This paper presents the analysis of the dynamic response of a sedimentary valley subjected to incident plane waves. The sediment-filled valley is composed by a fluid layer over a soft soil deposit whose effect is of importance in the design of long span bridges with high piers. The method of analysis makes use of decomposing the problem into interior and exterior regions. In the exterior region, the scattered wave fields are constructed with two linearly independent sequences of Lamb’s singular solutions which satisfy the traction free conditions at ground surface and radiation conditions at infinity. For the interior regions, the wave fields for the fluid layer and soft soil deposit are expressed in terms of wave functions which satisfy the equation of motion. The continuity conditions at the interface of the media are satisfied in the least square sense. The effects of geometric topography, soil amplification and fluid layer under different types of incident harmonic plane waves are analyzed and discussed in detail.