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Value of Geology
The
most remarkable geological events of Hạ Long 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 Hạ Long Bay is the result of this
long process of geological evolution that has been influenced by
so many factors. Some of the most remarkable are: the formation of
the limestone layer more than 1,000 m thick during the
Carboniferous and Permian periods (240 to 340 million years ago);
and the development of the Hạ Long 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 tourists now visiting Hạ Long 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.
At
the beginning of the Cambrian era (500 to 570 million years ago),
the area, which now forms Hạ Long Bay, was basically mainland,
submitted to a process of rain erosion. At the end of the period,
it was flooded, commencing the existence of Hạ Long Bay. During
the Odovic and Silurian periods (400 to 500 million years ago),
the area of north-east Vietnam 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
period and throughout the whole Devonian period (340 to 420
million years ago), the area was subjected to powerful forces of
erosion from the hot and dry climate. At this point, Hạ Long was
part of a wide mainland that comprised most of today’s East Sea
and Chinese continental shelf. Due to tectonic activity, the Hạ
Long 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 Cát Bà
layer of the early Carboniferous period (450 m thick); and the
Quang Hanh layer of the middle Carboniferous and the early Permian
period (750 m thick). These two layers constitute the majority of
the islands of the Bay.
Passing
into the early periods of the Contemporary era (67 million years
ago), Hạ Long Bay existed in the environment of a high mountainous
mainland due to the influence of strong mountain-forming phases.
The middle of the Paleocene 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
to the Quaternary era, the process of erosion began dissolving the
limestone-rich region of Hạ Long. The islands of today’s Hạ Long
Bay are basically remnants of these mountains flooded during the
early Holocene period. Rainwater 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 middle and late Pleistocene epoch
(11,000 to 70,000 years ago) marks the period when the famous
caves and grottoes of the area formed.
The
Holocene period (from 7,000 to 11,000 years ago) is notable for
the advance of the sea. This movement reached its peak 4,000 to
7,000 years ago and forming today’s Hạ Long Bay. After that,
3,000 to 4,000 years ago, with the sea in a steady process of
recession, Hạ Long 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.
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