Taunton Building Stones Walk; Fri 2Nd February 2018 a Leader's
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Taunton Building Stones Walk; Fri 2nd February 2018 A Leader’s Perspective Since I retired a few years ago I have been surveying building stones for the Somerset Heritage Trust. As Taunton town centre is a particularly good place for geologists to study building stones a walk here seemed a good way to start the AGM Weekend. Several people had done their homework and brought with them either Hugh Prudden’s book “Geology and Landscape of Taunton Deane “or Eric Robinson’s notes from a previous OUGS outing so I knew I had to concentrate! Taunton is located on Triassic Mercia Mudstone rocks which are unsuitable as a building stone. Rocks of many different lithologies, ranging in age from Devonian to Miocene, occur within a short distance of Taunton (see map) and have been used for the building stone in Taunton over many centuries. This variety of rock types and ages (see Table 1: stratigraphic column for the current BGS terminology and the equivalent traditional names of building stone) and the existence of buildings from many different historical periods mean that Taunton is a particularly good place for geologists to study building stones. We began our walk at the Taunton Castle which today is occupied by the Museum of Somerset. A wall by the approach path to the Castle Gatehouse is constructed of irregular pieces of chert which is used as a building stone in Taunton and southwards into the Blackdown Hills. It is the result of Miocene weathering of the chalk and today is found on the top of the hills and across the lower ground northwards towards Taunton, where it was probably transported in periglacial conditions during the Pleistocene Period. 1 Table 1: Approximate Stratigraphy of Building Stones showing traditional names used and up-to-date terminology Current BGS Group or Current and Traditional Local Names Formation Name Undifferentiated Building Stones Age Building Stones Building Stones - out of the area Taunton Deane Taunton Deane Miocene Clay with flints Chert LOCAL UNCONFORMITY Beer Head Limestone Beer Stone Cretaceous Upper Greensand Calcareous Grit LOCAL UNCONFORMITY Combe Down Oolite Bath Stone Doulting Stone or Inferior Oolite Jurassic Other Inferior Oolite Bridport Sands Ham Hill Stone Blue Lias Blue Lias White Lias White Lias North Curry Arden Sandstone Triassic Sandstone Helsby Sandstone Otter Sandstone TriasSandstones - Minor Intrusive Suite - Hestercombe Lamprophyre Diorite Permian conglomerates and breccias and conglomerates Thornverton Sandstone Permo Other Exeter Volcanics LOCAL UNCONFORMITY Carbonifer Westleigh Limestone Westleigh Limestone ous Pickwell Down Pickwell Down Sandstone Sandstone and Morte Slate Morte Slate Devonian Devonian Ilfracombe Slate Limestone Slates Cockercombe Tuff Hangman Hangman Sandstone Sandstone DevonianSandstones Other 2 In the archway leading to the Castle Courtyard the walls are largely red Triassic sandstones, some with conglomerate and breccias beds. The quoins (cornerstones) are distinctive golden-brown Ham Hill Stone (see Figure 1). This Jurassic bioclastic limestone, which is still quarried today at Montacute in South Somerset, was transported into the area along the river system from the 12th Century or earlier. Apart from Bath Stone, which arrived much later, it is the only stone from outside of the area that was used for building until the arrival of the canal and railway in the mid 19th century. Ham Hill Stone is easily carved and hence used widely in churches. Although fairly durable it weathers over time revealing cross bedding and many broken shell fragments but this does increase the interest for geologists. Fig 1: Quoins of Ham Hill Stone Within the Museum Courtyard we examined a section of the wall built of Lower Jurassic Blue Lias Limestone. Commonly used for building across the county this grey-blue fine-grained limestone outcrops to the south and east of Taunton. The ashlar (cut) blocks in the wall showed the blue centre and oxidised brownish edges which are characteristic of Blue Lias. To the right of the museum entrance stands the Wyndham Galleries, a 1930s extension to the Museum which must have been built with geology in mind as its walls contain ashlar blocks of five different rock types! Three we have already seen (Triassic sandstones, Blue Lias and Ham Hill Stone) plus Doulting Stone and Cockercombe Tuff. Doulting Stone is a cream to brown limestone from East Somerset which is readily cut and carved but is not often seen in the Taunton area. Cockercombe Tuff is a distinctive green stone consisting of volcanic dust. It outcrops over a very small area in the Devonian llfracombe Slate Formation of the Quantock Hills north of Taunton. There are only a handful of examples of its use as a decorative stone in Taunton and a few buildings constructed using it close to outcrop in the Quantock Hills. As a geologist one might prefer to think that the architects of this building sought to provide a point of interest for geologists but it seems more likely that they wished to match the variety of building stones seen elsewhere in the Castle! 