THE ENVIRONMENT
Observatory's home page for the public to synchronise the clock on their personal computers. The time-server performed over 58 million checks in 2000 or about 160 000 checks per day.
Topography and Geology
Hong Kong's natural terrain is dominated by mountains and hills with steep slopes, many of which descend directly into the sea. The seabed is relatively flat. The highest point in Hong Kong is Tai Mo Shan in the New Territories at 957 metres above Principal Datum. The deepest point is 66 metres below Principal Datum in Lo Chau Mun (Beaufort Channel) north of Po Toi Island.
The mountains consist primarily of volcanic rocks, with some of the lower hills formed of granite. Low-lying areas tend to be formed of granite or sedimentary rocks. In places, hill-slope debris forms a mantle over the bedrock and alluvium fills some of the valleys. Much of the seabed is covered by marine mud with some scattered sand banks.
Hong Kong lies on the southern edge of an ancient land mass. The oldest exposed rocks are Devonian fluvial sediments that were deposited 400 million years ago. The region was subsequently inundated by a shallow sea. Sediments from this period are represented by the carboniferous marble of Yuen Long and Ma On Shan. The sandy 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 two 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.
Detailed information of the geology of Hong Kong can be found in a series of 15 geological maps at a scale of 1:20 000 and six accompanying memoirs. These were produced by the Hong Kong Geological Survey, a part of the Geotechnical Engineering Office, which published in 2000 two new memoirs and a set of 1:100 000 geological and thematic maps that synthesise and summarise the current state of knowledge of the geology of Hong Kong.
Hydrography and Oceanography
Hong Kong's waters are characterised by the interaction of oceanic and estuarine water masses that vary in relative effect throughout the year. The variable freshwater discharge from the Pearl River has a marked influence on Hong Kong waters.
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