Correlation of the onshore Pleistocene alluvial deposits with the offshore Chek Lap Kok Formation is relatively straightforward (Table 11.2), but the stratigraphical treatment of the Pleistocene mass transport deposits (debris flow deposits, slope debris, etc.) is rather more problematical. They clearly interdigitate with the Pleistocene alluvial sequence onshore and have also been reported within the predominantly alluvial Chek Lap Kok Formation. As such, they can be regarded as members within the Chek Lap Kok Formation, although they do not necessarily have to be named individually. However, as these deposits are traced into more proximal environments, where they become the dominant component of the stratigraphy, it could be argued that they provide the basis for a separate, predominantly colluvial, formation. However, this solution has not been adopted mainly because the colluvial material is very heterogeneous and, more importantly, extremely laterally variable in thickness.

Type section

The type section of the Chek Lap Kok Formation (Figure 11.4) was described in offshore borehole B13/B13A (Strange & Shaw, 1986) which is now under the Chek Lap Kok airport platform, north of Lantau Island. The formation in the borehole comprises an 18.8 m interbedded sequence of sand, silt and clay. There, the sediments overlie weathered granite and underlie 2.9 m of soft clayey silt of the Hang Hau Formation and fill.

Four lithological units make up the type section. These comprise two units of mainly grey laminated mud and two units of bluish grey mottled clay (Shaw, 1985). The lower laminated mud unit is a broadly laminated silty clay that is sandy at the base of the formation where it sits directly on weathered granite. Black carbonaceous fragments are disseminated throughout the unit. The lower mottled clay unit is grey to buff in colour with red, ochre and yellow mottles, the most vivid colours occurring in the middle part of the unit. Fine to coarse sand and gravel bands within this unit include quartz clasts and rock fragments. The upper laminated mud is sandy at the base, and contains abundant organic fragments including twigs and rootlets higher up. The upper mottled clay unit has a similar mottling pattern to that of the lower unit and includes a 1 m-thick bed of fine to medium sand, which is gravelly at the base. The unit is capped by a thin palaeosol with a well-developed rootlet horizon, which indicates a former subaerial surface.

The palynological associations in this borehole have been subdivided into a number of zones (Shaw et al., 1986) (Figure 11.4). The lower mottled clay unit is devoid of palynomorphs, though the upper mottled clay unit contains several fern species. The lower laminated mud unit is dominated by Castanopsis pollen with numerous fern spores. Castanopsis, still common in the upper laminated mud unit, is accompanied by mangrove pollen, principally Sonneratia. This suggests that the lower laminated mud unit accumulated as a back-swamp deposit on an alluvial plain, while the upper laminated unit represents an intertidal sediment.

The type borehole was drilled on the Chek Lap Kok test embankment and, as a result, is not intersected by any modern seismic line. The track of a 1981 survey passes 100 m to the north of the borehole site but the record quality is poor. The units identified in the borehole cannot be traced laterally with any degree of certainty, though the formation as a whole comprises a series of parallel reflections cut by channel-like features.

The formation in the type section is interpreted as an alluvial plain and intertidal deposit. The fine to medium sand beds at the base and near the top of the section may be channel sediments, while the isolated bands of fine to coarse sand and gravel appear to be fluvial channel sands that were deposited on the alluvial plain during river flood stages. The concentration of vivid colours in the middle part of the mottled horizons suggests that oxidation was diagenetic and was controlled by the acidic waters in the carbonaceous laminated mud units above and below.

Depositional environment

The widespread distribution and variable lithology of the Chek Lap Kok Formation indicate prolonged deposition in several fluctuating sedimentary environments. In onshore areas the general geomorphology of the deposits provides valuable clues to their sedimentological interpretation. However, in offshore areas, where the formation is concealed below the Holocene Hang Hau Formation, interpretation depends on the identification of characteristic geometries using seismic reflection data, and of lithofacies in the few available boreholes.

