Investigation of high fluvial terraces along the Alabama River indicates that the highest terraces (115+ m above the floodplain) may predate the Citronelle Formation. Preliminary radiometric age determinations suggest that all of Quaternary history may be represented by those terraces less than 50 m above the present floodplain. The Citronelle surface in southwestern Alabama shows differential warping caused by subsidence of the Gulf Coast geosyncline and uplift of the adjacent upland. Rates of uplift were roughly 1.2 cm/10³ years.

The average rate of river entrenchment prior to North American Pleistocene glaciation was about 2.5 cm/10³ years. During glacial Pleistocene time, the maximum net rate of river entrenchment may have been as great as 7 cm/10³ years. Observation of fluvial features, such as meander scrolls, shows that, in terraces more than 60 m above the floodplain (i.e., more than 1.1 × 10⁶ years old), meander scrolls have been obliterated. Likewise, terrace surfaces are mostly destroyed where more than 1.6 × 10⁶ years old. In deposits more than 3.5 × 10⁶ years old, primary structures have been obliterated and the only evidence of these oldest terrace deposits is chaotic blankets of alluvium.

End_of_Article - Last_Page 1697------------