Book Notes pages 58-61 Bank & Ditch
Page 59 - Did Avebury's Ditches Hold Water?
In the the Keiller Room of the Red Lion pub in Avebury there is a well, disguised as a glass-topped table. In Sept 2010, when the water table was quite low, I put a tape-measure down the well and recorded its level as 14.3m below the floor level of the pub. The Red Lion sits at 155m above sea level, so the well's water level in Sept 2010 was about 141m. That was after a reasonably dry summer. Avebury's ditch was originally 9m deep, so its bottom would be at about 146m, Could it have been dug so deep so that it cut into the water table?
In February 2013 Jim Gunter, Dave Field and myself measured the well's level again, and concluded that the level of water was now some 4.5m higher than it had been in Sept 2010. Since levels had then begun to drop, we estimated that if we had measured it a week or two earlier it would have been more like a 5m difference.
The tricky part was then working out how that related to the height of Avebury's ditches. The modern OS map 157 shows all the centre of Avebury as being at 155m, but that is very rough. I consulted the 1900 edition OS map - the Red Lion's well is actually shown on it. The nearest spot height is 520ft, which is 158.5m.
I returned next day to put the lid back on the well, but first recorded the temperature of itswater using the pub's 'laser thermometer' that shows the temperature of anything it is pointed at. (It is designed for food hygiene.) The water at the bottom of the well was at 11 deg C. I also measured the water level again: it was 9.4m below the floor level. This time I let the tape all the way down below the water level - the well is 20m deep.
To find the correct altitude for the floor of the Red Lion's Keiller Room (where the well is) I calibrated my barometric altimeter to the 520ft spot height shown on the 1900 edition OS map, then checked it against the 525ft spot height. The floor around the well was then found to be at 521ft = 158.8m, so I rounded it to 160m.
Because the OS map's 5m altitude steps are too coarse to be of any use, I had made a map several years ago to show contours of the Avebury Henge in 1m steps. It was made using the same altimeter, taped to a walking stick at ground level. The map shows the ground surface of the henge next to the ditch as varying between 155 & 160m, so a ditch 9m deep would have its bottom at between 146 & 151m.
The Red Lion's well is 20m deep, and its bottom is at 140m; the two recorded water levels were at 145.7 and 150.6m. From this, it seems as though the ditch may indeed have cut into the water table, especially if the water table was higher than now.
Josh Pollard commented:
"Just looked at Gray's 1935 Archaeologia excavation report. A quick 'back of envelope' calculation using his heights published on the sections suggests the deepest part of the ditch he excavated (cutting IX, against the southern entrance) reached 149m OD, so your estimates are spot on. The ditch does of course vary a lot in depth around its circuit, but it really looks as though the intention at the S entrance at least was to reach/intersect with the water-table."
Dave Field added:
"I think it really needs to round to 159m which, minus the 9.4, leaves 149.6 - just half a metre lower than Gray's level for the bed of the Kennet. Still not sure what this means in terms of artesian factors.
Gray's illustration of course did not depict the water table just the level of the stream bed and we might expect higher rainfall to set a level somewhat higher than that.
However, the point remains that even with a small increase in the water table of the kind described in the Edmunds and Whitehead paper more of the ditch than expected might have been under water, particularly if it was seasonal."