Granite is the predominant bedrock
along the upper course of Shan Pui River basin. Hence
the ridges are generally flat and rounded, and slopes
on both sides are gentle. As granite is easily weathered
and eroded, a thick residual mantle of weathered rock
is formed. Large scale deforestation in the area has
also caused severe soil erosion. There are many badland
features such as rills and gullies. On the eastern
side of the watershed (Ho Hok Shan), the bedrock is
mostly volcanic rock – tuff, which is more resistant
to weathering, so there is less weathering and erosion.
Metamorphic rocks that originated from sedimentary
rocks such as metasiltsand, metasandstone and phyllite
occur to the south and west of the Yuen Long Plain,
as well as the northern hills including Kai Shan and
Chu Wong Ling.
The lower course of Shan Pui River
deposits sediments comprising clay and silt mixed
with gravel and sand onto Yuen Long Plain at Shap
Pat Heung, and forms an open alluvial plain. Granite
contains larger crystals, so deposits at the west
and south of Shan Pui River are mainly sand from weathered
granite and metamorphic sandstone. Volcanic rocks
contain smaller grains, and deposits in the east are
mostly silt and clay.
Geological analysis shows that Sham
Chung Tsuen and Lam Hau Tsuen at the lower course
of Shan Pui River were in a bay some 6,000 years ago.
The bay receded to become today’s Deep Bay, and sea-floor
deposits were exposed due to progadation. The sediments
were dark grey, soft clay mixed with broken shells,
with high salinity and low permeability.
Most earlier geological structures
of the lower course of Shan Pui River were buried
in the sediments and outcrops were few. Along the
upper course, granite bedrock covers several small
faults, which run mainly in the NNW/SSW and NE/SW
directions and parallel to the major faults (e.g.
Siu Lam Fault); upstream tributaries also follow these
lines of weakness.