## 5 Archaeogeological Discussion
### Assessing Stone Supply  
In summary, we reviewed the presumed original sources of stone materials used in building the ancient tombs near Hangang River according to construction period. The connected stone-mound tombs in the Seokchon-dong tomb complex, which were constructed at an earlier time during the early 4 th century, were mainly built using gneiss (particularly biotite gneiss), schists, and amphibolite. From our review of supply sites based on excavated materials, in Seokchon-dong connected stone-mound tomb, we consider the gneiss and schist to have been acquired from areas other than the known supply sites, South of the Hangang River and West of Namhan Mountain, but we concluded that the amphibolite was sourced from Namhan Mountain, which is \(>4\) km from the remains. We further assumed that stone materials were sourced from a relatively wider area to build the Seokchon-dong tombs. The Gamil-dong tomb complex, which was built during the late 4 th century through the early 5 th century, is composed mostly of amphibole schist with minor gabbro (as directly observed in the field). Amphibolite, schists, and diabase were collected for further study of the remains and of the current preservation status. Our results indicate that all component materials were sourced from Cheonma Mountain, which is within 1 km of these remains. However, although the origins of diabase from Gamil-dong tomb and Cheonma Mountain is similar, there are some differences in the microstructure. Additionally, since the provenance site of gabbro was not suggested in this study, it is necessary to examine the possibility that stones were supplied from Namhan Mountain, the largest mountain area around the remains. Bangi-dong tomb No.3, which was built in the mid to late 6 th century, consists mainly of gneiss, as was used in the Seokchon-dong tombs. It consists mainly of schist, but quartzite, and various igneous rocks are observed as well. Gneiss and schist are assumed to have come from Daemo Mountain, which is 4 km away from the remains. Although the Bangi-dong tomb was constructed at the latest among the tombs under study, it is confirmed that stone materials were supplied in a narrower area in Seokchon-dong tomb, which was built in the 4 th century. Whereas the distance between the two remains is close, the range of stone supply differs depending on the era, which may be due to the difference in the construction style of the tombs, but it may also be caused in different culture group as changing the hierarchy. However, as the Gamil-dong tomb complex is large in scale with many stone-chamber tombs and with a massive material source at Namhan Mountain to the east, the possibility of the stone materials being provided from Namhan Mountain needs further review. Additionally, as the current study compared rock samples collected in mountainous areas and remaining after the development of downtown Seoul, another possible source of supply to Seokchon-dong is Daemo Mountain, which needs further investigation along with a review of the ancient geography (Figure 9). A distinctive feature is that the gravel-shaped stones was found only in Seokchon-dong and Bangi-dong remains. When examining the proportions of rock types in each tomb and looking at the geologic distribution map around remains, igneous rocks were confirmed, it is assumed that these gravel-shaped stones were supplied from the Acha Mountain area in the northern part of the Hangang River by water system. Additionally, it is assumed that the Gamil-dong remains have no gravel-shaped stones due to their location. Thus, it appears that the stone materials used in building the ancient tombs in the Songpa district near the Hangang River were collected across a relatively wider area in the early 4 th century but also collected more locally (within 1 km) during the late 4 th and early 5 th centuries. In the 6 th century, stones were supplied from a point 4 km away from the remains, so it is confirmed that quarrying took place in a narrower range than in the 4 th century.