This beautiful specimen from Warwickshire Museum’s collection is part of a natural limestone nodule, collected about twenty years ago from the now-flooded ‘old quarry’ near Southam, formerly owned by Rugby Cement. It is studded with beautifully preserved fossil ammonites; the shells of extinct marine molluscs that were distantly related to modern octopus and squid. Many of the ammonite shells are partly replaced by pale-coloured calcite – a common mineral. Others have a dusting of iron pyrite – also known as ‘fool’s gold’. The scale bar is marked in centimetres, giving an impression of the nodule’s size.
Layers of tough clay
Nodules like this one were found within layers of tough clay within the quarry faces. The clay layers are nothing more than 200 million year-old Jurassic sea-floor mud, deposited on the floor of the shallow sea that once covered large areas of what is now southern Britain.
We think that these nodules formed through the action of Jurassic storms, causing sediment-laden currents to scour out natural hollows in the sea bed. These hollows later became filled with ammonite shells and other sea floor debris. Millions of years later the shells within the hollows became naturally cemented together by calcium carbonate, to form the nodules.
Examples of these Jurassic treasures are currently on display at the Market Hall Museum in Warwick.