Cordons pierreux are thin lines of fist-sized stones laid across fields. They catch water. When rain falls, it pushes silt (special dirt) across top of the field. The rocks slow down the water so it soaks into the ground more. The silt also makes a good place for seeds of local plants to grow. The plants slow the water even more, and their roots break up the soil, so it's easier for more water to soak in.