Mudstone, Leicestershire

The clay layers shown in the photograph below were laid down as muds during the Jurassic Period about 150 million years ago. They contain fossils such as ammonites (ancient relatives of squid and cuttlefish) which tell us the age of the rocks and show they were laid down in the sea. These particular clays (known as the Oxford Clay) are quarried extensively in Bedfordshire and Cambridgeshire for making bricks.

They contain some quartz and calcite as well as clay, that make the bricks particularly strong; they also contain organic remains which burn as the bricks are heated in the brick kiln. This helps to reduce energy costs in brick manufacture.