These people are walking on cement blocks.
The cement blocks are a wall.
The fact that they are walking on cement blocks doesn't imply that the blocks are a wall.  If anything, it implies that the blocks are a floor.
exists e1 x1 x2 x3.(n_people(x1) & on(e1,x2) & n_block(x2) & of(x2,x3) & n_cement(x3) & Actor(e1,x1) & v_walk(e1))
exists p1 x1 x2.(n_block(x1) & of(x1,x2) & n_cement(x2) & exists x3.((x1 = x3) & n_wall(x3)))