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
Queen Elizabeth II Quay simply "the harbor" and is Freetown's more centrally located harbor. Queen Elizabeth II Quay has sometimes been the called the "third largest natural harbor in the world"— this is inaccurate in two respects: 1.) the QE II Quay is not itself a natural harbor but a man-made port or "quay", and 2.) the quay is a very small feature of the river estuary on which it was built— it is this estuary which has carved out what is the largest natural harbor on the African continent. This still puts this harbor behind Port Jackson in Sydney, Australia, and several natural