In onshore areas, the formation is associated in large part with extensive floodplains, especially in the western and northern New Territories. These reflect fluvial and alluvial deposition within and alongside meandering channels, of varying size. The fluvial deposits include point bar deposits of widely varying grain size. The overbank deposits of the channels, generated during times of flood, are characteristically finer-grained. These are intercalated with organic rich clays and silts that were deposited in ephemeral lakes on the floodplains and probably also in oxbow lakes created in abandoned channel meanders. The fluvial and alluvial deposits are also extensively intercalated with, and may pass imperceptibly into, colluvium, particularly on steeper slopes and in valley heads.

The colluvium is the product of mass wasting with the material transported by gravitational processes. In this regard, the matrix-supported, rather than clast-supported nature of the material is characteristic. Water-initiation is generally inferred. The colluvial deposits have often been interpreted as debris flow deposits (e.g. Strange & Shaw, 1986; Langford et al., 1989 etc.). However, this represents only one of several transportational mechanisms that are likely to have been responsible for forming these deposits. Others include rock fall, rock slide, rock avalanche, debris slide, debris avalanche, earth flow, continuous creep and other compound and more complex mass wasting processes. The resulting deposits would be further dependent on the degree, or otherwise, of channelization of the debris, the degree of saturation of the debris, and the prevailing slope angle.

Interpretation of the Pleistocene environments of deposition has largely been carried out offshore, where the formation was first described ( Strange & Shaw, 1986). Extensive fluvial deposition occurred during the Pleistocene low sea level stands, which drew a close with the Holocene sea level rise. Offshore, seismic profiles show a large number of channel-fill features within the formation, some of which have been drilled and shown to contain upward-coarsening sequences. Elsewhere, the presence of flat-lying parallel reflection patterns suggests more regular deposition, such as floodplain, intertidal or possibly marine sedimentation. The paucity of fauna has hindered attempts to establish firmly the environments of deposition; this paucity may be either primary or a result of post-depositional decalcification during periods of subaerial exposure (Wang & Yim, 1985).

The formation lies on a comparatively subdued topography in which the broad pre-Quaternary drainage pattern can be distinguished. The oldest seismic unit is probably represented by the package of flat-lying basal reflections that are seen on some seismic profiles from the south. Seismic facies analysis provides limited information on sedimentary facies (Stoker et al., 1992). Nevertheless, laterally persistent sub-parallel reflections are consistent with low-energy conditions, such as those in floodplain, intertidal or marine environments. Poorly defined, flat-lying reflections may be seen on seismic profiles from the site of borehole A5/2. The sediments are interpreted as terrestrial sands overlying a thick sequence of lacustrine clays, derived from nearby granites, interbedded with alluvial plain sediments (Owen et al., 1995).

The wide channels identified on seismic records can be explained by several facies models. Channels may be cut during periods of fluvial entrenchment, as with those of the Tung Lung Member of the Hang Hau Formation (see Chapter 10 of Fyfe et al., 2000). These features must have formed as wide valleys in which fluvial sediments were deposited. Upward-coarsening sands are characteristic of nearshore sand-barrier and marine sand sheet sequences (Cant, 1992), though their occurrence in wide channels indicates that they probably formed as point bar sediments. Dominance of sand has been shown by Walker and Cant (1984) to indicate braided river systems with low-sinuosity channels (Figures 11.5 and 11.6).

In conclusion, the Chek Lap Kok Formation is clearly a complex unit. In broad terms, the onshore areas currently occupied by the formation were the sources of debris generated by mass wasting. The debris was fed into fluvial environments. These were essentially minor tributaries that in turn supplied sediment to the main fluvial channel systems, and onto the main floodplains that now lie in offshore areas of Hong Kong.

Extensive large-scale channelling is visible on the offshore seismic records. Consequently, any older surviving marine units will not have remained laterally continuous. It is believed that the formation comprises a number of depositional phases which are not clearly resolved on the seismic records. Resolution of the stratigraphy would require further seismic data specifically calibrated to give higher resolution in deeper levels of the Chek Lap Kok Formation. Interpretation would also benefit from the discovery of further macrofossils, similar to the oyster shells found in the East Lamma Channel area.

