Bricks were sun baked to harden them. These types of bricks are much less durable than oven-baked ones so buildings eventually deteriorated. They were periodically destroyed, leveled, and rebuilt on the same spot. This planned structural life cycle gradually raised the level of cities, so that they came to be elevated above the surrounding plain. The resulting mounds are known as tells, and are found throughout the ancient Near East. Civic buildings slowed decay by using cones of coloured stone, terracotta panels, and clay nails driven into the adobe-brick to create a protective sheath that decorated the façade. Specially prized were imported building materials such as cedar from Lebanon, diorite from Arabia, and lapis lazuli from India.