Petrological Characteristics and Procurement Route of the Rocks Used to Construct Munsusanseong, Gimpo

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Abstract

1. INTRODUCTION

Munsusanseong, a Temoe-type stone-built mountain fortress in the Ponae-ri area of Wolgot-myeon, Gimpo-si, Gyeonggi-do, is a designated historic site (No. 139). It surrounds the 8th and 9th ridges of Mount Munsusan (376 m). Built in the 20^th year of King Sukjong’s reign (1694) during the Joseon dynasty, Munsusanseong was a historically and geographically important government defense fortress. It faces the Yeomha River and views Ganghwado Island from Munsusanseong mountain (Jangdaeji). It is approximately 300 m long and has an internal area of 4,631 m². Although the shape, scale, altitude above sea level, and narrow interior space of Munsusanseong differ from those of mountain fortress of Silla Dynasty, they are similar in that their ramparts were built on the upper parts of cliffs (An et al., 2014; Shin, 2022). Although building these stone fortresses was labor-intensive because the rock had to be quarried, transported, and processed, they had the advantage of increased strength and ease of maintenance compared to mud-walled fortresses. The late Joseon period exhibited the mass production of lime and the development of mortarmaking technology (Lee et al., 2020). Various materials, including soil, rocks, lime, and roof tiles, were used as construction materials.

The remaining Munsusanseong ramparts appear to have undergone several contractions since their construction in 1694, and now, only the north and south gates, centered on Mount Munsusan, remain. Most of the western ramparts, which face the Yeomha River, have been washed away, leaving only diminished amounts of rampart stone on the ground. However, an antique map shows that the western rampart was about 1.2 km long and contained a west gate (Gonghaeru), a government office, and floodgate facilities (Gukto Institute of Cultural Heritage, 2020). It was also recorded that the stones required to construct Munsusanseong were procured from the island in the 20^th year of King Sukjong’s reign.

Identifying the sources of stones used to construct cultural assets can provide critical information on the economics, politics, and technology of the construction period. In recent years, the petrological characterization of stone cultural assets has been used to identify their origin and procurement route. In this study, we conducted a geological survey of the Munsusanseong area and a petrological analysis of excavated western rampart construction rocks to confirm their distribution. This study investigated the source and procurement route of the rocks used to construct the western rampart of Munsusanseong.

2. MATERIALS

Excavations of the western rampart castellation and Gonghaeru sites were carried out by the Gukto Institute of Cultural Heritage Foundation in 2021 and 2022 (Figure 1). A total of 25 samples were recovered from the sites during the second and third excavations (Figure 2). Nine samples were collected from the southern and northern sections of the western rampart castellation site. They were divided into five sampling points according to their location, for example, upper or lower. In addition, 16 samples were collected from the west gate and divided into floor stones, plate stones, upper plate stones, etc.

Figure 1.

(A) Location and excavation site photos of the (B) West gate(Gonghaeru) and (C) Western rampart castellation sites within the western wall of Munsusanseong.

Figure 2.

Rocks collected from the western rampart castellation and west gate (Gonghaeru) sites within the western wall of Munsusanseong.

3. METHODS

We identified the sampled rock types in hand picking using a magnifying glass and a visual inspection (Figure 2). Thin sections of the samples were then analyzed using a polarizing microscope (Eclipse LV100n, Nikon, JPN) to determine their semiquantitative and relative mineral compositions. X-ray diffraction (Empyrean, PANalytical Co., GBR) was used to precisely identify the constituent minerals. The X-ray target was Cu Kα, and a high-resolution Pixel 3D (256CH) detector was used. The accelerating voltage of the anode and the filament current were set to 40 kV and 30 mA, respectively, with a scan range of 5°–80°, a scan interval of 0.026°, and a scan speed of 0.04°/s. X-ray fluorescence (XRF-1800, SHIMADZU, JPN) was used to determine their geochemical characteristics. Pretreatment and primary component element content measurements performed at the Korea Basic Science Institute used 30 keV/100 mA with an Rh target. For the granitic rocks, whole rock magnetic susceptibility was measured using a magnetic susceptibility meter (SM-30, ZH Instrument Co., CZE) and expressed as 10⁻³SI.

