A method for producing a block having a flat bottom, a flat top which is parallel to the bottom and sidewalls which extend between the bottom and the top is disclosed. At least three cooperating mold parts are assembled on a first plate to produce assembled mold parts having interior surfaces which conform with the shape of the sidewalls of the block and flat, coplanar tops and bottoms. The assembled mold parts are clamped together, and a high slump concrete which is hardenable by hydration of cement therein is charged and worked as required into close proximity to the interiors of the mold parts and to the plate inside the mold parts. A foamable composition capable of forming a body of a thermoset, closed cell, urethane foam having a density of about 10 to 60 pounds per cubic foot which fills the interior of the assembled mold parts above the concrete therein is then charged, and a second plate is clamped over the tops of the mold parts. Preferably, there are small openings between the interior and the exterior of the mold through which excess water in the concrete can flow without appreciable loss of cement or aggregate from the mold.