ENG-1957 — Page 382

328

HONG KONG ANNUAL REPORT

structure of the valleys. The Tolo Channel is a notable example. The relative resistance of the different rock forma- tions to weathering is illustrated as follows. The highest peaks and the most prominent mountain ranges are all composed of Tai Mo Shan prophyry and the Repulse Bay volcanics. They tend to form smoother peaks than the Hong Kong granite which generally occurs at lower elevations with well-etched peaks and sharp ridge lines. The Tolo Channel sediments generally weather into lowlands and valleys except for the Pat Sin conglomerate which forms peaks and ridges along the crests of the Pat Sin mountain chain.

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Unlike the hills the plains are all alluvial and formed by deposition. Benches marking old sea beaches up to 400 feet or more above sea level indicate the deep submergence of the whole region within recent geological times. Progressive uplift has brought about marked changes on the shore-line. Submerged weathered rock surfaces overlain by peat and bog deposits drilled through in the harbour of Hong-Kong indicate that the former shore-line was at least 100 feet (16 fathoms) lower than now.

During the period of submergence valley heads were gradually filled, with sediment and this has been redistributed from higher to lower levels as elevation continued. The paddy fields along the lower reaches of the rivers, and the large semi-submerged plain around Yuen Long are alluvial deposits brought down by the local streams. At the brickyards on the Sheung Shui plain marine shells have been dug up fifteen feet below the alluvial deposits.

The alluvial origin of the plains is thus clear, and it is also evident that these plains are yearly growing seaward due to the deposition of the sediment brought down by the streams. It is interesting to note that an elevation of the land by 100 feet would restore the strand line approximately to the 16 fathom line and make all Hong Kong and the New Territories an integral part of the mainland. Thus across wide alluvial