ENG-1994 — Page 493

THE ENVIRONMENT

the development of a number of giant craters. At the end of each main phase of volcanic activity, these craters collapsed to form calderas, and the lava and ash pile was intruded by magma, which slowly crystallised to form granite.

During the Tertiary period, 60 million years ago, the area became more inactive, though the environment was still quite hostile. Rocks now seen on the island of Ping Chau represent sediments laid down in a temporary lake on the edge of a desert extending to the north.

During the Quaternary period, spanning the last two million years, major glaciations in polar regions affected the global sea level, which fell to 120 metres below the present level, leaving the site of present-day Hong Kong as much as 130 kilometres from the coast. At that time, when glaciers covered northern China, the flat-lying areas between what are now the islands of Hong Kong were part of the flood plain of the Pearl River. In interglacial periods, such as at the present time, the global sea level rose to its present level and higher while marine sediments were deposited around the territory.

Information about Hong Kong's terrain is contained in 55 maps and 12 reports of the Geotechnical Area Studies Programme. A description of the onshore and offshore geology can be found in a series of 15 geological maps published at a scale of 1:20 000 and six memoirs produced by the Hong Kong Geological Survey, a part of the Geotechnical Engineering Office.

Hydrography and Oceanography

The hydrography of Hong Kong waters is characterised by the interaction of oceanic and fresh water masses that vary in relative effect throughout the year. The discharge of fresh water from the Pearl River is the single most important factor influencing Hong Kong waters, especially in the summer months. Oceanic waters affecting the territory come from three different ocean currents with different characteristics.

During the summer, a northeasterly oceanic flow brings water of high salinity from the Hainan current into Hong Kong waters. This warm water mass interacts with fresh water from the Pearl River and divides the territory into three distinct sections. In the west, where the fresh water influence is greatest, the environment is estuarine and the water brackish. In the east, the water is dominantly oceanic, with relatively minor dilution from direct monsoonal rainfall and run-off from small streams. The limits of the central transitional zone vary, depending upon the relative influence of Pearl River flood waters and marine

currents.

During the winter, the Kuroshio oceanic current brings warm water, also of high salinity, from the Pacific through the Luzon Straits. The fresh waters of the Pearl River have a lesser influence than in summer and water salinity is more uniform across the territory.

In addition to the Kuroshio current, the coastal Taiwan current brings cold water down the South China coast, which affects inland waters, making the surface water cold during the winter months.

The normal tidal range in Hong Kong waters is between one and two metres. The tidal pattern is complex due to the relative effects of the diurnal and semi-diurnal components. The basic pattern during flood-tides is for oceanic water to flow north into Mirs Bay, west through Victoria Harbour and north through Kap Shui Mun and the Ma Wan Channel. Maximum tidal currents range from 0.5 to 1.5 knots. This flow is reversed during the ebb tide, though currents in some parts may be stronger due to the influence of the Pearl River.

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