  Crystals and other condensed matter systems described by density waves often
exhibit dislocations. Here we show, by considering the topology of the ground
state manifolds (GSMs) of such systems, that dislocations in the density phase
field always split into disclinations, and that the disclinations themselves
are constrained to sit at particular points in the GSM. Consequently, the
topology of the GSM forbids zero-energy dislocation glide, giving rise to a
Peirels-Nabarro barrier.
