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The formation is Holocene in age. Two 14 C radiocarbon dates have been obtained fromforaminifera in Borehole ESC 17 (Appendix 5). The first was near the base of the formation at-15.1 mPD and gave an age of 7,960 ±85 years BP. The second was 0.5 m below sea bed at -5.8mPD and yielded an age of 2,170 ±60 years BP. The older date accords well with a date of 8,080±130 years BP for the base of the formation at -19.70 mPD in the type section (BoreholeJBS1/1A) near Hang Hau (Strange & Shaw, 1986). Two radiocarbon dates were determinedfrom shells within the formation in Borehole A5/2 (Owen et al, 1995). The first date, of 3,950 ±95 years BP, was obtained from 3.29 m below the seabed at -21.60 mPD, and the second, of7,840 ± 120 years BP, was from 6.21 m below the seabed, or 1.09 m above the base of thefonnation, at -24.43 mPD. Elsewhere in Hong Kong, 14 C dates of 10,060 to 6,580 years BP havebeen obtained within the formation.Weathered rocks and sedimentsA widespread regolith, of variable thickness, covers the solid rocks of the district. Thisweathered mantle results from the differential breakdown of minerals in the bedrock. It comprisesresistant minerals, mainly quartz, within a matrix of clays derived from the decomposition offerromagnesian and feldspathic minerals. Regolith thickness depends mainly upon the underlyingrock type, its structure and the prevailing slope angle.*In general, the volcanic rocks are the most resistant to weathering and thus form, or cap, thehighest summits of Lantau Peak (934 m), Sunset Peak (869 m), Lin Fa Shan (766 m), Nei LakShan (751m) and the surrounding uplands. The granitic rocks are lower lying, and formsubdued, rounded hills, reaching a maximum elevation of 306 m at Lo Yan Shan on the Chi MaWan peninsula.The paucity of urban development on Lantau Island compared with the Hong Kong and Kowloondistricts (Strange & Shaw, 1986) has meant that there are fewer cut slopes in which deepweathering profiles can be observed. However, some sections are exposed in road cuts,catchwater slopes and stream sections around the island, and major excavations associated withthe North Lantau Expressway and the airport at Chek Lap Kok provided valuable temporarysections.An abundance of tors (Plate 49), boulder fields and boulder streams throughout the districtconfirms that deep weathering and corestone exhumation (Plates 50 and 51) has proceeded bothin the volcanic rocks, and the intrusive igneous rocks. In extreme cases, this results in theformation of pedestal rocks, where completely isolated corestones have been left resting on jointor contact surfaces. Examples occur on hillsides south of Lo Fu Tau 9 and along coasts, as at HeiLing Chau (Plate 52). The volcanic rocks have weathered to form elongate, ridge-like crests.Steeply inclined, dendritic drainage systems have developed on their flanks, with sharpinterfluves characterised by boulder accumulations, or ridge crest tors, with minor massmovements in the shallow weathering profile. Minor free faces, with thin talus slopes of blockyweathering debris, occur sporadically below ridge summits, usually near the heads of steams.Many channels are choked with boulder debris that has either rafted down from the adjacentslopes, or has been concentrated, by fluvial eluviation of the fines, from the weathering profilealong the stream lines, The Chi Ma Wan peninsula is the largest granite outcrop in the district, Itdisplays a distinctive landscape of summit and spur end tors, rounded granite corestones litteringthe slopes and sandy, eroded ridge crests. The topography is generally more rounded than in thevolcanic areas with gentler, shallower valleys that are commonly choked with valley trains of142