Square Barrows (a square-shaped kofun [tumulus]) (方墳)
The tumuli constructed from the beginning and the early phase of the Kofun period are considered an extension of the Yamato-style square tombs with moat surrounding and the Izumo-style tombs covered with a heap of earth, whose four corners are protruding outward, both of which were built in the Yayoi period. Their characters are also taken to be nearly unchanged. However, they are now regarded as a kind of 'square barrows' as a result of the epoch-making appearance of keyhole-shaped tumuli and the Haji pottery. Thus, even a coined term, 'the early-phase clustered tumuli,' has been created. A wealth of techniques already developed in the Kofun period is used in the tombs in Yayoi period which are covered with a heap of earth, whose four corners are protruding outward. Hence, they show the techniques of ancient Izumo.
The representative examples of the early-phase square barrows include Onari-kofun Tumulus in Yasugi City, Shimane Prefecture, and Tsukuriyama-kofun Tumulus (whose sides are 60 meters long and that is in Shimane Prefecture). They are the larger than any other tumuli in the country. Other square barrows are also concentrated in Izumo. There are also prototypes of square front, square back mounds. Most square barrows are relatively small. However, they were adopted as the shape of the tumuli of the influential persons by the seventh century, in which the construction of keyhole-shaped tumuli was stopped. Examples include followings: Osaka Kasuga Mukaiyama-kofun Tumulus (with the side lengths of 63 and 60 meters) specified as the mausoleum of Emperor Yomei.amada Takatsuka-kofun Tumulus (with the side lengths of 63 and 56 meters) specified as the mausoleum of Empress Suiko, Ishibutai-kofun Tumulus (with a competing view that it is a dome-shaped mound on a square base) believed to be the tomb of SOGA no Umako, and Ryukakuji Iwaya-kofun Tumulus in Chiba Prefecture (with the side lengths of 80 meters and the height of 13 meters). The Izumo region in the eastern part of Shimane Prefecture is also known as a region where particularly a large number of square barrows were located.