ENG-1958 — Page 350

GEOGRAPHY AND CLIMATE

293

through differential erosion are clearly recorded. As the region lies within the northern limits of the tropics, frosts even on Tai Mo Shan are of the rarest occurrence, and hence weathering depends almost completely upon the chemical action of the atmosphere aided by the alternation of wet and dry seasons. Erosion is likewise due to water action which is at a maximum during the torrential rains of the summer monsoon. Again denuda- tion is aided by the excessive wind velocities of the typhoons and to a much lesser extent by the gentler breezes of the dry winter monsoon from Central China. Because of the destruction of forest growth and vegetation wrought by the agricultural population by sickle and grass fires, the soil and rock mantle are left unprotected except by their own cohesion. Intensive efforts have been made to limit and control these nuisances by rigid regulations and systematic reafforestation. In consequence of this, bare rock surfaces and loose boulders occur commonly on the higher sur- faces and steeper slopes.

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 disintegration due to hydration, carbonation and temperature changes, has resulted in the formations 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 topo- graphy. 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 weather- ing 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.