In dry areas (which included most of the black-earth belt) strips were divided from one another by grass borders and access paths. The amount of arable occupied by these borders has been estimated as high as 7%. In moist and rainy areas (the non-black earth) the basic strip-units were formed as a ridge-and-furrow system. As in medieval western Europe, perennial ploughing had gradually produced a `corrugated' landscape of raised ridges or selions with ditches or furrows between them. The furrows acted as drainage. Such a system was necessary before the introduction of modern under-field drainage.