The Portland Group (defining the Portlandian) is a Late Jurassic (

Tithonian In the geological timescale, the Tithonian is the latest age of the Late Jurassic Epoch and the uppermost stage of the Upper Jurassic Series. It spans the time between 152.1 ± 4 Ma and 145.0 ± 4 Ma (million years ago). It is preceded by the K ...

) lithostratigraphic group (a sequence of

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

) in South East England. The name is derived from the Isle of Portland in Dorset where the

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

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 late Tithonian stage of the internationally used

geological timescale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...

Outcrops

The Portland Group crops out 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 Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

, and

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

. 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 The Portland Sand Formation is a limestone formation from the Tithonian stage of the Jurassic period on the Isle of Portland, Dorset, England. The formation is made up largely of dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate m ...

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 thick member of the Portland Stone Formation and is quarried on the Isle of Portland.British Geological Survey 2000. England and Wales sheet 341/342 ''West Fleet and Weymouth'' (1:50,000 scale geological map)

References

{{Reflist Geological groups of the United Kingdom Geologic formations of England Jurassic System of Europe Jurassic England Tithonian Stage Limestone formations Sandstone formations Geology of Dorset Isle of Portland