WERE TERRACES ALONG THE MIDDLE OHIO RIVER VALLEY A MIDDLE HOLOCENE CONSTRUCT?

Geoarchaeological investigations at the East Bend Bottom, Kentucky, showed that after glacial discharge ceased, the Ohio River apparently underwent >15m of downcutting. Reworked outwash and channel/bar lateral accretion characterized the Ohio Valley during the late Wisconsinan and early Holocene. Significant readjustment occurred during the middle Holocene and finer-grained, vertical accretion/overbanking and levee construction dominated. At East Bend, this interval is marked by a 200-400m-wide, >3m thick vertical accretion terrace. These sediments include stratified Archaic-age archaeological deposits buried up to 3m deep across the terrace. ¹⁴C ages and Late Archaic cultural material buried within the upper 50 cm of the East Bend terrace supports that only minimal sedimentation occurred since 3 kBP and that the Ohio River has been confined mainly to its existing channel belt during the late Holocene. A similar sequence of terrace formation and stratified archaeological deposits also occur downstream from East Bend at Mexico Bottom. Here deeply buried, stratified Middle to Late Archaic cultural material indicate that occupation begun on point bars greater >4 m deep just after 7 kBP. ¹⁴C ages from cultural deposits show that vertical sedimentation continued episodically until 4 kBP, while the age of near-surface cultural features indicate that the floodplain stabilized by 3.5 kBP.

This depositional pattern (above) may be general to bottom areas above the Falls of the Ohio and has significant implication for both understanding Ohio Valley development and archaeological site discovery and formation. The depositional pattern suggests initial, early Holocene erosion focused on cleaning outwash from the valley followed by a mid-Holocene interval of fine-grained, vertical accretion and levee formation 7-4 kBP. This depositional style adjustment may relate to Midwestern climate change after the Hypsithermal. The pattern developed during the early and middle Holocene also means that preservation of Early Archaic sites in the valley is low, that middle Holocene sites are more commonly buried and invisible, and that terminal Archaic and Woodland sites will be most abundant on the floodplain surface. Such a pattern will skew our understanding of human settlement in the Ohio valley