Abstract: A form assembly for cement or the like comprising interlocking form members. One form member is longitudinally straight and can be provided with horizontal stiffening ribs at its upper and lower edges. The other form member has upper and lower ledges at least one end and is resiliently flexible to permit bending into a longitudinally curved configuration. The ends of the form members are each cut away into complementary shapes to form an interlocking joint. The upper and lower horizontal ribs are provided with a first set of vertically aligned openings in the vicinity of the cut away joint and the upper and lower ledges are provided with a second set of vertically aligned openings in the vicinity of the cut away joint. The ribs and the ledges overlap each other so that the first and second set of aligned openings are in vertical alignment with each other when the cut away joints are interlocked, allowing a pin to be received by the aligned openings to fix the assembly into the ground.