ENG-1992 — Page 411

THE ENVIRONMENT

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Topography and Geology

The topography of Hong Kong is characterised by steep granitic and volcanic mountains. Much of the footslope terrain is blanketed by debris flow deposits and other forms of colluvium transported by erosion and mass movement from the hillsides. Some 40 per cent of the landmass is volcanic in origin, about 20 per cent is granitic, 15 per cent is colluvial and almost 10 per cent is alluvial in nature. The highest peak is Tai Mo Shan (957 metres) located in the central New Territories, and there are four peaks which exceed 750 metres, all on Lantau Island. Victoria Peak, the highest on Hong Kong Island and best known as a major tourist attraction, is ranked 18th in the territory with an elevation of 554 metres.

The territory lies on the edge of the ancient Sinian landmass formed more than 600 million years ago, and which now extends from Shandong in northern China to the Gulf of Hainan. The geological strata forming the landmass were deposited beneath the sea but were subsequently folded and faulted in mountain-building earth movements. A second period of mountain building, beginning about 160 million years ago, was accompanied by explosive volcanic activity that resulted in the deposition of thick layers of ash and lava culminating in the intrusion of large granitic bodies between 147 and 136 million years ago. The mountains formed from these deposits were subsequently eroded, with the material washed from the slopes into the valleys, plains and shallow seas.

During the Quaternary period of the last 1.6 million years, sea level has been fluctuating, falling as water was trapped in the expanding polar ice caps and rising as the ice caps melted. While sea level was low, Hong Kong was surrounded by broad plains crossed by distributary channels of the Pearl River delta which deposited large amounts of sand and gravel. During periods of high sea level, such as at present, the sea covers the earlier deltaic plain, and mud is deposited in the waters around Hong Kong. The fluctuations of sea level during the Quaternary have therefore resulted in a sequence of mud, sand and gravel up to 100 m thick in the present offshore area.

The erosion of the hills accelerated following destruction of the natural vegetation during the widespread colonisation of the area in the Song Dynasty (960-1279). Hong Kong's rocks are generally deeply weathered and much of the terrain is prone to landslips. More than 20 per cent of the terrain in the territory shows evidence of instability.

Generally, the granitic and volcanic rocks can be excavated quite easily for use as reclamation material. Sand and gravel dredged from offshore areas are also a valuable resource. Hong Kong has few mineral resources, although deposits of lead, zinc, quartz, kaolin, beryl and graphite have been mined in small quantities, and iron and tungsten were once extracted in significant amounts. Granites in Hong Kong have long been quarried for building purposes, and are now used as aggregates.

The natural landscape in the urban areas has been extensively modified as a result of site formation associated with development. Many of the natural granitic hills have been removed, and the material used as fill for the various reclamations. Almost 4 000 hectares of the developed land is reclamation.

Much of the undeveloped terrain in Hong Kong consists of steeply-sloping ground where soils are thin and nutrient deficient. These soils support only grassland or shrubland, except in protected valleys where small areas of broad leaf woodland survive, or in water catchments and country parks where re-afforestation has succeeded in establishing pines and deciduous trees.