The petrological, mineralogical, and geochemical characteristics of the rocks were then compared with those of surrounding rocks determined via a field geological survey.

4. RESULTS

4.1. Rocks used to construct Munsusanseong

4.1.1. Rock type classification

The rocks used to construct the western rampart castellation and the west gate were identified in hand samples as granodiorite, sandstone or pebbly sandstone, quartzite, and schist. Representative samples of each rock type were selected, and their constituent minerals were identified using a polarizing microscope and XRD (Figure 3 to 4).

Figure 3.

Thin section micrographs of the rocks used to construct Munsusanseong.

Figure 4.

X-ray powder diffraction patterns of the constituent rocks used to construct Munsusanseong. (Qz: Quartz, A-f: Alkali-feldspar, Pl: Plagioclase, Mi: Mica, Hb: Hornblende, Chl: Clinochlore).

The granodiorite (Figure 3A~ C) contained quartz, alkali feldspar, plagioclase, biotite, hornblende, and opaque minerals. Most of the sandstone and pebbly sandstone (Figure 3D~ H) contained quartz and mica, with chlorite also identified in some samples. The quartzite (Figure 3I) exclusively contained quartz. The schist (Figure 3J~ L) contained quartz, mica, and chlorite. Of note, the mineralogy of the sandstone and pebbly sandstone is the same as that of the schist; however, the schist is distinguished by the relatively large XRD peaks of the mica and chlorite (Figure 4).

4.1.2. Chemical classification

Principal component element analysis was performed to compare the geochemical characteristics of the different rock types (Table 1).

Table 1.

Major element compositions of the Munsusanseong rock samples

Harker diagrams show that the SiO₂ contents of the granodiorite and schist are similar, followed by the sandstone or pebbly sandstone and quartzite. However, although the SiO₂ content of the granodiorite and the schist are similar, they are distinguished by the differences in their CaO and Na₂O contents (Figure 5). Norm analysis using the principal component content (PCC) was performed to determine the mineral composition of the granodiorite. The granodiorite samples mostly plot as granodiorite on the quartz-alkali

Figure 5.

Harker diagrams of the major element oxides versus SiO₂.

feldspar–plagioclase triangular diagram (Figure 6). For the sandstone or pebbly sandstone, there is no noticeable change in the concentrations of MnO, CaO, Na₂O, and K₂O with changes in SiO₂ concentration; however, the concentrations of TiO₂, Al₂O₃, Fe₂O₃, and MgO tend to decrease (Figure 5). The mean value of the SiO₂ content was 79.92 wt.% (73.74–84.6 wt.%), which is similar to the arkose sandstone (77.10 wt.%) of Pettijohn (1975). The mean value of the Al₂O₃ c ontent w as 1 0.82 wt.% (9.41–13.01 wt.%), which is higher than the arkose sandstone (8.70 wt.%) of Pettijohn (1975). Al₂O₃ content is mainly dependent on the amount of clay minerals present as the substrate and is interpreted to be due to the weathering alteration of feldspars in sandstone or pebbly sandstone (Park et al., 1997; Choi and Lee, 2009).

Figure 6.

Modal quartz-alkali feldspar-plagioclase diagram of the granodiorite samples (blue dots).