Age

There are difficulties in dating the formation due to the general absence of calcareous fossils in the sediments. In addition, much of the formation is considered to be older than 40 000 years, and thus beyond the range of the radiocarbon dating technique. Dates have been obtained using thermoluminescence (TL) and optically stimulated luminescence (OSL) (Owen et al., 1995). Uranium-series dating (Yim et al., 1990; Owen et al., 1998) has provided the oldest ages in the Chek Lap Kok Formation, but these dates are on isolated shells and therefore must also be treated with caution.

- Radiocarbon ages

Several radiocarbon ages have been obtained from wood and peat in the formation. Radiocarbon dates obtained offshore have been reported from material from organic clay in the Shan Ha Tsuen Member at several localities in the northeast New Territories (Langford et al., 1989; Lai, 1997, 1998; Lai et al., 1996). These dates range in age from 16 289 ± 831 years BP to 33 575 ± 3 186 years BP (Beta Analytic; Guangdong Institute of Geological Sciences; South China Sea Institution of Oceanology). In addition, carbonaceous material within a colluvial deposit in a borehole (BGS 12, at 20.2 m depth) at Tin Shui Wai in the northeast New Territories, has yielded a radiocarbon age of 26 000 to 27 000 years BP (Frost, 1990).

At offshore localities, radiocarbon ages range from 16 420 years BP at Chek Lap Kok (RMP Encon Ltd., 1982) to >40 000 years BP (Kendall, 1975; Yim & Yu, 1993;. Strange & Shaw, 1986).

All of the radiocarbon dates may be subject to a young-age bias due to the introduction of a few percent of modern carbon, through CO2-rich groundwater having permeated the sediment during high sea level stands (Yim et al., 1990; Toscano & York, 1992). The work by Yim et al., (1990) shows that bivalves with radiocarbon ages of 39 460 ± 2 320 and 45 700 ± 2 088 years BP yielded ages of 142 000 ± 20 000 and 130 500 ± 5 300 years by the uranium-series method. This alone may cast sufficient doubt on the radiocarbon ages to make them all questionable. However, comparison of TL and OSL dates with radiocarbon dates from similar horizons within the Shan Ha Tsuen Member (Lai, 1998) has demonstrated good correlation, with overlapping uncertainty bands in all but one instance.

- TL and OSL ages

TL and OSL ages have been reported by Lai, (1998) from the Chek Lap Kok Formation in the northeastern New Territories (Figure 11.4). For the Shan Ha Tsuen Member, these ages range from TL dates of 23 800 ± 2 000 years BP (State Seismological Bureau, China) to 79 000 ± 6 300 years BP (Guangdong Institute of Geological Science). For the Wong Kong Shan Member, the dates range from 126 100 ± 10 100 years BP for a TL date (State Seismological Bureau, China) to an OSL date of 157 500 ± 36 300 years BP (University College of Wales).

Lai (1997, 1998) has reported a maximum age for colluvium thought to form part of the formation onshore, based on an OSL date of 196 100 ± 12 600 years BP (University College of Wales). The sample was obtained from a locality on the northern slopes of Victoria Peak in northwest Hong Kong Island.

Offshore, an OSL date of 31 743 ± 4 456 (HKU Radioisotope Unit) was obtained from -44.55 mPD in borehole C2/2, at the top of the formation, immediately below the Waglan Formation. A number of Weichselian ages have been recorded from boreholes. The oldest of these are from borehole A5/2, where an OSL date of 80 000 ± 9 000 years BP was obtained from sandy silt at -32.23 mPD, and a TL date of 78 000 ± 8 500 years BP from similar sediments at -32.27 mPD (Owen et al., 1998). These dates were obtained immediately below a horizon at -32.11 mPD, where a wood fragment had been radiocarbon dated at 35 000 ± 1 250 years BP (Beta Analytic).

