What is Blue Lias?
[[File:Blue lias cliffs at Lyme Regis.jpg|thumb|right|250px|This is a [[rhythmite]]: obvious repeat pattern, with [[limestone]] blocks with [[shale]] in between them. Blue Lias cliffs at [[Lyme Regis]], [[Dorset]]]] [[File:Lower Lias Nash Point Glamorgan.JPG|thumb|250px|Lower Lias at Nash Point, [[Glamorgan]], [[Wales]]]] The '''Blue Lias''' is a formation of rocks mostly in south west [[England]] and parts of south [[Wales]].<ref>Also some other parts of south and east England.</ref> It is part of the [[Jurassic Coast]] [[World Heritage Site]]. 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 [[fossil]]s, especially [[ammonite]]s. Also found were remains of a number of [[dinosaur]]s, and the pterosaur ''[[Dimorphodon]]''.<ref>''Coast and country: geology walks in and around Dorset, including excursions within the World Heritage Site'' 2003. Dorset Geologists' Association Group.</ref> 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 cycle]]s.<ref>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.</ref> 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 [[Ocean|marine]] [[environment]].<ref>Ambrose K. 2001. [http://www.ingentaconnect.com/content/geol/pga/2001/00000112/00000002/art00001;jsessionid=iyhnp71msoi2.alexan