The bedrock of Shing Mun Valley is tuff, and only in the central and north side of Shing Mun Reservoir can we find granodiorite and feldsparphytic rhyolite. Tuff is more resistant to weathering and erosion, resulting in high mountains such as Tai Mo Shan, Grassy Hill and Buffalo Hill. Weathering and river erosion will create concave slopes with large gradients, such as Tai Shing Stream, and steep, narrow and convex ridges such as Tung Yeung Shan.
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.
Most of the bedrock of Sha Tin Valley is granite, therefore the ridges are mostly low and round, though there are also some steep mountains such as Needle Hill, Beacon Hill, Lion Rock and Tate's Cairn. As granite is easily weathered and eroded, badland features such as rills and gullies can be seen.
The banks of Shing Mun River, from Hong Kong Heritage Museum to Shatin Sewage Treatment Works, originally belonged to the deeply indented bay of Tide Cove. The riverbanks were reclaimed from the sea. The natural estuary was at the present Sha Tin Tau. There is no substantial alluvial plain in the lower course of Shing Mun River; the largest of them are at Tai Wai and Tin Sam. Smaller ones can be found at Kau To Hang, Ma Lai Hau Hang and Man Hang. The colluvium comes from slope collapse and is distributed in river valleys such as Tai Shing Stream, Tso Shing Stream, Au Pui Wan Hang and Fa Sam Hang.