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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