Zambezia (1989), XVI (i). GULLY FORM AND DEVELOPMENT ON KAROO SEDIMENTS IN CENTRAL ZIMBABWE: A PRELIMINARY SURVEY

ABOUT ONE TENTH of Zimbabwe is characterized by sodic soils which are especially prone to sheetwash erosion and gullying associated with subsurface piping (Wendelaar, 1976). Although localized patches of sodic soil occur in poorly drained sites on granitic rocks, they are more widespread on Karoo sediments in the north-western, western and central parts of the country (see inset in Fig. 1). Previous research on these soils has been directed mainly at the dynamics of plant-habitat relationships, as a basis for devising suitable methods of reclamation (Dye, 1979; Dye and Walker, 1980), and at the definition of factors influencing the morphology and extension of headcuts (Stocking, 1977 and 1981). Consequently, a great deal is known already about the nature and current rates of erosion on sodic soils. However, very little basic geomorphological research has been carried out on the development and environmental significance of gully systems on the Karoo sediments or, more specifically, the fine sandy colluvium overlying these sediments. The present exploratory study was carried out in an 'island ' of Karoo sediments some 2 380 km2 in area, which lies to the south of Harare (Fig. 1). The objectives of the exercise were, firstly, to determine the distribution of active and stabilized gully systems in this locality and, secondly, to evaluate the morphology and sedimentary sequence of selected gully systems. Although it would be unrealistic, at this stage, to propose an evolutionary model and chronological framework comparable with that described by Smith (1982) for large gullies in northern Nigeria, there are good prospects for doing so in the future. In addition, it would be desirable to link such research with the picture of late Quaternary environmental changes identified from recent investigations in Swaziland (Price-Williams et ai, 1982). DISTRIBUTION OF GULLIE