Magnificent sea stacks of Ung Kong Group–Bluff Island (Sha Tong Hau Shan)

Magnificent sea stacks of Ung Kong Group–Bluff Island (Sha Tong Hau Shan)

Background Information

Bluff Island (Sha Tong Hau Shan), or Ung Kong Chau, lies west of Basalt Island and south of Town Island (Fo Tau Fan Chau). In the southern corner of this small island you can find one of the four biggest sea caves in our eastern waters ¾ Sha Tong Hau Cave. Viewed from above, Bluff Island resembles a turtle with four legs stretched out wide. Lushly wooded slopes of the rounded hills roll gently down to scenic sandy bays. This is an ideal place to study rhyolite. Formations in clearly defined layers are prominent across the island, and along the wavy shores, huge blocks tower by the water. Fan Tap Pai in the south is one of the most breathtaking coastal rock scenes in Hong Kong. This huge sea cave cuts right through the island and it is just wide enough for a boat to sail past. Not a bad idea for a leisure sea expedition. Bluff Island is also a diving hot spot. An increasing number of divers are coming here to explore the mysterious ocean.

Geological Information

Most parts of the exposed stratum of Bluff Island belong to the Early Cretaceous Kau Sai Chau Volcanic Group. From the Late Jurassic to the Early Cretaceous, this region was the site of earth shaking changes. It began about 146 million years ago (Late Jurassic) when there were violent magma activities. Fiery underground magma intruded upwards, melting ancient rocks along its path. It forced its way through cracks and thrust along crustal faults or weak bands until it reached the surface to cause great volcanic eruptions. Sometimes magma was pressed inside underground voids. It created such an immense force that when it finally broke out it made catastrophic impact. Explosion shot volcanic ash and gravels high into the air. Lava was vented out from the crater from where it coursed down the slopes, igniting fires and generating great clouds of ash and fume. According to data for determining the absolute age of rocks, volcanic eruptions and magma intrusion activities took a cyclical pulse pattern. In every cycle, major eruptions were always followed by great magma intrusions. Such cyclical volcanic magma activities can be categorised into four stages, and consequently there are four volcanic rock groups in Hong Kong ranging from old to new : Tsuen Wan Group, Lantau Group, Repulse Bay Group and Kau Sai Chau Group. Judging by the exposed stratum of Bluff Island, we know that eruptions took place in the 4th stage.

Stunning tubular rock columns and sea caves － Wang Chau

Stunning tubular rock columns and sea caves － Wang Chau

Background Information

Wang Chau is the smallest amongst the Ung Kong Islands. The highest point is merely 80 metres above sea level while the narrowest part measures only 500 metres. It is a barren island with poor soil and hardly any vegetation. Geological sceneries, however, are enthralling. On the north side of Wang Chau one finds a striking sea cave. To see it in true grandeur, visit a few days after a typhoon when the ravaging ocean surge has subsided but the sea is still a little rough. On such days, swirling swells beat up a spectacular resonance in the cave, testifying to the mighty power of nature. Sea cave generally means a pit or trough carved out by waves and currents at the foot of a sea cliff. It is a type of wave-cut landform. Wave-cut action is the damaging impact, erosion and abrasion effect on a coast caused by waves and the sand, gravels and small stones they carry. This process occurs in three forms: erosion, abrasion and dissolution. Erosion is the washing impact of waves and currents on a coast. When waves hit against the rock surface, air in the bedrock cracks is compressed and imposes great pressure on the rock. When the swell subside, compressed air expands suddenly. This continuous process of abrupt contraction and expansion made the rock disintegrate. Abrasion is the impact, chiseling and grinding effect on the bedrock made by sea surge laden with lithic shards and sandy gravels. It accelerates the wave-cutting action. Dissolution means that rock is dissolved by seawater. While carbonate rocks are most vulnerable to dissolution, other rock minerals like orthoclase and amphibolite dissolve more than ten times quicker in seawater than in freshwater. Sea caves, sea cliffs, sea arches, sea stacks, wave-cut platforms and sea terraces are typical wave-cut landforms.

Geological Information

Like Bluff Island, the outcropped stratum of Wang Chau is part of the Early Cretaceous Kau Sai Chau Volcanic Group. Sea caves on the island are well developed. As a result of geological factors and sea level movements, they are greater in depth than width. Major geological factors include the types of shore-forming rocks or substances and their resistance to erosion, attitude of rock beddings, geological composition and crustal movement of the coastal region. These factors influence coastal characteristics and evolution to a great extent.

