THE ENVIRONMENT
348
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, beryl and graphite have been mined in small quantities, and iron and tungsten were once extracted in significant amounts. Only kaolin is currently worked within the territory. 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.
An important agricultural area is the alluvial plain around Yuen Long in the north-west New Territories. These alluvial lowlands probably formed within the last 33 000 years, and some areas are still prone to flooding. More than 5 000 hectares of floodplain occurs in the territory and much of it is located in the Yuen Long district. The natural deposition of sediment is continuing around the Deep Bay area, where brackish fish ponds have been established successfully in areas that once were mudflats, mangrove swamps or salt-water rice paddies.
As Hong Kong lacks large rivers, lakes or underground water supplies, reservoirs have been constructed in larger valleys such as Shek Pik, Tai Lam Chung, and in coastal areas such as Plover Cove and High Island, where embayments and channels have been en- closed by large dams. In most instances, the catchment areas of the reservoirs have been designated as country parks.
New geological maps are being produced at a scale of 1:20 000 by the Geological Survey of Hong Kong, located within the Geotechnical Control Office, and maps for more than 60 per cent of the territory have been published. Detailed geological mapping is also giving rise to the publication of 1:5 000 scale maps, in particular for areas in Yuen Long, Ma On Shan and Northern Lantau. Information about the terrain is also contained in the 55 maps and 12 Reports of the Geotechnical Area Studies Programme. Published documents are available through the Government Publications Sales Centre.
Hydrography and Oceanography
Approximately two thirds of the territory of Hong Kong, or almost 1830 square kilometres, is covered by the sea. Historically, the sea has been very important, as a highway for international shipping and in the form of a large and sheltered anchorage. Hong Kong is now one of the busiest ports in the world. Offshore areas have assumed greater importance in recent years with the increasing number of offshore engineering projects in the marine environment, including reclamations, tunnels, pipelines, cables and especially as a source of fill.
Maximum water depths reach a little over 30 metres in the south-eastern corner of territorial waters to the south of Mirs Bay. More commonly, water depths range between