The mean values of the MgO and Fe₂O₃ contents were 0.49 wt.% (0.21–0.84 wt.%) and 2.70 wt.% (1.11–4.99 wt.%), respectively, which are similar to the MgO (0.50 wt.%) and Fe₂O₃ (2.2 wt.%) contents of the arkose sandstone of Pettijohn (1975). The content of the MgO and Fe₂O₃ tended to decrease with increasing SiO₂ content, which has been interpreted as indicating that the grains of the arkose sandstone were largely derived from granitic plutonic rocks (Park et al., 1997). When plotting on Herron’s (1988) sandstone classification, which is often used to categorize lithic sandstone and feldspathic sandstone, some of the samples correspond to quartz sandstone (i.e., wacke) and lithic sandstone (i.e., lithic arenite), but most correspond to feldspathic sandstone (i.e., arkose) (Figure 7).

Figure 7.

Geochemical classification of the sandstones (red dots) (Herron, 1988).

4.2. Rocks surrounding the Munsusanseong

The variety of rock types found at the western rampart castellation and west gate sites suggests that they were sourced from multiple sources rather than just one. In this study, an extensive geological survey was conducted in the area surrounding Munsusanseong and the southern Ganghwa region to determine the source of the construction rocks. Rocks from the Munsusan Formation (Figure 8) were collected and compared using petrological research.

Figure 8.

(A) Location and (B) Geological map of Munsusanseong and Bunori Dondae in southern Ganghwado Island.

4.2.1. Munsusan formation

The Cretaceous-aged Munsusan Formation is widely distributed around Munsusanseong and is estimated to be about 850 m thick. It comprises layers composed mainly of red conglomerate, greenish-gray pebbly sandstone, milky conglomerate, and red pebbly sandstone interbedded lenticularly with red mudstone or fine-textured sandstone (Chwae et al., 1997). During this investigation, we identified pebbly sandstone (sampling points 1 and 2) and sandstone (sampling point 3) within approximately 3 km of the western rampart castellation and west gate sites (Figure 9A). The polarized microscopy and XRD analyses of the rocks from each sampling point showed that sampling point 1 and 2 rocks are pebbly sandstone, and the constituent minerals are quartz, alkali feldspar, and mica, while the sampling point 3 rocks, located between the western rampart castellation site and the west gate, are mostly sandstone, and the constituent minerals are quartz and mica (Figure 9B~ E). A comparison of the surrounding rocks with the rocks used to construct the western rampart castellation and west gate show that the petrological and mineralogical characteristics are similar, especially with the rocks from sampling point 3, which is closest to the excavation site (Figure 4 to 6). Therefore, it is likely that the sedimentary rocks used for the construction of Munsusanseong were quarried from the widely distributed Munsusan Formation in the surrounding area to supply all the rocks needed for construction.

Figure 9.

(E) Results of the thin section and X-ray diffraction analysis of the Munsusan Formation rocks surrounding the Munsusanseong.

4.2.2. Granodiorite

Because granodiorite was confirmed to have been commonly used for the construction of the western rampart castellation and the west gate, the scope of the survey was expanded to the southern Ganghwa region to determine the source and procurement route of the granodiorite. The southern Ganghwa region in the vicinity of Bunori Dondae, another historic fortress (Figure 10A), is underlain by gneiss and schist of the preCambrian Gyeonggi metamorphic complex and intruded by Mesozoic igneous rocks. The granodiorite distributed throughout the Bunori Dondae area is also distributed in the center of Seokmodo Island and Mount Manisan on southern Ganghwado Island. The hornblende granodiorite of Seokmodo Island extends to Geonpyeong-ri, Ganghwa Island, but it does not extend to Mount Manisan, Ganghwa. Although it appears to be Jurassic-aged Daebo granite, it has been U–Pb dated to the Cretaceous (Hwang and Kim, 2005).

Figure 10.

Visual observational features and rock formations in the Bunori Dondae area.

Granodiorite was sampled in Sagi-ri, Hwadong-myeon, Ganghwa-gun, (site 185-1) from a sea cliff and shore platform, where Bunori Dondae, built before Munsusanseong in the 5th year of King Sukjong’s reign (1679), is located. The adjacent sandy beach is about 10 m wide and 200 m long, and tidal mudflats are present on the tideland meeting the Yeomha River. The sampled rock is neutral to coarse-grained hornblende granodiorite, with characteristic oval-shaped mafic microgranular enclaves varying in size from a few centimeters to tens of centimeters.

