found at: https://youtu.be/j5LJ2sGcKOA