Narrative accounts and floristics suggest a broader historical distribution of Piedmont upland savannas and woodlands with a prairie-affinity flora than today, although dates of emergence, spatiotemporal extent, and historical dynamics remain unclear. To help address the question whether remnant prairie-affinity patches in the state represent at least historical, if not ancient, grasslands, we analyzed stable carbon isotopes from soil organic matter from two well-known localities hosting prairie-affinity heliophytes, both within the boundaries of historically mapped “Grande Savane”. Soil cores for δ13Corg analysis and radiocarbon dating were collected from five sites, hosting different present-day plant communities and spanning three soil orders. Recovered δ13Corg values suggest historical grasslands, likely savanna-type with some fluctuations in cover, were present at both localities essentially continuously over the past 2000 years until the more recent canopy closure over the past century. These findings are consistent with historical narrative accounts, although significant additional sampling is needed to determine its spatiotemporal extent. While the general trend at our localities transitions from open to closed systems, pronounced fluctuations are apparent in all profiles, particularly between 1254–1468 CE. Precipitation does not appear predominantly responsible, but available climatic reconstructions are from an adjacent basin. The timing of the pronounced vegetation class fluctuations is intriguingly coincidental to the timing of Siouan occupancy and intertribal movements, suggesting a need for more vigorous interdisciplinary investigations.