An investigation is made into the interaction between a rapidly expanding planar H II region, bounded by a weak R-type ionization front, and a contact discontinuity which separates two regions of unionized gas of differing densities in the H I region. In order to determine a unique resultant flow configuration it is found necessary to supplement the equations of mass, momentum, energy, and radiation flux with Axford's ionization front stmctural analysis. This is particularly important when strong D-type fronts are evolved. It is found that an interaction with high density gas results in the evolution of a slow moving weak D-type front with an isothermal shock reflected towards the illuminating star. This result is used qualitatively to deduce the gasdynamics of elephant trunks which, it is suggested, may be formed after a collision between an H II region and a relatively small globule of high density gas. The bright rims which have been observed to bound these structures are regions of gas compressed by the passage of the reflected isothermal shock.