Yim and Yu (1993) tentatively ascribed their lower marine deposits (LM) to oxygen isotope stage 7 (Holsteinian age). In borehole NLC4, 1 km north of Lamma Island, (Owen et al., 1998) recorded the presence of oyster shells, one of which has been uranium-series dated at 248 ± 12-16 kA (Guangzhou Institute of Geochemistry; P.C.T. Cheung, personal communication). This is an isolated date, but it indicates that part of the Chek Lap Kok Formation may be as old as pre-Holsteinian.

Stratigraphical variation

Two members have been described within the Chek Lap Kok Formation, both for the first time: the Wong Kong Shan Member and the Shan Ha Tsuen Member. In addition, colluvial deposits that have been mapped in onshore areas are considered to be un-named members within of the formation. In offshore environments, the seismic character of the formation, underpinned by borehole control, ideally demonstrates the stratigraphic variation of the formation.

- Wong Kong Shan Member

This member was formally described in Fyfe et al., (2000) for the first time, and the following description is based largely on that of the 'Wong Kong Shan Formation' by Lai (1998). The 'Lam Tsuen Formation', also described by Lai (1997), is essentially a synonym of the 'Wong Kong Shan Formation'.

The type section of the Wong Kong Shan Member is located on the plain to the west of Wong Kong Shan (832700 838900 s-1). Here the member mainly comprises yellowish grey to mottled red and yellow silty medium sand with subangular to subrounded gravel to boulders of moderately to completely decomposed tuff, granodiorite and fresh vein quartz. The member is up to 3.7 m thick. Another section through the member is exposed at Wai Tau Tsuen (833330 836000 s-2), about 2 km northwest of Tai Po, near the junction of Lam Kam Road with the Tai Po Road (Figure 11.8). The member is mainly exposed on the second alluvial terrace at elevations between +80 and +7.4 mPD (Addison, 1986). Typically, the member comprises subrounded cobbles and boulders, up to 0.8 m in diameter, set in a mottled red, yellow and brown, clayey silty sand matrix. Individual boulders display a weathered rind up to 5 mm thick.

In the area of the type section, the formation overlies completely decomposed granodiorite and is in turn overlain by debris flow deposits which are themselves part of the Chek Lap Kok Formation. Along the Lam Tsuen Valley, the member thins downstream to only 1 m thick at Wong Kong Shan .

Lai (1998) has reported several OSL dates from the Wong Kong Shan Member (Figure 11.4). These ages range between from 126 100 ± 10 100 years BP (State Seismological Bureau, China) up to 157 500 ± 36 300 years BP (University College of Wales) and indicate a Middle Pleistocene age. The member is always overlain either by deposits of the Shan Ha Tsuen Member, described below, or by colluvium of Late Pleistocene age.

- Shan Ha Tsuen Member

The following description of the member is based on that presented for the 'Shan Ha Tsuen Formation' by Lai (1997, 1998)

The type section of the Shan Ha Tsuen Member (Figure 11.9) is 600 m southwest of Shan Ha Tsuen village (819200 832100 s-3), southwest of Yuen Long. Here, the member is 3 to 15 m thick. At Shan Ha Tsuen, the member (Plate 11.1) overlies Pleistocene slope wash and is overlain by a debris flow deposit of the Holocene Fanling Formation.

The lithology of the member varies with topography. Near the foot of hills, it comprises yellowish brown, gravelly silty sand, interbedded with dark grey, organic-rich silty clay layers. The clays are interpreted as lacustrine or marsh sediments. In upper valley sections, the member may pass laterally into coarser colluvial deposits. In more distal plains, fluvial terrace deposits are mottled red to brick red, yellow, brown and white, well sorted clayey sandy silt with sand layers (Figure 11.8).

Radiocarbon dating of organic-rich samples from the type section has yielded ages of 32 500 ± 490 years BP for the lowest exposed clay layer (+15.7 mPD) and 16 289 ± 831 years BP for the highest layer (+20.5 mPD). A clay sample from a locality 150 m northeast of the type section has yielded a radiocarbon age of 33 575 ± 3 186 years BP. These dates are supported by TL dates (Figure 11.10) that range from 23 800 ± 2 000 years BP to 29 300 ± 2 300 years BP (State Seismological Bureau, China), and an OSL date of 30 400 ± 8 000 years BP (University College of Wales). These dates indicate a Late Pleistocene age for the member as a whole, However, a further, significantly older OSL age of 81 000 ± 13 500 years BP (University College of Wales) has also been reported from near the base of the member (Lai, 1998).

