In front of the east coast of the delta of the Red River, in Tonkin, an archipelago formed by rocks and islets delimits the Blue Sea, of which Ha Long Bay represents the western end located off the city of Hon Gay.
The legend of the dragon
It is a landscape of rare beauty, having such incredible shapes that its origin, in the Vietnamese tradition, has been attributed to the work of the dragon, superior genius of the waters, from which the name of Ha Long or Along, which defines the place in which the dragon came down to.
Among these unexpected forms include the Cave of Wonders, the Customs Tunnel, the Island of the Great Monkey, the Island of Surprise, the Insane, the Puppets and the Udders.
Such a landscape represents only the southernmost part of a grandiose complex which extends over part of southwestern China and extends into Tonkin.
This formation is comparable to an extensive mass of permo-carboniferous limestones, dating back to primary age, shaped by the erosion processes typical of these rocks.
Karst erosion
It is karst erosion that gives rise to relief forms, whose originality derives from the property of limestone to dissolve under the action of the carbonic acid contained in the water, which in turn comes from the carbon dioxide released by the atmosphere and plant soil.
Calcium carbonate, the main component of insoluble limestone, is transformed by carbonic acid into calcium bicarbonate, which on the contrary is soluble.
The particular shapes of the limestone landscapes are due to this phenomenon of chemical dissolution.
Spectacular forms
There is a large variety of shapes with more or less spectacular karst reliefs, depending on the period spent, the type of limestone and the amount of rainwater that bathes them, which in turn is influential for its degree of carbon dioxide.
Under these conditions, the dissolution processes are so pronounced that it reaches the extreme stage of karst evolution, with numerous sinkholes developed to reduce the limestone mass in residual peaks, whose height, between 100 and 300 meters, testifies to the power of the 'erosion.
These peaks are in turn shaped by the karst dissolution, with the presence of grooves on the sides and caves.
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A karst relief of this species is called a karst with cones or towers, whose extreme type, the Turmkarst, is made up of the residual reliefs of the Ha Long Bay.
Islands and islets
But the element that makes this bay of exceptional beauty is the flooding of the entire base of this karst, which occurred during the last marine transgression, which transformed the limestone towers into a multitude of islands and islets, characterized by vertical walls, and the depressions in deep bays or in salt lakes at first sight closed, but actually reached by the tide, dating back through the tunnel caves dug at the base of the towers.
Marine erosion is thus combined with what remains of karst erosion, thus accentuating the fantastic character of this landscape.
The numerous shelters created naturally by the relief itself, at a sea depth that does not exceed 20 meters, make this place one of the largest fish reserves in Vietnam.
