Ancient alluvial deposits are believed to be incomplete because flood-plains undergo relatively infrequent episodes of sedimentation for which the cumulation of time is geologically limited. However, many alluvial sequences contain superposed paleosols that preserve not only the record of deposition but also the geologically lengthier record of nondeposition and nonerosion. The only time that is not represented in an alluvial sequence with paleosols is that time represented by sediments and paleosols that have been eroded. Erosion can reflect channel migration, which produces coarse sediment filled scours and is a process common to both degrading and aggrading fluvial systems. In contrast, deep and laterally extensive scours, filled dominantly by fine sediments, are evide ce for lengthy periods of degradation. This second type of scour, whose recognition

End_Page 275------------------------------

is enhanced by color differences between paleosols developed on the scoured and scour-fill deposits, is a valuable indicator of climatically and/or tectonically controlled base level changes in continental basins. Because paleosols preserve the record of nondeposition and nonerosion and are useful in identifying periods of degradation, their analysis is essential to reconstructing the complete geologic history of alluvial sediments.

End_of_Article - Last_Page 276------------