Very high quality Jurassic limestone from the Chalfield Oolite Formation quarried in a variety of locations east of Bath. Many of the Bath Stones come from the Bath Oolite Member which is a true freestone, lacking fossils or lamination. The quality of the best Bath Stones is such that they have been mined deep underground.
Individual examples may be difficult to distinguish from higher quality Cotswold Stones. The lack of fossils in many examples and generally paler colour are the most distinguishing features. There are various named variants, some of which may show some distinguishing features. Stoke Ground (quarried in Limpey Stoke, Wiltshire) has a base bed with distinctive circular patches of fossil and cement material. Box Ground (mined in the Box Tunnel Mine) is sometimes characterised by calcite veins running perpendicular to bedding. Many of the mines and quarries in Wiltshire only became major concerns after the completion of the Kennet and Avon Canal in 1810 and so the date of building can serve to eliminate them as a possible source.
There are several sources still operating of which Stoke Ground is probably the best known.