River Thames Thames Estuary), the river is subject to tidal activity from the North Sea. Before the lock was installed, the river was tidal as far as Staines, about upstream. London, capital of Roman Britain, was established on two hills, now known as Cornhill and Ludgate Hill. These provided a firm base for a trading centre at the lowest possible point on the Thames. A river crossing was built at the site of London Bridge. London Bridge is now used as the basis for published tide tables giving the times of high tide. High tide reaches Putney about 30 minutes later than
River Thames of the river. These included Putney Bridge, Westminster Bridge, Datchet Bridge, Windsor Bridge and Sonning Bridge. Several central London road bridges were built in the 19th century, most conspicuously Tower Bridge, the only Bascule bridge on the river, designed to allow ocean-going ships to pass beneath it. The most recent road bridges are the bypasses at Isis Bridge and Marlow By-pass Bridge and the motorway bridges, most notably the two on the M25 route Queen Elizabeth II Bridge and M25 Runnymede Bridge. Railway development in the 19th century resulted in a spate of bridge building including Blackfriars Railway Bridge and