Both the East and West Streams follow faults; the former the Tung Chung Fault, while the latter has eroded a course along Shek Pik Fault. Tung Chung Fault runs northwest/southeast and across Tung Chung Bay to Pak Kung Au; Shek Pik Fault runs northeast/southwest and across Tung Chung Bay via Dong Shan Fa Mun to Shek Pik Reservoir.

The solid geology of Tung Chung River basin is mostly volcanic, particularly rhyolite lava and tuff. There is only a small outcrop of granite, with feldsparphyric rhyolite and metamorphic rocks, on the small hill to the west of the lower course. Volcanic rocks are more resistant to weathering and erosion, hence forming higher hills such as Lantau Peak and Sunset Peak. Weathering and erosion by the river result in slopes that are steep and concave, as well as slender and raised ridges with steep sides. On the ridges and valley sides one can frequently find tors and boulder debris; while dendritic depositions of colluvium fill the upper course, especially around Nei Lak Shan and Pak Kung Au.

 


Sunset Peak formed from volcanic rocks



V-shaped valley


Erosion of Tung Chung River's East Stream along Tung Chung Fault

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