Halong Bay, its adjacent islands and seashore contain a multi-layers system of carbonate sediment with a long geological history commencing nearly 500 million years ago recording great events of the geological processes in this area. It can be seen through the colors, material composition, stone layer formation, and fossil relics that have been preserved.
There are sediment layers containing paleontologist relics under different fossil forms of which a number of fauna and flora group that have been all or almost annihilated in all over the world. Therefore, it can be said that this area possesses a treasure that has not been discovered in terms of evolution and growth of the life on the earth in this area.
With regard of geological structure, Ha Long Bay is located in the Carboniferous period under island formation process and inverse tectonic movement about 340 to 286 million years ago.
The Quaternary and marine geological value
Ha Long Bay area has many typical characteristics of the Quaternary geology, namely: The sediment layers, the rising sea shelf, terraced plains submerged in the bay, the ancient rivers, grottos and caves and their sediment, the traces of the old sea level and the selfish found high on the cliffs. These are living proof of the geological events during the Quaternary.
To the view of offshore geological values, Ha Long is considered as a modern agglomerated basin which was formed by a system of preventive islands, not protruding capes. Here, the seashore that is eroded by carbonate chemical process develops very well under the marine environment creating the unique marine notch caves forming multi-forms and interesting strange shapes of limestone islands on Halong Bay.
Karts morphological value
Halong Bay is also an ideal model of a mature Karts landscape developed during a war and wet tropical climate. The sequence of stages in the evolution of a Karts landscape over a period of 20 million years requires a combination of several distinct elements including massive thickness of limestone, a hot wet climate and slow overall tectonic uplift. There are five stages of Karts formation: the first stage is remnants of either an older plain or a flat landscape with inherited relief; the second stage is the development of the Karst funnels; the third stage is the formation of clusters of the conical and pyramid shapes; The fourth stage is the formation of individual high towers having abrupt slopes; The final stage is the formation of Karts plain.
The system of caves and grottoes in Halong Bay is varied and multiform. To date, over 30 caves and grottoes have been found with the length of tens to hundreds meters. They are divided into three following groups:
Remnants of old pancreatic caves on Halong Bay are: Sung Sot, Tam Cung, Lau Dai, Thien Cung, Dau Go. Most of these caves and grottoes were the underground channels that carried drainage from the Karst funnels. Today, they are located on islands at various altitudes. They are distinguished from other cave types by their sloping passages and considered vertical range.
Old artistic foot caves such are: Trinh Nu, Bo Nau, Tien Ong and Trong. This type of caves is a ubiquitous feature of karts landscapes which have reached a stage of widespread lateral undercutting at base level. They may be stream caves draining from larger cave systems within the limestone. They are distinguished by the main elements of their passages being close to horizontal, and they are commonly related to rock or sediment terraces that were a function of base level erosion or deposition.
Marine notch caves are Luon Cave, Ba Hang and Ba Ham lakes. This is an outstanding karts feature of Halong Bay. All rocks are eroded by wave and tide action to create cliff notches. Dissolution of the limestone allows the cliff notches readily to be deepened and extended into caves; many of these sea-level extend right through the limestone hills, into drowned do-lines which are now tidal lakes or adjacent bays. A distinguishing feature of these marine notch caves is an absolutely smooth, flat and horizontal ceiling cut through limestone. This type of caves is not only simply formed at the present sea level but also largely created by dissolution in times past when the old sea levels during marine transgression process in Holocene times and old sea levels in the Pleistocene times.
In sum, the karts landscape of Halong Bay is of international significance and of fundamental importance to the science of morphology. The morphology of Halong Bay may be described as unique because its karts landscapes is the most extreme form of limestone landscape development with tower karts invaded and eroded by the sea.