Portland Group (geology)

Portland Group
Stratigraphic range: Late Jurassic
Cliffs of the Portland Stone Formation on the western side of Portland Bill
Type	Group
Underlies	Purbeck Group
Overlies	Kimmeridge Clay Formation
Location
Region	southern England

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.

Rocks of this age have in the past been called the Portlandian stage by geologists which corresponds with the Tithonian stage of the internationally used geologic timescale.

Outcrops

The Portland Group outcrops in Dorset on the Isle of Portland, on and near the south coast of the Isle of Purbeck and to the north of Weymouth.

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

The Portland Freestone is a 5–15-m thick member of the Portland Stone Formation and is quarried on the Isle of Portland.