ENG-2020 — Page 275

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The Environment

Physical Characteristics, Flora and Fauna

Topography, Geology and Landforms

Hong Kong's natural terrain is characterised by rugged uplands flanked by steep slopes. The highest point is Tai Mo Shan (957 metres above Principal Datum) in the central New Territories, and the lowest point (66 metres below Principal Datum) is in Lo Chau Mun (the Beaufort Channel) to the north of Po Toi Island. The mountains are predominantly formed of volcanic rocks, whereas the lower hills and low-lying areas are generally underlain by granite or sedimentary rocks. A layer of soft, weathered rock covers the bedrock in most places, slope debris mantles the natural hillsides, and alluvium fills many of the valleys. Offshore, the seabed is covered with marine mud, with sand sheets occurring near the coast and in channels.

The oldest exposed rocks in Hong Kong were deposited as river sediments about 400 million years ago. Between 360 and 300 million years ago, the region was occasionally inundated by a shallow sea, during which limestones (now marble) and siltstones accumulated. From 170 to 140 million years ago, violent eruptions from several volcanic centres deposited thick ash layers. At deeper levels, molten magma intruded and slowly crystallised to form granite. Volcanism ended with a colossal eruption from the High Island Supervolcano centred in southeastern Hong Kong. Layered rocks on the island of Ping Chau are younger sediments, laid down in a lake on the edge of a desert about 50 million years ago. During the last 2.6 million years, several major glaciations occurred, causing successive lowering of the global sea level. Widespread river floodplains surrounded Hong Kong, and were later covered by marine mud when the sea level rose during interglacial periods.

Despite its small size, Hong Kong has a great variety of coastal landforms, including sea cliffs, sea caves, sea arches, geos, tombolos, wave-cut platforms, sea stacks, notches and blowholes.

The Civil Engineering and Development Department's Hong Kong Geological Survey Section has produced a series of fifteen 1:20,000-scale geological maps and six accompanying geological memoirs. In addition, two summary memoirs and a set of 1:100,000-scale geological and thematic maps in Chinese and English, covering various aspects of Hong Kong's geology, have been published. Geological information is available on the department's website.

Flora

Hong Kong is situated near the northern boundary of the distribution of tropical southeast Asian flora, sharing similar species and structure with the flora of Guangdong. Despite its small size, Hong Kong has a rich flora with about 3,300 species of vascular plants, of which 2,100 are native to the city.

The major types of vegetation cover comprise woodland, shrubland and grassland. Remnants of the original forest cover can still be found in steep ravines or behind traditional villages in rural areas. They have survived as a result of their location in precipitous topography and the moist micro-climate, or because they are protected for cultural reasons.

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