Trang An area has undergone a series of sea transgressions and regressions corresponding to global glacial and interglacial periods from the Early Pleistocene to the Late Holocene, especially the Flandrian sea transgression. The alternating sea transgressions and regressions affecting the Ninh Binh area are: Cat Lam, Bim Son and Vinh Phuc transgresions in Pleistocene, followed by Dong Da transgression (also called Hai Hung transgression and corresponding with Flandrian transgression in countries such as Belgium, Holland, etc.) which reached the maximum in Early-Middle Holocene. Erosion notches in the limestone mark the Quaternary sea levels at different elevations. The results of C14 dating of oyster shells in the erosion notches at 2 m a.s.l give the age of 4300 to 6500 years BP, corresponding with Early-Middle Holocene (Hai Hung transgression - Flandrian transgression). The caves in Trang An area are mostly through karst caves and fossil caves formed in four periods: Period I lasted from the end of Early Pleistocene to the beginning of Middle Pleistocene with the formation of the oldest caves at 60 m a.s.l, such as Tr6ng, B6i, Cho. Period 2 was in Late Pleistocene with the formation of the caves at 10-15 m, 20-30 m a.s.l, such as Thung Binh and Moi. Period 3 was in Early-Middle Holocene with the formation of the caves at 2-9 m a.s.l, such as Yang, Ong Hay, Oc Trau Bai Dinh, Thien Ha, etc. In period 4 were formed the caves at below 2 m, mainly though caves such as Sinh, Seo, Thuoc, Dia Linh, Quy Hau, etc. Stone implements of ancient people have been found in many caves, such as Oc Thien Ha, Yang, Ong Hay, Moi, which were used as shelters during Hai Hung (Dong Da) transgression in Early-Middle Holocene when the sea water submerged the Trang An area as a whole to the height of 2 to 10m. Some caves like Trong, Boi and Cho might have been occupied by ancient people in the Late Pleistocene (23,000 years BP).