OSL Chronicles: Questioning Time in the Geological Tale of the Avon Valley

December 24, 2023 Trebor2510

Contents

1 Introduction
2 UPDATE
- 2.1 More Empirical Evidence of Post-Glacial Flooding and a Flooded Stonehenge
3 Further Reading
4 More Blogs

Introduction

When we explore the Quaternary deposits of the Avon Valley, a complex geological story begins to emerge.
This story challenges traditional interpretations and demands a more careful understanding of the landscape.

Across the valley, we find many different deposits.
These include clay with flints, head deposits, gravelly head, river terrace gravels, brickearth, and occasional peat.

One deposit stands out in particular.
Clay with flints.

This material is a residual deposit.
It formed through the breakdown of older Palaeogene sediments and the gradual dissolution of the chalk beneath them.

The precise age of these deposits remains uncertain.
However, many geologists believe that clay-with-flint deposits in southwest England date to the Pleistocene period.

These deposits sit on hilltop flats.
They are often found alongside older head deposits.
Those head deposits formed through processes such as solifluction and the slow movement of material downslope.

Together, these features show us a landscape shaped by long-term natural processes.
They offer a glimpse into deep geological time.

As we move down into the valley itself, we encounter new deposits.
These include head gravel, gravelly head, and head.

These layers are the result of water movement and slope processes.
They reflect fluvial transport, hill wash, hill creep, and solifluction.

Each layer records how water once moved across the landscape.
They show how the river interacted with its surrounding terrain.

One of the most striking features of the Avon Valley is its river terraces.
There are fourteen distinct terraces.

These terraces are wide and elevated above the modern valley floor.
They suggest that sediments were laid down broadly across the landscape, rather than cut into narrow channels.

The terraces maintain a consistent thickness along the valley.
This points to a system overloaded with sediment.

That sediment came from upstream sources and from tributary rivers.
Lateral erosion and repeated redeposition played a major role in shaping these features.

This behaviour does not fit neatly with simple explanations tied only to Milankovitch climate cycles.
Instead, it suggests a more complex interaction between river dynamics and landscape change.

When we look deeper into the geological history, the picture becomes even more complex.
In the Hampshire Basin, older terraces are labelled T5 through T10.

(OSL Chronicles: Questioning Time in the Geological Tale of the Avon Valley) — River Avon River Terraces showing the size of the River in the Past

Many of these terraces remain poorly studied.
Their ages are often uncertain due to a lack of detailed investigation.

Organic remains found within some terrace deposits have been linked to different interglacial periods.
These remains provide clues about past climates and environments.

However, recent research has introduced new questions.
Optically Stimulated Luminescence dating — known as OSL — has been applied to the Avon Valley sediments.

The results challenge earlier interpretations.
OSL dates from different terraces often appear inconsistent or even random.

This raises important concerns about accuracy.
The situation is similar to early debates surrounding radiocarbon dating.

Further complications arise when OSL dates do not match carbon-14 results.
These discrepancies suggest unresolved problems within current dating frameworks.

Differences between upper and lower catchments add yet another layer of uncertainty.
Together, these issues show that terrace dating is far from straightforward.

In conclusion, the Quaternary deposits of the Avon Valley reveal a highly dynamic landscape.
They challenge simplified geological narratives.

From clay-with-flint deposits on hilltops, to complex river terrace systems in the valley, every layer tells part of the story.

The uncertainties exposed by OSL dating highlight the need for continued scrutiny and refinement of dating methods.

The Avon Valley remains an open geological record.
One that invites us to look again, question assumptions, and read between the lines of deep time.

And there is one conclusion that now seems unavoidable.

The river terraces of the Avon were flooded in relatively recent times.
This would have had a profound effect on the people living in this region.

It also offers an important clue.
Many ancient sites appear to have been built along the shorelines of these raised rivers.

UPDATE

More Empirical Evidence of Post-Glacial Flooding and a Flooded Stonehenge

Prehistoric Levels and Widths for the River Avon

Take a close look at this illustration. It is not speculation, it is empirical science — mapped and measured river terraces from the Avon Valley, published in Egberts (2016), Pleistocene terrace formation and the Quaternary evolution of the Hampshire Basin, Bournemouth University.

What are we looking at?

These are the terrace steps cut by the River Avon over multiple glacial–interglacial cycles.
Each “T-level” marks a former stable floodplain where the river held its height for centuries, often millennia.
The heights are measured in metres OD (Ordnance Datum) and tied to known quarry and pit sites (e.g. Hatchet Gate Farm, Woodgreen, Somerley, Ashley).

How much bigger was the Avon?

Today, the river meanders with a width of just ~50 m near Salisbury.
At its maximum (T11), the Avon floodplain stretched ~12 km across.
That is ~240 times wider than the river today.

What does this mean for Stonehenge?

Phase 1 of Stonehenge (Car Park Postholes) sits on T9 (~90 m OD).
Phase 2 (ditch, Aubrey Holes, bluestones) cuts into T8 (~75 m OD).
The terraces show that the palaeochannel not only flooded up to the old car park, but at times overtopped the entire Stonehenge site.

Why this matters:

Terraces are not theory — they are empirical geomorphological evidence.
They prove that the Avon has flooded to multiple levels, sometimes far higher than the monument itself.
This is not about “if” water could reach those heights — the terraces prove it already has, repeatedly, over many Ice Age cycles.

So when critics dismiss the role of high water tables or argue “the site couldn’t have been wet,” they are ignoring the most basic geological record in front of us. The terraces are the diary of the river — written in gravel, chalk, and silt — showing that water rose and fell, over and over again.

The real question is not if Stonehenge was surrounded by water. It is when, and how many times it happened during its long prehistory.