ABSTRACT
Three-quarters of Sri Lanka lies less than 150 metres above sea level. Its northern half is lowland broken up by north-south ridges separating the drainage basins. The whole of this lowland surface is covered by a mantle of deeply weathered rock called kabouk which conceals much of the underlying solid rock so that its nature is imperfectly known. It would appear to be largely of ancient metamorphosed material and of low mineral-endowment. There is neither coal nor oil and little indication of any metalliferous or non-metalliferous deposits of consequence with but one exception. Rich deposits of gem-bearing gravels yielding rubies, sapphires, garnets, amethysts and opals have been worked over the centuries in the hills of the south-west around Ratnapura. Out of this lowland of hills and valleys rises abruptly a central core of mountains with plateaux of 900 to 1400 metres and peaks rising from 2000 to 2400 metres. This area accounts for less than 10 per cent of Sri Lanka's area but it is an important tenth since it carries the tea plantations. The increase in altitude reduces the temperatures from the 27°C which prevails throughout most of the year on the coast to a range of between 15 and 20°C in the mountains, and favours not only the growth of tea but also some temperate crops. Sri Lanka experiences both a south-west and a north-east monsoon with the former being
the more significant of the two, bringing heavy rains to the mountains and the south-west where everywhere precipitation exceeds 1300 mm and wide areas experience more than 2500 mm. To the north and east of the central highlands the south-west monsoon yields less than 500 mm and most of the rain comes from the weaker north-east monsoon from November to February. This area is referred to as the Dry Zone because of its high evapotranspiration losses and the greater variability of the rains even though its rainfall exceeds 1300 mm. The Dry Zone extends over two-thirds of Sri Lanka and is a significant physical component in development planning. The wet south-west and the central highlands are green throughout the year and, as the focus of commercial cropping, have the greatest density of road and rail networks, and support the bulk of the population (Figs 29 and 30).
