Caves: Cave system in the cave include threaded Chang'an (now close to the water, which is the basis of the current local erosion) or fossil cave at some higher level. Hang usually develop in the fault system NW-southeast and northeast-southwest. Age cave formations is estimated on the basis of the respective steps. The cave has elevations below 2 m was formed in the Late Holocene, for a round of Quang Xuong transgression. The cave at a height of 2-9m been formed in the early-mid Holocene, with variable advance Flandria. The cave at a height of 10-30 m was formed in the Late Pleistocene, for a round of Vinh Phuc transgression. The cave at a height of 40-60 m can be formed in the early-middle Pleistocene, for a round of Yen Mo transgression. The higher caves can be formed earlier than in continental conditions.
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