The average depth or traveltime in which the acoustic impedance of marine muds exceeds surrounding sands is not clearly established but is important for a host of applications, such as geohazard analysis for deepwater drilling, slope stability, and prospecting for natural gas hydrates. We have established a practical depth and time for the mud-sand crossover. We have assembled the largest bulk density and compressional velocity data set for marine muds in the first 1200 m below the seafloor (mbsf) from drilling data. The marine mud data set is combined with laboratory data for sands at pressure to determine the physical property trends and crossovers in the average marine sediment column. We have used a simple lithologic model, the average marine mud and sand trends, and a Ricker wavelet to link the physical properties to marine seismic data. We found that a seismic mud-sand crossover occurs, on average, at 875 mbsf, which is approximately two-way time below the seafloor. More practically, the impedance contrast between sand layers and marine mud layers may only be sufficiently visible to 500 mbsf or approximately two-way time due to the similar acoustic impedances of muds and sands below that depth.