ENG-1996 — Page 489

THE ENVIRONMENT

No. 1 was hoisted on two occasions during the approach of tropical cyclones Lisa and Niki.

The total rainfall of 604.0 millimetres recorded in September 1996 was the sixth- highest for the month and was more than twice the normal figure of 299.7 millimetres. Typhoon Sally rapidly traversed the northern part of the South China Sea, bringing thundery and frequent squally showers to Hong Kong and the No. 8 Gale or Storm Signal was hoisted early on September 9. Gales were recorded at Waglan and Cheung Chau with the maximum hourly winds exceeding 80 kilometres an hour.

October was warmer and drier than normal. The mean temperature of 26.0 degrees was the sixth-highest for October. With only 44.8 millimetres recorded, the monthly rainfall was 69 per cent below normal.

With less frequent continental cold air affecting Hong Kong, November 1996 was warmer than normal. The mean minimum temperature of 21.1 degrees was the highest for November. The month was also rather dry, with 3.5 millimetres of rainfall recorded against a normal 35.1 millimetres.

December was generally fine and dry. Only a trace of rainfall was recorded and Fire Danger Warnings were in effect on 27 days in the month.

Topography and Geology

Hong Kong's natural terrain is dominated by mountains and hills with steep slopes, many of which descend sharply 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. The deepest point is in the Lo Chau Mun (Beaufort Channel) north of Po Toi Island, in which a sounding of 66 metres is shown on the Admiralty chart.

The mountains consist primarily of volcanic rocks, with some 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 overlying bedrock. Much of the seabed is covered by marine mud with scattered sand banks.

The territory lies on the southern edge of the ancient Sinian land mass. The oldest exposed rocks are Devonian fluvial sediments that were deposited 300 million years ago. The region was subsequently inundated by the sea and shallow marine sediments laid down, represented by the Carboniferous marble of Yuen Long and Ma On Shan and the sandy and muddy sediments of the Permian rocks of Tolo Harbour.

During the Jurassic and Cretaceous periods, 170 to 135 million years ago, while dinosaurs roamed the land to the north, Hong Kong was the scene of violent volcanic activity, with lavas and volcanic ash being deposited over the territory. The eruptions were associated with the development of a number of giant craters (calderas). At deeper levels, the lava and ash pile was intruded by molten magma, which slowly crystallised to form granite.

During the Tertiary period, 60 million years ago, the area became less active. Rocks now seen on the island of Ping Chau represent sediments laid down in temporary lake on the edge of a desert extending to the north.

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,

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