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corestone boulders. Granite cliffs and exposed sheeting joints (Plate 53) are characteristic of thecoast and in several places vertical and sub-vertical joints have been selectively weathered anderoded by wave action, to form narrow joint caves.The sandstones and siltstones of the Lok Ma Chau Formation have been differentially weatheredand eroded to form minor escarpments, the more resistant sandstones producing sharp ridgelinesstanding above the vales in the softer, interbedded siltstones. Both rock types generally weatherto sand or silt sized material and produce very little coarse debris. Scarps are particularly welldeveloped along the coastal strip (066 153) northeast and southwest of Sham Wat Wan.In the northeast of Lantau Island the feldsparphyric rhyolite dyke complex has differentiallyweathered to create a very lineated topography, which is dominated by upstanding ribs, ridgesand ledges oriented in a westsouthwest to eastnortheast direction. The topographical grain is veryclear on high level aerial photographs, which show that the drainage pattern has a subparallel,trellis-like geometry that contrasts with the dendritic, radiating patterns on the other hills. Thechilled margins of the dykes have been preferentially weathered out to leave the central, relativelyless weathered, portions of the dykes upstanding. The ridges weather out to produce coarseblocky debris which topples and collapses to feed debris sheets and streams on the adjacentslopes. This weathering phenomenon is clearly displayed on the slopes (465 m) (181 179) of LoFu Tau. To the south, granite rocks crop out. The more readily weathered granites form a lower,less angular topography, as exemplified by the area around the low col (180 m) (170 156) nearWo Sheung Au, where the sharper and higher peak is developed on the feldsparphyric rhyolite.At outcrop scale, there are many examples of differential weathering, particularly in the volcanicrocks. The clearest example is the distinctive weathering-out of the finer-grained matrix of theSunset Peak Member, leaving the more resistant blocks protruding from weathered faces. This iswell displayed on boulders on the flanks of Lin Fa Shan (Plate 16). Less pronounced, but stillevident, is the etching out of the softer bands in the finely flow-banded lavas of the LantauFormation. These produce sub-parallel, ridged surfaces with interspersed, raised cross-joints(Plate 8) or irregular, wavy banding, emphasising primary flow features (Plate 10). Pitting iswell developed in the lapilli-bearing tuffs in which flattened, aligned lapilli are preferentiallyweathered leaving a honeycombed surface (Plate 17). On the granitic rocks, weathering pits onthe upper surfaces of boulders, as on the Chi Ma Wan peninsula, and, more rarely,pseudo-karren flute the sides of some boulders and ridge top tors. Alveolar weathering, which isrestricted to the granites, is rare in the district, but an extremely well developed example occurson a coastal outcrop at Ha So Pai (Plate 54).The effects of weathering are important in distinguishing the ages of the transported superficialdeposits in the district. Late Pleistocene alluvium occurs as high fluvial terraces that have beenincised by contemporary streams. The alluvium comprises weathered, oxidised, mainlyyellowish-brown to orange debris. Similarly, the older Pleistocene debris flow deposits areoxidised and stained red and brown, or dark yellowish-brown to orange-red. Large clasts displayrinds up to about 50 mm thick which must result from post-depositional weathering as they aretoo fragile to have survived transport. Weathering of the matrix has caused cementing in somedeposits, and, where the matrix is silty or clayey, it has been dessicated to produce a stiff to verystiff materialA weathered mantle, of variable thickness, also occurs beneath offshore superficial deposits ofthe district, This feature can be recognised on seismic profiles on which the fresh to moderatelyweathered rock usually has high amplitude reflectors, whereas the overlying, highly to completelyweathered rock material of the weathering profile is characterised by more moderate amplitudereflectors. The rock surface has an undulating topography similar to that developed onshore butwith a lower amplitude, and in places the weathered mantle is thin or discontinuous.143