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Present SurveyGeological field mapping in the district started on Tsing Yi in May 1986, but most was carriedout between May 1988 and February 1991. Additional geological information used in thismemoir includes data acquired during construction of the new Chek Lap Kok airport and otherinfrastructural developments up to, and including, the middle of 1992. Geological information foronshore areas was plotted directly onto 1:5000 scale topographic base maps. It was thentransferred onto 1:10000 scale maps, and provisional geological maps \vere prepared at thisscale. These formed the bases for the published 1:20 000 scale geological map sheets.Traverses w r ere made along most roads and footpaths, and accessible stream courses and coastalsections (Figure 2). Northeast Lantau Island is generally sparsely vegetated and access wasrelatively easy. However, the much more rugged areas of central Lantau are heavily vegetated,and traverses were restricted to ridge lines and recognized footpaths. Helicopters, provided by theGovernment Flying Service (formerly Royal Hong Kong Auxiliary Air Force), enabled access tothe more remote and mountainous areas. Most of the coastline of north Lantau Island was easilytraversed at low tide. Islands and remote coastal sections were reached using commercially-hiredboats, with some additional assistance from the Royal Hong Kong Police Marine Region.However, the coastlines of the Chi Ma Wan peninsula, Cheung Chau, Shek Kwu Chau and theSoko Islands are steep and inaccessible.Temporary sections were common in the urban areas, but relatively uncommon elsewhere. Theywere examined during the field mapping as sites developed. They provided much additionalgeological information for Tsing Yi, Discovery Bay and, latterly, Chek Lap Kok. Numerousground investigation boreholes have been sunk in north and northeast Lantau Island, many ofwhich were sampled. A desk study, undertaken between 1985 and the middle of 1988, involvedthe examination of available borehole logs, many of which contained colour photographs ofsufficiently high quality to allow rock types to be determined. The logs were partlycomplimentary to the numerous cores examined, and were particularly useful for areas which areeither poorly exposed, or are covered with fill, or marine deposits.Photogeological interpretation of the distribution of superficial deposits and lineaments in thedistrict was undertaken, mainly using 1963, 1964 and 1983-86 photographs. The 1949 and 1955photographs were also useful for examining the pre-development topography of areas such asShek Pik Reservoir, and these were used, together with old topographic maps, to establish theextent of fill. The extent of reclamation shown on the maps is based on data supplied by theSurvey and Mapping Office, Lands Department.High resolution seismic reflection profiling was the principal technique employed for offshoresurveying. Sequences were interpreted from continuous seismic records calibrated by the loggingof sediment and rock from boreholes and, in some localities, by cone penetration tests and vaneshear testing. Figure 3 illustrates the geophysical track lines for the records interpreted for thismemoir and the accompanying published maps, and Figure 4 shows the positions of boreholesand cone penetration tests. The boomer gave a depth of penetration beneath the sea surface ofabout 100 metres. A sparker source, with a lower frequency than the boomer, was run on twosurveys over some parts of the area, and was capable of penetrating and recording reflectorsfrom greater depths. A marine magnetometer was also deployed along some of the sparker lines,and a Territory-wide gravity survey was also carried out (Busby et a/., 1992; Electronic andGeophysical Surveys, 1991; Evans, 1990; Busby & Langford, 1994) in which marine gravitymeasurements were taken at stations spaced on a 2 km grid.19