ENG-1955 — Page 256

GEOGRAPHY AND CLIMATE

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The uplands and mountains are eroded remnants of rock formations, in which relative resistance of rock and structure through differential erosion is clearly recorded. As the region lies within the northern limit of the tropics, frosts, even on Taimoshan, are rare, and weathering depends almost com- pletely 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, denudation is aided by the excessive wind velocities of typhoons, and to a much less extent by the gentler breezes of the dry winter monsoon from Central China. Because of the rural customs of cutting grass for fuel and lighting grass fires on hills, the soil and rock mantle are left unprotected except by their own cohesion.

The laterite-type product of decay is locally such, how- ever, 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, carbona- tion 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 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 Taimoshan 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 form 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