The Norfolk Broads owe their existence to the large-scale extraction of peat and clay during the Middle Ages. They were once deep pits, up to in depth, from which an enormous amount of peat was dug over a period of centuries. Peat extraction may have begun during the Anglo-Saxon period, but the first evidence of the industry in Norfolk is from mediaeval abbey records. John of Oxnead is the first chronicler to record the major floods that recurred during this period, as the sea breached the vulnerable east coast and devastated the land. As a result of flooding, the extraction of peat declined and the records change from peat-digging accounts to descriptions of fen, marsh, fisheries and the importing of sea-coal from the north-east of England. Over a long period the battle to obtain peat and clay from the pits was steadily lost, as they become water-logged and then permanently flooded and all memory of the origin of the new 'broads' was lost.