page_title: Hạ Long Bay
section_title: Karst geomorphology value
hierarchical_section_title: Hạ Long Bay / Geology and geomorphology / Karst geomorphology value
caption_attribution_description: English: Hạ Long Bay, in Vietnam. Français&#160;: La Baie de Hạ Long, au Viêt Nam.
context_page_description: Hạ Long Bay is a UNESCO World Heritage Site and popular travel destination in Quảng Ninh Province, Vietnam. The name Hạ Long means "descending dragon". Administratively, the bay belongs to Hạ Long city, Cẩm Phả city, and is a part of Vân Đồn District. The bay features thousands of limestone karsts and isles in various shapes and sizes. Ha Long Bay is a center of a larger zone which includes Bai Tu Long Bay to the northeast, and Cát Bà Island to the southwest. These larger zones share a similar geological, geographical, geomorphological, climate, and cultural characters.
  Hạ Long Bay has an area of around 1,553 km², including 1,960–2,000 islets, most of which are limestone. The core of the bay has an area of 334 km² with a high density of 775 islets. The limestone in this bay has gone through 500 million years of formation in different conditions and environments. The evolution of the karst in this bay has taken 20 million years under the impact of the tropical wet climate. The geo-diversity of the environment in the area has created biodiversity, including a tropical evergreen biosystem, oceanic and sea shore biosystem.
context_section_description: Due to a simultaneous combination of ideal factors such as thick, pale, grey, and strong limestone layers, which are formed by fine-grained materials; hot and moist climate and slow tectonic process as a whole; Ha Long Bay has had a complete karst evolution for 20 million years. There are many types of karst topography in the bay, such as karst field.
  Hạ Long Bay is a mature karst landscape developed during a warm, wet, tropical climate. The sequence of stages in the evolution of a karst landscape over a period of 20 million years requires a combination of several distinct elements including a massive thickness of limestone, a hot wet climate and slow overall tectonic up lift. The process of karst formation is divided into five stages, the second of which is the formation of the distinctive do line karst. This is followed by the development of fengcong karst, which can be seen in the groups of hills on Bo Hon and Dau Be Inland. These cones with sloping sides average 100m in height with the tallest exceeding 200m. Fenglin karst is characterised by steep separate towers. The hundreds of rocky islands with form the beautiful and famous landscape of the Bay are the individual towers of a classic Fenglin landscape where the intervening plains have been submerged by the sea. Most towers reach a height of between 50 and 100m with a height to width ratio of about 6. The karst dolines were flooded by the sea, becoming the abundance of lakes that lie within the limestone islands. For example, Dau Be island at the mouth of the Bay has six enclosed lakes including those of the Ba Ham lakes lying within its fengcong karst. The Bay contains examples of the landscape elements of fengcong, fenglin and karst plain. These are not separate evolutionary stages but the result of natural non – uniform processes in the denudation of a large mass of limestone. Marine erosion created the notches which in some places have been enlarged into caves. The marine notch is a feature of limestone coastline but, in Ha Long Bay, it has created the mature landscape.
  Within Hạ Long Bay, the main accessible caves are the older passages that survive from the time when the karst was evolving though its various stages of fengcong and fenglin. Three main types of caves can be recognized in the limestone islands (Waltham, T. 1998):
  Remnants of old phreatic caves
  Old karstic foot caves
  Marine notch caves
  The first group consists of old phreatic caves which include Sung Sot, Tam Cung, Lau Dai, Thien Cung, Dau Go, Hoang Long, Thien Long. Nowadays, these caves lie at various high levels. Sung Sot cave is on Bo Hon island. From its truncated entrance chambers on a ledge high on the cliff, a passage of more that 10m high and wide descends to the south. Tam Cung is a large phreatic fissure cave that developed in the bedding planes of the limestone dividing the fissure cave into three chambers. Lau Dai is a cave with a complex of passages extending over 300m opening on the south side of Con Ngua island. Thien Cung and Dau Go are remnants of the same old cave system. They both survive in the northern part of Dau Go island at between 20 and 50m above sea level. Thien Cung has one large chamber more that 100m long, blocked at its ends and almost subdivided into smaller chambers by massive wall of stalactites and stalagmites. Dau Go is a single large tunnel descending along a major set of fractures to a massive choke.
  The second group of caves is the old karstic foot caves which include Trinh Lu, Bo Nau, Tien Ong and Trong caves. Foot caves are a ubiquitous feature of karst landscapes which have reached a stage of widespread lateral undercutting at base level. They may extend back into maze caves of stream caves draining from larger cave systems within the limestone. They are distinguished by the main elements of their passages being close to the horizontal and are commonly related to denuded or accumulated terraces at the old base levels. Trinh Nu, which is one of the larger foot caves in Ha Long Bay with its ceiling at about 12m above sea level and about 80m in length, was
caption: Hạ Long Bay
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