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.