Walls are often covered with non-adobe materials and have a hefty, rounded appearance. Adobe dwellings were discovered to be ideally adapted to the harsh heat of the Southwest by early Americans. The large walls collect the sun's heat throughout the day and gently release it at night. They were also found to be very durable and resistant to wind and water. Adobe structures are made of crushed rock or earth that has been mixed with straw or manure and then packed into thin layers between two sheets of wood or metal. The mixture is then dried in the sun or under a shelter and burned when no longer needed. The word "adobe" comes from a Mexican language and means "dry ground". In New Mexico, Arizona, and California, these buildings were used primarily from about A.D. 1200 to 1820. After that time, more modern buildings were built using similar techniques but with different materials. People began building with adobe about the same time they started making bricks. Bricks are easier to work with and last longer than adobe, so most ancient buildings are made of adobe. But because bricks can be reused, people started making them later and keeping them inside their homes instead of burning them after they were done with use. By the time adobe housing was being built, people had already learned how to make brick easy!