Abstract

The granite in Hong Kong is frequently weathered to a depth of more than 60 m (200 feet). The soil (or A and B horizons of pedology) is only a few feet thick. The very thick C horizon is considered as the weathering profile and is divided into four zones roughly parallel to the surface. Division is based on the percentage of solid rock and the character of the residual debris.

Local variations in the weathering profiles are due to conditions of slope, subsurface water, vegetation, structure, and type of granite. Widespread modifications are the result of a varied climatic and geomorphological history.

At Lai Chi Kok a wave-cut platform truncates a complete weathering profile. A Han tomb, discovered in the terrace, provides evidence of considerable subsurface eluviation in the upper weathering zone. Eluviation is shown to be important in the differentiation of local weathering debris and may give rise to a well-sorted residue.

Studies in three areas show that thick, complete profiles were formed on gently sloping surfaces and are now variably dissected. Adjustment to structure has taken place within the weathering profiles in a small vertical range of denudation. Large pockets of core stones in residual debris of the upper zones are explained as abandoned portions of the weathering profile.

Most gullying in Hong Kong occurs on slopes from 10°–40° in the region of deep weathering. It is due to instabilities in the upper weathering zones after partial dissection and deforestation. Surface erosion bares the subsoil to drying winds, which with partial eluviation from available free faces causes a contraction of the debris. Deep cracks are formed, and accelerated eluviation aided by storm runoff develops these into gullies.

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