They are usually made of local stone, varying from limestone and flint to granite and sandstone. However, the quality of building stone varies greatly, both in terms of its endurance to weathering, resistance to water penetration and ability to be worked into regular shapes before construction. Worked stone is usually known as ashlar, and they are often used for corners in stone buildings. Granite is thus very resistant to weathering, while some limestones are very weak. Some limestones, however, such as Portland stone have a deserved reputation for resistance to the weather.
Large structures are usually made of very thick walls, so that castles and cathedrals possess walls which may be up to 12 feet thick. They normally consist of a layered stone exterior and rubble infill.