Layering at the Moho as determined from seismic reflection data is a commonly observed phenomenon in the Basin and Range for which we propose a porous-flow melt model. Analysis of seismic reflection data from the 1986 PASSCAL Nevada experiment indicates that the Moho in the Nevada Basin and Range is layered and has unusually high reflectivity. This reflectivity is difficult to explain in terms of a velocity contrast caused by different rock types, and, in fact, a lower velocity of less than 6.5 km/s suggests the presence of partially molten material at the Moho in a zone containing 20% to 25% of low-velocity layers. To explain the seismic observations, we have developed a viscous, porous-flow model for basaltic melt ascent. Such a model produces systematic variations in volumetric melt flow with depth and close correspondence between seismic interpretation and modeling of partial melt ascent. This suggests that such layering involving viscous, porous flow of partial melt is a plausible explanation of the layering of the Moho in the Basin and Range and may have more general application.