An assembly (10) for forming a trench (T) includes one or more frame members (16) that cooperate with a removable form (12) around which a moldable trench forming composition (C) is poured. The form is made of deformable buoyant material such as expanded polystyrene. Rebar legs (18) are attached to the frame members to provide secure anchoring of the frame members to both the ground (E) and in the trench forming composition. Preferably, each frame member includes a horizontal leg that is received in a slot (22) formed in one of the opposing side walls of the form. The cooperating engagement between the frame member (16) and the form provides longitudinal support to prevent the buoyant form (12) from lifting within the trench composition (C) while also maintaining the proper spacing and planar alignment of the frame members. Wire strands (28) are placed within cooperating guide tracks (24) in the opposed ends of the form. The wire strands are pulled through to deform the form after the composition has hardened. To form the trench, a ditch (D) is prepared. The trench form assembly is prepared and then secured in the ditch. The moldable trench forming composition is poured around the form assembly and allowed to set. The form is then deformed by pulling the deforming wire strands through the form. The segmented form is then removed to reveal the finished trench.