Mudstone, N Yorkshire

Muds, deposited in the seas of the Jurassic Period (about 180 million years ago), contain many fossil sea-creatures such as ammonites (below), which are ancient relatives of squid and cuttle-fish.

Most of these shells were crushed flat as the clay particles were compressed (under the weight of sediment on top) to form mudstone, but some fossils are found intact in hard nodules like the one shown. This happened because the decaying organic matter in the sediment caused it to harden quickly, before compression took place.