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Boreholes (A5/1, A5/2, B2/1, ESC17 and B15; see above) drilled through the Chek Lap KokFormation sediments show that they were mainly deposited subaerially. The effects of theirsubaerial weathering range from sporadic orange oxidation mottles and streaks, to intensiveweathering to a deep red colour and the development of nodular concretions. The latter areassociated with palaeosols on former exposed surfaces. Generally, the intensity of weatheringdecreases downwards for about 2 m> below which it is usually represented by intermittent yellowor orange brown streaks and mottles. The weathering occurred during, or soon after deposition,and prior to deposition of the overlying grey, reduced, marine muds of the Hang Hau Formation.The Sham Wat Formation is weathered to a limited extent. It still retains its grey groundmasscolour but this is superimposed with sparse orange-yellow mottles and streaks. It is also firm andrelatively dewatered, and macroscopic shells are soft and slightly corroded, suggesting subaerialexposure of the sediments.144