Prehistoric Rivers – Page 3 – Prehistoric Britain

Rivers were higher in the past – Now UPDATED here: https://prehistoric-britain.co.uk/case-study-river-avon

At the outset, the clay-with-flints, a vestige of ancient weathering and erosion, stands as a testament to the relentless forces of nature that sculpted the landscape. Born from the remnants of Palaeogene sediments and the dissolution of chalk, these deposits serve as silent witnesses to the Pleistocene’s cold embrace. Their presence on the hilltop flats signifies a chronological anchor, predating the rhythmic succession of river terraces that stitch the valley’s quilt. (The Mesolithic River Avon)

Rivers were higher in the past (The Mesolithic River Avon)

As one descends the slopes, a mosaic of older head deposits unfolds, their genesis tied to the ancient processes of solifluction and solution. These sediments, bound to the clay-with-flint, narrate a tale of gradual descent and transformation, shaping the valley’s upper reaches with a subtle, yet profound, hand.

Further down the valley, the narrative evolves with the introduction of head gravel, gravelly head, and head deposits. These characters in the valley’s story are borne of fluvial transport, hill wash, hill creep, and solifluction—agents of change that have, over millennia, contributed to the valley’s sculptural form. The river terraces, numbering fourteen, ascend like steps from the valley floor, each a plateau from which to view the passage of time. The highest terraces, perched up to 100 meters above the valley, offer a broad vista extending 12 kilometers across, while the lower terraces, more intimate in their proximity to the present-day river, mark the recent chapters of geological history.

The consistency of thickness across these terraces speaks to a dynamic equilibrium of erosion and deposition, influenced by sediment overloading and tributary contributions. This interplay suggests a complex narrative of landscape evolution, one not solely dictated by the simplistic rhythm of Marine Isotope Stage cycles but enriched by a multifaceted process of lateral erosion and sediment redeposition.

Amidst this discussion of terraces and quaternary deposits, the narrative briefly diverges to contemplate the pre-Quaternary geology, where terraces from the River Avon linger in the Hampshire basin, their ages enshrouded in mystery. The challenges of dating these terraces, and by extension, understanding the full scope of the valley’s geological history, are underscored by recent findings that question traditional dating methods. Such inquiries not only deepen the mystery but also invite a reevaluation of our understanding of the Earth’s past.

Thus, we are reminded that the study of the Avon valley’s quaternary deposits is not merely an academic exercise but a profound exploration of the human quest for knowledge and understanding. It is a journey that connects us to the very essence of the natural world, revealing the intricate interplay of forces that have shaped not only the valley but also the broader tapestry of Earth’s geological history. (The Mesolithic River Avon)

Figure 5 – OSL Results Avon River

(The Mesolithic River Avon)

The intriguing findings presented in the diagrams regarding Optically Stimulated Luminescence (OSL) dating within the Avon valley unearth a complex narrative of sediment deposition and geological processes that challenges traditional understandings. The OSL results, as depicted in Figure 5, illuminate the temporal relationship between terrace formations and Marine Isotope Stages (MIS), while Figure 6, based on a three-dimensional model constructed from borehole data, offers a visual cross-section of the valley’s superficial geology.

The OSL ages for terraces T10 through T7, indicating deposition during or before MIS10/9, including the Last Glacial Maximum (LGM), suggest a timeline that not only aligns with but also refines previously established chronological frameworks. This refinement has significant implications for interpreting the archaeological record associated with Terrace T7 and recalibrating regional uplift and incision rates, which are crucial for understanding landscape evolution over geological timescales.

However, the apparent inconsistencies in the OSL dating results, particularly the dating of Terrace T7 before Terrace T10 and the identification of a Loess Terrace laid during the LGM, introduce a paradox into the sedimentary record. These anomalies challenge the linear progression implied by the terrace hypothesis that has guided interpretations of the valley’s geological history.

