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 are mainly formed of granite. 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 are composed of Devonian river sediments that were deposited approximately 400 million years ago. The region was subsequently inundated by a shallow sea, in which were deposited Carboniferous limestones, the parent material of the Yuen Long and Ma On Shan marble. Between 170 to 140 million years ago, during the Jurassic and Cretaceous periods, Hong Kong was the scene of violent volcanic activity that deposited thick accumulations of ash and lava. These eruptions were associated with the development of several calderas (giant craters). At deeper levels, molten magma was intruded and slowly crystallised to form granite. Igneous activity ceased 60 million years ago. Layered rocks now seen on the island of Ping Chau are younger sediments, laid down in a lake on the edge of a desert.

During the last two million years, the Quaternary Period, several major glaciations affected the polar regions. These caused successive lowering of world sea level of up to 120 metres, leaving the site of present day Hong Kong as much as 130 kilometres from the coast. During the interglacial periods, such as the present time, global sea level returned to its present level and marine sediments were deposited.

Information about the geology of Hong Kong is presented in a series of 15 1:20 000-scale geological maps and six accompanying geological memoirs that were produced by the Hong Kong Geological Survey, a part of the Geotechnical Engineering Office of the Civil Engineering and Development Department. The Hong Kong Geological Survey has also published two summary memoirs and a set of 1:100 000-scale geological and thematic maps that synthesise current interpretations of the geology of Hong Kong.