EROSION,BURKINA FASO, West Africa, Yatenga Province, Kalsaka village. . Every Monday and Tuesday in the dry season stone lines are built to prevent erosion. The lines, like very low dykes, slow down rain running off the land and dam it back uphill leaving it time to soak in and noursh root crops. The stones have to be aligned. exactly along the level contour, by means of a cheap housepipe level. The lines increase crop yields by 50% on average, much more in drier years.