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
Dartford Crossing bridge piers were constructed in the Netherlands. Each caisson was designed to withstand a bridge strike of a ship weighing up to 65,000 tonnes and travelling up to The bridge deck is about high, and it took a team of around 56 to assemble its structure. During construction of the approach road, a World War II bomb was found in its path, which required closure of the entire crossing. The bridge was opened by Queen Elizabeth II on 30 October 1991. The total cost of construction was £120 million (£ in ), including £30 million (£ million in ) for