Blue Lias

geologic formation in southern, eastern and western England and parts of South Wales

The Blue Lias is a formation of rocks mostly in south west England and parts of south Wales.[1] It is part of the Jurassic Coast World Heritage Site.

This is a rhythmite: obvious repeat pattern, with limestone blocks with shale in between them. Blue Lias cliffs at Lyme Regis, Dorset
Lower Lias at Nash Point, Glamorgan, Wales

It is part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers. It was laid down in late Triassic and early Jurassic times, between 195 and 200 million years ago. The Blue Lias is famous for its fossils, especially ammonites. Also found were remains of a number of dinosaurs, and the pterosaur Dimorphodon.[2]

The Blue Lias alternates between limestone, (with some clay), and mudstone. These alternations are caused by short-term climatic variations, thought to be caused by Milankovitch cycles.[3] The blue/grey colour is given by iron pyrite.

A bit higher, these limestone-mudstone alternations change into a clay member. The deposition of a clay-rich mudstone member shows it was laid down in a deeper marine environment.[4]

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  1. Also some other parts of south and east England.
  2. Coast and country: geology walks in and around Dorset, including excursions within the World Heritage Site 2003. Dorset Geologists' Association Group.
  3. Small, slow but regular changes in the Earth's orbit round the Sun, and the tilt of the Earth's axis. The dynamics are complex. The changes affect the 'insolation' (sunlight falling on parts of the Earth). This leads to cycles of climate on Earth, at about 21,000, 41,000 years, 100,000 and 400,000 years. This whole field is still under active research.
  4. Ambrose K. 2001. The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands Archived 2006-03-07 at the Wayback Machine. Proceedings of the Geologists' Association 112 (2) pp. 97-110.