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Illustration of the principle of progradation of sediments over time. At any one instant in time, the seafloor becomes the depositional site for sediments. Normally, with an increase in water depth, progressively finer-grained sediment is deposited. In this picture, pink is sand, brown is silt, and red is mud. Time 3 in the picture shows the present-day ocean floor in front of a delta. Because the delta builds outward as well as upward with time as a result of progradation, the positions of some older seafloor surfaces are shown (Times 1-3). As the sediments prograde with time, each of the sedimentary deposits is laid down in a flat or horizontal manner, even though the time lines (or old seafloor surfaces) are inclined. The seaward buildup of the delta deposit is the result of the process of "progradation."
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