  We discuss differences in shapes, expansion velocities and fragmentation
times of structures created by an energy deposition from a single Gamma Ray
Burst (GRB) or an OB association to the ISM. After the initial inflation,
supershells produced by GRBs are almost static or slowly expanding, contrary to
more rapidly expanding supershells created by OB associations. We discuss the
position of the energy source relative to the symmetry plane of the galaxy:
observed arc-like structures can be the most dense parts of structures formed
by an expansion from a source above or below the galactic plane. Arcs may also
form, if the expansion takes place inside a giant HI cloud. We try to reproduce
the size, the age and the average distance between OB associations in the
Sextant region at the edge of LMC 4.
