ENG-1998 — Page 488

THE ENVIRONMENT

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and muddy sediments of the Permian rocks of Tolo Harbour are of alternate marine and continental deposits.

From the Jurassic to Cretaceous periods, between 170 and 140 million years ago, Hong Kong was the scene of violent volcanic activity. Thick accumulations of lava and ash were deposited. The eruptions were associated with the development of several giant craters (calderas). At deeper levels, the volcanic deposits were intruded by molten magma, which slowly crystallised to form granite.

Igneous activity had ceased by the beginning of the Tertiary period, 60 million years ago. Rocks now seen on the island of Ping Chau represent sediments laid down in a lake on the edge of a desert.

During the Quaternary period, spanning the last 2 million years, major glaciations in polar regions affected 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 the flat areas between what are now the islands of Hong Kong were part of an extended Pearl River flood plain. In interglacial periods, such as at the present time, the global sea level rose to its present level and higher, and marine sediments were deposited.

Information about the 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

Hong Kong's waters are characterised by the interaction of oceanic and estuarine water masses which vary in relative effect throughout the year. The variable freshwater discharge from the Pearl River has a marked influence on Hong Kong

waters.

During the summer, an oceanic flow from the south-west to the north-east brings the warm, high-salinity water of the Hainan Current into Hong Kong waters. This interacts with fresh water from the Pearl River and divides Hong Kong into three distinct sections. In the west, where the fresh-water influence is greatest, the environment is estuarine and the water is brackish. In the east, the water is mainly oceanic with relatively minor dilution from rainfall and runoff from streams. The limits of the central transitional zone vary depending upon the relative influence of Pearl River water and marine currents.

During the winter dry season, the Kuroshio oceanic current brings warm water of high salinity from the Pacific through the Luzon Strait. The fresh water discharge of the Pearl River is much lower than in the summer and salinity is more uniform across Hong Kong. The coastal Taiwan current also brings cold water from the north-east down the South China coast affecting inshore waters.

The normal tidal range in Hong Kong waters is between one and two metres, depending on the relative influence of the moon and sun. 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 and west through Lei Yue Mun into Victoria Harbour and through Kap Shui Mun and the Ma