In their upper courses all the southern affluents of the Ghazal flow across a plateau of ferruginous laterite, their valleys having steep banks.
Being composed largely of red clays and laterite, the soil is not generally rich, and calls for the patient cultivation of the Chinese gardener to make it really productive.
The humid climate causes the foliage here, as in other parts of Malaya, to be very luxuriant, and the contrast presented by the bright green on every side and the rich red laterite of the roads is striking.
The eastern portion of the district is the ordinary alluvial plain of the Gangetic delta; the western part consists of undulating beds of laterite resting on a rock basis, and covered with small scrub jungle.
Laterite is a ferruginous and argillaceous rock, varying from 30 to 200 ft.
Laterite may be roughly divided into two kinds, high-level and low-level laterites.
Geologically, Banka resembles the Malay Peninsula, its formations being mainly granite, Silurian and Devonian slate, frequently covered with sandstone, laterite (red ironstone clay) of small fertility, and alluvium.
Masses of laterite, buried in hard ferruginous clay, crop up as rocks or slabs.
Laterite, shale and sandstone occur all over the district.
It is probable that these are related to bauxite and certain kinds of laterite.