The mound of "Pleistocene gravels" at Silbury Hill

Chapter 8
SILBURY HILL: A MONUMENT IN MOTION
Jim Leary
From Leary, J. Ch 8. Silbury Hill: a monument in motion
In: Leary, Darvill and Field. 2010, Round Mounds and Monumentality in the British Neolithic and Beyond. Oxbow Books.

https://www.researchgate.net/publication/308084271_Silbury_Hill_a_monument_in_motion

The first clear evidence for construction activity is a low, fairly unimpressive, gravel mound
overlying the old ground surface in the centre; it measured just less than a metre high and nearly
10 metres in diameter (Fig. 8.3 and Colour Plate 1). The material used for this mound was
Pleistocene gravels, suggesting that people would have had to quarry the material or found it
exposed in a river valley, for example the side of the River Kennet; either way, it was clearly very
deliberately imported and used here. Environmental evidence suggests that the material was
extracted from an open grassland environment.


Bedrock: chalk, with a mantle of clay-with-flints which has been mobilised downslope.


The gravel mound is something of a mystery. Could it have been an in situ Pleistocene landscape feature. or maybe even something relating to past glaciation -- for example a low morainic remnant, or a small patch of glaciofluvial material? Jim Leary clearly thinks that the gravels might be river gravels from the banks of the river Kennet or from another river valley -- but we know nothing about their characteristics or about the presence (or absence) of bedding features. The literature suggests that the gravels were imported and laid onto a pre-existing and prepared old ground surface -- but I am not sure where the boundary lies between hard evidence, carefully described, and supposition.........


If the gravels really were imported onto the site, why did the builders do that, if they then had to put up with gravel flowage or settling, requiring containment by timber and stones around the circumference of the circle? Was this strange little mound a ritual feature? That seems a bit far-fetched. Or maybe a drainage feature designed to deal with spring seepage and to stabilise the mound as it was built higher and higher?


The current view seems to be as follows:1. The Neolithic builders stripped the ground of its natural topsoil and turf, leaving behind a sterile layer of clay. This was then trampled and compacted by foot traffic.
2. The first act of construction was to pile gravel, possibly sourced from the nearby River Kennet, into a modest mound approximately one meter high.
3. This initial gravel heap was then contained by a circular revetment—a kerb—of wooden stakes and large sarsen boulders. The presence of this deliberate, human-made boundary confirms that the gravel core was a specific building phase, not a random, pre-existing geological deposit.


This is all very interesting. I'm trying to dig up the hard evidence that underpins this story --- at the moment it all seems to be based on speculation rather than hard published data.

Happy New Year!

 

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Late Glacial to Holocene climate oscillations

Below I reproduce the sequence of climate oscillations from the time of the LGM ice wastage period through to the present day.  After Prof Mike Walker and others.  In spite of a vast amopunt of researchj effort over the years, there is still doubt about the preciose dates allocated in this table -- radiocarbon ages and calendar ages are still being revised.  Among the questions still being mulled over:  To what extent is this sequence fully representative of various parts of the North Atlantic arena?  Was there really an Older Dryas phase that can be picked up in the field or the laboratory in multiple dispersed locations?  How widespread was the Allerod Interstadial?  Where do the Alftanes and Budi stages in Iceland fit into this table?  The Little Ice Age fits into the Sub-Atlantic phase, but where does the Neoglacial fit?   Should that label actually be used at all?

From Wikipedia:  The neoglaciation ("renewed glaciation") describes the documented cooling trend in the Earth's climate during the Holocene, following the retreat of the Wisconsin glaciation, the most recent glacial period. Neoglaciation has followed the Hypsithermal or Holocene Climatic Optimum, the warmest point in the Earth's climate during the current interglacialstage, excluding the global warming-induced temperature increase starting in the 20th century. The neoglaciation has no well-marked universal beginning, in particular not in the Greenland Icecore temperatures: local conditions and ecological inertia affected the onset of detectably cooler (and wetter) conditions.

13. Sub-Atlantic -- 2500 yrs BP to present

12. Holocene -- Sub-Boreal 5,000 -- 2500 yrs BP
11. Holocene -- Atlantic 7,500 - 5,000 yrs BP
10. Holocene -- Boreal c 9,700 - 7,500 yrs BP
9. Holocene interglacial -- Pre-Boreal c10,300 - 9,700 yrs BP
8. Sharp warming -- onset of Holocene c 10,000 yrs BP
7. Younger Dryas (Zone III) cold snap / Loch Lomond readvance c 11,000 -10,000 yrs BP
6. Allerod (Zone II) "interstadial" -- warming 11,800 - 11,000 yrs BP
5. Cooling erratically to Older Dryas (Zone I) 12,000 - 11,800 yrs BP
4. Sharp climate oscillations: mid-Weichselian substage 14,000 - 13,000 yrs BP
3. Bolling "interstadial" -- warming to almost interglacial level, started c 14,700 yrs BP (some dates place this event around 12,500 yrs BP)
2. Continuing cold -- sparse tundra vegetation 20,000 - 15,000 yrs BP
1. Deglaciation around 20,000 yrs BP