ABSTRACT

Investigation of high fluvial terraces along the Alabama River indicates that the highest terraces (120+ m above the flood plain) may predate the Citronelle Formation. Preliminary radiometric age determinations suggest that all of Quaternary history may be represented by those terraces less than 55 m above the present flood plain. The Citronelle surface in southwestern Alabama shows differential warping due to subsidence of the Gulf Coast geosyncline and uplift of the adjacent upland. Rates of uplift were roughly 1.2 cm per 10³ years.

The average rate of river entrenchment prior to North American Pleistocene glaciation was about 2.5 cm per 10³ years. During glacial Pleistocene time, the maximum net rate of river entrenchment may have been as great as 7 cm per 10³ years. Observation of fluvial features such as meander scrolls show that for terraces more than 60 m above the flood plain, (i.e. more than 1.1x10⁶ years old), meander scrolls have been obliterated. Likewise, terrace surfaces older than 1.6x10⁶ years are mostly destroyed. For deposits older than 3.5x10⁶ years, primary structures have been obliterated, and the only evidence of these oldest terrace deposits is chaotic blankets of alluvium.