A rough crust-mantle boundary can produce amplitude versus offset characteristics similar to those of a second-order discontinuity (i.e., a gradient), namely a reduction in precritical PmP. Basin and Range reflection and refraction data can be simulated as well with rough Moho models as with gradient models. A rough Moho presents a different view of the interaction between the mantle and crust, where tectonic and magmatic structures are preserved and the crust and mantle are distinct. The rough Moho supports a crustal evolution model in which the crust is injected by mantle-derived mafic dikes and sills, but not to the extent that the lower crust is a continuous sequence of mafic/ultramafic intrusions, as commonly inferred from gradient models. As such, in the Basin and Range the Moho represents a well-defined chemical boundary.