- Un-named colluvial members

Colluvial deposits of Pleistocene age are included within the Chek Lap Kok Formation. Most occur onshore and where greater than 2 m thick, are depicted on the 1:20 000-scale geological maps and described in accompanying Hong Kong Geological Survey memoirs as debris flow deposits, slope debris or slide deposits. On some of the maps in the 1:20 000-scale series, these deposits were not differentiated from similar deposits of Holocene age whereas on other maps in the series, they also include some talus deposits. The colluvial deposits are generally poorly sorted and commonly comprise a reddish brown or dark yellowish brown to orangish red, slightly mottled slightly clayey gravelly sandy silt matrix containing subangular, slightly to moderately decomposed boulders and cobbles. The grain size can vary between predominantly silt, sand, gravel, cobbles or boulders, depending on the original topography, parent rock type and distance travelled. Based on observations first made in the Mid-Levels of Hong Kong Island by Lai (1982), GCO (1982) and Lai and Taylor (1984), and then elsewhere in Hong Kong (e.g. Lai, 1997, 1998), deposits of this type have been interpreted as colluvial deposits of Late Pleistocene age (Plate 11.2). They have been differentiated from older colluvial deposits of Early to Middle Pleistocene age that comprise red to brick red, brown or white clayey sandy silt with highly to completely decomposed boulders and cobbles. For larger boulders, the extent of development of highly to completely decomposed material is restricted to a distinctive rind, commonly up to 20 mm thick (Lai & Taylor, 1984; Lai, 1997), but which can be up to 100 mm thick (Lai, 1998). Thin, white, kaolin infillings (Plate 11.3) are often observed within boulders in the older colluvial deposits in particular. The Early to Middle Pleistocene colluvium, also contrasts with Late Pleistocene equivalents in terms of its engineering strength, being stiff to very stiff as opposed to firm to stiff.

Lai (1997, 1998) has reported typical examples of Late Pleistocene colluvium, up to 10 m thick, from slopes below Lion Rock (838188 823268 s-4) in north Kowloon, and Mount Butler, on Hong Kong Island. In addition, he has described examples of colluvium of Early and Middle Pleistocene age, at elevations of between +150 and +100 mPD, below steep slopes such as at Fei Ngo Shan (841513 822962 s-5), Castle Peak (812645 827407 s-6), and Victoria Peak (833322 815775 s-7) where the thickness of deposits in the Mid-Levels area is up to 16.7 m thick. All of these deposits are included here within the Chek Lap Kok Formation.

Whereas colluvium within the Fanling Formation of Holocene age characteristically infills drainage lines in the present topography, the equivalent deposits within the Chek Lap Kok Formation do not exhibit such a strong relationship and often occur within spurs in the present topography.

The colluvial deposits interdigitate with and, to varying extents, grade laterally and vertically into alluvial and fluvial deposits of the Chek Lap Kok Formation as a whole, and locally with the Wong Kong Shan and Shan Ha Tsuen members in particular.

Regional lithological variations

Onshore alluvial deposits of Pleistocene age, incorporated here within the Chek Lap Kok Formation, include extensive terraced alluvium in the western, northern and central parts of the New Territories. In other onshore areas of Hong Kong, Pleistocene terraced alluvium is less widespread.

Considerable variation has been observed in the characteristics of the colluvium of Pleistocene age assigned here to the Chek Lap Kok Formation. To a large extent this reflects the lithology of the source of the deposit. In addition, variations in maximum grain size and of the ratio of coarse to fine material reflect changes in topography. Coarser debris is associated with steeper slopes and valleys (Plate 11.4).

