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[MZZ48-P01] Rock species of the cornerstone of the earthen wall in the northeastern part of Ninomaru earthworks of Yamagata Castle
Keywords:cornerstone of earthen wall, rock species, Ninomaru earthworks, Yamagata Castle
The northeastern part of Ninomaru earthworks at Yamagata Castle encompasses the area from Higashi Otemon (Eastern Gate) to the North Gate. During an archaeological excavation conducted by the Yamagata City Board of Education from 2016 to 2018, stone walls of Ushitora Yagura and Sakanamachimukai Yagura were detected. The cornerstone of the earthen wall was successfully excavated in almost the entire area, and a geological survey was conducted on the stone walls(Ohtomo, 2020). Two periods of old and new were assigned to the cornerstone of the earthworks. It was judged that the old earthworks were updated with new construction inside because the earthworks are tilted owing to unequal subsidence(Yamagata City Board of Education, 2020). The stone materials surveyed during this project are of the new period.
Most of the cornerstones of the earthen wall are composed of andesite. In particular, 1270 of the 1383 cornerstones are andesite, accounting for 92.03%. Other major materials include dacite, granite, and rhyolite in 47, 40, and 18 cornerstones, amounting to 3.40%, 2.89%, and 1.30%, respectively . In addition, five are composed of basalt, and three are composed of tuff breccia, green tuff, and green andesite. Most occur as subcircular gravel derived from riverbeds. Most of the dacite and some of the rhyolite and granite occur as angular rock masses.
Most of the andesite is gray to black; of these, 30.39% are inclusion rocks presumed to be basalt, and 4.7% contain conspicuous volcanic gas voids. In addition, 1.89% is reddish-purple in color owing to oxidation at high temperatures.
Of the dacites, 33.33% are characterized by thin, black stripes; 31.11% have no such stripes; and 35.56% some did not distinguish between them. Both lithologies are found in the dacite bodies distributed around the Mount Sakazukiyama, although the striped lithology is limited in distribution(Ohtomo, 2016). Most of the dacite is in the form of angular rock masses having the same lithology as the rock found in the stone walls of Sakanamachimukai Yagura and Ushitora Yagura. It is believed that the dacites remaining from the stone wall construction were used in the cornerstone foundation of the earthen wall.
The granites are gravels composed of 65.00% coarse-grained biotite granite, 32.50% medium-grained biotite granite, and 2.50%, fine-grained granite originating from the granite body distributed upstream of the Mamigasakigawa River(Ohtomo et al., 2019).
The gravels assumed to be composed of basalt contain large plagioclase phenocrysts approximately l cm in diameter and numerous phenocrysts. Because this lithology is characteristic of Ryuzan basalt, the gravels of this lithology were classified as basalt.
The tuff breccia and green tuff are from the Miocene Narisawa Formation, and the green andesite is from the Houzawa Formation. The tuff breccia contains granite fragments, and the voids of the green andesite are filled with agate.
The proportion of each of these rock species is almost constant throughout the northeastern earthworks of Ninomaru; no large bias was found among the districts. In District 7, which is adjacent to Ushitora Yagura, 11 dacites having the same lithologies as those present in the stone wall are distributed.
References
Yamagata City Board of Education, 2020, Survey report on buried cultural properties in Yamagata City, Yamagata Prefecture, 39, 14-110. *
Ohtomo Y., 2016, 123rd Annu. Meet. Geol. Soc. Japan, Abstr., 190.
Ohtomo Y., 2020, IV Survey results of stone walls and cobblestone of Sumiyagura in Ninomaru. 39, 111-115. *
Ohtomo Y., et al., 2019, Applied geol. Yamagata, no.39, 89-93.
*English translation from the original written in Japanese
Most of the cornerstones of the earthen wall are composed of andesite. In particular, 1270 of the 1383 cornerstones are andesite, accounting for 92.03%. Other major materials include dacite, granite, and rhyolite in 47, 40, and 18 cornerstones, amounting to 3.40%, 2.89%, and 1.30%, respectively . In addition, five are composed of basalt, and three are composed of tuff breccia, green tuff, and green andesite. Most occur as subcircular gravel derived from riverbeds. Most of the dacite and some of the rhyolite and granite occur as angular rock masses.
Most of the andesite is gray to black; of these, 30.39% are inclusion rocks presumed to be basalt, and 4.7% contain conspicuous volcanic gas voids. In addition, 1.89% is reddish-purple in color owing to oxidation at high temperatures.
Of the dacites, 33.33% are characterized by thin, black stripes; 31.11% have no such stripes; and 35.56% some did not distinguish between them. Both lithologies are found in the dacite bodies distributed around the Mount Sakazukiyama, although the striped lithology is limited in distribution(Ohtomo, 2016). Most of the dacite is in the form of angular rock masses having the same lithology as the rock found in the stone walls of Sakanamachimukai Yagura and Ushitora Yagura. It is believed that the dacites remaining from the stone wall construction were used in the cornerstone foundation of the earthen wall.
The granites are gravels composed of 65.00% coarse-grained biotite granite, 32.50% medium-grained biotite granite, and 2.50%, fine-grained granite originating from the granite body distributed upstream of the Mamigasakigawa River(Ohtomo et al., 2019).
The gravels assumed to be composed of basalt contain large plagioclase phenocrysts approximately l cm in diameter and numerous phenocrysts. Because this lithology is characteristic of Ryuzan basalt, the gravels of this lithology were classified as basalt.
The tuff breccia and green tuff are from the Miocene Narisawa Formation, and the green andesite is from the Houzawa Formation. The tuff breccia contains granite fragments, and the voids of the green andesite are filled with agate.
The proportion of each of these rock species is almost constant throughout the northeastern earthworks of Ninomaru; no large bias was found among the districts. In District 7, which is adjacent to Ushitora Yagura, 11 dacites having the same lithologies as those present in the stone wall are distributed.
References
Yamagata City Board of Education, 2020, Survey report on buried cultural properties in Yamagata City, Yamagata Prefecture, 39, 14-110. *
Ohtomo Y., 2016, 123rd Annu. Meet. Geol. Soc. Japan, Abstr., 190.
Ohtomo Y., 2020, IV Survey results of stone walls and cobblestone of Sumiyagura in Ninomaru. 39, 111-115. *
Ohtomo Y., et al., 2019, Applied geol. Yamagata, no.39, 89-93.
*English translation from the original written in Japanese