A JWKB asymptotic expansion describing inplane elastic waves is used to approximate a Rayleigh-like wave guided within a curved elastic waveguide whose curvature is small and changes slowly over a wavelength. The two lowest eigenmodes in a curved guide, taken together, constitute the Rayleigh-like wave. It is shown that this wave lies in the shadows of four, closely spaced, virtual caustics, two caustics per constituent eigenmode. If the curvature becomes too large one or more of the caustics ceases to be virtual and enters the guide after which a Rayleigh-like wave cannot propagate. The overall disturbance is shown to have an amplitude that is modulated because the wavenumbers of the constituent eigenmodes differ by a small amount. Moreover, the disturbance is shown to propagate with a wavenumber that, to leading order, has a linear dependence on the curvature causing the phase to be modulated, as well. Passing from a thin guide to a very thick one suppresses the amplitude modulation, making the phase modulation evident. Propagation into an environment of increasing curvature, for both thin and thick, shallowly curved guides is studied so that the modulations may be observed.