Beware of the Overturned Cliff. It is a place where the earth dramatically shows its age. Here at the Overturned Cliff, we’re standing on a testament to Hong Kong’s tumultuous geological past.
The Overturned Cliff’s story begins millions of years ago. This area was a hotbed of volcanic activity. Specifically, during the Mesozoic era, cataclysmic eruptions shaped most of Hong Kong’s landscape. Igneous rocks formed, including the granitic and volcanic rocks we see today. They make up 85% of Hong Kong’s land surface.
The geological history of Hong Kong began in the Devonian period. That was around 420 million years ago. Placoderm fossils found in northeast Hong Kong mark this era. The youngest rocks are from the Paleogene period, about 50 million years ago. You can see them in Tung Ping Chau.
Look around you. Notice the unique features. Igneous rocks formed hexagonal columns in Sai Kung. Sedimentary rocks created wave-cut platforms and sea stacks in Tung Ping Chau. Metamorphic rocks led to iron ore deposits in Ma On Shan. Each rock type tells a different story about Hong Kong’s formation.
Faults are a crucial part of this story. They mainly run from the northeast to the southwest. You can find deformation features near major faults. Examples are sheared rocks, folds, and faulted rocks. Past fault activities shaped the Overturned Cliff’s landscape.
From the Devonian to the early Jurassic period, this area was in a pre-volcanic sedimentary phase. The environment alternated between a river plain and a shallow sea. Rocks from this period contain various fossils. They also feature heavily folded strata and steeply tilted beds.
Later, from the middle Jurassic to the early Cretaceous period, Hong Kong became a volcanic hotspot. Volcanic lava, ash, and granitic rocks covered the area extensively. From the middle Cretaceous onward, the post-volcanic sedimentary period began. Reddish-colored sedimentary rocks indicate an arid tropical climate.
Igneous rocks dominate Hong Kong’s geology. During the middle Jurassic to the early Cretaceous period, Hong Kong was at a convergent plate boundary. The Paleo-Pacific oceanic plate subducted beneath the Eurasian continental plate. This process led to the formation of magma chambers. Volcanoes then erupted, expelling volcanic ash and rocks. These materials cooled into volcanic rocks.
Volcanic rocks are widespread in Hong Kong. They form many of the highest mountains. Examples are Tai Mo Shan and Lantau Peak. In eastern Hong Kong, these rocks formed hexagonal columnar cooling joints. You can see them from the High Island reservoir.
Around 140 million years ago, volcanic activity ceased. The magma cooled and became granitic rocks. These rocks cover about 35% of Hong Kong’s land. They are mainly in Kowloon, north Hong Kong Island, east Lantau, and Tuen Mun. Kowloon granite formed Victoria Harbour. This is where Hong Kong got its name, “scentful harbour.”
Consider the Sai Kung hexagonal columns. These formed when volcanic materials cooled and contracted. A volcano once centered east of the Sai Kung peninsula. Its final eruption emptied the magma chamber. The remains became a caldera. Volcanic ash settled and cooled. This created hexagonal-shaped cracks.
There are about 200,000 hexagonal columns in Hong Kong. They cover 100 square kilometers. Most columns tilt and dip towards the northwest. Some columns are curved by tectonic force.
The columns are light brown due to their silica-rich chemistry. They contain about 76% silica. Such a large group of silica-rich hexagonal columns is very rare. Therefore, the hexagonal columns are the most important feature of the UNESCO Global Geopark of Hong Kong.
The International Union of Geological Sciences recognized the “Early Cretaceous rhyolitic columnar rock formation of Hong Kong” as a geological heritage site.
As you stand here at the Overturned Cliff, remember the deep history beneath your feet. This isn’t just a scenic spot. It’s a window into millions of years of geological activity. It is a reminder of the powerful forces that shaped Hong Kong.