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When a mixture of dry particles with different diameters is disturbed, a segregation will occur with the finest particles moving downwards and the largest particles moving upwards. The objective of this paper was to determine whether this principle could be used to explain the high stoniness of topsoils of cultivated fields in the Mediterranean. A dense cover of rock fragments reduces soil erosion and promotes infiltration and hence, maintaining a coarse top layer may be an important management practice in areas threatened by desertification. Field experiments were conducted to determine the change in vertical rock-fragment size distribution after a certain number of tillage passes, until steady state (no more change) was reached. Soil pits were prepared and filled with four layers of rock fragments. Each 4-cm-thick layer contained a known distribution of rock-fragment sizes. One series of pits contained the coarsest layer initially at the bottom and the finest on top, while a second series showed an even distribution of coarse and fine particles throughout the profile. The pits were subjected to different tillage frequencies (max. of eight passes, all to a depth of 16 cm) by a caterpillar tractor pulling a chisel with a duckfoot. After the tillage operations, rock-fragment content (by mass) of each layer was determined by sieving and weighing, and rock-fragment cover (percentage) at the soil surface was assessed by photographical interpretation respectively. The experiments, conducted under dry conditions, demonstrated that there was a strong tendency for the coarsest fraction to accumulate in the top layers and for the finest fraction to accumulate at the bottom of the tilled layers. After two events, the coarsest fraction was already the dominant fraction in the surface layer for the first series of experiments (coarsest rock fragments initially at bottom). For the second series (equal distribution of rock-fragment sizes with depth), this already occurred after one event. The photographical interpretation of the surface rock fragment cover showed initially a rapid increase of intermediate sized fragments (0.6-5.0 cm) during the first series of experiments. After five tillage passes, the coarsest rock fragments were dominant at the surface. The rapid grading due to tillage observed in these experiments may explain the high rock fragment content of the topsoil in many cultivated shallow soils in the Mediterranean area. These results also suggest that in soils in which rock fragments on the surface are known to improve soil and water conservation, these benefits may be obtained after only one or two tined tillage operations in dry soil. (C) 1997 Elsevier Science B.V.