The River Path The River Path
Building Bridges
When a friend of mine in the Geologists' Association told me about Common Ground's Confluence Project and asked if I could help, an idea that had been sitting in the back of my mind was re-awakened. I am a geologist specialising in the stones used in historic buildings. After looking at all the villages in Dorset I have realised that the river valleys were important routes for the transport of stone. I have recorded the stone used in a few bridges as part of a study for English Heritage but a study of all the stone bridges in the Stour catchment area would be an individual project which could be added to Common Ground's work.
During this summer (2000) I have managed to get reasonably close to all the bridges cows, nettles, electric fences and so on permitting! There are, unfortunately, very few footpaths along the river, so I was 'exploring' I hope the farmers will forgive an earnest student. From past experience I know that the Stour can hide much of the stonework during wet weather, so the summer was essential. From the river bank one can see the structure of each bridge, and in many cases identify the different phases of work. While some are recorded as 16th or 17th century in origin, the 18th century seems to have seen a great deal of bridge building. This would have been part of the roadbuilding programme of that most prosperous century. Some bridges were replaced and others widened in the 19th century, and of course the coming of heavy motor transport has meant that the 20th century saw more replacement or strengthening work.
Now part way through my study, a pattern is beginning to emerge in the use of stone for the bridges, and transport is a clear factor. Stone is a heavy commodity; mostly it was carried by horse and cart, where the river valleys provide an easier route, but I believe that it was also carried by water. There are many records of stone quarried in Purbeck or Portland being carried by sea, and the Stour would have been navigable for some distance before the mill weirs were built. However, that requires an historical study which is phase two of the project.
The pattern that appears depends on the stone available in each locality, and on its suitability for bridges. There are many limestones and sandstones used for building houses and churches, but only certain ones will survive standing constantly in water. Within the Stour catchment area they are the Forest Marble limestone quarried south of Sherborne, the Upper Greensand sandstone quarried at Shaftesbury, the Tisbury Sandstone quarried in the Vale of Wardour, the Lytchett Matravers sandstone quarried west and north of Wimborne, and the Purbeck and Portland limestones.
The Shaftesbury, Tisbury and Lytchett Matravers sandstones were obtainable in large blocks, and were mostly cut as ashlar (cleanly squared blocks with very thin mortar). Portland limestone can also be cut as ashlar, but only one 19th century bridge has been wholly built of it. The Purbeck and Forest Marble limestones tend to be used as smaller blocks, though the size and workmanship depend a great deal on when the bridges were built.
As you would expect, the bridges over the Stour from Marnhull to Blandford have been built of Shaftesbury or Tisbury Greensand. The green colour is due to the presence of crystals of glauconite (dark green) scattered in the sandstone. The smaller bridges over the tributaries in the Blackmore Vale are mostly of Forest Marble, though Greensand has been used nearer to Shaftesbury. Several that are mostly brick have foundations or coping stones of Greensand.
Below Blandford the Shaftesbury Greensand has been taken as far as Crawford Bridge (top), but here it is mixed with flint from the Chalk and the Lytchett Matravers Sandstone (a warm brown colour, caused by iron oxide). White Mill bridge at Sturminster Marshall is entirely made from the Lytchett Matravers Sandstone, except for ornamental blocks of Purbeck Burr limestone in the outer arches. All the bridges below Sturminster Marshall, except Canford Bridge, which was entirely replaced in 1813, have the same sandstone in the original arches, often hidden underneath by later widening. Purbeck limestones are used in most bridges from Iford up to Wimborne, and these may have been brought by sea. Portland limestone, used for Canford Bridge, is found in 19th century work to widen or improve existing bridges. - The project continues.
This article originally appeared in Confluence News no. 9 (Sept-Oct 2000)
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