Superficial Geology

containing subrounded cobbles and boulders which are usually highly to completely weathered The
intermediate deposits, 'Class 2 Colluvmm', have a firm to stiff sandy clay matrix with subangular to
subrounded, moderately to highly weathered cobbles and boulders The youngest material, 'Class 3
Colluvium , consists of soft to firm, uniform, pale brown clayey sandy silt with angular to subrounded,
slightly to moderately weathered clasts Figure 23 indicates the present thickness of the debris flow deposits
around the Mid-levels In most places their average thickness is about 2 m, but particularly thick deposits
occur along probable former valleys, a borehole near Robinson Road penetrating some 33 m of this
material The debris flows are believed to have extended beyond their present downslope boundary above
Sheung Wan and Central, and would formerly have continued to the natural shoreline or even beyond
Howat (1985) noted the presence of a lobe of terrestrial material into marine mud at a depth of about
-15 mPD in excavations for Sheung Wan MTR station (3382 1644) The deposit was examined during this
survey and closely resembles debris flow material seen higher in the Mid-levels, with cobbles and boulders of
volcanic rocks A wood sample collected from the base of the underlying marine mud has yielded a date of
8 600 ±270 yr BP (Howat, 1985) M D Howat (oral communication, 1986) has observed similar debris flow
deposits resting on marine mud m excavations beneath Johnston Road, Wan Chai (359 153)
Debris Flow Deposits up to 10m in thickness
Debris Flow Deposits
in excess of 10m thickness
Figure 23 - The Distribution of Debris Flow Deposits in the Mid-levels of Hong Kong Island
Debris flow deposits are present on the western slopes of High West, and these are thickest along stream
courses South of Victoria Peak, m the Pok Fu Lam valley (3310 1440), there is a widespread veneer of debris
deposits between 2 and 5 m thick, with only isolated exposures of bedrock visible m stream beds Similar
valley infilling is common in many stream courses through the Aberdeen Country Park, and in the valleys to
the south of Mount Nicholson and Wong Nai Chung Gap Debris flow deposits are not widespread m the
Stanley, Tai Tarn and Shek O areas, but occur as isolated accumulations on valley floors Most valleys in the
Chai Wan area have a 1 to 3 m infill of debris flow material, consisting of subangular to subrounded volcanic
clasts set m a clay matrix On the D'Aguilar Peninsula an unusual debris flow deposit is derived from the
quartz syenite pluton centred on D'Aguilar Peak (4370 0870) This quartz-deficient plutonic rock has
weathered to a fine silt but contains abundant subrounded corestones, some up to 5 m across Debris flows
consisting of this material are common on the western slopes of D'Aguilar Peak (4329 0832), subsequent
erosion has removed much of the fine matrix, leaving piles of quartz syenite boulders (Plate 49)
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