  The effects of initially uniform magnetic fields on the formation and
evolution of dense pillars and cometary globules at the boundaries of H II
regions are investigated using 3D radiation-magnetohydrodynamics simulations.
It is shown, in agreement with previous work, that a strong initial magnetic
field is required to significantly alter the non-magnetised dynamics because
the energy input from photoionisation is so large that it remains the dominant
driver of the dynamics in most situations. Additionally it is found that for
weak and medium field strengths an initially perpendicular field is swept into
alignment with the pillar during its dynamical evolution, matching magnetic
field observations of the `Pillars of Creation' in M16 and also some cometary
globules. A strong perpendicular magnetic field remains in its initial
configuration and also confines the photoevaporation flow into a bar-shaped
dense ionised ribbon which partially shields the ionisation front and would be
readily observable in recombination lines. A simple analytic model is presented
to explain the properties of this bright linear structure. These results show
that magnetic field strengths in star-forming regions can in principle be
significantly constrained by the morphology of structures which form at the
borders of H II regions.
