EROSION DUE TO CULTIVATION OF CALCAREOUS CLAY SOILS ON THE HILLSIDES OF SOUTH-WEST FRANCE .1. EFFECT OF FORMER FARMING PRACTICES

Between the principal river valleys of south west France, the tertiary molasse outcrops have produced a deeply undulating surface relief. So il distribution in this upland landscape is highly disordered and the spatial organisation can only be understood from detailed examination of soil profiles. The considerable modifications to which the soil cov er has been subjected may be explained by factors other than natural a gents of erosion. Careful examination of former gullies and the hillsi de relief reveals the important erosive effect of the methods of culti vation used on these sloping soils. A simple model is proposed to test the effect of former contour ploughing in which soil was turned over towards the base of the slope. Simulation over a broad time scale gave the same results as those observed in the present landscape.