THE ENVIRONMENT
An important agricultural area is the alluvial plain around Yuen Long in the north-west New Territories. These alluvial lowlands were 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 large 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 enclosed 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 Engineering Office, and maps for more than 80 per cent of the territory have been published. Detailed geological maps at 1:5 000 scale have been published for areas in Yuen Long and are being prepared for Ma On Sha, Northern Lantau and Tsing Yi.
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 10-15 metres, with a generally flat and featureless muddy seabed that slopes gently southwards. Several deeper channels occur in constricted tidal pathways, as for example in Urmston Road, the East Lamma Channel, Lei Yue Mun, Sheung Sz Mun and Lo Chau Mun.
During the Quaternary geological period, the last 1.6 million years, world sea level fell and rose several times during each of the four major glaciations when water was taken up to form great ice sheets. In south-east Asia, the sea-level fell to between 120-150 metres below its present level, which would have exposed an area of continental shelf about 130 kilometres wide to the south of Hong Kong. During this low sea-level period the Pearl River deposited alluvial sand and silt as channels over this wide plain.
As the ice began to melt at the end of the last glaciation, sea-levels rose reaching their present level about 6 000 years ago. The returning sea deposited fine clays and silts (marine mud) over the earlier alluvial deposits. Recent boreholes drilled offshore have revealed a sequence of up to 20 metres of marine mud overlying up to 80 metres of predominantly alluvial deposits which in turn overlie bedrock.
353