Inferior Oolite – Middle Jurassic (Lead author: JT)
The Inferior Oolite was so-named in 1813 by Rev. Joseph Townsend, a collaborator of William Smith, as it lies below the Great or Main Oolite, the Bath Stone. The Portland Stone was termed the “Superior Oolite”. The Inferior Oolite Formation comprises condensed, marine [R1] limestone (4 - 44 metres thick) deposited 174-167 million years ago (Aalenian, Bajocian and the lowest part of the Bathonian stages) but much of this time-span is not represented by rock, instead the formation consists of thin beds each of which represents a geologically short time span, separated from adjacent beds by breaks in deposition of varying duration. The recognition of over 30 ammonite subzones has enabled a refined time-scale for the deposition of the Inferior Oolite.
The Inferior Oolite comprises cream-orange bioclastic and nodular, often ferroan, oolitic limestones and micrites, with inter-bedded lime mudstones, ferruginous pelletal beds, oolitic & glauconitic horizons - all richly fossiliferous (small ammonites are a diagnostic feature).
The Inferior Oolite comprises cream-orange bioclastic and nodular, often ferroan, oolitic limestones and micrites, with inter-bedded lime mudstones, ferruginous pelletal beds, oolitic & glauconitic horizons - all richly fossiliferous (small ammonites are a diagnostic feature).
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The larger fossils give rise to weak areas in the building stone so, generally, only those beds with few, or smaller, fossils have been used. In South Dorset the Burton Limestone (Parkinsoni Zone, Upper Bajocian), provided the best building stone that could be cut as ashlar, notably in the Bridport area. These were known as the Top Limestones by quarrymen. Many of the beds with small fossils are seen as hand-dressed block stone in cottages and barns, especially in the Beaminster to Bridport area. To the west of Bridport around Symondsbury and Chideock, shelly limestones of the Scissum and Murchisonae zones have also been used as here they are formed of compact shelly, ironshot oolitic limestones of reasonable thickness for building purposes.
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In North Dorset, particularly around Sherborne, the best quality stone is Sherborne Building Stone of Garantiana Zone age from quarries in the Oborne area. This yellow stone is largely macrofossil free and contains few voids. To improve the appearance of high status building in the area made from Sherborne Stone, the corners are constructed from Upper Lias, Ham Stone of the Bridport Sand Formation. This freestone is a well-cemented fine-grained sand and is easily identified due to its make-up of small shell debris and cross-bedding.
Colfox Estate. This shows blocks of ‘Wild Bed’ mostly of Scissum to Murchisonae zones but possibly as high as Sauzei Zone. There are snuff-box horizons at lower levels but the typical occurrence is Discites Zone. Its ironshot oolitic, pisolite and oncolites welded bioclastic stone. In my charts this is beds 3-5. Image: R.B.Chandler
Text and images by JT and WGT March 2017. Updated June 2017 with further contributions by R.B.Chandler.
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Caption as photo left : R.B. Chandler
The equivalent of the Burton Limestone of Parkinsoni Zone age has been quarried on the northern side of the A30 road at Nether Compton. Apart from being an excellent building stone the large exquisitely preserved ammonites were exploited as a cottage industry locally. The stone can be examined in the grounds of Yeovil Pen Mill Station just beyond the Dorset border.
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