ENG-1959 — Page 371

316

HONG KONG ANNUAL REPORT

sickle and grass fires, the soil and rock mantle are left unprotected except by their own cohesion. In consequence of this, bare rock surfaces and loose boulders occur commonly on the higher surfaces and steeper slopes. Intensive efforts have been made to limit and control erosion by rigid regulations and systematic reafforestation.

The laterite-type product of decay is locally such, however, as to provide an impervious mantle for the underlying rock. In colour and composition the products of weathering accurately reflect their rock origin. Although frost action is absent, mechanical disintegra- tion due to hydration, carbonation and temperature changes has resulted in the formation of gravel and boulders over the surfaces of some rock types. The net result of the erosion cycle is that of an upland system with rocky mountain peaks and well-defined ridges giving an impression of partly matured topography. In some areas the topography shows that adjustment to rock structure and resistance to weathering and erosion are very complete. This is evidenced particularly in the general anticlinal structure of the valleys. The Tolo Channel is a notable example. The relative resistance of the different rock formations to weathering is illustrated as follows. The highest peaks and the most prominent mountain ranges are all composed of Tai Mo Shan porphyry 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.

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