- New Territories

The most widespread alluvial deposits occur in the New Territories. They are particularly well developed along the Tuen Mun prior to reclamation (814644 827432 s-8) and the Ngau Tam Mei Valley (826166 837431 s-9) and on the Yuen Long, Kam Tin and San Tin plains (826558 832364 s-10) in the northwestern New Territories, and in the central New Territories, along the Sham Chun, Ng Tung Ho and Sheung Yue Ho valleys (835549 841861 s-11), and particularly on the Sheung Shui and Fanling plains (831682 840390 s-12). The largest areas of colluvium of Pleistocene age occur on the northwestern slopes of Tai Mo Shan (829300 831498 s-13), below Castle Peak (813688 826704 s-14) in the Tuen Mun Valley and around Tai To Yan (828312 834630 s-15), and Tai Shek Mo (829377 842013 s-16). The thickest accumulations are at the bases of steep slopes, especially around the high hills comprising volcanic rocks.

In the Tuen Mun Valley (815254 827239 s-17), Late Pleistocene fluvial terrace deposits, locally interbedded with colluvium, underlie most of the low-lying land in the broad valley prior to reclamation. The valley has a central watershed and extends 11 km from Tuen Mun in the south to Tin Shui Wai in the north. In the main Tuen Mun Valley, the deposits vary in thickness from 8 m in the south, to 1.2 m near the watershead, and thickening to 16 m in the north. Although generally poorly exposed, the deposits locally comprise yellowish brown and grey clayey silty sand with thin layers of gravelly coarse sand and boulders in the beds of buried former channels.

Pleistocene colluvium occurs on the footslopes of Castle Peak (814057 827556 s-18), on the western side of the Tuen Mun Valley, and further north towards the coast. Here, it is preserved as terraces that form the interfluves between the present drainage lines and larger fan-shaped bodies. The deposits are typically 1 to 10 m thick, generally poorly-sorted and comprise moderately to completely decomposed boulders and cobbles and gravel of sandstone, siltstone, vein quartz and granite in a variably mottled red and ochre sandy clay matrix. Deposits on the lower slopes also include debris of tuff breccia and fine ash tuff derived from the Tuen Mun Formation. Several depositional mechanisms have been interpreted. These include debris flow, mud flow and alluvial torrent. The granite boulders are highly to completely decomposed. On the east side of the valley, the colluvium is also mainly of Pleistocene age.

The Yuen Long Plain (820418 833099 s-19) is a broad alluvial valley surrounded by low hills, except towards the north where the plain passes into coastal mudflats. The valley is filled by Late Pleistocene fluvial terrace deposits overlain by Holocene alluvium (Plate 11.1). The elevation of the Pleistocene terraces varies from +23 mPD in the south, to -2.5 mPD in the north. They vary in thickness from 3 to 7 m near the margins of the plain to over 15 m in its central region with a maximum of 31.5 m in a buried channel. The deposits typically comprise firm to stiff, mottled red and yellow clay with some sand. Towards the base of the deposits they generally comprise gravelly sand with cobbles. Up to five layers of dark grey organic clay may be present. Deposits in the south and west, derived from granite and sandstone, have a high sand content whereas those in the east, derived from tuff, have high silt and clay and vary to yellowish brown colour.

The Kam Tin plain (826558 832364 s-10) is a broad valley lined with widespread Late Pleistocene fluvial terrace deposits whose surface elevation varies from +27.6 mPD in the east to -3.9 mPD in the west. At the head of the valley, Pleistocene colluvium, interpreted as debris flow deposits (Langford et al., 1989) are mostly derived from volcanic rocks. The deposits are up to 18 m thick and comprise moderately to highly decomposed boulders, up to 4 m diameter, and cobbles of coarse ash tuff or tuff breccia in a matrix of yellowish brown gravelly silty sand. The colluvium grades into, or underlies, Pleistocene fluvial terrace deposits in the upper part of the Kam Tin Valley. The fluvial deposits comprise yellowish brown gravelly sand, about 1 to 3 m thick, with thin layers of mottled red and brown silty clay. In the central part of the valley these deposits are up to 14 m thick, but are more typically 8 m thick. Here, the lower part of the deposit comprises silty sand with gravel, with a layer of gravelly sand with cobbles at the base. The upper part is mainly mottled red, brown and grey sandy silt or silty clay. At the western end of the valley, where it passes into mudflats, boreholes have proved the thickness of the Pleistocene alluvium to be up to 11.5 m. Three units have been identified: a lower layer up to 2.4 m thick of gravelly sand with cobbles; a middle layer of 5 to 7 m of yellowish brown silty sand; and an upper layer of 3 to 4 m of mottled red, yellow and grey silty clay (Langford et al., 1989).

