Throughout history, a number of bridges named London Bridge have spanned the River Thames between the City of London and Southwark, in central London. The current crossing, which opened to traffic in 1973, is a box girder bridge built from concrete and steel. This replaced a 19th-century stone-arched bridge, which in turn superseded a 600-year-old medieval structure. This was preceded by a succession of timber bridges, the first built by the Roman founders of London.The current bridge stands at the western end of the Pool of London but is positioned upstream from previous alignments. The traditional ends of the medieval bridge were marked by St Magnus-the-Martyr on the northern bank and Southwark Cathedral on the southern shore. Until Putney Bridge opened in 1729, London Bridge was the only road-crossing of the Thames downstream of Kingston upon Thames. Its importance has been the subject of popular culture throughout the ages such as in the nursery rhyme "London Bridge Is Falling Down" and its inclusion within art and literature.The modern bridge is owned and maintained by Bridge House Estates, an independent charity of medieval origin overseen by the City of London Corporation. It carries the A3 road, which is maintained by the Greater London Authority. The crossing also delineates an area along the southern bank of the River Thames, between London Bridge and Tower Bridge, that has been designated as a business improvement district.