3 Outside the Castle we examined a section of its 12TH Century walls which contained, within a small area, Triassic Sandstones, Blue Lias, Ham Hill Stone, chert and a single piece of a dark purple sandstone (see Figure 2). The latter is Upper Devonian Pickwell Down Sandstone from more than ten miles to the east of Taunton. As often is the case in geology this section of wall poses questions which are difficult to definitively answer for example why are there so many different rock types in this wall? We could also ask, why transport Pickwell Down Sandstone ten miles over 12th Century roads? Was stone being reused from earlier buildings? Were people from the surrounding area asked (compelled?) to contribute stone as a form of tax? Does it represent many different repairs over the years? It is probably a question more for historians than for geology. This is not an isolated example and during my building stones survey I have seen several examples of very old buildings, usually churches, with walls consisting of multiple types of building stone. Fig 2: Taunton Castle Wall: Triassic Sandstone, Blue Lias, Ham Hill Stone Close to the Castle stands the Winchester Arms. This is built of North Curry Sandstone, a greyish-brown medium to coarse grained sandstone of Triassic age. In parts of the walls the stone is deeply weathered revealing cross-bedding and beds containing broken pieces of a fine grained mudstone indicating that it was deposited in a fast flowing stream. North Curry Sandstone outcrops within the Mercia Mudstone to the west and to the east of Taunton and despite its tendency to weathering damage has been used in prominent buildings such as Queen’s College, Taunton and Norton Fitzwarren Church. 4 After walking a short distance to Corporation Street we see Devonian Morte Slate and Ham Hill Stone in the walls of the Corporation Hall which was built in 1522 as Taunton’s first Grammar School. The Morte Slate Formation is predominately a cleaved mudstone with some sandstone beds. Although a poor building stone the closeness of the outcrop to Taunton means that it is often seen in buildings usually as rubble stone walls with an another stone (here the Ham Hill) as quoins. Adjacent to the Corporation Hall is a building, originally the Taunton Technical School and currently not in use, constructed of Carboniferous Westleigh Limestone with Ham Hill Stone. Westleigh Limestone is a hard grey stone with distinctive chert bands which was imported to the area by canal and railway from the 1850s and seen in several buildings including the Railway Station in Taunton. Across the road can be seen the former library building of Triassic Otter Sandstone and the former Art College with a base of very large ashlar blocks of Westleigh Limestone and upper stories of fine cream coloured Bath Stone. Bath Stone is also seen in the former TSB Bank where, taking care not to disturb the diners (now a restaurant!), the oolites which characterise this stone could clearly be seen using a hand-lens. Fig 3: Church Tower of St Mary Magdalene Walking across the road and down Hammet Street we approached the magnificent church tower of St Mary Magdalene which was rebuilt in the 1860s using red Triassic sandstone and a darker red stone, Hestercombe Diorite (see Figure 3). This comes from a Permian aged intrusion located 5 miles north of Taunton. It can be seen in a few buildings near to its source and only here at St Mary’s in Taunton. 5 We examined the older parts of the church and noted here another example of the oldest walls containing the greatest number of different building stones (see Figure 4). In addition to its use for decorative stone, unusually, Ham Hill Stone can be seen to be good effect as the main building stone for part of the church. Fig 4: St Mary Magdalene church walls including Ham Hill Stone as a building stone We made our way back to the Museum, stopping on the way to examine a buttress of the Castle Hotel constructed of Cretaceous Calcareous Grit from the Blackdown Hills. Using a hand lens small black glauconite crystals, as occur in the similar aged Greensand, can be seen in the rock. In case you are still not convinced that geologists need to study building stones, Calcareous Grit was the fourteenth different rock type we saw in our short walk around Taunton! Garry Dawson 6 .