Shallow‐water propagation experiments conducted on the Scotian Shelf in 1978 produced very high transmission losses at low frequency for a range of only 4 km. The bottom along the propagation track consisted of a thin layer of coarse sand over granite bedrock which protruded through the sediment in certain regions. The sources of the high losses at low frequency are investigated by varying the acoustic properties of the geoacoustic model. It is shown that neither scattering nor shear effects in the granite basement accounts for this loss, but that a third mechanism, excitation of shear waves in the sediment, can cause such high losses.