 It was important also for the export of anthracite, which was mined on its west bank and shipped from Hook.The combined estuary – the Daugleddau - from Picton Point to the Blockhouses guarding the harbour entrance, is a massive ria which is deep and wide, but sufficiently serpentine to be sheltered from high winds and rough seas, and is thus an excellent natural harbour.Because it can easily accommodate supertankers of 300,000 tonnes and more, it became from 1957 an important centre of the oil industry, with Esso, BP, Texaco, Gulf Oil and Amoco operating terminals and oil refineries.