Masonry generally refers to stone or brick units that are separated by a mortar cushion, effectively transferring the loads evenly from one masonry unit to the next without point loading. Masonry is a particularly durable building material, but is limited by the geology of its components. In short, it is important to learn from geology about the physical and chemical nature of different types of stone and aggregates. Stone in particular begins to change and deteriorate from environmental factors (thermal stresses, biological and acidic attack) as soon as it is removed from the lithosphere (in the earth) and placed in the atmosphere (the weathering environment.) The durability of masonry can also be a weakness in that it is often neglected for long periods, sometimes going a hundred years before the first serious maintenance. With changes in modern construction and the increased use of veneers in the latter half of the 20th century, builders are often unaware of the particular nature of different stones or the types of mortar better suited to porous masonry units common in the past. Stone, not unlike wood, has pores, grain direction and, in the case of sedimentary a laminar structure (bedding planes), that have implications for structural and working properties. Knowing the specific types of materials and their weaknesses is important when making decisions about masonry repair materials and procedures.