Blue Lias

The Blue Lias is a geologic formation in southern, eastern and western England and parts of South Wales, part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers, laid down in latest Triassic and early Jurassic times, between 195 and 200 million years ago. The Blue Lias is famous for its fossils, especially ammonites.

Its age corresponds to the Rhaetian to lower Sinemurian stages of the geologic timescale, thus fully including the Hettangian stage. It is the lowest of the three divisions of the Lower Jurassic period and, as such, is also given the name Lower Lias. Stratigraphically it can be subdivided into three members: the Wilmcote Limestone, Saltford Shale and Rugby Limestone.[1]

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Contents
[hide] *1 Lithology and facies
 * 2 Occurrence
 * 3 Use in construction
 * 4 Paleofauna
 * 4.1 Dinosaurs
 * 4.2 Pterosaurs
 * 5 See also
 * 6 References
 * 7 External links
 * }

[edit] Lithology and facies
The Blue Lias comprises decimetre scale alternations of argillaceous limestone and mudstone. These alternations are caused by short-term climatic variations during the Early Jurassic attributed to orbital forcing (Milankovitch cycles). These limestone-mudstone alternations pass up into a clay member formerly known as the Lower Lias Clay now the Charmouth Mudstone Formation. This lithology consists of monotonous mudstones weathering to clay at the surface. Sparse thin limestone and nodule bands are seen where the rocks are exposed. The deposition of a clay-rich mudstone member normally indicates deposition in a deeper marine environment.

[edit] Occurrence
The Blue Lias is a prevalent feature of the cliffs around Lyme Regis and Charmouth, on the Jurassic Coast in Dorset, where it exists in layers of limestone interspersed with softer clay. It is also present in Somerset, particularly around the Polden Hills and Glastonbury, and it forms a broad plain across the East Midlands. It also appears near Whitby in Yorkshire and Southam in Warwickshire where a pub is named after it. There are outcrops along the coast of South Wales, notably that of the Vale of Glamorgan.[2] The type section of the Blue Lias is at Saltford near Bath.

[edit] Use in construction
Lytes Cary, Somerset, built of Blue Lias with Ham stone dressings around the windowsBlue Lias is useful as a building stone, and as a source of lime for making lime mortar. Because it is argillaceous, the lime is hydraulic. Since the mid-nineteenth century, it has been used as a raw material for cement, in South Wales, Somerset, Warwickshire and Leicestershire. The cement plant quarry at Rugby, Warwickshire is probably the best exposure of the formation: more than 100 layers can be seen.

Occurrences of Blue Lias can be found in buildings local to its source as well as the local churches and cemeteries (where it is used in tombstones). A prime example of a 'Blue Lias' town would be Street, near Glastonbury. Other local examples of Blue Lias buildings can be found in the nearby towns of Somerton & Ilchester.

It is still popular in more modern-day surroundings where it is still used in the construction of new housing developments and extensions for existing buildings in conservation areas. Blue Lias is mainly used in flooring, walling and paving slabs - both coursed and layered. It is evident too in the making of flagstones and cobbles.

There are only three quarries in the country quarrying Blue Lias at present. Ham & Doulting Stone Co Ltd operate one of these, Tout Quarry near Somerton.[3]

[edit] Paleofauna
[4]

The rock is rich in fossil remains from the Jurassic period. The blue-grey colour is provided by its iron content, enclosed to a large extent in pyrites.