Field trip to the building stones of Canterbury

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Trevor Devon (UK)

Eleven members of the Hastings and District Geological Society (HDGS) assembled in front of the Canterbury Law Courts on a fine Sunday morning in June 2010 to meet up with our guide for the day, Geoff Downer. Geoff had previously given a talk to HDGS in the spring on the building stones of St Augustine’s Abbey and clearly had a great passion for this subject (he calls it a “hobby”).

The day was spent on a gentle walk around the eastern part of Canterbury, largely taking in St Martin’s Church, St Augustine’s Abbey and Canterbury Cathedral. Geoff provided a fascinating commentary on the geology, history, archaeology and architecture of the area, and stopped at appropriate sites to explore and identify the building stones more fully. Given that Canterbury had been an important major Roman town, and given its subsequent ecclesiastical history from Saxon to Norman and medieval times, there was no shortage of material to see.

After a brief introduction to the geology of the Ouse Valley, we took a short walk to look at the thirteenth century Conduit House (Fig. 1), a well-preserved example of medieval water technology that was used to collect groundwater from the natural springs of the surrounding hills and gravity feed it down to St Augustine’s Abbey using lead pipes. The reservoir and tunnels are constructed of all sorts of stone, using some reclaimed material from the nearby city and the structure would originally have borne a circular roof. From this fascinating feature, we then joined a famous footpath that had once served as the main track from the port of Fordwich (on the Wantsum Channel) to Canterbury, along which all the imported building stones were transported. Geoff explained that building stones had to be imported because Kent had so little good building rock (too much chalk).

Fig. 1. Remains of St Augustine’s thirteenth century Conduit House.

This track brought us down to St Martin’s Church (Fig. 2), the first of Canterbury’s three World Heritage Sites (the Cathedral and St Augustine’s Abbey being the other two). This church was built in the sixth century on the site of a Roman building that had been used as a chapel by King Ethelbert’s Christian wife, Queen Bertha. It is the oldest such church in continuous use in the British Isles and was used to welcome St Augustine to Canterbury in 597 AD. The church has been enlarged several times, notably in the seventh to eighth centuries and in the thirteenth century, so it was interesting for us to examine the stones used in the walls.

Notable of course was the reuse of Roman building materials, especially the long orange bricks – on one wall, Geoff pointed out a massive Roman lintel composed of a single stone, which was a blue ragstone typical of Hythe. Apart from the wide use of flint – common to Kent – other local stones identified in the walls included brown, grey and yellowish Thanet sandstones and greenish Kent ragstone from the Maidstone area. The other main ingredients were pale yellowish Caen limestone blocks and pale grey Marquise stones, both from France. A couple of rarer stones were pointed out – tufa, which is an irregularly shaped vesicular limestone that has formed by crystallisation out of ambient temperature saturated waters, and Purbeck Marble. The latter is a stone normally used for interior decoration, because its high iron content gets oxidised by weathering and, over time, this leads to disintegration of the rock where it is used externally. The church wall was a really great introduction to early building stones and would prove to stand us in good stead when we continued our walk down to St Augustine’s Abbey.

Fig. 2. Geoff Downer explaining the stonework used in the construction of St Martin’s Church.

Our route took us down onto the Richborough Road, which runs due east and leads to the Roman harbour of that name, close to present day Sandwich. Richborough featured a castle at the southernmost end of the Wantsum Channel and was almost certainly the primary landing point for the invading Roman forces. As we walked west along this road (called the Longport) towards Canterbury city centre, we stopped briefly outside Canterbury Prison, built as the city goal in 1808, and looked at the impressive gateway and very high brick walls. Next door was an impressive white building constructed in blocks of Portland Stone, which had been the Old Sessions House, but now belonged to Christ Church University.

