A 500-million-year-old foundation The famous limestone pillars of Ha Long Bay began their story over 300 million years ago, during the Carboniferous and Permian periods.%%CITE_1%%%%CITE_2%% At that time, the region was a warm, shallow sea.%%CITE_3%% Over millennia, the skeletal remains of marine life accumulated on the seafloor, compacting into a massive limestone deposit more than 1,000 meters thick.%%CITE_1%%%%CITE_4%% This bedrock is composed of two main layers: the early Carboniferous Cát Bà layer (450 meters thick) and the middle Carboniferous to early Permian Quang Hanh layer (750 meters thick).%%CITE_1%% These ancient marine sediments form the raw material for the bay's dramatic landscape.
Tectonic forces later lifted this immense limestone block above sea level. Once exposed, the rock became subject to the erosive power of a hot, wet tropical climate. For the last 20 million years, rainwater, made slightly acidic by atmospheric carbon dioxide, has seeped into fractures, slowly dissolving the calcium carbonate. This chemical weathering carved the solid plateau into a mature karst landscape, a process that continues today.
Towers, caves, and a rising sea
The landscape of Ha Long Bay shows two classic forms of tower karst. The first, called fengcong, consists of clusters of conical hills with shared bases, representing an earlier stage of erosion. The more advanced stage, fenglin, is what defines the bay: tall, isolated pillars with steep, vertical sides, separated by a plain. Many of these towers rise 50 to 100 meters from the water.
The decisive event in the bay's formation was the end of the last ice age. During the Pleistocene epoch, global sea levels were over 100 meters lower than they are today, and Ha Long Bay was a dry valley filled with these limestone towers. As continental glaciers melted, the sea began to rise, a process known as the Holocene transgression. Around 11,000 years ago, seawater flooded the valley, submerging the plain and transforming the limestone peaks into more than 1,600 islands and islets. The sea continues to shape the landscape, carving distinctive wave-cut notches at the base of the cliffs, which contributes to their precipitous nature. The bay also contains three main types of caves: ancient phreatic caves formed below the water table, old karstic foot caves, and marine notch caves created by wave action.