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. 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.