Most unconsolidated ocean sediments have been reported to possess small, but finite, rigidity. In sandy sediments rigidity is due to sliding and rolling friction resulting from the interlocking grain structure; in silt clays rigidity is due to cohesion. For many marine environments acoustic propagation is not measurably affected by sediment rigidity. In some environments, however, the effect of sediment rigidity is the dominant mechanism in modal attenuation. Two coastal regions on the Scotian shelf will be contrasted by comparing the effects of sediment shear wave excitation on transmission loss at low frequency. The key environmental factor in unconsolidated sediment shear wave excitation is not the intrinsic sediment properties but the degree of elastic contrast found in the acoustic field.