The polarized microscopic analysis showed that the constituent minerals include quartz, plagioclase, alkali feldspar, biotite, hornblende, chlorite, and opaque minerals, and the minerals are generally euhedral or subhedral. Plagioclase phenocrysts exhibit albite or Carlsbad twinning. The XRD analysis also showed a similar mineral composition, which is consistent with the granodiorite used for the construction of the western rampart castellation and the west gate sites (Fig. 10).

5. DISCUSSION

While the sandstone or pebbly sandstone used to construct Munsusanseong was probably sourced from the Munsusan Formation found in the surrounding area, the granodiorite was not available locally. This is supported by a Border Defense Council of Joseon record in the Bibyeonsa registry that states that stonecutting for the construction of Munsusanseong should start from Ganghwa. We examined the petrological homogeneity of the granodiorite used to build Munsusanseong and the rocks found in the Bunori Dondae area. The granodiorites showed similar whole rock magnetic susceptibilities, and the CIPW norm values calculated using the PCCs plotted as granodiorites on the normative Q-Or-Pl triangle. Furthermore, based on their chemical composition, the granodiorites correspond to sub-alkaline and calc-alkaline series, interpreted as products of the middle phase of an eruption, based on the total alkali diagram for the SiO₂ component of Le Maitre (1976) and the AFM(Na₂O + K₂O-FeO-MgO) of Irvine and Baragar (1971) plots (Figure 11).

Figure 11.

Petrological homogeneity of the granodiorites used to construct Munsusanseong and Bunori Dondae.

The granodiorite used to construct Munsusanseong is petrologically identical to the granodiorite found in the Bunori Dondae area. Therefore, it is highly likely that the Munsusanseong granodiorite was quarried from sites on many shore platforms below Bunori Dondae. This is supported by an antique map (Jeopyeokjido) that shows a waterway leading from the southern part of Ganghwa to Munsusanseong (Figure 12B). The tidal range of the nearby Yeomha River is about 6 m wide, so it is possible to transport rocks by boat via this river. Therefore, the granodiorite used to construct Munsusanseong is interpreted to have been sourced from the Bunori Dondae area and transported by boat via the Yeomha River (Figure 12A).

Figure 12.

Estimated procurement route of granite used in the construction of Munsusanseong Fortress and sea route marked on old map (Intersection map, Kyujanggak Institute for Korean Studies in Seoul National University).

6. CONCLUSION

This study aimed to identify the types of rocks used to construct the western rampart castellation and the west gate of Munsusanseong and their source and procurement route. Petrological, mineralogical, and geochemical analyses were undertaken on the sampled castellation and gate rocks recovered from excavations and surrounding rocks.

A combination of various rock types, including granodiorite, sandstone, pebbly sandstone, schist, and quartzite, was used to construct the western rampart castellation and west gate. While the sedimentary rocks could have been easily sourced from the widely distributed Munsusan Formation in the surrounding area, the granodiorite is interpreted to have been sourced externally.

This study has demonstrated that the granodiorite used to construct Munsusanseong is identical to the granodiorite from the Bunori Dondae area in terms of occurrence, whole rock magnetic susceptibility, constituent mineral types and textures, and geochemical characteristics. This is supported by a historical record that indicates that the Munsusanseong construction stone was sourced from Ganghwa.

In addition, many traces of quarrying suggest that the granodiorite was quarried from a shore platform in the Bunori Dondae area in the southern part of Ganghwado Island. The procurement route is interpreted to have been by boat via the Yeomha River. This waterway from the southern part of Ganghwa to Munsusanseong is clearly marked on an antique map. The rock would have been transported at high tide across the low-tide mud flats. This procurement route indicates that the late Joseon dynasty’s economic power and area of labor mobilization were centered in Ganghwa and its surrounding areas.

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