HIGH-LEVEL STREAM TERRACES IN THE RIDGE AND VALLEY OF SOUTHEASTERN TENNESSEE AND IMPLICATIONS FOR DRAINAGE EVOLUTION

Surficial mapping along the Hiwassee and Ocoee Rivers in western Polk County shows more than 100 square kilometers of alluvial surfaces. By far most extensive is high-level alluvium resting on limestone bedrock. This old alluvium is highly weathered, with B horizons having clay contents exceeding 50 percent and Munsell hues of 2.5YR or 10R. Sandstone clasts generally are decomposed. Such old alluvium occurs at elevations of 20 m or more above modern river level (AMRL) and may be found at elevations exceeding 60 m AMRL. (Maximum elevations AMRL appear to be somewhat lower than maximum elevations reported at some other locales in the Ridge and Valley.) Surfaces here are rolling rather than flat. Deposits below 20 m AMRL are somewhat less weathered and occur on relatively flat surfaces. The most definitive clasts in the alluvium are well rounded pebbles and cobbles of vein quartz, which attest to the origin of the alluvial sediment in the Blue Ridge province. Such clasts become rarer with increasing height AMRL, and at the highest levels “alluvial” deposits consist of scattered quartz clasts mixed with residuum. Alluvial deposits are most abundant within 4 km of the main rivers. An exception of interest is a band of old alluvium extending from the Ocoee south southwesterly to the Conasauga River near the Tennessee-Georgia border. This band crosses a low divide where only small, local streams flow today. Presently the Conasauga flows southwesterly to the Coosa and thence to the Gulf, whereas the Hiwassee and Ocoee flow to the Tennessee River. The distribution of old alluvium, however, suggests that the Conasauga once flowed north northeasterly to the Ocoee or Hiwassee, and that subsequently it was diverted to the Coosa system, representing a signficant loss from the Tennessee River basin. Alternatively, the band of old alluvium might simply represent selective preservation on favorable rock units.