  We determine the local density of states (LDOS) of one-dimensional
incommensurate charge density wave (CDW) states in the presence of a strong
impurity potential, which is modeled by a boundary. We find that the CDW gets
pinned at the impurity, which results in a singularity in the Fourier transform
of the LDOS at momentum 2k_F. At energies above the spin gap we observe
dispersing features associated with the spin and charge degrees of freedom
respectively. In the presence of an impurity magnetic field we observe the
formation of a bound state localized at the impurity. All of our results carry
over to the case of one dimensional Mott insulators by exchanging the roles of
spin and charge degrees of freedom. We discuss the implications of our result
for scanning tunneling microscopy experiments on spin-gap systems such as
two-leg ladder cuprates and 1D Mott insulators.
