Prime agricultural land in developing countries has been taken over for the production of cash-crops which tends to push peasants onto marginal lands which, more often than not, is totally unsuitable for farming. The results have frequently been disastrous. Peasant farmers have been forced to cultivate mountain sides so steep that it can not be cultivated except by hand and the resulting erosion has been devastating. In Africa nomadic farmers have been pushed into grazing the arid and inhospitable margins of the Sahara desert. Marginal lands are often especially vulnerable to disasters, such as drought, landslides or flooding (whether river or tidal)