Adjacent to this is an extension consisting of a modern circular building that has been sympathetically built in Portland Stone as well, except, in this case, the stone blocks have been made from quarry residues cemented together (much cheaper). On the opposite side of the Longport was an attractive row of almshouses, built of brick in 1597 in the Dutch style. This brought us to St Augustine’s Abbey, built towards the end of the Saxon dynasty in the tenth century. We did not actually need to enter the premises, as its outer wall in Longport provided plenty of scope for looking at the building stones used. By now, we were able to spot the “obvious candidates” – Caen stone, flint, ragstone, and sandstones, and found the rarer Marquise stone, tufa and Purbeck Marble.

Our next stop on our itinerary was a modern (1960s) 5-story office block that had been dressed with some interesting ornamental stone claddings. The panels identified included some metamorphic schist with a bright sheen from the mica, darker panels of gabbro, possibly Rustenburg granite (but actually a gabbro) and panels at street level of attractive brecciated serpentinite with veins of calcite. Next door, the building featured a black plinth of orbicular granite, with pinkish feldspar rosettes – the sort of stone used for kitchen tops. This one is known commercially as Rapakivi granite and is from Finland.

Along Lower Bridge Street, we were now following the eastern city wall, where we stopped at the Quenin Gate to inspect the stonework in the wall (Fig. 3).

Fig. 3. Geoff explaining the stonework on the Quenin Gate.

Identified here were several original Roman stones and orange bricks (that partly outlined the original gate), blocks of Caen limestone, pieces of Kentish ragstone and sandstone. The Quenin Gate, also known as the Queen’s Gate, was on Bertha’s route from the city to St Martin’s Church. On the other side of the road, we could see the beautiful Renaissance brickwork of Fyndon’s Gate, the main entrance to St Augustine’s Abbey. In front of the gate, a pleasant little park has been created, Lady Wootton’s Green, in which life-size bronze memorial statues of King Ethelbert and Queen Bertha have been erected recently (Fig. 4). This was a nice spot for a rest and lunch.

After lunch, we convened in the cathedral precincts for an account of its construction (Fig. 5).

Fig. 5. Canterbury Cathedral.

Geoff had to contend with the bells that pealed for some ten minutes, but on the plus side, being Sunday afternoon, there was no admission fee to pay. The cathedral as we know it today is over twice the length of the first Norman cathedral, and that was built on the site of a smaller Saxon cathedral. The design of two towers at the western end with a taller tower towards the eastern end extends throughout the five structures that make up the cathedral, although, by the late twelfth century, the “eastern tower” had now become more central.

During the twelfth and sixteenth centuries, various extensions were introduced, towers were taken down and reassembled, walls restructured with larger windows and reinforcing flying buttresses and additional chapels constructed at the eastern end. The major construction stone evident was the Caen limestone from Normandy quarries owned by Duke William, but, as we walked around the cloisters and outbuildings, Geoff showed us examples of tufa, sandstones, Purbeck Marble, Marquise stone, onyx marble and ferricrete that had been used in walls, columns, doorways and towers.

It was also evident from our walkabout how much restoration has been done in more recent times (including the present), with Caen stone remaining the main restoration stone, although there has been some problems with the quality of some of the supplies. A west country shelly limestone from the Mendips – Doulting stone – has been used in restoration for the past 100 years, where shaping and carving is significant (for example, monuments, doorways and windows).

In a trick question, Geoff asked us to identify the building stone of the high central tower called the Bell Harry Tower. It looked like Caen stone, but it was explained that the construction material was actually brick, clad with Caen stone. The brick was used because the stone would be too heavy for a tower of that height. Bringing the walk full circle, in one sense, to the north of the cathedral buildings, we saw the circular water tower that was at the monk’s end of the water pipes from the Conduit House (Fig. 6).

Fig. 6. The circular water tower used by the monks, which was fed from St Augustine’s Conduit House shown in Fig. 1.

The group enjoyed this outing very much and was very grateful to Geoff for his guidance notes, and for providing such a well-paced and fascinating account of these remarkable buildings.

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