  We present Smoothed Particle Hydrodynamics (SPH) simulations of the response
of gas discs to a spiral potential. These simulations show that the commonly
observed spurs and feathering in spiral galaxies can be understood as being due
to structures present in the spiral arms that are sheared by the divergent
orbits in a spiral potential. Thus, dense molecular cloud-like structures
generate the perpendicular spurs as they leave the spiral arms. Subsequent
feathering occurs as spurs are further sheared into weaker parallel structures
as they approach the next spiral passage. Self-gravity of the gas is not
included in these simulations, stressing that these features are purely due to
the hydrodynamics in spiral shocks. Instead, a necessary condition for this
mechanism to work is that the gas need be relatively cold (1000 K or less) in
order that the shock is sufficient to generate structure in the spiral arms,
and such structure is not subsequently smoothed by the gas pressure.
