ZAMBIA VALLEYS & HILLS
Several deep rifts traverse the eastern and southern parts of Zambia, forming the southern end of the great East African rift system. These rifts, or troughs as geologists prefer to call them, vary in depth. The two deepest are the valleys of Lakes Tanganyika and Malawi, which, with Lake Baikal, are world’s deepest lakes. The valleys of the middle Zambezi and the Luangwa and its tributaries, the Lukusashi and Lunsemfwa, are all approximately 300m in depth. The Kafue Flats form yet another valley trough, although the altitude, about 975m, is only slightly less than the surrounding plateaux, and there are only minor escarpments. All of these valleys have been formed by down-faulting. The rocks of the valley floors date from the Karroo period. Fossil bones of mammal-like reptiles (Therapsids), which preceded the dinosaurs, have been found in a few places. Soils derived from the Karroo sandstones generally have a higher mineral content than those derived from the basement complex of the plateaux, and the contrast in the vegetation is sharp. The Luapula valley is not part of the rift valley system and belongs to the basement complex. Its natural vegetation is not miombo, and although it resembles the vegetation of the other valleys in structure, the species are quite different. Valley vegetation consists of complex mosaics. It is affected by the drainage pattern and soils, and also by large herbivores which are concentrated in these nutrient-rich areas. Deciduous thickets commonly occupy the well drained sites. The banks of rivers and lagoons have riverine fringing forest. The slopes between the thickets and the riverine forest are frequently covered with mopane woodland. Grassy plains occur on cracking clay soils. Lagoons, which are frequently formed as ox-bow lakes, have a rich variety of aquatic vegetation. At the lower end of the mopane slopes there are frequently large termite mounds covered with forest species. These mounds are often partly or completely encircled by pans, which hold water for several months into the dry season. These pans begin as wallows and are extended as more mud is carried out on animals using them.
Steep slopes and rocky outcrops are generally protected from fires by the sparseness of the grass cover. The good drainage on slopes ensures that the soil remains friable and free from compaction. These areas are consequently rich in many of the more fire-sensitive plant species. Smooth-barked trees and thicket clumps are characteristic features of the vegetation. Hills of limestone and other basic rock types may develop deciduous thickets, with pockets of rich herbaceous vegetation. Characteristic of these hill slopes are the smooth-barked species of Brachystegia B. bussei, B. glaucescens and B. microp hylla, and the white-barked Sterculia quinqueloba. Miombo species can only be supported where there is sufficient soil to sustain them. Elsewhere deciduous species predominate.