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.



Weathered Granite

The lower course of Shan Pui River was a bay 6,000 years ago. (Taken at Kai Shan)

Rounded ridge with gentle slopes is typical landform of granite bedrock. (Taken at Wong Nai Tun Irrigation Reservoir)


An interesting landscape “Fairy Ruts” formed by a pair of parallel rills (Taken at Wong Nai Tun Irrigation Reservoir)


Gully is a landform associated with headward erosion. (Taken at Wong Nai Tun Irrigation Reservoir)


A stream erodes along a line of weakness. (Taken at Tsuen Yuen Ancient Trail)
   
Sediments in streams at Tai Tong Valley are mainly sand.

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