ENG-1997 — Page 477

THE ENVIRONMENT

December was also warmer than normal with a mean temperature of 18.9 degrees, the seventh-highest for the month. However, the total bright sunshine duration of 100.4 hours was the fourth-lowest for December.

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 some 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 laid down during this time are represented by the Carboniferous marble of Yuen Long and Ma On Shan. The sandy and muddy Permian rocks of Tolo Harbour are of marine and continental origins.

During the Jurassic to Cretaceous period, 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 a number of 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 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, 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 Section of the Geotechnical Engineering Office.

Hydrography and Oceanography

Hong Kong's waters are characterised by the interaction of oceanic and fresh water masses which vary in relative effect throughout the year. The variable freshwater

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