Wednesday, 22 April 2009

Greenwich medieval tide mill

Southwark and Lambeth Archaeological Society recently hosted a talk on the Greenwich medieval tide mill. They have been kind enough to allow us to publish their account of the talk - and also thanks for permission from Simon Davis

There was great excitement when the medieval tide mill was found at Greenwich in 2008 – with more surviving than any in Southwark. Early contact was made with Simon Davis of MoLAS, who, after a period of doubt when work was stopped due to the developer of the site being ‘credit crunched’, agreed to speak to the Southwark & Lambeth Archaeological Society on 14th April 2009. Moreover, he arranged for Damian Goodburn, generally regarded as London’s premier archaeological timber expert, to speak too.
The site of the mill is about half a mile downstream of the (erstwhile) Royal Naval College, where the Thames turns north towards the O2 (Dome). The Naval College is now Greenwich University, but was previously: the Royal Naval Hospital (Asylum) for Seamen; the Tudor Palace of Placentia; Duke Humphrey’s house; and the Lewisham house of the Abbey of St Peter of Ghent – who were there from 918 to 1414 and (probably) built the mill. The site is known as Granite Wharf.
Mr Davis began with a plan of the site, showing bore holes and evaluation trenches – none of which had revealed the mill. Then he showed a picture looking north to the 1694 Naval Powder Magazine (surrounded by a square wall), thought to be near where Enderby House is now. In the foreground the scene shows peat cutting - suggestive of good damp conditions for archaeological survival.
The mill was found during site watching, when a machine exposed some massive timber beams, some 30m from the present river front. Two were 3 ft wide, across the centre of a 6m square timber floor frame (denoted FF1 herein*), pointing towards the River. Digging revealed further floor framing (FF2) towards the river, doubling the length of the mill building. It was possible to tell that there had been two phases of flooring, and evidence for braces showed how at least the lower part of the superstructure was built. The surviving timbers had been secured to the underlying ground by piles, four of these being cradle timbers supporting the corners of FF1. There were chalk foundations, with some mill stone sherds mixed in.
There was still a plank on one side of the entry channel to the mill, leading to the head race which ran over the floor of FF1 above one of the 3 ft wide beams; FF1 would also have supported the sluice. A woven hazel panel was found - at first it was thought to be a filter, but it had a very dense weave and would have impeded the water flow. As it passed into FF2 the channel dropped down into the wheel pit, which extended across FF2 into the tail race beyond.
It is hoped that access to dig the tail race will be granted later this year. There was no sign of the mill pond, which was possibly above the levels seen in the bore holes or evaluation trenches.
Mr Goodburn described the woodwork of the mill, and how it fitted in with the changes in technology - at that time carpentry was being introduced from France (it had an Arabic origin), replacing Saxo-Norman tree wright techniques. Carpentry includes prefabricated framing, usually made flat, the joints having carpenters’ marks.
All the wood was oak, and apart from the base timber of the wheel pit, carpentry techniques were used. The large beams were felled in 1194 (dendro date), smaller timber being similarly dated. Timber conversion (cutting a trunk to size) could be just trimming square for a large beam, or splitting in half, quarters etc. Some beams were tapered, the whole trunk from the base to the first branch having been used. Boards were not sawn but cleft, giving a characteristic feathered shape; the broad end can be grooved. (Only later did mills and similar structures use elm boards.)
The base timber of the wheel pit was dug out from a single three foot wide oak log (like a dug-out canoe), curving down from the entry to the level of the tail race at the exit, which was set at -1.15m OD. Above the base timber the wheel pit had tongue & groove boarding. A part of the mill wheel was found in the pit – enough to postulate a diameter of about 5½ m, with 12 spokes and 60 paddles. The wheel was a lightweight design, having a single rim with the paddles extending either side of it. Maintenance of the wheel would have been straightforward. One paddle survived.
Mortices in the timbers for vertical posts had no residual signs of the tenons, and floor boarding had been carefully removed, indicating that the mill had been dismantled after it went out of use.
The timbers have been taken up and sent for conservation, some to York where their 4m long facility made it necessary to cut the 6m timbers. They should be impregnated with polyethylene glycol in 2-3 years.
Tidal Range. Mr Goodburn said there must have been a tidal range of 5 to 5½ m to operate the mill. Preparatory building work for the mill would have been at a spring low tide, that level being well below the tail race. Other tide mills, wharves, etc. along the Thames would also have needed such a tidal range. (Sea level has risen over the centuries, but he said that the tidal range must always have been much as it is now – current received wisdom not withstanding.)
Mr Davis, in summing up, said that the Greenwich Tide Mill was of national significance. The developer showed great interest in the mill, and was very helpful while it was being excavated. He even suggested building a replica on site, though probably not full size, as a feature to lure buyers to the proposed housing.
Apologies for any confusion caused by the FF1 and FF2 designations, which will inevitably differ from the nomenclature in any proper publication.
Richard Buchanan

1 comment:

The Greenwich Phantom said...

Fascinating stuff! If the developer is sympathetic, does that mean the original will be preserved?