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Rock andweathered rockFigure 12 - Schematic Section Relating Onshore to Offshore Superficial Deposits.Colours vary from grey to yellowish-brown, depending on the nature and proximity of thebedrock source and the degree of weathering. Where the source is granitic or dyke rocks,sand-size material predominates, while clayey, clast-rich alluvium derives from volcanic rocks.Two ages of alluvial deposits are recognised. The late Pleistocene deposits are found in extensive,elevated, fluvial terraces, with the Holocene alluvium largely confined to the recent streamcourses that are incised by up to 7 m into the terrace surface. The lithofacies of the latePleistocene fluvial terrace deposits vary across the outcrop. Near the foot of hills, the depositsconsist mainly of semi-sorted, yellowish-brown to orange, gravelly, silty sand which is probablyderived by slopcwash from the adjacent slopes. In the upper valley sections these depositscommonly pass laterally into more bouldery Pleistocene slope deposits. In the distal plains thefluvial terrace deposits are characterized by mottled red, yellow, brown and white, well-sortedclayey sandy silt with some sand layers.In general, deposits of Holocene alluvium are relatively low-lying, reaching a maximum height ofc.10 mPD, and are usually narrow, cutting into the surrounding Pleistocene terraced alluvium.The deposits comprise gravelly sand in the upper stream courses with some subrounded boulders,and yellowish-brown, well-sorted clayey sand, or silt, in the lower courses. At several localitiesalong the coast, the streams are dammed by sand bars or spits, behind which lagoonal depositshave formed, as for example at Pui (X Small patches of hillside alluvium have also been mapped.These occur in narrow valleys in restricted areas, are generally only 50 to 100 m wide, and areless than a kilometre long.Because of the steep, rugged topography which characterises the district, alluvium tends to beonly locally developed, Lantau Island is characterised by short, steep valleys, mostly choked withbouldery debris deposits. With the exception of the Tung Chung and Shek Pik valleys, alluviumis generally limited to small deposits at seaward ends of valleys, occurring either as short linearvalley fills, or wide fans along the coastal zone.109