Ngau Tam Mei (826166 837431 s-9)is a narrow valley with Pleistocene fluvial terraces. They vary from elevations of +26 mPD in the east to -1 mPD in the west. The deposits are 1 to 10 m thick and comprise silty coarse sand with some boulders in proximal environments and silty sand with thin layers of clay distally. Deposits of similar thickness occur in the Sheung Yue Ho Valley.

The Ng Tung Ho (837312 842685 s-20) rises from a source on Pat Sin Leng and is fed by a major tributary from the Lam Tsuen Valley to the south and southeast. Colluvium, mainly of Pleistocene age, occurs on both sides of the alluvial plains and in the valley heads. It comprises highly decomposed cobbles in a gravelly clayey silt and is typically 1 to 4 m thick, but is locally up to 10 m thick. The Lam Tsuen Valley (Figure 11.11) contains an 800 m wide Pleistocene alluvial terrace containing two units (Plate 11.5). These units are also recognized further down river beneath the Fanling plain. The lower unit is 1 to 5 m thick and typically comprises dark yellowish brown, varying to mottled red and grey, sandy silt, coarse sand, gravel, cobbles and subrounded, highly to moderately decomposed boulders. The upper unit, which is 1.5 to 4.7 m thick, contains pale grey, fine to medium sand and clay, yellowish brown sandy silt and dark grey to black organic clay and silt. In the lower part of the Ng Tung Ho, the Pleistocene deposits are up to 12 m thick.

Late Pleistocene alluvium, up to 15 m thick, comprising mottled red and yellow clayey silt, is widespread beneath the San Tin Plain (826558 832364 s-10). The plain incorporates the estuaries of the Shenzhen River and the Shan Pui River. The alluvial deposits underlie Holocene marine deposits of the Hang Hau Formation.

Colluvium of Pleistocene age commonly occurs as narrow deposits infilling the dendritic river systems in the northeastern New Territories. However, deposits are not generally widespread. They are commonly 1 to 3 m thick and are composed of yellowish brown silty sand with boulders, cobbles and gravel of tuff, conglomerate and siltstone, depending on the underlying geology.

A few Pleistocene fluvial terraces are present in the eastern New Territories. Their scarcity probably reflects the generally limited extent of alluvial tracts in this part of Hong Kong. In addition, intensive agricultural terracing may have obliterated some natural terrace features (Strange et al., 1990), for example at Ho Chung.

- Kowloon, Hong Kong Island and neighbouring islands

The extent of Pleistocene alluvium in Kowloon is very limited although such deposits may have existed in East Kowloon and Junk Bay prior to agricultural terracing and building developments. Some probable Pleistocene terraces were observed by Ruxton and Berry (1957) on aerial photographs taken prior to development in the So Uk, Lei Cheng Uk and Shek Kip Mei areas of northwestern Kowloon. The terraces in this area are approximately 3 m above the level of the Holocene alluvium. A Han dynasty tomb was discovered within the terrace at Lei Cheng Uk (Strange & Shaw, 1986). Alluvial terraces thought to be of Pleistocene age also occur at Yung Shue Wan on Lamma Island. Pleistocene alluvium has not been recognized on Hong Kong Island or on the Po Toi Islands (Strange & Shaw, 1986).

