Blue Lias

The Blue Lias is a formation of rocks mostly in south west England and parts of south Wales. 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 fossils, especially ammonites. Also found were remains of a number of dinosaurs, and the pterosaur "Dimorphodon".
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. 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.