Cob is a carefully blended mixture of wet earth, containing enough lime to enable it to set hard. Also added are chopped reeds or straw, animal dung, sand, gravel and small stones as ballast. Cob walls are seldom less than two and as much as four feet thick. These were built up in layers of about six to twelve inches thick and each had to set before the next was added. A two-story building could take anything up to two years to finish. The building up was done by eye, leading to the classic rounded and undulating appearance of cob walls.