Portland Group (geology)
Stratigraphic range: Late Jurassic
Cliffs of the Portland Stone Formation on the western side of Portland Bill
|Overlies||Kimmeridge Clay Formation|
- For the computer company, see The Portland Group
The Portland Group (or Portlandian) is an Upper Jurassic lithostratigraphic group (a sequence of rock strata) in south-east England. The name is derived from the Isle of Portland in Dorset where the strata are exposed and have been extensively worked.
Lithology and stratigraphy
The rocks include limestones, dolomitic sandstone, siltstone and mudstone. It is divided into two formations; the Portland Stone Formation (or 'Portland Limestone Formation') and the underlying Portland Sandstone Formation (or 'Portland Sand Formation'). The Portland Stone Formation is further divided into a lower Portland Chert Member and an upper Portland Freestone Member, both of which are limestones. The Portland Sand Formation is made up largely of dolomites but includes siltstones and fine-grained sandstones in its lower parts.
Use in construction
- Barton et al. 2011 Geology of south Dorset and south-east Devon and its World Heritage Coast. Special memoir of the British Geological Survey. Sheets 328, 341/2, 342/3 and parts of 326/340, 327, 329 and 339 (England and Wales)
- British Geological Survey 2005. England and Wales sheet 327 Bridport (1:50,000 scale geological map)
- British Geological Survey 2000. England and Wales sheet 342/343 Swanage (1:50,000 scale geological map)
- British Geological Survey 2000. England and Wales sheet 341/342 West Fleet and Weymouth (1:50,000 scale geological map)