«Sai Kung Geopark Tour Commentary Special Thanks: Agriculture, Fisheries and Conservation Department and San Cheung for their invaluable support The ...»
Geological Information on Sharp Island Near the pier of Sharp Island, there is evidence showing that the place was once sitting at the edge of the ancient volcano. The rocks at the right hand side of the pier are mostly pyroclastic rock (火山碎屑岩) including volcanic agglomerate (火山集塊岩), volcanic breccia (火山角礫岩) and tuff (凝灰岩) etc. During the volcanic eruption, large, coarse, rock fragments associated with ash and lapillus (火山礫) are ejected from the crater, they quickly descended in sequence of larger size rock / fragments are closer as heavier while tiny size fragments are further away from the crater. They cooled off and solidified to form rocks. Agglomerates are pyroclastic rocks that consist almost wholly of angular or rounded fragments in relatively large size (64 mm in definition) and varying shape.
Further down the beach near the tombolo, a very rich varieties of volcanic rocks can be seen easily. Rhyolite (流紋岩), a type of rock formed from viscous lava (or silica-rich) that carries obvious flowing marks on the body. These marks proved that when the viscous lava extruded the land surface, the semi-molten mineral crystals within the lava were elongated and twisted, forming a pattern of parallel ‘lines’ and the texture was preserved when the lava turn into rock after cooling down. Hence, traces of flowing can be clearly seen on the surface of the rock.
Rhyolitic lava is rich in silica content making it high in viscosity that normally unable to travel a long distance. As a result, it will also stay close with the volcanic crater. Hence, it resembles another evidence of Sharp Island is near the edge of the ancient volcano.
Among the rhyolite and many other boulders, there are rocks suffered from serious physical weathering and resembles very much like a piece of Pineapple Bun ( 菠 蘿 包 ), visitors are all attracted by the interesting appearance and taking lots of photos on them. This is a rock pretty close to granite but the mineral content is slightly different. You can still call it a granite, but specifically and scientifically, it has another name called Quartz Monzonite (石英二長岩). In the case of Sharp Island, the quartz monzonite here was formed from magma intruded along cracks of the earth crust and nearly made it to the surface. Like granite it was also solidified underneath the ground.
The Quartz monzonite was finally exposed to physical weathering after the above surface being weathered away. The sun energy kept heated up the rock surface to expand during day time and it cooled off at night, or being splashed by sea water or rain, making the surface layer gradually peeling off from the center body, and at the same time, the surface layer cracking into pieces.
Hence, before peeling off, the Pineapple Bun was created.
Page 7 Kiu Tau is the name of an island just off the beach. During time of high-tide, the island is isolated from Sharp Island by sea water in between. However, at times of low-tide, there is a solid passage linked up the island to the beach. Originally, Kiu Tau was really an independent islet.
But the Monsoon wind help transporting the sediments depositing along the island and the beach.
Eventually a long sand-spit was built and connected the ends – a Tombolo (連島沙洲) was created.
This tombolo would be extremely fun for visitors who must follow the tidal information in order to walk across to Kiu Tau and back during low tide.
Ung Kong Group (甕缸群島) Among the veteran outdoor travelers in Hong Kong, there is one boat trip being considered as a “must do” item on their list when travelling around country-side of Hong Kong –visiting the “Four Great Sea Arches of the East Sea” (東海四大奇洞 - 火石洲欖灣角洞, 沙塘口山沙塘口洞, 橫洲橫 洲角洞 and 吊鐘洲吊鐘洞). This is actually talking about the Ung Kong Group of Islands. The Ung Kong Group consists of three main islands and a few smaller ones. They are namely the Bluff Island (甕缸洲或沙塘口山), Wang Chau (橫洲) and Basalt Island (火石洲). These islands are scattered over the actual ancient caldera. As a matter of fact, the rocks related to this ancient caldera have got special shapes – Hexagonal Rock Columns. The tall hexagonal volcanic columns on the southeast coasts of the islands have developed into numerous steep cliffs and amazing sea arches under the impact of the relentless waves and winds of the West Pacific Ocean. Beside the Bluff Island sea arch, the 30m-high sea arch at Wang Chau, the 45m-high sea arch at Basalt Island, and the Tiu Chung Arch at Jin Island are four most famous Hong Kong sea arches.
