The geological formation of Halong Bay
Finding out Halong Bay Geology, since ancient times the Vietnamese people have associated Halong Bay with national origin of the Dragon and the Nymph. At the beginning of the Cambrian era (500 to 570 million years ago), the area which now forms Halong Bay was basically mainland, submitted to a process of rain erosion. At the end of the period, it was flooded by the advance of the sea westward, commencing the existence of Halong Bay
During the Odovic and Silurian periods (400 to 500 million years ago), the area of north-east Viet Nam was basically a deep sea, submitted to the constant activity of tectonic plates. At the end of the Silurian period, it underwent a phase of inverse-motion that created mountains deep under the water. From the end of this pe-riod and throughout the whole Devonian period (340 to 420 million years ago), the area was subjected to powerful forcese of erosion from the hot and dry climate. At this point, Halong was part of a wide mainland that comprised most of today's East Sea.
Due to tectonic activity, the Halong area and the entire north-east region were raised from the depths at the end of the Devonian period. In the later Carboniferous and Permian periods (240 to 340 million years ago), a shallow and warm sea reformed, which existed for approximately 100 million years. It created two kinds of limestone: The Cat Ba layer of the early Carboniferous period (450m thick); and the Quang Hanh layer of the middle carboniferious period (750m thick). These two layer constitute the majority of the islands of the bay.
Passing into the early periods of the Contemporary era ( 67 million years ago), Halong Bay existed in the environment of a high mountainous mainland due to the influence of strong mountain-forming phases. The middle of the Palaeocene period saw these motions remain continuous and stable, while strong processes of erosion began, and after millions of years, a form of semi-highland topography took shape. The continuation of this erosion has progressively cut the highlands into blocks with altitudes similar to today's mountains.
Into the Quaternary era, the process of erosion began dissolving the limestone rich region of Halong. The islands of today's Halong Bay are basically remnants of these mountains flooded during the early Holocene period. The middle and late Pleistocene epoch (11,000 to 70,000 years ago) mark the period when the famous caves and grottoes of the area formed. Rain water flowed into crevices in the limestone that had formed from tectonic activity. This steady erosion constantly widened the cracks, eventually creating today's formations.
The Holocene period (from 11,00 years ago to today), is notable for the advance of the sea. This movenment reached its peak 3,000 to 4,000 years ago, forming today's Halong Bay. After that, with the sea in a steady process of recession, Halong culture began to develop. At the beginning of the late Holocene epoch, the level of the water once again increased, forming a marshy floor of canals and streams, and creating the water marks that can be seen on the stone cliffs of today.
The most remarkable geological events of Halong Bay's history in the last 1,000 years include the advance of the sea, the raising of the bay area and the strong erosion that has formed coral and pure blue and heavily salted water. This process of erosion by seawater has deeply engraved the stone, contributing to its fantastic beauty. Present day Halong Bay is the result of this long process of geological evolution that has been influenced by somny factors. So of th most reemarkable are : the formation of the limestone layer more than 1,000m thick during the Carboniferous and permian period (240 to 340 million years ago); and the development of the Halong depression during the Neogene period (10 to 26 million years ago). The erosion process forming the limestone plain was most active in the Quaternary Pleistocene epoch (11,000 to 2 million years ago).
It is because of all these factors that visitors now visiting Halong Bay are not only treated to one of the true wonders of the world, but also to a precious geological museum that has been naturally preserved in the open air for the last 300 million years.
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