metallurgy developed into a science capable of amalgamating metals into bronze and other compounds. Bronze Age (3000 to 600 BC) Norfolk was a centre of metalworking and by the Late Bronze Age northern Norfolk had developed into a major area of production. There were also throwbacks to the earlier ages and one such type were the monuments. A significant find was discovered in 1998 between the high and low tidal limits off the north west coast of Norfolk at Holme-next-the-Sea. A ring of timbers was found and within it an upturned tree stump with its roots above the sand. The remains were excavated in 1999 and the samples taken dated it to 2049 BC. A find at Sutton (near Stalham) on 7 July 1875 revealed a copper alloy shield buried under peat lying on, and covered by, white sand. There have been numerous finds from all periods of the Bronze Age but amongst the most notable were a find of 141 axes heads at Foulsham and pottery including collared urns from the Witton Wood barrow. Grimes Graves is a large and well-preserved group of Neolithic flint mines at a site near Brandon, consisting of 400 pits. It was first named Grim's Graves by the Anglo-Saxons and was first excavated in 1870 when the pits were first identified as mines dug over 5,000 years ago. Iron Age There is evidence that much of Norfolk was intensively farmed by people during the Late Iron Age. The Cenimagni tribe who submitted to Caesar in 54 BC were settled in both lowland and upland Norfolk by this time. They may have been centred in north-west Norfolk, where hoards of coins and torcs have been discovered such as that found at Snettisham. Coins found in the south of the county indicate that the Iceni tribe may have been centred around Thetford by the mid-1st century AD. Roman Norfolk Norfolk's coastline is markedly different from the coastline that existed when the Romans first occupied what is now the county of Norfolk. The northern coast of Norfolk has eroded over the last two millennia, parts of it perhaps having retreated by up to . The eastern coast was dominated by an enormous estuary, the island of Flegg and a peninsula (modern Lothingland). The Wash was much larger in size and the area now known as the Fens was impenetrable marsh containing isolated islands surrounded by water. The