Portland Stone Formation - Wiltshire (Chilmark) (Lead Author: WGT)
The Portland Stone Formation (Portland Group) is the formal name for limestones in Wiltshire and Dorset deposited in shallow seas about 149-145 million years ago (Late Tithonian/Middle Volgian). In the Vale of Wardour, the Portland Stone Formation (Fm) is divided into three Members (Mbr), from bottom upwards, the Tisbury Mbr, the Wockley Mbr and the Chilmark Mbr. In Dorset the Portland Stone Formation is divided into the Portland Chert Mbr overlain by the Portland Freestone Mbr. In both Wilts and Dorset the marine Portland Group is overlain by the non-marine Purbeck Group.
In the Vale of Wardour, the Portland Stone has long been quarried between Tisbury in the west and Chilmark in the east. Names have evolved since the outcrops were first described over 200 years ago but two quality freestones, the Lower/Main Building Stone (Tisbury Mbr) and the Upper Building Stone (Chilmark Mbr), have been used widely in Wilts, North Dorset and Hants, e.g. Salisbury Cathedral. The Ragstone (Tisbury Mbr) has also been used locally. The Wockley Mbr is a soft lime mudstone unsuitable for building. Thin hard basal Purbeck limestones have also been used locally.
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Lower/Main Building Stone (c.12m): Once quarried extensively in the Chicksgrove and Chilmark area, this building stone is a fine-grained sandy limestone (quartzose glauconitic biocalcarenite, 20-40% quartz). The glauconite content (up to 6%) gives a green-buff appearance which can resemble Upper Greensand. Serpulids, ammonites & sponge spicules are common and trough cross-bedding is seen in outcrops. Some cross-cutting calcite veins occur.
Wardour Lower: Buff-coloured fine-grained glauconitic quartzose bioclastic limestone. Image: WGT
Sometimes referred to as “Chilmark Stone”, it is not to be confused with the Upper Building Stone, extracted only from Chilmark. The Lower or Main Building Stone is best designated as Wardour Lower or, if necessary, Chilmark Main. Currently quarried at Upper Chicksgrove only.
The area of rock shown in this XPL photomicrograph is
0.55mm x 0.35mm.
CHILMARK LOWER (Main) BUILDING STONE (under XPL) – (for number key see below)
1.Pastel shades of sparitic calcite – cementing this glauconitic quartzose limestone 2.Quartz grains vary between white and dark grey 3.A grain of plagioclase feldspar from older weathered rock, probably volcanic 4.Dissolved shell cavities which, if greatly interlinked, create porosity. 5.Brown micrite envelope enclosing minerals replacing a fragment of shell 6.Originally a fragment of shell, replaced here by large bright calcite crystals 7.Pellet of green/yellow glauconite – an alteration product of North Sea volcanic ash 8.Patches of muddy micritic calcite in stronger sparite (sparry calcite) cement |
Upper Building Stone (0-4m): This has a limited extent, being best developed in the Chilmark area where the Chilmark Oolite has long been quarried (surface & underground). This is a white-cream, shelly, cross-bedded oolitic limestone, similar to the oolitic limestones of the Isle of Portland. It is more durable than the Wardour Lower/Main and is best designated as Chilmark Oolite or Wardour Upper. This building stone is no longer quarried.
Chilmark Oolite: Whitish medium-grained moderately-cemented oolitic limestone (sawn slab).
PB collection, courtesy John Needham. Photo WGT
Chilmark Oolite: Porous oolitic grainstone, well compacted. Quartz nuclei, part coated. Srt. mSL 300u. Early cement rim only, rest pore space. Ooids mostly micritised - few good lamellae. Thin section by WGT. Photomic by PB.
NB. The commonly used term “Chilmark Stone” does not distinguish between these two very different rock types.
The Wardour/Chilmark stone can be similar to Upper Greensand, for a comparison click here Text by WGT & JT. Photos and images by JT and PB Feb 2017 |