Excellent exposures of colluvium were present in cut slopes along the old Clear Water Bay Road (Plate 11.6 and 11.7), 300 m east of Choi Wan Estate, but unfortunately the slopes have now been covered. The deposits, up to 10 m thick, were described (Strange & Shaw, 1986) as being poorly sorted and comprising subangular to rounded volcanic boulders and cobbles, typically highly decomposed, in a well consolidated light orange-brown clay matrix. These deposits were interpreted as Early to Middle Pleistocene by Lai and Taylor (1984).

At several localities around northern Kowloon, for example on the slopes below Lion Rock, the colluvium is up to 10 m thick. Temporary exposures here have revealed colluvium of presumed Pleistocene age, underlying more recent colluvium. Commonly, the Pleistocene colluvium occupies ridge spurs, in contrast to the more typical valley-fill morphologies of the recent colluvium. On the southern slopes of Piper's Hill, Whiteside (in Strange & Shaw, 1986) reported that just above weathered rock, the matrix at the base of the colluvium, was mostly a white clay (kaolinitic?) while above that, elongate white flecks, 20 to 40 mm long, interpreted as caused by iron reduction, and containing rootlets, occurred within the more typical brown matrix.

On Hong Kong Island, a widespread mantle of colluvium covers the northern slopes of Victoria Peak (833322 815775 s-7). During the Mid-Levels Study (GCO, 1982), this was examined in detail. At least three cycles of colluvium were identified of which the older two, described above as Late, and Early to Middle Pleistocene age deposits, are assigned here to the Chek Lap Kok Formation. The debris are almost entirely composed of volcanic rock derived from the source area around Victoria Peak. The colluvium, including deposits of Holocene age, is up to 33 m thick in this area. The deposits extend downslope to Sheung Wan reaching the former coastline (Howat, 1985b), and possibly beyond.

- Lantau Island and neighbouring islands

On Lantau Island, extensive elevated fluvial terraces of Late Pleistocene age are present. Similar lithofacies variations to those observed in the western, northern and central parts of the New Territories occur on Lantau Island. Near the foot of hills, deposits mainly comprise semi-sorted, yellowish brown to orange, gravelly silty sand that was probably derived from hillwash from the adjacent slopes. In upper valley sections the deposits grade into more bouldery colluvium whereas in the distal plains, the deposits are typically mottled red, yellow, brown and white, well-sorted clayey sandy silt with some sand layers.

Alluvium of the Chek Lap Kok Formation infills the western Tung Chung Valley (811310 815581 s-21) (Plate 11.8) to form a raised terrace from 400 to 500 m wide. The deposit extends inland for about 2000 m and reaches an elevation of almost +20 mPD. The terraces grade up into coarse bouldery debris on the surrounding slopes. Although rarely exposed, the partially indurated deposit is coarse and comprises boulders and cobbles within a mottled red and yellow sandy silt with some sand lenses and layers. More isolated remnants of terraced alluvium occur in the eastern Tung Chung Valley and in the Sham Wat Valley (806493 814122 s-22).

Colluvium of the Chek Lap Kok Formation, i.e. of Pleistocene age, has not generally been distinguished on Lantau Island from colluvium of Holocene age. Typically the colluvium occurs in isolated upland valleys, and on lower slopes as more extensive accumulations that usually merge imperceptibly into the alluvium. Colluvium is mainly developed on the lavas and tuffs and is more restricted on the granites and feldsparphyric rhyolites. The thickest and most extensive deposits fill entire valley systems, extending from the coast to dendritic accumulations that reach ridge crests in the headwaters. Good examples include dendritic networks of deposits feeding down from Nei Lak Shan, Tung Chung Au and Sunset Peak (811921 812898 s-23) towards Tung Chung. These deposits, comprising coarse boulders within a stiff mottled sandy silt matrix, grade downslope into the extensive, alluvial terrace deposits. Other laterally extensive deposits of colluvium blanket the western slopes of Sze Shan above Tai O, and on the northwestern slopes of Por Kai Shan, crossing the coast near Lau Fau Shan. Prior to construction of the airport, there was a large area of colluvium at Chek Lap Kok, which was proved to be at least 4 m thick.