Portland Stone Formation - Dorset (lead author: WGT)
The Portland Stone Formation (Portland Group) is the formal name for limestones in Dorset and Wiltshire, deposited in shallow seas about 149-145 million years ago (Late Tithonian/Middle Volgian). In Dorset the Portland Stone Formation is divided in two Members (Mbr) the Portland Chert Mbr overlain by the Portland Freestone Mbr. In both areas the marine Portland Group is overlain by the non-marine Purbeck Group. In Wiltshire (Vale of Wardour), it is divided in three, from bottom upwards, the Tisbury Mbr, the Wockley Mbr and the Chilmark Mbr. [See Portland Stone - Wilts, this website]
The Portland Chert Mbr is a cherty shelly lime wackestone-mudstone, rich in calcite casts of Rhaxella sponge debris. It is of little use as a building stone but is currently crushed on the Isle of Portland as aggregate. The chert was used for Stone Age tools (qv Chert). This member varies in thickness (10-30m) but underlies the Freestone Member throughout the outcrop from Durlston Head in the east to Portesham in the west and Portland to the south. To read about the Chert occurring in the Portland Stone Formation please click HERE.
2. Dorset Central: Portland & Ringstead Bay to Lulworth Cove. Here the limestones are mainly shelly cross-bedded oolitic limestones, extensively quarried on the Isle of Portland and exported to London since 1650. On Portland the quarrymen’s subdivisions (Base/Best Bed, Curf, Whit Bed, Roach) apply only in the northern area and it is only in this northern area that the distinctive alga Solenopora is common (diagnostic), as are algal/oyster patch reefs and their debris. The Roach is a local northern layer capping the Whit Bed containing empty moulds of bivalves and gastropods (unless silicified) – again diagnostic. This is a very durable rock used for harbour facings and sea defences (Lyme Regis Cobb, Portland Harbour). On the mainland, the oolitic limestones are fractured and used only very locally.
3. Dorset West: From Poxwell (3km from the oolites at Ringstead) to Portesham the Freestone Mbr is entirely lime mudstone with some chert layers and shell beds. Although used locally (e.g. Hardy monument) this is not a durable rocktype. The overlying basal Purbeck limestones are better building stones (Cypris Freestones, qv).
The Portland Freestone Mbr varies considerably in thickness (10-20m) and in composition across the outcrop - and in detail when exposed in 3D on the Isle of Portland (6-9m) There are three main rock types: Oolitic Limestones, Bioclastic Limestones and Shelly Lime Mudstones. There are three areas:
1. Dorset East: Coastal Isle of Purbeck. Known as “Purbeck Portland” the freestones long quarried on the coast are shelly bioclastic limestones (biocalcarenites, non-oolitic), with occasional chert. Calcite-filled shell moulds are common (“spangle”). The uppermost beds are shelly lime mudstones, capped by basal non-marine Purbeck limestones. From Worth to Bacon Hole, bioclastic limestones are interbedded with oolitic limestones.