Abstract: A method and mold assembly for forming concrete or another moldable composition. A plurality of elongate elements, such as PVC pipes, are stacked in rows in gravity-stable arrangement to form a contour, such as an arch. The radius of the arch may be controlled by arranging the pipes as desired. Particulate matter, such as sand, may be used between the stacked pipes. Also, an anchor assembly can be used to secure one or more of the pipes to the earth. After the pipes are stacked and a bed of sand has been placed thereon, a waterproof cover is laid over the top of the stack, and layer of concrete is spread. After the concrete is set, the pipes, the cover and the anchor assemblies are removed. The removal will be expedited by washing the sand from the stack. Once removed, the pipes and anchors can be reused indefinitely.