Stratigraphy: Carboniferous metamorphosed sedimentary rocks with ages ranging from approximately 354 to 280 million years before present. The Lok Ma Chau Formation stratigraphically overlies the Yuen Long Formation.

Distribution: The rocks of Lok Ma Chau Formation are present in the northwestern New Territories in a northeast-trending outcrop and subcrop stretching from Tuen Mun to Lo Wu, and also between the three islands of West Brother, East Brother and Reef Island.

Lithology: The Lok Ma Chau Formation consists of metamorphosed sandstone and carbonaceous siltstone with graphitic interbeds and conglomerate.

Depositional Environment: The Lok Ma Chau Formation has been interpreted in terms of deposition in neritic swamps within a prograding deltaic environment.

(Graphitic siltstone of the Mai Po Member, Lok Ma Chau Formation, West Brother island)