The mechanical composition of clastic deposits is usually compared either with an ideal statistical distribution or with the postulated end product of a particular depositional environment. It would seem preferable to employ the parent material as the standard of comparison since it determines the range of grain sizes potentially accessible to its daughter deposits. This approach is practicable in the Mediterranean area and Iran, where many valleys contain two alluvial fills of which the younger consists largely of material derived from the older. Relative silt/clay depletion of the younger fill suggests that, although the parent deposit was laid down by intermittent floods, the second phase of aggradation was characterized by sustained flow during which suspended load could leave the basin preferentially.

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