Blue Lias

  Blue Lias is a sequence of layers of limestones and shales, laid down in Jurassic times, between 195 and 200 million years ago. In geology, it corresponds with the Hettangian stage of the Jurassic. It is the lowest of the three divisions of the Lower Jurassic period and, as such, is also given the name Lower Lias. It 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.

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Blue Lias was used wherever it occurs 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. It also appears near Whitby in Yorkshire and Southam in Warwickshire where a pub is named after it.

The rock is rich in fossil remains from the Jurassic era. The blue-grey colour is provided by its iron content, such as pyrites.