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 Fm is divided into the Portland Chert Mbr overlain by the Portland Freestone Mbr. See “Portland Stone - Dorset” elsewhere in this website. 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 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. N.B. The Lower Building Stone is also sometimes referred to as the Main Building Stone 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.
Sometimes referred to as “Chilmark Stone”, it is not to be confused with the Upper Building Stone, extracted only from Chilmark. The Lower Building Stone is best designated as Wardour Lower or, if necessary, Chilmark Lower. Currently quarried at Upper Chicksgrove only.
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 and is best designated as Chilmark Oolite or Wardour Upper. This building stone is no longer quarried.
Right (under XPL) – CHILMARK LOWER (Main) BUILDING STONE (for number key see below and right)
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
Lower 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 (qv). Serpulids, ammonites & sponge spicules are common and trough cross-bedding is seen in outcrops. Some cross-cutting calcite veins occur.
Chilmark Main Building Stone
A sandy limestone in the Tisbury Member of the Late Jurassic Portland Stone Formation of the Vale of Wardour.
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
Text and images by WGT and PB, updated August 2017