Sherborne Building Stones
More than most perhaps, this day was one to be there rather than read about. All more so as we came out of Sherborne Abbey and the sun broke through, so the little party that had shivered in the chill wind as we gathered started to relax and the photographers to smile.
That is jumping the gun a bit. This was a journey mostly through the early Jurassic oolitic limestones, these awkward ones which, from the colour, texture, and cross-bedding, the brash beginner can so easily declare simply to be 'sandstones' in buildings. We met the local stone first in the monument on the green, the outside of the abbey, and the building round the Close. We found that there were two local sources of subtly different quality.
The town itself is built on the Inferior Oolite, Inferior being of course simply a stratigraphic term. The building stone version is ‘Ham Stone’; though this can be worked decoratively, as we saw in the monument, it tends to appear in building facings as roughly faced rubble The fully finished ashlared stone is generally Sherborne Building Stone, which comes from higher up the hill, in a thicker formation. As we saw later, the upper layers of the Sherborne Stone are also rubbly and these were burnt for lime mortar, so the entire mineral structure of a building can come from this one origin.
Our Leader, Jo Thomas, did not let us go into the Abbey out of the cold until we had also studied the ‘cobbles’, not chert but the Forest Marble sandstone from a little later in the Jurassic and from the hill to the S of the town across the river. We learnt how the intelligent cobble path maker will put the slablets with the bedding vertical and across the line of the path, both for maximum resistance to wear and for the best grip for the walker; we saw later in the town how the ignorant put the bedding lines running downhill along the path direction.
Inside the Abbey, we found Ham Hill stone preponderant, but Forest Marble in some detailing and the late Jurassic Purbeck Marble, from South Dorset, as en ‘exotic in the tombs, a font base, and as one shaft in the Lady Chapel - other shafts were Forest Marble with a somewhat similar appearance. An interesting variant was the vaulting stone ot the interior of the Abbey roof, which was a yellow limestone tufa, the limestone redeposited in running watcr This could have been of local Sherborne Park stream origin but might also have come in with Caen stone from the Seine area.
Walking up towards the old quarries we were surrounded by Sherborne and Ham stone of all ages. Most of the quarries have been filled by tipping, the sad fate of so many geologically interesting exposures but in the relandscaped tip one face has been left and cleared of hanging ivy. A much better exposure, a little higher up the bill, is the result of the work of Dorset geologists, who used public and private funding and much effort to clear a fly-tipped quarry and ensure that it was secure against further despoliation to be preserved as a Regionally Important Geological Site (RlGS).
Lunch was had on the Fullers’ Earth Clay River terrace, now playing fields. Perhaps fortunately the Fullers’ Earth here is not the industrially important variety, or we might have been sitting on another reclaimed tip, while we admired Doreen Smith’s Stour Valley display board funded through, amazingly, Help the Aged. The OU’s Third Age contingent might make more use of this category for pursuing publication?
Our afternoon country tour first took in Cornford Bridge, where we admired the mix of Forest Marble and Ham Stone in the facing and tutted at the insensitive use of French stone for part of the coping during the recent renovations. We were now in Forest Marble and Cornbrash country and went to admire the oldest working post-box in mainland Britain, in front of a Forest Marble cottage. We saw clearly here how the yellow-grey Forest Marble, in the right exposure, becomes characteristically faced with red lichen making it resemble an ironstone.
Our final stop, at Sturminster Mill, took us to the top of our early Jurassic sequence, skipping the Oxford Clay into the Corallian oolite. The mill was a mixture of this stone, found all about the river cutting (some specimen hunters had much satisfaction behind the Men’s toilets!) and Forest Marble, again in relatively thin slabs and camouflaged with red lichen.
With the lower courses of the Mill we skipped a major Unconformity above the Corallian. The rest of the Jurassic and through to the middle Cretaceous either was never deposited or was eroded away in these parts; nearby, in Shafiesbury, the sequence restarts with the Gault and Upper Greensand. The Greensand makes a building stone, which is deemed more resistant to occasional flood immersion that the limestones and so was brought in for the susceptible part of the Mill.
Finally, we strolled across the water meadows to the bridges, admiring the calcite crystals in some of the stones sparkling in the afternoon sun, and ensured no members of the party made themselves liable for transportation, as the plaque on the bridge warned us, by damaging the structure.