Since all these islands have got tall and linear columnar rocks, the weathering and wave erosion have actually sculpted spectacular scenic spots here. There are sea cliffs, including the 140-metre cliffs of eastern Bluff Island - the highest sea cliff in Hong Kong, as well as Hong Kong's highest sea arches - a 45-metre high arch through Basalt Island (火石洲欖灣角洞) and a 30-metre arch through Wang Chau (橫洲橫洲角洞). Besides, a good coral community is found at northwest Bluff Island inside a sheltered cove. Hence a boat tour out here is really stunning and be memorable to every visitors. However, the tour could only make possible during the hot Summer months when it’s not blowing the Easterly wind. Other months in a year is always involving winds coming directly or indirectly from the West Pacific Ocean which would make the boats extremely rough and not ordinary people could bear with it.
(Note: The Chinese word 洞 means sea-cave which is incorrectly used to descript these physical features. These four Sea Arches should really be called in Chinese 海蝕拱. So the phrase should be correctly changed to 東海四大奇拱.) Page 8 Ninepin Group (果洲群島) Ever since the Sung Dynasty, the Nine Pin Island was already being regarded as a dangerous sailing spot to those early day merchants due to its rough sea waves especially during Winter season. The name of the Islands was being called Guo Parn Zhou (果盤州) as the group of islands looked like a platter of fruits on the sea. 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 colonial-colour name. Local people still call the islands Guo Chau (果洲). The Ninepin Group is made up of North Ninepin Island, South Ninepin Island, East Ninepin Island and 26 other islets.
The Ninepin Islands is situated East of Sai Kung and Clearwater Bay peninsula are completely exposed to this natural ocean spreading 900 km across the West Pacific with no land barrier until reaching North of the Philippines. Imagine if there is strong wind generated somewhere in the West Pacific and blowing directly towards Hong Kong, after travelling hundreds of km picking up moistures from the ocean surface adding its power along the way. When reaching Sai Kung, the wind power sometimes could be really damaging. The weather in Hong Kong is heavily influence by the Easterly wind with almost 8 months in a year. Only the few Summer months are less affected by this ocean wind. Even though volcanic rocks are essentially hard and resistant, wind and waves have taken their toll over the ages to shape the terrain of East Sai Kung rugged and imposing.
Given such challenging condition, visitors to the Ninepins can only make their trips during the two or three summer months every year. Located at the middle part of the South Ninepin there stands the “Tiger Mouth Cave” (虎口大洞). Watching from the boat, one will marvel at nature's brilliant craftsmanship and truly appreciate the greatness of nature's power relative to our own.
Further up to the hilltop above Tiger Mouth Cave, there is a Tin Hau Temple in mini size. Since the harsh condition of this region, there’s no inhabitant on the island. But on the contrary, underneath the water it is surprisingly rich habitat for water creatures. Fishermen then built here the smallest Tin Hau Temple to get blessing from their water god to protect themselves from the rough sea while working around here with their fishing nets. The water here also attracts groups of scuba-divers in search of underwater heaven.
The rocks on the Ninepins are remarkable. They are in fact mother nature's masterpieces. About 140 million years ago in the early Cretaceous period, the last stage of our volcanic eruption took place in Sai Kung. The violent eruptions had literally driven huge amount of underground magma as well as the ash fragments, leaving the underground magma chamber empty. Without proper support, the entire volcano collapsed to form a gigantic caldera. This big bowl-shape cavity at the end appeared like a lake containing thick layers of volcanic ashes and acidic lava mixture. When these molten volcanic matters slowly cooled down, the contraction effect resulted from heat lost gave rise to polygonal cracks, some of them in hexagonal shape. These cracks called ‘joints’ in geology, net work of polygonal joints cut through the rock mass vertically downward leading the formation of columnar rocks. The Ninepins Island seems to be sitting near the center of the caldera where the rock strata is about 400 meters thick in estimation. As the cooling process might take a bit longer time than those near the edges, the Hexagonal Rock Page 9 Columns are seen bigger in sizes. Columns nearly 2 meters in diameter could be found quite easily.
