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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 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 from this period are represented by the carboniferous
marble of Yuen Long and Ma On Shan. The sandy and muddy sediments of the
Permain rocks of Tolo Harbour are of alternate marine and continental
deposits.
From the Jurassic to Cretaceous periods, 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 several 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 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 two
million years, major glaciations in polar regions affected global sea
level, which fell to 120 metres below the 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, the global sea level rose to its present
level and higher, and marine sediments were deposited.
Detailed information of the geology of Hong Kong can
be found in a series of 15 geological maps at a scale of 1:20 000
and six accompanying memoirs. These were produced by the Hong Kong Geological
Survey, a part of the Geotechnical Engineering Office. The Hong Kong Geological
Survey has also recently published two new memoirs and a set of 1:100
000 geological and thematic maps that synthesise and summarise
the current state of knowledge of the geology of Hong Kong. |
