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.