Topography and Geology

Hong Kong's natural terrain is dominated by mountains and hills with
steep slopes, many of which descend directly 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 above Principal Datum. The deepest point is 66
metres below Principal Datum in Lo Chau Mun (Beaufort Channel) north
of Po Toi Island.

The mountains consist primarily of volcanic rocks, with some of the
lower hills formed of granite. Low-lying areas tend to be formed of
granite or sedimentary rocks. In some places, hill-slope debris forms a
mantle over the bedrock and alluvium fills some of the valleys. Much of
the seabed is covered by marine mud with some scattered sand banks.

Hong Kong lies on the southern edge of an ancient land mass. The oldest
exposed rocks are Devonian fluvial sediments that were deposited 400
million years ago. The region was subsequently inundated by a shallow
sea. Sediments laid down during this time are represented by the
Carboniferous marble of Yuen Long and Ma On Shan. The sandy and
muddy Permian rocks of Tolo Harbour are of marine and continental
origins.

During the Jurassic to Cretaceous period, between 170 and 140 million
years ago, Hong Kong was the scene of violent volcanic activity. Thick
accumulations of lava and ash were deposited. The eruptions were
associated with the development of a number of giant craters (calderas).
At deeper levels, the volcanic deposits were intruded by molten magma,
which slowly crystallised to form granite.

Igneous activity had ceased by the beginning of the Tertiary period, 60
million years ago. Rocks now seen on the island of Ping Chau represent
sediments laid down in a lake on the edge of a desert.

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, leaving the site of present-day Hong Kong as
much as 130 kilometres from the coast. At that time the flat areas
between what are now the islands of Hong Kong were part of an extended
Pearl River flood plain. In interglacial periods, such as at the present time,
global sea level rose to its present level and higher, and marine sediments
were deposited.

Information about the terrain is contained in 55 maps and 12 reports of
the Geotechnical Area Studies Programme. A description of the onshore
and offshore geology can be found in a series of 15 geological maps
published at a scale of 1:20 000 and six memoirs produced by the Hong
Kong Geological Survey Section of the Geotechnical Engineering Office.

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