Iron Age salt works (known as red hills) have been found in large numbers around the Essex coast, including several large concentrations located in the salt marshes close to Camulodunon in the Colne Estuary, on the Roman River near Fingringhoe, in Alresford Creek, on Mersea Island, the Pyefleet Channel, the Blackwater River and around the Tendring Peninsula. Two large groups existed at Peldon and Tolleshunt D'Arcy. Camulodunon may have been an at the centre of the local trade in this important preservative.