Mud Day 2011, Westland, Michigan, United States

Mud is a liquid or semi-liquid mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone (generally called lutites). When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds. Mud is closely related to slurry and sediment.
Mud, in the construction industry, refers to a fluid material used to coat or adhere together items that dries hard such as plaster, stucco, concrete or other similar substances.
Mud that is mostly clay, or a mixture of clay and sand may be used for ceramics, of which one form is the common fired brick, or dried with the inclusion of straw reinforcing to form an unfired adobe brick. Adobe walls are frequently finished with a mud plaster, seen above in the introductory illustration. Such buildings must be protected from groundwater, usually by building upon a masonry, fired brick, rock or rubble foundation, and also from wind-driven rain in damp climates, usually by deep roof overhangs. In extremely dry climates a well drained flat roof may be protected with a well-prepared and properly maintained dried mud coating, viable as the mud will expand when moistened and so become more water resistant.