Devonian-Triassic tectonic unconformity boundary and pillow basalt on Khau Khoang Pass, Cam Khang commune, Thach An district. The site shows a clear unconformable relationship between limestone of Na Quan formation (D1- D2nq), pillow basalts of Bang Giang formation (P3-T1bg) and siliceous-clayish shales of Song Hien formation (T1sh). Surrounding is the karst terrain of peak-cluster depression form on limestone and the denudation-erosion terrain in the form of hills on clayish shale and basalt that are dissected into erosive grooves and saddles on fractured zones.
This location is a highlight for visitors to Cao Bang Geopark. The unconformity boundary between Devonian and Triassic rocks expresses tectonic spreading and continental rifting with the base being limestone of the ancient Paleo-Tethys Ocean in Northeast Vietnam-South China, filled by terrigenous sediments and acidic volcanic rocks and tuff in the late Permian-early Triassic period.