Typical coastal terrain–Basalt Island

Typical coastal terrain–Basalt Island

Background Information

Basalt Island is an island south of Town Island (Fo Tau Fan Chau) and Wang Chau, east of Bluff Island (Sha Tong Hau Shan) and north of the Ninepins. Only 174 metres at its highest point, it was the site of an air disaster in 1948. An aircraft coming into Hong Kong from Shanghai crashed on the island, killing all 30 crew members and passengers. Sixty years have since passed and all traces have been erased by swells and typhoons. Only an air disaster monument remains on the island to recall this tragic accident.

Geological Information

Like Bluff Island and Wan Chau, the outcropped stratum of Basalt Island is part of the Early Cretaceous Kau Sai Chau Volcanic Group. The island has very well developed sea stacks, island reefs and wave-cut bays which clearly demonstrate the impacts of wave erosion on the environs. Waves, in forms of erosion, abrasion and dissolution, cause devastating damage to the shores. Given that the Kau Sai Chau Volcanic Group is full of columnar joints and fissures, impacts of pounding waves on rocks are particularly obvious. Meanwhile, continuous abrasion by sand and gravel carried by waves has brought changes to the shores and submerged bedrock, sculpting an amazing array of precipitous cliffs and spectacular landforms. In geology, wave cut action is one example of exogenous effect. The islands and wave-cut terrains in the Basalt Island region are excellent showcases that reveal how of this geological process modified geomorphology.

Spectacular joint columns–Ninepin Group (Kwo Chau Islands)

Spectacular joint columns–Ninepin Group (Kwo Chau Islands)

Background Information

Located about 15 km southeast of Sai Kung Town Centre, the Ninepin Group comprises South Ninepin Island, North Ninepin Island and East Ninepin Island, along with several small rock islets. It is located in the open sea and is thus subject to the relentless impact of strong wind and waves, resulting in diverse peculiar coastal landforms, including steep cliffs, sea arches and various strangely-shaped rocks. Famous scenic wonders on North Ninepin Island include Cannon Rock, Moon Rocks and Big Stove Arch; and on South Ninepin Island include Stone Arch, Jacob's Ladder Cave, Backwash Cave and Y-shaped Cave. In the Sai Kung Volcanic Rock Region, the hexagonal rock columns exposed on North Ninepin are the most magnificent. The columns may have a diameter of over 2 m, making them the largest in the area. As the Ninepin Group is within the core protection area, and in view of the strong wind, big waves and dangerous terrain, sightseeing is suitable only on calm summer days, and only boat tours with no land excursion. The name "Ninepin" is inspired by an old British game similar to modern day bowling. When British seamen first saw the array of these islands, the familiar bowling game sprang to mind and they gave the group this colourful name. The Chinese name Guo Chau, meaning fruit island, is even more innovative. It is said that the Ninepin Group (Kwo Chau Islands) was originally called Guo Pun Chau which means fruit platter. It is so named because viewed from above, the group looks like scattered fruit from an overturned platter from heaven. The Ninepin Group is made up of North Ninepin Island, South Ninepin Island, East Ninepin Island and other islets. North Ninepin includes Sai Mei Chau and Hok Tsai Pai. South Ninepin includes Tai Chau, Tai Chau Mei, Ta Long Pai and Lai Chi Pai. East Ninepin includes Tuen Chau Mei, Shue Long Chau and Lung Shuen Pai. Riding on a small boat to South Ninepin Island, the sea is calm and beryl. After going ashore, the first thing that comes to view is the imposing "Tiger Mouth Cave". Hong Kong's eastern coast has its fair share of famous caves, and this is one of them. In geology, this "cave" is actually a sea arch. Hong Kong is influenced by easterly wind most part of the year. The islands east of Sai Kung and Clearwater Bay peninsula are completely exposed to this natural force. Even though volcanic rocks are essentially hard and resistant, wind and waves have taken their toll over the ages. The resultant terrain is rugged and imposing. Given such challenging condition, visitors to the Ninepins can only get ashore during two or three summer months every year. Standing below the giant arch, one will marvel at nature's brilliant craftsmanship and truly appreciate the greatness of nature's power relative to our own.