The Hexagonal Columns in Hong Kong is considered to be the best of its kind in the world. First of all, the rock columns here are spreading over a scale of almost 100 sq km from the Ninepin group all the way to the High Island. Secondly, our light colour acidic volcanic rock is rare compare to those dark colour basaltic rock columns found in various parts of the world. Lastly, the average size of rock columns is found much bigger in average size. From The Ninepin Group, we can see columns well over 2 meters in diameter, whereas in Northern Ireland from the world famous Giant Causeway, those columns are on average only 0.5 meter.
One way to appreciate this natural wonder is to sail from South Ninepin to North Ninepin, from the boat visitors could appreciate the breath taking scene of unique volcanic rock arranged in a striking formation of giant rock columns. Just the turn from South Ninepin to North Ninepin, an islet off the east shore called Yuen Shek Pai (圓石排) offers the most amazing sight. Numerous rock columns rise up neatly around the centre of the islet and form a natural staircase in spiral shape. As the shape is really interesting, some people simply rename it the “Spiral Staircase” (迴 旋的階梯).
Bluff Island (Sha Tong Hau Shan) Bluff Island (local Chinese call it Sha Tong Hau Shan 沙塘口山), or Ung Kong Chau, lies west of Basalt Island and south of Town Island (Fo Tau Fan Chau 伙頭墳洲). In the southern corner of this small island you can find one of the four biggest sea caves in our eastern waters - Sha Tong Hau Cave (actually it should be correctly called a Sea-arch 海蝕拱). Where as sitting up on the center North of the island, there is a white sandy beach called Ung Kong Bay(甕缸灣) which is a very popular spot for swimmers from Sai Kung. Hikers like to call this island as Cliff Island (峭壁洲).
During their hiking, they will encounter cliffs almost in all directions of the island and the highest cliff is measured some 140 meter above sea-level. Moreover geologist finds it a fantastic location for the studying of rhyolite. Formations in clearly defined layers are prominent across the island, and along the wavy shores, huge blocks tower by the water. These were evidence showing lava was flowing around Bluff Island during the Early Cretaceous Period which was the very last stage of the volcanic activity in Hong Kong. Fan Tap Pai in the south is one of the most breathtaking coastal rock scenes in Hong Kong. When approaching, you will first see a huge sea cliff which is wide and tall (140 m) appears like a giant cathedral organ, followed by a huge sea arch cuts right through the island and it is just wide enough for a boat to sail past (but it’s risky as it might have rock fall from above). It’s not a bad idea for a leisure sea expedition. Bluff Island is also a diving hot spot as a large coral community is found all around the island. An increasing number of divers are coming here to explore the mysterious ocean.
Basalt Island (火石洲) Basalt Island is an island south of Town Island (Fo Tau Fan Chau 伙頭墳洲 ) and Wang Chau (橫 洲), east of Bluff Island and north of the Ninepins. The Island is to certain extent rather well known to local people. First of all, during the British administration and before 1985, the surrounding area of Basalt Island was a restricted area for the British Force to practice Cannon Fire. Since fishermen in the old days were mostly poor people. Some fishermen risked themselves to land on the island after every cannon practices to pick up the bomb-shell as well as those gun powder that left-over for making small fishing use explosives. Sometimes it could be Page 10 very unfortunate that some dynamite might go off just when they pick it up and getting hurt was quite a common thing for the fishermen. Then Basalt Island was also the site of the first air-crash that ever happened in Hong Kong in 1948. It was a flight coming into Hong Kong from Shanghai crashed on the island due to heavy fog weather, killing all 30 people onboard including the grandson of the American President Mr. Quentin Roosevelt. More than 60 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. Moreover, there are modern tragedies happened to some aggressive Shore Fishing guys as well. Since Basalt Island is sitting at the most Eastern location among the Ung Kong Group and receiving most severe wave erosion that broken rocks and debris kept going down under to the sea by the edge of the island, hence creating lots of living habitats for the corals and water creatures. It would then become a good spot for predator fishes to look for food.
Shore Fishing here is becoming popular as large fishes are always available. Unfortunately, both the big waves and the unstable hanging rocks could be deadly threats to these fishing guys. In June 2010, one experienced shore fishing guy was being headed-on by a several ton rock and went dead immediately.