## 4 Discussion  
### Geographic Context Controlling the Spatial-Temporal Distribution  
The study area presents a diverse landscape of canyons and prominent river valley basins, including the Shupu, Zhijiang, Yuanma, Anhong, and Qushui basins [27; 34; 35; 48]. This is primarily the result of the Neocathaysian tectonic system's influence, which plays a key role in shaping its unique geomorphic environment [45]. Bordered by majestic mountains to the north, west, and south, the area contrasts sharply with the expansive Dongting Lake Plain to the east (Figure 1). The stunning Wuling Mountains and the Yunnan-Guizhou Plateau, both known for their snow-capped peaks, further enhance the landscape. A significant geographical feature is the intersection of the Xuefeng and Wuling mountain ranges, forming a distinctive Chinese \"eight\" shape around Yuanling County. This configuration has facilitated the development of two prominent ancient cultural groups: the \"Wushui Ancient Cultural Group\" from the middle Yuanjiang catchment and the \"Lower Yuanjiang Ancient Cultural Group\", as depicted in Figure 1.  
Our literature analysis indicated that the first to fourth-level terraces were widely distributed along the lower reaches of the Yuanjiang River, while the fifth and sixth terraces were less common [34; 35; 40]. Taoyuan County was notable for its well-developed terraces, serving as a key reference point for analyzing surface height ratios, which were measured at 8-14, 14-21, 34-46, 71-82, 80-110, and 120-140 m above river level (a.r.l.) based on our field survey, consistent with previous studies [25; 34; 35; 47; 49] (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**River** & **Terrace Level** & \begin{tabular}{c} **Height/m** \\ **a.s.l.** \\ \end{tabular} & \
\begin{tabular}{c} **Height/m** \\ **a.r.l.** \\ \end{tabular} &
\begin{tabular}{c} **Age/ka** \\ \end{tabular} \\ \hline \multirow{6}{*}{Maoxi River from the middle Lishui River [47]} & T\({}_{7}\) & 311 & 136 & 928 \(\pm\) 92 (ESR) \\  & T\({}_{6}\) & 297 & 122 & 574 \(\pm\) 57 (ESR) \\  & T\({}_{5}\) & 272 & 94-100 & 689 \(\pm\) 68 (ESR) \\  & T\({}_{4}\) & 256 & 78–83 & 347 \(\pm\) 34 (ESR) \\  & T\({}_{3}\) & 235 & 57–62 & 151.05 \(\pm\) 12.84/201.24 \(\pm\) 17.11 (TL) \\  & T\({}_{2}\) & 206 & 28–33 & 60.95 \(\pm\) 5.18 (TL) \\  & T\({}_{1}\) & 188 & 10–15 & Q\({}_{h}\) \\ \hline \multirow{2}{*}{Suoxi River from the middle Lishui} & T\({}_{4}\) & – & – & Q\({}_{p}\)\({}^{2-2}\) \\  & T\({}_{3}\) & 205 & 55 & 104.45 \(\pm\) 8.88 (TL)/117.62 \(\pm\) 9.99 (TL) \\  & T\({}_{2}\) & 188 & 28 & \(\text{Q}_{p}\)\({}^{3}\) \\ \multirow{2}{*}{River [46; 47]} & T\({}_{1}\) & 173 & 13 & Q\({}_{h}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dating of fluvial terraces in the Yuanjiang River and neighboring regions. The terrace deposits exhibited a binary phase structure, reflecting the complex geological and hydrological processes that have shaped the region over time. Sites on the fourth level and some third-level terraces might date back to the late early-Paleolithic period or early mid-Paleolithic period, while those on the second level and certain third-level terraces correspond to the middle or late mid-Paleolithic period. The first and some second-level terraces are associated with the late Paleolithic period. Distinct vertical migration patterns in Paleolithic culture were attributed to tectonic movements. Previous studies suggested that high river marshes and low terraces near rivers were most conducive to human survival, offering low-lying terrain, abundant water sources, lush vegetation, and ample stone resources [19; 25; 27; 40]. Consequently, Paleolithic humans typically inhabited high floodplains and low terraces near rivers. Based on these findings and the analysis in Figure 3 and Table 1, it could be inferred that the lower terraces from the Paleolithic era would rise to higher elevations due to subsequent tectonic uplift.