A rugged and barren hill path leads to Nam Yue Tau, passing through many natural rock arches that may suddenly collapse any day. Looking east, the seascape is boundless. With the nearest land being the Philippines, no wonder fishermen built a Hin Hau Temple here on this last outpost. Along the way, the soil is barely covered except for some grasses, shrubs and coastal plants. Beach Naupaka (Scaevola taccada) and Screw Pine (Pandanus tectorius) are two key species. The rocks here are marked by interesting graphics. At first glance, you may think that some idle impressionist or post-modernist painter has dressed them up with his works. They are in fact mother nature's masterpieces. About 160 million years ago, an earth-shaking eruption caused fundamental changes to the nearby coasts. Along with scorching volcanic ash, great acidic lava was spewed out from the volcano, covering slopes and the land. When large areas of volcanic ash and lava slowly cooled down to become solid rock, the contraction effect gave rise to extremely uniform hexagonal cracks and joints. What followed were climatic changes over millions of years. The combined impact of wind, rain and ocean surge brought out various minerals which were once hidden within, decorating the pale rock face with a dash of colour. Sailing from South Ninepin to North Ninepin, the spectacular hexagonal volcanic rock on the island is visible even from quite a distance. Volcanic rock made up of black basic basalt is not particularly rare yet such large coverage of hexagonal joint volcanic rock formed by cooled and solidified pinkish acidic rhyolitic volcanic rock is very seldom seen. This unique natural wonder features volcanic rock arranged in a striking formation of giant hexagonal columns. An islet off the east shore called Yuen Shek Pai offers the most amazing sight. Numerous rock columns rise up neatly around the centre of the islet and form a natural spiral staircase.

Geological Information   The outcropped volcanic rock with columnar joints of the Ninepins is a type of acidic rhyolite volcanic rock. The rock is relatively consistent and has significant contents of potash feldspar and quartz phenocrysts. This rock set, known as the High Island Formation, is part of the Early Cretaceous Kau Sai Chau Volcanic Group. Rocks of the Ninepins date back to the age of dinosaurs about 140 million years ago. It is believed that this group of islands were created by a major volcanic eruption which produced about 70 cubic kilometres of volcanic ash. A series of peaks emerged abruptly on the broad plain as a result. Given that Mount St. Helens spewed out only 2 cubic kilometres of ash when it exploded while the catastrophic Krakatau eruption of Indonesia in 1883 produced 12 cubic kilometres of volcanic substances, one can imagine the devastating magnitude of this ancient eruption. After rising, these peaks faced severe erosion over millions of years. Gradually they became the magnificent rugged landscape we see today.

Oldest rock in Hong Kong － Wong Chuk Kok Tsui

Oldest rock in Hong Kong － Wong Chuk Kok Tsui

Background Information

The oldest stratum in Hong Kong, as proven by existing fossil evidence, is a bed of Devonian sedimentary rock. It is mainly outcropped on both sides of Tolo Channel. Sedimentary pyroclastic rocks found around Pak Sha Tau and Wong Chuk Kok Tsui on the northern shore are called Wong Chuk Kok Tsui Formation. They are primarily sandstone, pebbly sandstone and conglomerate. The best outcropped areas of this formation can be found around Wong Chuk Kok Tsui at the northern tip of the channel. Influenced by the folding and staggering action of geological movements, the beddings are almost vertical. It is a breathtaking sight. The rich Devonian fossils found in Tolo Channel provide a strong basis for determining geological age and sequence stratigraphy. They also help us understand how Hong Kong's natural environment evolved during that time.

Geological Information

The oldest rocks of Hong Kong (Devonian) are distributed on both sides of Tolo Channel, i.e. Ma On Shan on the southern shore and the region south of Plover Cove Reservoir on the northern shore. The latter are known as Wong Chuk Kok Tsui Formation. The faults of this formation, some 800 metres thick, have contacts with other strata both in the north and south. The entire Wong Chuk Kok Tsui Formation is made up of clastic sedimentary rocks like sandstone, siltstone and conglomerate. While rock property variations are insignificant from bottom to surface, ages of fossils within cover the entire Devonian sedimentary sequence, from its early stage to the late stage. By analysing the sedimentation and living organisms of the Devonian in depth, we can see that the region around Plover Cove Reservoir on the northern shore of Tolo Channel today was primarily foreshore, river delta or inshore sedimentary shallow sea during the Early and Middle Devonian. Bivalvia, archaeostraca and conchostracan fossils have thus been discovered in this area. Fossils from the Late Devonian are mainly terrestrial primitive ferns. This indicates the sedimentary environment changed at this stage. The sea receded and land was exposed.

