 The largest is Foulness, which is internationally important for wildfowl and waders, and has 71 nationally rare invertebrate species.The smallest is Holland-on-Sea Cliff, a geological site which throws light on the course of the River Thames before it was diverted south by the Anglian glaciation around 450,000 years ago.Hangman's Wood and Deneholes has deneholes, shafts created by medieval chalk mining which are now used by hibernating bats.