Stratigraphy: Late Cretaceous post-volcanic sedimentary rocks.

Distribution: The Port Island Formation is only exposed on small islands in Mirs Bay including Port Island, Round Island, Channel Rock and Shek Ngau Chau.

Lithology: Reddish-brown thickly bedded conglomerate and sandstone, with thinly bedded reddish siltstone.

Depositional Environment: The sedimentary rocks of the Port Island Formation are interpreted as distal alluvial fan or proximal braidplain deposits.

(Thickly-bedded volcanic conglomerate and sandstone of the Port Island Formation, Port Island)