Youngest sedimentary rock in Hong Kong － Tung Ping Chau

Youngest sedimentary rock in Hong Kong － Tung Ping Chau

Background Information

Ping Chau, or Tung Ping Chau to differentiate from Peng Chau, sits in Mirs Bay of the northeastern waters. It is the easternmost outlying island of Hong Kong. This crescent-shaped island, measuring 600 metres long and 200 metres wide, is renowned for strange rock formations. Unlike most other rock types across the territory, the island is made up of sedimentary rock including siltstone, dolomitic siltstone, mudstone and chert. This sedimentary rock strata is known as Ping Chau Formation. Many extremely well preserved fossils have been found within the formation. Fossil studies, investigations of rock properties and regional geophysics, as well as research in climatic changes during ancient times have confirmed that this formation is the product of sedimentation which took place in Early Tertiary. Layer upon layer of rock form a bewildering structure. The strata looks just like a huge book, guarding great mysteries of geological history within its numerous pages. Since no sediments after Early Tertiary have successfully solidified into enduring rock, Ping Chau Formation is the youngest rock in Hong Kong's geological history. Tung Ping Chau is a popular holiday destination. Its many fascinating attractions, such as wave erosion landscape, the shale that resembles a layered sponge cake and the unusually flat lay of the island itself, have drawn thousands of visitors to the island.

「Three Flats and One Wonder」

Three flats are flat sea, flat island and flat rock. Tung Ping Chau is only 1.1 square kilometres in size. An island of extremely low elevations, it really lives up to its name. The highest point in the southeast, Hok Ngam Teng, rises a mere 48 metres above sea level, while the highest peak in the northwest, Au Kung Shan, is only 37 metres. As for the wonder, it refers to the famous shale.

「Top Rock」 The Tung Ping Chau shale is rated as the No. 1 rock in Hong Kong for the following reasons : Clearly defined bedding and distinctive sheet conformation This is the most significant characteristic of shale. Every layer is 1 to 5 mm thick. Overlapping each other, the coarser-grained siltstone layers and the finer-grained mudstone layers have clear lamination and sophisticated structure. These shale beds are believed to date from Late Cretaceous to Early Tertiary. It is a safe guess that Mirs Bay and Tung Ping Chau were a remote salt lake detached from land. As a result of sedimentation over long ages, shale resembling a layered sponge cake emerged in the hypoxic, highly salt and still water. Attractive and colourful Shale is usually made up of pyrite, geolite and augite. The vivid colours come from micrograined calcium, iron and magnesium particles that the rock contains. After formation, the rock was exposed to weathering and wave action, creating eye-dazzling plum, ochre, charcoal and bottle-green outcrops. Vague ripple marks Close observation will reveal vague or faint ripple marks in every layer. We can thus infer that the rock was close to the lake surface or submerged in relatively shallow water at the time of formation. In such setting, even small ripples would leave detectable marks. Emulsion and regeneration of vertical lamination At Chau Mei Kok, you can find vertically fused or dotted reticulation on the shale sections caused by emulsion. As shale has high calcium content, when it is subjected to tidal, wave and sea current erosion, as well as impacts of salinization, wind, sun and rain, it develops melted calcium sections similar to stalactites. Later when the melted calcium is released, it flows out vertically at the outcrop tips or agglomerates into visible dotted reticulation. This is called vertical emulsion.

Internationally rare acidic polygonal volcanic rock columns － High Island

Internationally rare acidic polygonal volcanic rock columns － High Island

Background Information

High Island Reservoir is one of Hong Kong's most popular geological sites, and home to our most spectacular rock wonder. Here, volcanic rock composes neat arrays of giant polygonal joint columns along the shore. Amongst these the hexagonal rock columns are most typical examples. Catching your eyes with striking angular facets, these giants are rare natural prodigies and invaluable geo-tourism resources that draw great interest. Around High Island Reservoir, especially the East Dam, you can observe the most typical hexagonal columnar joints from different angles. There is also a chance to examine the rock composition and structure at close range. Looking down from the East Dam rock cliff, you have a clear view of buckles in the hexagonal columnar joints. Judging by the s-shape section, we know the rock columns buckled under gravity before it could cool down completely and solidify, and thus the scene we see today. With such evidence, it is easy to trace the formation of volcanic extrusive rock. At the observation point for the hexagonal columnar joints, you can also find outcropped basic lamprophyre veins. Lamprophyre is a dark dyke comprising mainly biotite, amphibolite, augite and light minerals like plagioclase and orthoclase. It is usually in vein attitude and wide distribution. Po Pin Chau just off the East Dam is another interesting place to see extraordinary formations. This is where you find giant stacks that emerged when a complete hill was slashed into two by natural forces. It is undoubtedly a fine example amongst similar geo attractions. Geology enthusiasts will certainly cherish the opportunity to see towering rock stacks and fascinating hexagonal joints. Maritime explorers will find Po Pin Chau intriguing and unforgettable. After admiring nature's exquisite works, don't forget to see High Island Reservoir, the impressive man-made wonder. This water storage facility is the biggest in Hong Kong. It is also a water project of key importance. The dramatic East Dam and West Dam are great attractions in their own right. The seafront is protected by concrete dolosse units. Functional as it is visually appealing, this cofferdam attracts many visitors.