Abstract: A composite masonry building block (1) that has an inner core (20) of lighter density than an outer face and includes upper most protrusions (4, 73) and matching lowermost cavities (5, 76) to have an interlocking arrangement with other like blocks. Also disclosed is an inner lighter density material which includes an expanded plastic such as polystyrene with a cement binder. The upper (2) surface of the block (1) may include a plurality of protrusions and lower (3) may have a plurality of cavities (5). The arrangement of the protrusions (4) in relation to the upper surface (2) and the arrangement of cavities (5) in relation to lower surface (3) are such that the protrusions (4) will locate within and interlock with the consistently aligned and placed cavities of the adjacent block.