CHRIS KNAPTON / SCIENCE PHOTO LIBRARY CHRIS KNAPTON / SCIENCE PHOTO LIBRARY
Dry stone wall. This type of wall is called dry because no mortar or cement is used to keep the stones together. The gaps between the stones have been filled with straw. The wall's builder must pick each stone and its position before setting it, so as to keep the wall from tumbling down. Some dry stone walls have lasted for hundreds of years. Photographed in Northumbria, England.
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