  Let F be the fundamental group of S, where S is a compact, connected,
oriented surface with negative Euler characteristic and nonempty boundary.
  (1) The projective class of the chain \partial S in B_1(F) intersects the
interior of a codimension one face of the unit ball in the stable commutator
length pseudo-norm. (2) The unique homogeneous quasimorphism on F dual to this
face (up to scale and elements of H^1) is the rotation quasimorphism associated
to the action of F on the ideal boundary of the hyperbolic plane, coming from a
hyperbolic structure on S.
  These facts follow from the fact that every homologically trivial 1-chain in
S rationally cobounds an immersed surface with a sufficiently large multiple of
the boundary. This is true even if S has no boundary.
