Auditing the claim of Holocene flooding of Stonehenge Bottom

Robert John Langdon has often claimed that the area around Stonehenge was flooded during prehistoric times, his latest Facebook post claims the evidence is in a borehole record and is auditable:

So I took him up, with an independent audit of what the borehole record actually shows.  

Borehole records available from https://mapapps2.bgs.ac.uk/geoindex/home.html?layer=BGSBoreholes

It's a long report, but the summary is: 

No direct evidence of submersion or flooding in the Holocene. The site appears to have been stable dry land since the end of the Pleistocene, consistent with the formation of chalk dry valleys through periglacial erosion and chalk dissolution.

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Borehole Report: BGS Borehole 17111365 (SU14SW62), Stonehenge Bottom

Executive Summary

This report summarises the key findings from the British Geological Survey (BGS) borehole log for Borehole 17111365 (reference SU14SW62), located in Stonehenge Bottom, Wiltshire, UK. The borehole was drilled as part of the A303 Stonehenge Ground Investigation project for the Highways Agency. It reaches a depth of 50.00 m and primarily encounters chalk formations with a thin superficial layer of topsoil and gravelly clay. No groundwater strikes were recorded during drilling, though borehole flushing medium was used.

Regarding the specific query on whether this location was under water in the last 10,000 years (the Holocene epoch), the borehole log shows no direct evidence of Holocene aquatic deposits such as alluvial silts, clays, or peats that would indicate prolonged submersion or flooding. The superficial deposits appear to be periglacial in origin (from the late Pleistocene), consistent with colluvial or head material common in chalk dry valleys. Mainstream geological interpretations suggest that dry valleys like Stonehenge Bottom have remained largely dry since the end of the last glacial period (approximately 11,700 years ago), formed by meltwater erosion under periglacial conditions. However, some alternative archaeological and palaeoenvironmental interpretations propose higher water tables and seasonal or tidal influences in the Mesolithic period (around 10,000–6,000 years ago), potentially leading to temporary flooding in low-lying areas. These views are based on core samples from nearby sites and historical depictions, but they remain debated and are not supported by this specific borehole log.

Borehole Details

• Borehole ID: 17111365
• BGS Reference: SU14SW62
• Location: Stonehenge Bottom, near Amesbury, Wiltshire. National Grid Reference: 412924.00 E, 141917.00 N (OSGB36).
• Ground Elevation: 96.00 m Ordnance Datum (OD).
• Drilling Method: Rotary cored using 150 mm triple tube wireline techniques.
• Drilled By: Noble (logged by JCKLB, checked by SJS).
• Drilling Dates: Not specified in the log, but associated with the 2001 project.
• Total Depth: 50.00 m.
• Project: A303 Stonehenge Ground Investigation, carried out for the Highways Agency.
• Remarks: Continued on multiple sheets (6 in total). Core recovery varied, with some reduced diameter cores due to catcher and core loss. No strikes for groundwater; flushing medium used for borehole stability.

Strata Summary

The borehole penetrates a thin superficial deposit overlying extensive chalk bedrock. The strata are dominated by various grades of chalk, typical of the Seaford Chalk Formation in the White Chalk Subgroup (Upper Cretaceous). Descriptions include structureless chalk, fractured chalk, and chalk with flint nodules or fragments. No significant organic or alluvial layers indicative of recent (Holocene) water bodies were noted.

The following table summarises the key strata, depths, thicknesses, and descriptions (interpreted from log sheets, with depths in metres below ground level):

Depth Range (m)	Thickness (m)	Level (m OD)	Legend	Description
0.00–0.10	0.10	95.90	C	Topsoil: Brown slightly silty sandy clay with rootlets.
0.10–1.00	0.90	95.00	B	Brown slightly silty sandy gravel: Gravel is fine to medium angular to subangular flint in a clay matrix. Medium density. Likely head deposit (periglacial colluvium).
1.00–5.20	4.20	90.80	Chalk (Grade V)	Structureless chalk: White, low to medium density, with fine to medium gravel-sized chalk and flint fragments. Occasional yellow staining.
5.20–9.11	3.91	86.89	Chalk (Grade IV)	Fractured chalk: White, medium density, with subhorizontal and subvertical fractures. Some orange staining and flint nodules.
9.11–18.50	9.39	77.50	Chalk (Grade III)	Blocky chalk: White to pale yellow, high density, with closely spaced fractures. Includes flint bands and nodular flints.
18.50–28.45	9.95	67.55	Chalk (Grade II)	Firm chalk: White, very high density, with occasional fractures and fine flint pebbles. Some grey marl partings.
28.45–47.50	19.05	48.50	Chalk (Grade I)	Hard chalk: White, massive, with sparse fractures. Includes yellow-brown staining and rare fossil fragments.
47.50–50.00	2.50	46.00	Chalk (Grade I)	As above, with increased drilling fluid loss noted. Exploratory hole end at 50.00 m.

Notes on Strata:

• Chalk grades follow the CIRIA classification (Grades I–V, where I is intact hard chalk and V is structureless/soft).
• Flint horizons and fragments are common throughout the chalk, typical of Cretaceous marine deposits.
• Core recovery was generally good (70–100%), but some intervals showed loss due to fracturing.
• No samples or tests for palaeoenvironmental indicators (e.g., pollen, diatoms) are mentioned in the log.

Groundwater and Hydrogeology

• Groundwater Strikes: None encountered during drilling.
• Behaviour: Borehole made using flushing medium (likely water or polymer-based). Remarks indicate "groundwater made at borehole flushing medium," suggesting artificial introduction rather than natural inflow.
• Implications: The chalk aquifer in this region is highly permeable, but the absence of strikes suggests the water table was below the drilled depth or not intersected. Current water table in the area is typically 20–40 m below ground, but historical variations are possible.

Analysis: Evidence of Water in the Last 10,000 Years

The borehole log provides insights into the geological history but focuses on engineering geology rather than palaeoenvironmental reconstruction. Key points:

From the Borehole Log

• Superficial Deposits: The top 1.0 m consists of topsoil and gravelly clay with flints, interpreted as head (colluvial/periglacial deposits). These are typical of late Pleistocene solifluction under cold climates, not Holocene aquatic environments. No laminated silts, clays, shells, or organic matter indicative of lakes, rivers, or flooding were recorded.
• Bedrock: Entirely chalk from ~1.0 m down, formed in a Cretaceous marine setting (80–100 million years ago). Fractures and staining may indicate groundwater flow, but no recent sedimentary overlays.
• Conclusion from Log: No direct evidence of submersion or flooding in the Holocene. The site appears to have been stable dry land since the end of the Pleistocene, consistent with the formation of chalk dry valleys through periglacial erosion and chalk dissolution.

Broader Geological Context

Dry valleys like Stonehenge Bottom are a hallmark of chalk landscapes in southern England, including Salisbury Plain. Their formation is attributed to:

• Pleistocene Periglacial Processes: During the last glacial maximum (Devensian stage, ~20,000–11,700 years ago), permafrost and meltwater carved valleys. Fluvial incision occurred under frozen ground conditions, leading to deep erosion without permanent rivers. Post-glacial warming caused springs to dry up as the water table lowered due to chalk permeability and reduced precipitation.
• Holocene Stability: Colluvial deposits in nearby dry valleys (e.g., east of River Till) accumulated from postglacial times through the medieval period, primarily via slope wash rather than fluvial action. No widespread evidence of Holocene rivers or lakes in these valleys; they have remained dry, with occasional surface water only in historical times (e.g., 19th-century depictions of ponds in Stonehenge Bottom).