Stratigraphy: Pre-Middle Jurassic volcanic rocks.

Distribution: Rocks of Tuen Mun Formation are restricted to the northwest of the New Territories where they outcrop to the west and north of Tuen Mun.

Lithology: Andesite lava and lapilli lithic-bearing fine ash crystal tuff with intercalated tuff breccia.

(Andesitic metatuff of the Tuen Mun Formation, Tuen Mun, western New Territories)