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Mui Wo. Extensive alluvial flats occupy the valley floor (177 147) at Mui Wo. Borehole records indicate that thealluvium is 9 to 13 m thick, and comprises gravel, cobbles and boulders in a sandy silt, or sandy matrix. Thisalluvial deposit is the distal portion of the slope deposits filling the valley behind Mui Wo, and it thins rapidly tothe east, ranging from 4 to 13 m in thickness. It is dominated by gravel, cobbles and boulders, but also containssand layers. The sand and silty sand matrix is greyish-brown to yellowish-grey. The clasts are dominantly roundedto subrounded, weathered volcanic rocks derived from the slopes of Lin Fa Shan. The alluvium north of Luk TeiTong (174 138) is a brownish-grey to pale brown sand. The stream bed is wide, covered in boulders and cobbles,and is inset 2-3 m into the surrounding alluvium.)Pui O - Tong Fuk Several thin and isolated deposits of alluvium occur along the steep stream courses that draini slopes of Sunset Peak.Shek Pik. An extensive alluvial deposit fills the valley occupied by the Shek Pik Reservoir. A series of coalescinglacustrine fans mantle its eastern slopes, which are commonly exposed during periods of low water level to beeroded and reworked. The material is a coarse gravelly sand, with some pebbles and cobbles, although sparseboulders occur.Slope DepositsJSlepe* Deposits are defined as accumulations of mass-transported material, formed bywater-mobilised, gravitational processes (Varnes, 1978). These mass movement deposits includecreep debris, rock falls, debris slides and debris flows. Previously in Hong Kong, they have beencollectively referred to as colluvium., although this term has not been rigorously defined andembraces the full range of slope deposits (Lai, 1982). They are diverse in composition andtypically poorly sorted. Depending on the original topography, source rock and distance travelled,they comprise variable mixtures of silt, sand, gravel, cobbles and boulders. Most commonly, theslope deposits consist of boulders and cobbles embedded in a gravelly, sandy silt matrix.Boulders up to 10m occur, but they average around 0.5 m across. They vary considerably inroundness, depending on the parent rock type and age of the deposit. Lai & Taylor (1983)presented criteria for the subdivision of colluvium into three classes, based on superposition,differences in clast-matrix ratios and the degree of decomposition of the clasts. They discussedthe probable ages of the deposits, suggesting Early Pleistocene for the older debris flows and aHolocene age for the younger deposits. During the present survey it was found that therepresentation of the three classes of slope deposits was impractical at the mapping scale.Generally, the fine debris occurs on the gentle slopes and in the upper sections of low gradientvalleys, whereas the boulder deposits lie at the foot of steep slopes, or fill the heads of steepvalleys. The thickest accumulations usually occur at the base of steep slopes, lining upland valleyfloors and filling lower valleys, with significant deposits often more than 20 m in thickness.Slope deposits are very extensively developed on the lavas and tuffs of the Lantau Formation.They have a more restricted occurrence on the granites of the Chi Ma Wan peninsula, and occuronly as isolated patches on the feldsparphyric rhyolites of northeastern Lantau. Quaternary slopedebris is manifested in several morphological forms. It develops as isolated linear fills in theupper sections of minor drainage lines, such as on the northern and southwestern slopes of NeiLak Shan (087 138). It also occurs as more complex, digitating but isolated networks belowridge lines and at the head of larger drainages, such as at the inception of the Sham Wat drainagewest of Nei Lak Shan (080 142), or north of Sunset Peak (130 138). The deposits also occur aslaterally extensive sheets, indiscriminately blanketing hillsides, for example on the western slopes(045 132) of Sze Shan above Tai O, and on the lower, northwestern slopes (125 165) of For KaiShan, crossing the coast near Lau Fau Shan. However, the thickest and most extensive depositsfill entire valley systems from their headwaters to the coast, as major dendritic accumulationswith fingers reaching up to the ridge crests. Of particular note are the deposits feeding down toTung Chung from Nei Lak Shan (095 146)ffimg Chung Au (121 116) and Sunset Peak (127144). Boulders tmd to accumulate in drainage lines, either by rafting and rolling down from theadjacent slopes, or as a result of duviation of the fines by stream action, to form linear boulder^' 111