The deposition of stony colluvium on clay soil as a cause of gully formation in the Rif Mountains,Morocco

V-shaped gullies are formed on slopes in the Rif Mountains where stony colluvium covers a truncated Luvisol in finegrained, early pleistocene slope deposits. The colluvium resulted from large-scale deforestation of summit areas in recent times. A number of properties related to the response of soil material to rainfall were investigated. Colluvium has a high infiltration capacity compared to the Luvisol. Consequently, the deposition of colluvium reduced overland flow and erosion by surface wash. Gully-forming processes on the other hand were activated by the superposition of permeable over impermeable material.