The present invention is a method for predicting the grain size distribution at a designated location within a water-lain sedimentary deposit. Initially, the vertical thickness of the sedimentary deposit at the designated location must be determined, as well as the vertical thickness and grain size distribution at a second location different from the designated location. Second, a distance parameter corresponding to the two locations must be determined. Finally the distance parameter is used, along with the initially determined vertical thickness at both locations and the grain size distribution at the second location to calculate the grain size distribution at the designated location.