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Norwich was mined for chalk and flints from the Middle Ages until the beginning of WWII but some of the chalk mines are much older, the earliest are believed to date from the 1100s. The oldest mines are located closest to the centre of Norwich and more mines were dug further out as the city grew. The chalk was used for liming in agriculture and in building mortar. The flints that can be seen embedded in layers in the chalk were used to build the city's walls and some of Norwich's finest buildings such as the Guildhall. Initially the chalk was excavated from an open hole. Later tunnels were dug from the side of the resulting pit, following the richest seams of flints. The last chalk mine to close was at Harford Hills, to the south of Norwich. There are no detailed maps of all the mines located within the city boundary but it is known that the great majority were privately owned and dug between the 12th and 18th centuries, at times when record keeping was not thought necessary. The only existing records date from later times when the Council documented the locations where collapses have occurred that were deemed to be due to mine workings. (Much of this information was taken from the County Council's website.)