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Ogmion
The Landscape Channel Ltd.
Landscape Studios
Crowhurst
East Sussex TN33 9BX

20th May 1996

Dear Nick,

Thank you for sending the copy of the report concerning the archaeological excavations undertaken at Upper Wilting Farm. There are a number of issues arising from that report which require further consideration:

1. Surveying.

On page 7, paragraph 3.5 it is stated that the surveying was tied-in with Ordnance Datum using the "nearest spot height." There could be problems using spot heights rather than bench marks. Bench marks provide a height to the nearest centimetre, but spot heights, if taken directly off an O.S. map give heights rounded to the nearest metre. This means that there is a potential error of about 0.5 metres when surveying from a spot height, unless the surveyors have acquired more accurate data directly from the Ordnance Survey. A further error is introduced if the exact location of a spot height cannot be found, and an additional error occurs if the spot height's original position has been disturbed.

The report does not state which spot height was used, nor does it discuss the possible error attached to this form of surveying. Therefore, the elevation figures given in the report on pages 17-13 should be treated with caution, at least until Wessex Archaeology clarity the reliability of their surveying using spot heights. They have provided figures to the nearest 10 centimetres, although the figures are also expressed to two decimal places, that is to the nearest centimetre (e.g. .."c.3.50 and 7.2Om AOD" (paragraph 3.5, page 18)). It is usually safest to assume that at best surveying from spot heights will give you elevation data no more accurate than to the nearest metre.

2.Silting-up of the channels.
On page 18, paragraph 8.4, the report argues that channels were rendered virtually inaccessible by slope wash by the Roman Period. This conclusion is largely derived from our own publications (Smyth & Jennings 1988,1990), but is one that we certainly did not draw. We argued that much of the slope wash material was deposited on the flood plain, not in the channels. Whether channels become silted-up or not is largely dependent on their hydraulic efficiency. I suggest that the hydraulic efficiency of the channels within the Combe Haven valley was reduced as a consequence of the accretion of large volumes of gravel, via long shore drift, at the entrance to the valley during the l3th/l4th centuries. This would have reduced the volume of flow within the valley by eliminating tidal current activity. Therefore, I doubt that there had been severe silting of channels by the Roman Period.

3. Trench G.
This is the trench in the Redgeland area, and is the one which probably has the most archaeological potential. Indeed, the report discusses the presence of a 'buried soil', with charcoal, burnt clay and slag. I am surprised that the archaeological report did not make more of these finds, especially as they resulted from a small trial trench. Their concluding paragraph 8.6.7. is phrased in a very conservative manner. Clearly this area is worthy of a more detailed investigation.

I am not sure how they have concluded that the present surface relief is a consequence of rotational slipping or slumping. In order for this process to occur the geology must comprise a material such as clay overlain by a more massive rock, e.g. sandstone, and the toe of the slope must be undercut. I am not sure what the detailed geology is for that area. The report describes the 'natural' (bedrock?) as being a "yellowish brown silty clay." Figure 4 of the report, which illustrates Trench G in section, does not contain any of the geological detail necessary to conclude that slipping occurred. Also, it is not clear what process may have induced undercutting, and therefore instability, at the toe of the slope. One problem here is that the authors of the report do not indicate when the slipping took place.

Yours sincerely,

Simon Jennings

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