Karst landform in Non nuoc Cao Bang UNESCO global geopark

Karst is the weathering condition causing the erosion of Limestone Mountain which is very popular in mountainous areas. The erosion is not caused by physical contact but by CO2 in the air combined with water and H+ to produce Carbonic acid (H2CO3). It is the main factor causing the limestone erosion. It is the erosion that results in caves with stalagmites, stalactites, and underground stream, etc.

The limestone outcrops, accounting for more than 60% of Non nuoc Cao Bang UGGp, are mostly Devonian and Carboniferous-Permian to Late-Permian in age. Devonian limestone is found east of Cao Bang city in the districts of Thach An, Trung Khanh, Quang Hoa and Ha Lang. The Carboniferous-Permian limestone is more common northwest of Cao Bang city at Ha Quang District, and it extends in a NW-SE direction and is narrowed in the southeast of the Phuc Hoa area. The late Permian limestone is in narrow strips extending NW-SE west of Cao Bang city. Karst landforms in Cao Bang can be divided into four periods including: Early youth karst landforms; Young karst landforms;  Mature karst landforms; and Old karst landforms (residual).

Early youth karst: The early youth karst landforms, or karst ranges, have not been subject to dissolution, leaching or erosion that result in sinkholes, funnels or depressions, are found in Ha Quang and Nguyen Binh districts. Due to fault activities, landform disruption in karst ranges has occurred, therefore, steep cliffs were formed along the ranges, and also continuous triangular facets are common.

Young karst: Young karst landforms are mainly distributed on the Carboniferous-Permian limestone outcrops at Ha Quang district, with fewer in Quang Uyen, Phuc Hoa, and Trung Khanh. Karst valleys, sinkholes, dolines formed by dissolution, leaching or erosion are mainly of isometric form, not yet deepened to the local corrosion base level, leaving conical spikes connected to each other, with 45-60o cliffs, forming peak cluster-depressions.

Mature karst: is characterized by isometric limestone towers, with peaks and steep cliffs, scattered on the surface of the local corrosion base, which is relatively flat. The sinkholes, valleys, and dolines, after developing down to the local corrosion base, develop horizontally, expanding and linking at the bottom to create open, vast, and flat valleys, especially along large faults. These valleys are filled with large volumes of terra-rosa soil on the surface, and have many surface flows, which is beneficial for farming. Cave systems are well-developed, with large caves that develop horizontally. Most common are caves with waterways developing at the local corrosion base, connected with each other to link the valleys. There are various types of valley such as pocket valley, through valley, and blind valley, especially found at the boundary between karst and non-karst areas.

Mature karst is common within the Non nước Cao Bang UGGp, typically at the Trung Khanh, Quang Hoa, and Thach An. Blind valleys are commonly found in Ha Quang, Hoa An, Quang Uyen, Tra Linh, Phuc Hoa, especially in Tinh Tuc, forming large tin ore accumulation traps. Through valleys are in Na Nguom village (Canh Tien, Trung Khanh), Ma Phuc pass, and in the west karst block of Hoa An. Pocket valleys are distributed in Pac Bo (NE Cao Bang), Tien Thanh (SE Cao Bang), Phuc Sen (Quang Uyen), Ha Quang, Hoa An, Quang Hoa, and Trung Khanh.

Old karst: This is the final stage of karst evolution with karst towers becoming increasingly smaller both in size and height, caves collapsed and valleys expanded. Many places have only very small residual karst towers on flat and large fields, and such places are called karst fields. Within the area of Cao Bang Scenic Geopark, old karst is commonly found in the areas of Ha Lang, Quang Uyen, and Trung Khanh, with karst fields in Phuc Sen (Quang Uyen ), Diem village, Ga village (Tra Linh), south of Phong Chau (Trung Khanh), and Ha Thon (Ha Quang).

Owning to the diversity of Karst landforms, Non nuoc Cao Bang UGGp has been endowed with manificient and breathtaking landscapes such as Ban Gioc waterfall, Karst valleys in the western route of the geopark such as Phong Nam, Ngoc Con, and Thang Hen lake system, which is unique in the world, and a system of astonishing caves such as Nguom Ngao, Doi (bat), and Nguom Puc, etc.