The highest terrace, T10, positioned at 102 meters above ordnance datum (OD) as illustrated in Figure 6, spans an unexpectedly broad temporal range of over 200,000 years, according to OSL dating. This finding disrupts the presumed chronological order, especially when juxtaposed with the dating of Terrace T7 at 58 meters OD, which, perplexingly, predates T10. Additionally, the Loess Terrace, situated at 77 meters OD and undifferentiated in the terrace sequence, laid down during the LGM, along with Terrace T4, which harbors the youngest dates, further complicates the scenario.

These results hint at a more intricate story of terrace formation and sediment deposition than previously thought. The seeming randomness and inconsistencies in the dating challenge the traditional terrace hypothesis and suggest that other factors, perhaps related to climatic variations, tectonic activities, or both, played significant roles in shaping the valley’s geomorphology.

The evidence points to a dynamic and possibly non-linear process of terrace formation, where episodes of sediment deposition were influenced by a combination of environmental conditions, rather than a simple chronological succession. This complexity underscores the need for a reassessment of the methods and models used to date and interpret terrace formations, advocating for a more nuanced understanding of the interplay between geological processes and climate change over the Pleistocene.

Thus, while the OSL dating provides valuable insights into the timing of terrace deposition, it also raises critical questions about the reliability of traditional chronological frameworks and the factors driving landscape evolution in the Avon valley. These findings invite further investigation and a reevaluation of existing hypotheses, highlighting the ongoing dialogue between past and present in the quest to decipher Earth’s geological history.

Figure 6- Avon River Terrace Levels – Avon River

(The Mesolithic River Avon)

The authors’ observations highlight significant discrepancies and anomalies in the OSL dates that raise questions about the method’s reliability in certain contexts, particularly when compared to other dating methods like radiocarbon dating. These discrepancies are not merely academic curiosities; they fundamentally challenge our understanding of the temporal and environmental context in which these sediment layers were deposited.

The attempt to explain the notable discrepancy in the age estimates of Terrace T4 across different locations within the Avon valley suggests that sediment reworking due to recent fluvial processes or the presence of compound terraces exhibiting different depositional behaviors might be responsible. This acknowledgment of variability within the depositional environment underscores the dynamic nature of fluvial landscapes and the complexity of accurately dating such contexts.

The variability in OSL dates for samples taken at the same soil level (e.g., GL14039, GL14041, GL14038, GL14040) further complicates the narrative. The presence of nearly contemporaneous dates within error limits, juxtaposed with the significantly different sedimentation rates observed just below the topsoil, suggests that the depositional history of the Avon valley is more nuanced than previously understood. These findings indicate that relying solely on visual stratigraphy for dating purposes can lead to inaccuracies, reinforcing the need for a multi-methodological approach to construct a reliable chronological framework.

The comparison between OSL and radiocarbon dating, as discussed in the Gaigalas (2000) study, exemplifies the potential for significant age discrepancies between different dating methods. The observation that OSL dates can be substantially older than their radiocarbon counterparts highlights the need for caution and cross-validation when interpreting chronological data, especially in contexts where sediment exposure and reworking may have occurred.

The discussion of Holocene river flooding and its impact on the dating of river terraces introduces an additional layer of complexity. Flooding events can lead to the deposition of silt and other materials that obscure the original depositional sequence, potentially leading to out-of-sequence terrace dates. This phenomenon complicates attempts to use uplift modeling or the Palaeolithic record as reliable chronological markers, as evidenced by the discrepancies in age estimates for Terrace T7.

The passage concludes by emphasizing the potential of terrace deposits to provide a valuable chronological framework, albeit one that must be approached with caution. By integrating chronometric age control with detailed modeling of deposit height and thickness, researchers can gain a more nuanced understanding of the Avon valley’s landscape evolution. This approach not only enhances our interpretations of past hominin landscape use but also improves the predictive modeling of Palaeolithic sites. The challenges and discrepancies encountered in OSL dating underscore the importance of adopting a holistic and critically engaged approach to understanding the geological past, one that acknowledges the inherent complexities and uncertainties of dating dynamic fluvial landscapes.

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.