Hong Kong's natural terrain is dominated by mountains and hills with steep slopes. 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.

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. 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 the 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. 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.