The majority of the bedrock in the upper course of Kam Tin River is volcanic, with tuff predominating. Most ridges formed by the volcanic rocks are tall and thin. Therefore, the mountains in the upper course of Kam Tin River, such as Kai Kung Leng and Tai To Yan are steep-sided. To the east (Kwun Yam Shan) and southeast (Tsing Tam Village to Kap Lung) of Kam Tin River, the main bedrock is granodiorite; to the southwest (Ma On Kong) the bedrock is mainly granite. As granite is easily weathered and eroded, the southwestern sector of the Kam Tin River catchment is at lower altitudes, with scant vegetation. Badland landforms such as gullies can be found within this region.

Along the middle course of Kam Tin River, alluvium deposits form the extensive Kam Tin Plain in the vicinity of Kam Tin and Pat Heung. The major components of alluvium are clay, sand and gravel. According to geological analyses, some six thousand years ago a bay occupied the area that's now the lower course of Kam Tin River, from Nam Sang Wai to Kam Tin Shi. The bay has since retreated to the current shoreline within Deep Bay, exposing marine deposits on the former seabed. The major component of these deposits is soft clay, mixed with shell fragments.

The main stream of Kam Tin River probably follows the fault that runs northwest-southeast, from Shek Kong to Nam Sang Wai. Also, there are two other faults in the Kam Tin River drainage area, running northeast-southwest in parallel. Erosion along Sham Tseng Fault has formed a saddle in between Tai To Yan and Kwun Yam Shan. On the other hand, erosion along Sui Lam Fault formed a saddle near Ma On Kong, and another one between Kai Kung Leng and Tai To Yan. Kam Tin River has eroded between these intersecting faults, giving rise to the rhombus-shaped Kam Tin Plain.

Kai Kung Leng and Tai To Yan are formed by volcanic rocks. (photographed in Tai Mo Shan)

V-shaped valleys on Kai Kung Leng

The watershed at western Ma On Kong is made of granite.

Marine mud mixed with shell fragments, along the lower course of Kam Tin River (Tai Sang Wai)

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